Lombardi Echo Mixer containing Binson Echorec Magnetic Drum Recording System

[nevetslab]

I'm intrigued enough with the Echorec system that I'd love to hear this working as it should, and not the overwhelming RF hash that dominates the audio. I have the unit opened up again, and removed the power supply chassis from the base, flipped it over while the two harness connectors are still attached. That forces the chassis cattywampus to allow the harness cables to remain attached and not stressed. Reading thru the Binson Echorec memory system document, which has numerous links to reveal loads of into, I see I need to get the disc cleaned & lubed, as well as the drive wheel rim, idler wheel cleaned. I can't yet bring myself to believe the hash noise is coming from lack of the fine film of oil (sewing machine oil is similar to that used by Binson), so I need to stop off at the local supply store to see if I can find some oil as well as a degreaser based on carbon tetrachloride as recommended by Binson.

I've found the interface wiring between the playback heads and the wiring passes thru what I assume is some rubber bushings, and connects to what looks like some optical interface. The back side of these bushings that the head wires feed thru couples to a machined aluminum block that have four of the five cavity holes drilled out, and are firmly mounted. But, the entire aluminum block is loose. I only see a pair of mounting screws/nuts with loctite compound on the threads on the top side of the Echo plate.



I'll have to get some additional close-up photos to show the details of this playback head interface. Look down the hole in the aluminum block that's open. Is this some sort of optical coupling, with the other four parts mounted into the black plastic housing photo transistors?

I don't yet have approval to purchase the replacement power supply electrolytics. There's also a couple transistors that appear to be part of the supply regulators that look wrong. All old parts, so one substitution wasn't made correctly.



The oscillation problem I found, I have to wonder if that's from all of the non-shielded wire between the preamp wiring, then down to stand-alone amplifier/buffer stage PCB's in the power supply PCB, then back up to the three master volume pots below the meters, then back down to the output amps just above the AC Mains connectors.



At present, I'm still waiting for repair work to arrive from CenterStaging, since not being in the same building where all of it is located, this 'waiting game' held up by personnel and truck for delivery is a real PITA! Didn't have that problem when I was in the building with all of it.

[J M Fahey]

The thin oil film is *essential* for operation.
The heads stay at proper distance from the magnetic media by "surfing" on it.

If drum is not rotating heads touch it, but at speed they dynamicaly "float".

The GENIUS Binson idea was to avoid tape which wears fast and must be replaced regularly by winding very very fine magnetic wire around non magnetic aluminum.

[drewl]

Yeah, there's a bottle of oil with the vintage tube one I got in.
they sound sooo much better than tape echoes when working properly.

[nevetslab]

I find it so tiring not being able to find low SAE oil, such as sewing machine oil or degreaser fluid based on carbon tetrachloride, being able to walk in off the street and pay for it without having to order from two separate locations on the internet, and get hit with UPS shpg charges! Corporate America specifies WHAT Mr and Mrs America can purchase in stores. sigh.............

[nevetslab]

Further digging after another fruitless stop at a hardware store, I found Michaels Arts and Crafts carried Singer Sewing Machine oil, and was able to buy a bottle of it for $5.50 on the way home. And, I had forgotten having a full can of Freon in my stash, so I'll sparingly use that in cleaning the magnetic disc / heads when I get back to the shop in the morning.

I also found what was causing the 'RF Hash' when the Echo System was turned on. The wire harness passing around the corner in the power supply chassis had come free from the adhesive-backed tie-wrap hold-down blocks, and was in contact with the rotating large appendage of the capstan motor. I had placed new hold-down blocks on Monday, and yesterday, assessing where the noise sources were, I forced that harness back into contact with the motor, and that returned the RF Hash. So, that was a simple fix. The Preamp boards don't appear to be a major source in the residual noise at present. It's all in the System Master circuitry, and the power supply filters. The noise level didn't change at all when I turned all the preamp channels off or on, with faders at max. Of course, turning up the HF EQ yielded oscillation, so that issue has to be sorted out. Turning up and down the Master Volumes did also change the noise level.

