Lombardi Echo Mixer containing Binson Echorec Magnetic Drum Recording System

[nevetslab]

While I'm nearly finished restoring this 45 year old Lombardi 8-Ch Input Mic Mixer, it having a Binson Echorec Magnetic Disc Delay System built in, I've discovered the source of the total lack of LF response on Ch 4, which was found prior to my pulling all the modules out to replace the 45 year old Ducati Electrolytic Capacitors throughout the mixer.

When I had the mic preamps removed, which was where I thought I'd find the problem, all eight preamps measured the same. When I finally re-installed them, I again found this lack of LF response on Ch 4. Clipping my Fluke 8060A DMM in RMS dB mode across the secondary of the input xfmr, and swept the response, I found it was the transformer itself having this fault.

The first item I found with regards to the transformers was this HF boost added in the preamp, but never found any de-emphasis network in any of the modules. I had been measuring near flat frequency response on the output. But after looking at the response of the input transformer in the HF range, I also found the transformers have a serious HF rolloff.

I just got collecting data on the Ch 4 input xfmr, as well as one of the other typical input xfmrs for comparison. Ch 4's HF response is also vastly different than the others. Mechanically, it looks the same, but measures way differently. I measured the input impedance of the xfmrs, using a 2 ohm source impedance, passing thru an ESI decade resistance box, with a Fluke DMM in AC RMS mode, dB mode, REL mode, and at each spot frequency, adjusted the output for -6dB, which would occur at the input impedance of the load....load being the input xfmr + the secondary load, which is more than 2 decades higher in resistance than the input impedance, so negligible.

What would account for this severe loss of LF response on Ch 4's Mic Input xfmr? I wondered if, due to the low overall system gain between mic input and mixer output being only 38dB, and plugging a typical dynamic mic into the mixer, such as a Shure SM57, I barely got any meter movement on the VU meter. I had to resort to plugging in a Symetrix SX-202 Mic Preamp ahead of the mixer, so I could give a listen with a typical condenser mic (SX-202 having Phantom Power). Could this mic xfmr have been severely overloaded by a similar set-up, and the problem not found right away, finally having such input level do damage to the transformer?

The lack of HF response on all of the transformers also is puzzling. I can't see any rational reason for selecting an input transformer for a professional mixer to have severe HF rolloff, unless that was a production mistake from the transformer mfgr, and the production fix was adding a HF shelving boost across the emitter resistors of Q1 input xfmr (C4 20nF across R7 2.2k + R6 47ohms).

I've attached the measurement data recorded today below:

LOMBARDI MIC #4 INPUT TRANSFORMER DATA.pdf

LOMBARDI MIC INPUT TRANSFORMER DATA.pdf

Lombardi Mic Preamp Schematic-3A.pdf

[nevetslab]

After finally receiving the TCH Road Case Hardware I ordered 12/07/21, I first had to seep wood glue into the broken corner and lid top joints that were damaged when this mixer, still inside the case, which had been taped shut took a fall in transit from Italy to LAX. That was when the client brought it to me, potentially seeking damages from the unit being dropped. Somehow, the only internal damage I had found in the mixer was at a harness connector at the back of the power supply chassis, and the AC Mains wire harness that wrapped around the corner where the Echo Drum motor assembly that spins was in contact with that harness, and it injected RF hash into the output. The rest was just mechanical damage to all the tiny latches on the road case, and separation of several wood joints around the road case itself. The mixer somehow wasn't damaged....though the Ch 3 fader might have sustained damage from that fall, as I had to pull it apart and repair the wiper to the slide actuator to restore it.

So, I spent a couple days prying open some of the wood joints to seep wood glue into them, then clamped those tight while I drove in #6-1/58" Drywall screws along those obvious joints. Then, went thru the four corners of the upper half of the case, laying in construction lines at the center of the 5/8" plywood used in the case fab, center-punching locations for the screws, and drove in screws for what must have been hours until I had everything now solid.

Next, I positioned the rear take-apart hinges at the back end of the case, scribing the outline of the hardware plates, and center-punching where the 1/4" holes would be drilled for pressing in #10--32 T-nuts from the inside of the case walls. Then, did the same with the dished latch plate assemblies. I had thought about using dished recessed latch plates, but not having a functional wood shop to tackle that amount of labor, I elected to use the inverted dished latch plates that protruded outwards from the case surfaces. Never had seen nor used them, but they were easier to install than just using latches and catches. So, got those positioned and holes center-punched using layout tools to get them all into place. Finally, clamping 2 x 4's on the insides of the case walls, I drilled the mtg holes for the T-nuts using a spur-point 1/4" drill bit, then pressed the T-nuts in with a C-clamp. Finally checked the overall fit with all the hardware to check that everything aligns, and fits properly with the mixer installed.

That done, I removed all the case hardware, and threaded in FHMS's into the T-nuts (to prevent paint from getting into the T-Nuts threads), and wheeled the case lids out to the parking lot to spray pain a couple coats of Gloss Black. Had to mask off the original handles, after finding the attempt to remove them was an issue with the hex nuts hidding inside the case walls, so had to mask them off. Did the same with the Lombardi Logo plate. Painted them yesterday around mid-day when it was as warm as it would get (mid-60's), and brought them back in around 4PM. Installed the hardware on the base, after putting it's large rubber feet on. Put the rest of the case hardware on this morning.

Did final check on the fitting, and thought I had done something wrong, as the lid wasn't closing, but found I had not properly positioned the mixer into the bottom cleats properly. After correcting that, I removed it to staple loose case wall felt pieces that I hadn't yet taken care of.

I thought I had taken photos of the damaged case corner joints, but found I didn't. You can see a little bit of the damage in the bottom half of the case, and also took photo of the tiny bent-up case hardware.....which for something that size is a total joke!



Now the restored case:



These next photos show some of the arrays of drywall screws I had to drive in to pull the cabinet back together. The entire lower case had all four corners and the perimeter of the bottom joints drilled in with those drywall screws.

[nevetslab]

This morning, having pondered the use of #8-15 Truss Head SMS for mounting the two spring-loaded handles on the sides of the lid above the latches, I got to thinking.. If anyone were to grab hold of those to try and lift the loaded cabinet by them, or use them to pull the case in the back of a carpeted van or SUV, those screws may not hold, they being like wood screws. Even though there's five of them. I checked to see if I had enough #10-32 T-nuts left, and did, so I removed those #8 SMS's, drilled out to 1/4" and pounded in the T-nuts. All of the cabinet hardware is using #10-32 Stainless Steel Truss Head screws & T-nuts. I really didn't want to use the pair of flat washers in that offset present on the handle mounting flange. I was sure I still had what was left over on a 12" x 12" sheet of neoprene Rubber, so looked thru the McMaster-Carr listing in my previous purchases, found I did purchase a sheet of that, found the date, looked up the service note, and found I had only ct out a couple large circles for power xfmr toroidal mtg pads. So that material was still here in the shop....someplace. Finally found the box, with it listed on the outside of the box.

Pulled up the pdf drawing of the handles, cut the material to the size of that offset, marked the hole locations, punched those thru with my Whitney punch, and then removed the hardware & wasners used for spacers, and slipped in the 3/32" thick neoprene rubber. Love it when the right material is used!