When I get the go-ahead, I'll order all the replacement caps, replace the main filter caps and all the electrolytics in the power supply/amplifier boards in the power supply chassis and then assess what we have. I've found the shielded wiring has fragile spiral wrap shields, which break easily. Replacing caps on all the small boards in that chassis, I'll be dealing with that wiring, so hopefully that doesn't turn into having to replace that wiring. I do have new Mogami cable that size, if needed.

I'll have to punch new mounting holes for the snap-in 2200uF/100V Electrolytics, which will require mounting brackets. Thankfully, there's already 1/4" holes at the centers of the 40mm dia Ducati Electrolytics that are being removed, so punching thru the Greenlee chassis punches will be easy.

I won't start into the preamp surgery until I see that I have made major differences in the residual noise there in the power supply chassis.

What I had thought might have been Beyer input transformers.....what's in this chassis are much larger, though also using a threaded bushing/lock nut to mount the input transformers. I thought I saw heat marks on one of the xfmr cases....Plastic?? Haven't gone back to look. I'd be VERY surprised if the case of the xfmrs were plastic.

I'm also hearing cyclic noise when I have the Echo system turned on. That suggests there's a dent in the rubber idler wheel, which transfers the capstan rotation to the magnetic disc. Binson warned about leaving the idler wheel in contact with the capstan if the mixer is to be standing unused for long periods of time...removing the tension spring. I hope that's NOT what I'm hearing. That is one of many components in the system that ae not replaceable.

Onward....

[nevetslab]

I got the go-ahead to order parts, which I did Saturday morning, so tomorrow, I should receive the parts needed. After ordering them, it dawned on me I hadn't looked at the mounting hardware for all of the discrete PCB's in the power supply chassis. The thought if having to juggle screw heads on one side of the chassis, while trying to unthread the nuts, all with thread-lock compound on the joints and cured for the past 46 years, made me think of male-female stud-standoffs. I looked close at the photos I took & posted to see if I could get any clues to whether they just used long screws, spacers and nuts with thread-lock on the top side of the PCB's. I then saw a hex nut at the floor of the chassis, so they at least locked the screws into place, doing what a male-female stud-standoff would do. Assuming the top nut on the PCB doesn't make the entire assembly rotate when trying to unthread the nut.

The shielded wire they used, I found to be fragile, and have already had some break in moving the connectors. So, this project will resume later this week.

[nevetslab]

I received the electrolytic caps for recapping this unit. I removed the power supply/Echo Unit chassis from the mixer, and began by unsoldering the three 2200uF/100V Ducati Buss Caps from the chassis. That took using my 100W American Beauty to unsolder the folded cap can flaps, then unsoldered the three BLU supply wires from the buss caps. I had to remove the regulator PCB up in the corner behind the buss caps, as I had enlarge the 1/4" holes from each buss cap to 3/8", allowing punching 7/8" dia holes thru the chassis with a Greenlee Punch. After deburring those holes, I basically bomb-sighted the mounting clamps for the three 35mm dia Snap-In 2200uF/100V Buss caps, then marked up the chassis using an adjustable machinist square, scribe and calipers, replicating the pattern I bomb-sighted offset by 43mm, the spacing set by Lombardi. Punched three of those holes with a Whitney Punch, but had to drill the holes closest to the chassis floor, being too close for the Whitney punch to fit in. Mounted the new caps.



Then, unsoldered the wires from the bridge rectifier, formed copper buss wire to span the three buss caps, soldered that into place, adding the middle ground terminal to the buss. Installed a Chassis Ground terminal to tie the buss caps to. There was a Static Shield wire from the power xfmr that had been connected to the (-) terminal of the bridge rectifier. Then a long 22AWG wire connected from there to the mounting terminal serving as Chassis Ground where two of the three buss caps' (+) terminals tied onto those insulated terminals to. There was a heavy Ground Wire on the first amplifier PCB having BC301 NPN TO-5 Xstrs on it. I began swapping caps on that board while having removed the clumsy ground wiring I found in the chassis.

I haven't yet measured the supply voltages, but I see one power supply regulator board...that behind the buss caps, whose pass xstr is the heat-sink mounted 2N6259 NPN Pwr Xstr. The output from the regulator connects to the left buss cap (marked Y-BLU inside on the chassis wall). That BLU wire leaving that terminal makes it's way over to the Solenoid on the Echo chassis. The other side of that Solenoid connects to the RED supply wires of all the small single-stage preamp boards packed into this chassis, as well as feeds a control board on the Echo chassis, as well as feeds the preamp DC Supply connections thru the 20-pin I/O connector via WHT wire.



That RED wire from the Solenoid feeds to the cascaded (+) terminals of each of the small single-stage PCB's thruout the chassis, along with BLK wires for Ground.

The Output XLR connectors installed were 4-pin Male's, with insulated Cliff TRS phone jacks wired in parallel with them. I replaced those with 3-Pin Switchcraft D3M connectors, and cleaned up the wiring on that panel. Took that GRN/YEL Chassis Ground wire, tied the IEC-320 and 220V Ground terminals together and attached them to a dedicated Chassis Grounding Screw I installed between the XLR's and the 220V connector, removing the path below in the wiring I found.



From this point, it was carefully removing each of the small PCB's to replace the electrolytic caps. Plus there was the one board that had the BD601 Power Xstrs that feeds the output connectors. One xstr had been replaced with a BC550 TO-92 xstrs. I replaced it with a TIP31C, it being close to the specs of the BD601. Those two boards also feed a separate board that I assume is the Meter Driver board (yet to be confirmed, but an educated guess), outputs from it thru 8.2k resistors feeding unshielded wires to the 20-pin I/O connector.



I'll connect this back up to the Mixer, after I first try and figure out how the two power switches are wired up, as well as ring out supply connections between this chassis and the mixer to see just how this is all interconnected. I still haven't found any information on what appears to be an optical pickup head interface from the magnetic drum. I also still have to clean the magnetic drum and heads. I see the motor is rated 220V/50Hz. I seem to recall some discussion about the AC Mains frequency setting the delay between the heads in the Echo System.

The single-stage 3-Ch Amplifier board sitting between the power xfmr, and the Echo Chassis, just behind the 20-pin I/O connector...there's only five 10uF/50V coupling caps on it. All the others have six. One shielded cable passes thru a grommet into the Echo Chassis. I"m unsure what that board is in the system.

Then, I'll power it up, get some supply voltage measurements, and give a listen to the residual noise in the mixer, now that all but the preamps of the mixer have been re-capped. I found previously the preamps didn't appear to be contributing to the overall system noise. The three Master Volume controls do, of course, affect that, but with the masters turned up, turning on or off the preamps, as well as turning down the faders didn't change the noise floor. Hoping what I've done here has made significant changes so the changes yet to be made on the preamp boards will likewise improve matters.

Onward.....

[nevetslab]

This morning, I went hunting for an Adhesive-backed U-shaped Cable Clamp to hold down the wire harness bundle that wraps around the corner where the Capstan Motor is positioned. The one tiny cable tie hold-down block Lombardi used had come off, and with the chassis in normal position, that harness bundle was in contact with the rotating motor assembly, and had found that was the source of the RF hash noise that was rendering the Echo system useless.

I only found one mfgr listed in Mouser's listing of Wire & Cable Management parts, and it was non-stocked, 48-wk lead time. Same with Digi-Key and Newark. I googled Adhesive-backed U-clamps, found the mfgr that Mouser had, found their range of parts but when I got to the order page, found those were 1000 pc min for around $430. I thought about using a Keystone steel U-shaped clamp and punching mtg hole thru the chassis, but....needing a couple of them, the motor assy is too close to let me use the Whittney Punch. I tried ebay, and lucked out...found several vendors that had a range of usable parts, with adhesive backing. Bought a pkg of 25 pcs for $10, local California source, so I'll see those in about a week.

[nevetslab]

The adhesive-backed cable clamps that have an adjustable diameter clamp to lock a cable form down arrived, and I installed those to the AC mains harness that pass around the Capstan motor assembly in the chassis, so that keeps it away from the motor, where contact with it yielded RF hash. I finally got the Mixer back out of the shipping container and back up onto the bench, and connected the two harness connectors, so I could finally check the DC levels and regulation. Unregulated DCV at 120VAC input is +82.9VDC. Output from the regulator is +51.5VDC. And taking the drop thru the solenoid of the Echo System capstan drive, which is then the Preamp DC Supply voltages for the system....+41.4VDC. I can drop the AC Mains down to 80VAC before the supply falls out of regulation.

Back to 120VAC, I didn't take noise readings previously (that I can recall), and should have, so I'd have more to go on than recollect, but the residual hum/noise level after re-capping and revising the wiring what's in the power supply chassis has made significant improvements. Now, apart from not hearing Echo, turning that system on and off no longer has any obvious change in the noise level, where signal level on the meters are at 0VU, and output level is 0dBm (0.775V). Feeding burst pink noise, the dead time between 2.5 sec bursts is also 2.5 sec, and it sounds dead quiet. It's not, of course, as I can turn up the Amber 3501a Audio Analyzer 40-50dB, and you can then hear residual noise and what hum there is. Hum is down -60dB relative to 0dBu signal level. Not great, but way better than it was. I still have the original electrolytic caps in the preamp cards, so I think I can move on to tackle that task. There are several tone pots that need cleaning, LF EQ on one channel not working, low gain on another, the usual stuff one finds with a near-50 year old mixer.

Oh.....while I had it powered up, and the power supply chassis was still open to me, switching on the Echo Playback heads, each switch turns on a lamp in that aluminum block assembly, which has the pickup wiring feeding into. So, this head pickup system appears to have an optical path. I haven't yet identified individual head pickup preamps, though there's one preamp card that seems to be dedicated to the Echo System...sits between the transformer and the Echo System in the power supply chassis, behind the I/O cable connector.

Onward

[nevetslab]

Finally getting a break from the onslaught of repairs, and still pondering just how I'd deal with the power supply distribution busses (V+ & GND) that presently pass thru each board from one end to the other, making maintenance on a given Preamp module a total PITA. After disconnecting the start of the two buss lines, at the Ch 8 end (left side viewed from the insides), I found after sliding two insulator sleeves back onto the feed wires of the wiring harness bundle, they used 20AWG solid buss wire, with similar sleeves between each board. I unsoldered the start, then at each module, unsoldered the buss wire connections, and pulled it thru until I had them removed from the seven modules, leaving the buss wires connected to the last module.



With the buss wires pulled thru, removal was fairly simple. Each set of rotary pots has a ground buss wire soldered to the case of each pot (even though the pots are mounted to a grounded chassis), so unsoldered those pot connections to the Bass and Treble pot, leaving the buss wire in place, as it passes from the Echo Send Pot at the top down to the case of the Slide pot below. Then, the shielded wire from the input transformer at the top, and the shielded wire carrying the output from the preamp to the slide pot gets unsoldered from the top of the pot and case ground. That signal also ties to the Mute Switch for each channel. The Wiper of the slide pot feeds each of the Echo Send Pots, as well as the mix buss. While there are three Meters on this mixer, there aren't three mix busses. Mono mixer, with three metered channels. Kind of odd. And, due to the vintage, no Phantom Power buss for condenser mics.



So I removed the Bass and Treble pots, then, one by one, fumbled with a 6mm open end wrench while reaching in with a short slotted screw driver to first hold the M3 x 6mm Cheese Head screw from turning until I got the hex nut loosened. Once the first two boards were removed....first one being the pain, having to instead reach in with bent nose pliers to hold the screws from turning, I was just able to finish unthreading the nuts with my fingers. The last board I left the power supply buss wires attached, as there's no access for hands or soldering iron at that end with the inside wall and another smaller harness running up the wall. I now had a pile of boards on the bench.





Next tasks were to replace the aged Ducati electrolytic caps with Nichicon electrolytic caps. Then, pull the tone control pots apart to clean/lube and reassemble. The pots had a punched aluminum spacer inserted onto the threaded bushing, and no flat washer nor lock washer under the large 14mm brass hex nut. I found after prying the spacers off, the bushings were NOT 3/8"/32 but 10mm threaded bushings. My large ITL washers fit on ok, but I had to hand-select flat washers to fit under the brass hex nut when I reassembled the boards/pots back onto the panel.



After getting thru with the boards and pot cleaning, and finding a couple resistors that had broken, along with 220k resistors tacked across the back side of the PCB to load/discharge the input coupling cap from the input transformer that also broke during unsoldering the old caps, soldering in the new ones, I came up with a solution for installing the power supply buss wires that would allow using the same busses, while still allowing removal of the preamp boards. I removed the long 0.025" square wire-wrap pins from some 12-conductor in-line sockets I had, cut off the socket contacts, folded that end over enough to pass thru the PCB, then folded the protruding side over and clamped it down to lock the stiff square pin in place, and soldered them into place. Then formed a hook with round nose pliers to cradle the power supply buss wires when I reassemble them.

I think I need to attach the PCB mounting screw to the PCB (not having any Pem Stud inserts for PCB's on hand), and just deal with threading on #4-40 Keps Nuts, and not have to deal with the loose screw and nut where there's just no hand access. Bad enough to fumble with hex nuts that far in where a child's hand could barely fit in. I'll have to fumble about with that soon enough.

Next task is to power these boards up and see where the problems are on two of them. One had lower gain than the rest, the other had no LF EQ. I left that 4.7uF tall Tantalum Capacitor in place (believe that's what it is). The circuit looks simple enough to cobble a schematic together after a little effort, then draw it up on one of my PCB Layout Program's Schematic Capture routine. More to come.

[nevetslab]

I got the Preamps powered up, one at a time. The DC Supply voltage in the mixer chassis supplies 41.4VDC to all of the amplifier modules, so I dialed that up on one of my power supplies to check each of the modules. When they were all installed in the mixer, I had one module that was low in gain, and another that had no LF EQ response on the Bass Control. The rest worked. After re-capping the modules, replacing some resistors, finding some having broken physically, they now all have the same relative gain. I didn't try to couple the mixer's input transformer to the input of the module, but just fed it from my transformer-coupled step attenuator, driven from the Amber 3501a Audio Analyzer, and connected the output wire that normally feeds the 100k slide pot to the 100k input terminals of the analyzer, parked the oscillator at 1kHz, adjusted the output for 1V output, finding 25dB gain nominal (not sure what the step-up is thru the mic input xfmr yet.

I then drew up the schematic, got DC voltages at all the essential circuit nodes, then this morning loaded it into Multisim 11 to have something to load into my files, and post here.

Lombardi Mic Preamp Schematic-2.pdf

When I looked at the relative frequency response, re 1kHz, I found 20khz was up 12dB. So, there's a HF weighting to this section of the mixer. I haven't drawn up the schematics for any of the single stage amplifier boards in the power supply, but I'd guess there is a reciprocal curve being applied someplace before the output. I don't recall there being this HF boost from injecting signal into any of the mic inputs and looking at/listening to the output from any of the three channels. I wonder if that HF boost, which is applied to the Echo Send controls to that Binson Echorec Magnetic Disc Echo System, is using that HF boost curve for improving the S/N of the Echo system.

At least I found all eight mic preamps responding the same, same gain. So, that was reassuring. There's a 4.7uF Dip Tantalum cap on the mic preamp...strange looking color-coded part. I removed one, and checked it on my GenRad Digi-Bridge and it measured fine, but I haven't checked it on the network analyzer. I didn't replace those.

So, forward progress being made.

I briefly looked at the photos of the individual single-stage amplifier boards mounted in the Power Supply chassis, and don't see evidence of any film caps that would be used for a de-emphasis curve to flatten the HF response curve. So, there may be a network parked up in the section where the meters live.

[nevetslab]

After coming up with the decision to install #4-40 x 3/8" PHMS onto the PCB's, locking them in place with tiny SS flat washers and 3/16" thin hex nuts, and tweaking the spacing of them in the slightly elongated holes, aligning them with the mounting holes of the mixer chassis, I installed them yesterday afternoon, slipping them into place and fumbling with my fingers to start the 1/4" mating Keps Nuts on the back side of the mounting brackets, tightening them up with a tiny 1/4" ignition wrench. Getting the first one mounted, which had no space for fingers between the PCB and the cabinet wall was no fun, but eventually succeeded. Originally, the PCB component side board was mounted against the mounting bracket. I found in looking at the mechanical fit with the original mounting method, there were components on the Ground side (of the solder pads) making contact with the mounting bracket, so I wanted to elevate the board and move it to the other side of the bracket to prevent that, defining how components get grounded...that being in the PCB layout and NOT random contact with the chassis.



So, I got them mounted, and ready for the restoration of the input and output wiring, as well as re-installing the power supply buss wires. Chopped off the ends of the input cables to start with fresh wire and solder, so prep'd those ends. Verified the color code, as Lombardi had used dual core hifi spiral wrap shielded cables between the input xfmr secondary and the input to the preamp board. Tinned the fresh shield and core wires, then installed those at the top of the PCB's.



Getting the EQ Pot's ground buss wires re-soldered to the body of the pots was a nightmare. I had already wire-brushed the surface of the pots when I had the pots apart, but didn't think to pre-solder the pot covers when I had the pots apart. Big mistake. Thinking I'd get the wires resoldered to the cases using hot iron (ended up using 900 deg F with 3/16" chiesl tip) was an exercise in futility, with hot solder blobs rolling off the cases and landing on the fuse-style lamps below!! Colorful language was clearly heard coming out of the shop at this point. Never understood why the ground buss was used, since all of the pots are mounted to a fully grounded/shielded case. Did the best I could, which looking back at earlier photos prior to disassembly, didn't look any better than what the factory achieved.

I had to do clean-up work on the shielded wires that landed at the top of the slide faders, using the case to ground that end of the cable, and during removal of the preamp output cables, I had to heat up those solder joints, so a couple of the cables that fed the Echo Send pots' shields came up in the process. Got those back into place before going thru and re-soldering the preamp board's output cables to the slide pots. And finally, I got to see how the change in routing the power buss wires worked out, using formed stiff hooks soldered into place at the bottom edge of the preamp boards. That worked out quite nicely.....easy to install the wires, and made removal of the modules possible.





So, next task when I get in tomorrow is to power this mixer back up. I still have to clean the magnetic disc & heads, now that I have the fine machine oil and a full can of Freon for cleaning the surface of the magnetic drum.

I never did find any de-emphasis network that I'd expect to be used to counter the HF boost seen on the 1st stage of the Mic Preamp. Onward.

[nevetslab]

I first installed all of the control knobs to the EQ and Echo Send pots so the mixer was ready to use.

With my DMM connected across the DC Power Supply buss of the Preamps, I powered this up this morning to see if everything works and if I made improvements from where it was. In a word, YES. Hum is gone (haven't connected it to my spectrum analyzer to really show the dirt). I connected my B & K 1027 Sine/Random Generator to the input, thru an HP 353A Step Attenuator/Xfmr box, set the Frequency to 1kHz with 1V output to it, dialed that down 30dB to apply to the mic inputs, connected the Output from Ch 1 Output (three separate outputs, metered, separate Master Level controls), and set a nominal 0 VU on the mixer's meter, output on the Amber 3501a Audio Analyzer, with scope monitoring the analyzer, and used the Relative trim on the analyzer to set 0dBV on it.

I started with moving the input cable to each channel, running the slide fader all the way up, EQ Flat, Echo System turned off, and verified I had signal appearing at the same level on all of the inputs. I had no signal on one of the channels, but found on Ch 5 & 6, there are slide switches on the rear panel marked BASSA in the up position, ALTA in the down position, with IMPED stated between the two positions. It seemed to work like a Channel On/Off switch, so left it in BASSA, and had signal again. I got thru all eight channels, having the same relative gain.

Checked the other two Outputs, finding when I feed all three Output Meters, it loads down that output signal some.

Next, going back to Ch 1 input, I manually swept the generator up from 1khz, finding the output was down 3dB at 15.3kHz, then swept down to 200Hz, finding it down 3dB, and as I went lower, seeing some rise and fall, found -3dB again at 93Hz, and again at 31Hz.

I did the same with the rest of the channels, and when I got to Ch 4, I found the LF rolled off as it had before. I had already swept the preamp cards free-standing, noting there was a 12dB boost at 10kHz. I wasn't seeing that with the EQ controls set flat, so I connected my Fluke DMM across the secondary of the input xfmr, RMS/dB Rel mode at 1khz, and swept up to 20khz, and found the reason for that pre-emphasis HF curve. The input xfmr rolls off HF, finding it was down 15dB @ 20kHz. It compensates for this HF rolloff of the input xfmrs! Then, swept back to 1khz, where I had 0dB, and swept lower. I found the LF response of Ch 4's input xfmr 13.3dB down at 100Hz, 21.3dB down at 50hz, and 30dB down at 25Hz.

None of the other xfmrs had that problem, so I have a bad input xfmr (or something else between it and the input to the preamp).

I went on to the rest of the channels, and then after stopping at Ch 8, again finding what appeared to be a flaky input connector, it was the mic cable connector causing issues, so swapped cables and made that go away. Odd I didn't find that on the other inputs. With 0dB set on the Analyzer at 1khz, I then changed to 10khz to see what the boost/cut was of the Shelving EQ. +8dB and -6dB. Looked at 15khz, got the same thing. Then went to 50Hz, finding +9dB and -15.5dB with the LF control.

I grabbed my Shure SM57 mic, plugged it in and dialed it up. With the channel fader at max, and Output Master at max, I was barely getting any response on the VU meter, so I disconnected the mic, and set up the instrumentation to measure the overall gain of the system, with Channel Fader at Max, Output Master at Max. Adjusted the input signal to show 0dB on the mixer's meter, then set a relative 0dB level on my analyzer. From there, I repatched the analyzer to read the source level at the output of the HP 353A. 38dB down, so overall gain input to output is 38dB. That would explain why I couldn't get any reading on the VU meter with that mic. Mic works fine.

I then turned on the Binson Echorec system, and played with it a bit using my voice and the Shure SM57 mic. With low overall gain, the system seemed a bit noisy, so I'd say an additional mic preamp would be needed. And, with no Phantom Power, you can't plug your favorite condenser mic into the system (without adding Phantom power).

Still, the overall sound of the system is a whole lot better than what I heard to begin with. I still have to clean the magnetic drum & heads, add the lubrication to the drum.

I'll have to open up the rear panel to look again at input xfmrs. I had it open earlier this morning to fix a broken solder joint on the Male XLR Output, which is fed thru a xfmr and a level pot. I didn't get any output from that earlier, but had before I pulled the preamps out. I know all of the preamps worked the same, so at least I've found the LF problem on Ch 4. What to do about a 'bad' input xfmr....dunno. Move that xfmr to Ch 8, or just not use Ch 4? More digging to do.

I do have a Symetrix SX-202 Mic Preamp on my bench, so I haven't yet plugged the SM57, or any condenser mics into it, and out of it to the input to the mixer to see what it sounds like with a hotter mic signal.

Onward...........

[nevetslab]

Today I went to try and run frequency response simulations of the Preamp PCB circuit, and have found I believe a mistake in how the 220nF C6 capacitor is shown connected in the schematic I cobbled and then loaded into National Instruments' Multisim 11. I have the preamps all mounted again in the chassis, but close inspection of the photos I took and published here, one side of C6 appears to be connected between the bottom side of VR1 / YEL wire and R9. The other side may be between VR1's wiper and R10 6.8k that goes between the two wipers of the LF and HF eq pots. Running the frequency response plots in the simulator thus far isn't yielding the usual HF shelving response I would expect, so.....apart from the present placement in the schematic I published, that isn't yielding a HF curve either.

I did get some curves on the response with and without the HF boost (that having C4 across the emitter resistors of Q1), which are there to compensate for the input transformer's poor HF response. I can hear HF variation with the HF EQ pot, which sounds normal, so my schematic must be wrong.

I checked the connections on the preamp PCB's, and found the error. C6 220nF DOES connect between the bottom of the LF EQ pot (Yel wire) and to the wiper (BLU wire). I've updated the schematic here:

Lombardi Mic Preamp Schematic-3A.pdf

LOMBARDI MIC PREAMP FREQ RESP-1.pdf

LOMBARDI MIC PREAMP FREQ RESP-2.pdf

[nevetslab]

Having corrected the Preamp schematic, I spent a little more time with that circuit in National Instrument's Multisim 11 to see if I could get typical HF Shelving EQ plots on the circuit. I wasn't able, though got response plots on the LF EQ pot settings. I know in the past, I had been successful in creating multiple plots on the same graph, and sort of found how to do that, but each time, the software reverted to 0 to -150dB scale, instead of where I had set the upper/lower limits based on the graph it gave me. I finally gave up, as time was fleeting.

I still had Ch 4's Mic Input Xfmr with no LF response. The lead wires on these input xfmrs are extremely tiny, made me wonder if they were Litz wire.



There was evidence of the phenolic terminal strip having broken and was glued back together in the past. But, that wouldn't cause a LF loss. With only having 38dB overall Gain from Mic input to Line Output, and with a Shure SM57 plugged in, all at max gain, and barely getting any movement on the meter....well below the -20dB mark on the VU meter, it makes me wonder if somebody in the past plugged in a preamp and overloaded the channel with LF to where the xfmr crapped out. It passes signal, but, with HF rolloff that they all have, plus this lack of LF response, that transformer isn't anything like the others.

I also have to wonder why would Lombardi select an input xfmr with no flat HF response? On the preamp PCB, there ARE solder pads for C4 which is the HF Boost circuit to compensate for the HF rolloff of the input xfmr. Was there a major mistake made by the transformer mfgr, and the only way out was to alter the preamp to compensate for that? The 'fix' would certainly add NOISE. If I have to replace that input xfmr, anything I select would have flat HF response, so I'd have to remove C4 from the PCB. And, of course, have the mounting issue to deal with, unless I were to find one with a threaded bushing as is used here.

I moved my Symetrix SX-204 Mic Preamp over to the mixer, fetched a TRS to Male XLR cable from the rack and patched it in. I have an AKG C3000B shop mic here so plugged it in and dialed up the system to give a listen, NOW having 0VU meter response. All sounds very nice. Powered up the Echo System, and dialed in a dual delay mix with their controls to add a bit of that effect. Nice sounding mixer/Echo System. I still have to do the maintenance on the Magnetic Disc & heads, so I'll tackle that in the morning.

Heading home to watch the LA Dodgers flounder, now that Max Scherzer's arm gave out and can't pitch today.....probably going to end up being an Atlanta Braves/Houston Astros World Series.