Vox Beatle Super Reverb V1143 Restoration Project

[nevetslab]

Instead of re-using that nasty galvanized steel plate with the two XLR connectors on it, I scrapped that and drove down to Industrial Metal Supply a few miles away and brought back a small sheet of 6061-T6 0.062" thick aluminum, and cut a small piece out to make a new plate to fill that opening. I mounted the IEC-320 AC mains connector on that plate, and next to it a Switchcraft B3MG 3-pin Gold pin output connector. This simplifies the AC mains wiring, bypassing that screwed-down cable clamp on the floor in that right rear corner.



While I was removing the short XLR output pigtail with it's in-line connector, I found on the preamp end of the harness connector one of the output wires' terminals was buggered, the result having left a burn mark in the nylon. I was able to extract that pin, and restore it's original shape and clean up the residue before re-installing the terminal. Minor tweaking on the XLR pigtail connector pins, and now that wiring is restored. Today, I'm installing some insulated turret terminals on the preamp rear wall to install the AC coupling cap, followed by a 20dB resistor pad with 100 ohm source that will feed that added 1/4" insulated connector on the control panel. It had been wired with a 22k source/10k shunt from the output connector....first feeding that Switchcraft D3M connector next to the Cannon 3-pin male output, with a kluged bunch of wires that fed that output jack on the control panel.

I followed RG's suggestion of adding an output fuse holder/fuse in the output line. I installed a fuse holder on the circuit board in the power amp, where the original discrete diode bridge was installed. While I was back in that chassis, I installed the 150VAC MOV across the input to the power xfmr primary.



I probably won't see the AC Mains barrier strip (along with some other parts ordered) until Monday, so installing the attenuator network on the preamp chassis, then wire up the 6-Pin DIN male cable and build the 5-switch pedal substitution box that will allow turning on/off the Distortion, Repeat Percussion and Midrange Boost circuits, along with muting/unmuting the Reverb and Vibrato functions. So, as the project unfolds towards completion, lots of little steps. I still haven't even turned this amp on, though it did have a note on it stating 'HUMS', so I have to assume it was working to some extent. I should have listened to it before doing all this work, just to have a frame of reference!

[nevetslab]

The Mouser parts order arrived this morning, so that provided the 3-terminal barrier strip and some new cable clamps to work with. I took the preamp top cover and after seeing the flanges fit on the inside of the chassis, I scribed lines indicated where the cover is, and proceeded to install the barrier strip, then the single insulated terminal posts to which I mounted the 20dB line output attenuator. After making the calculations of how much current could be flowing thru a 111 ohm load, I had to dig thru my power resistors, finding 100 ohm 5W and 10 ohm 2W, so the attenuation is closer to 22dB than 20dB. Close enough. Punched all those mounting holes, then mounted the parts and wired up the attenuator first. Fed from the rotary switch terminal that feeds the output connector.



I didn't use the third position on the terminal block, opting instead to take that AC Mains cord ground wire directly to chassis ground, underneath the 1/4" fast-on terminal that receives the chassis ground wire from the power amp chassis. The other end of the AC mains cable is fitted with 1/4" insulated quick-disconnect terminals to plug onto the back side of the IEC connector. After I got all of the decades-old adhesive tape used to bundle wires on the reverb cables, as well as the preamp/power amp harness cables, I don't know which of the two cables (thick or thin) for the Reverb Tank is which. At present, both appear to be the same length, though should be harnessed so that it's obvious which goes where. From R.G.'s book, it appears the Send cable is the thinner one, with the Return cable being the shielded cable (thicker one). I'll have a look at the circuit board diagram as well to verify that.



I still have a small chassis box to wire up, it containing five toggle switches and an LED/current limiting resistor to test the foot pedal functions. I thought I was going to punch the cover panel of the box, then wire it up today, expecting the Mouser order coming in tomorrow. So, I'm now close to being able to put the two chassis' together and see if this powers up.

[nevetslab]

In reviewing the 9-pin connector wiring, BRN/WHT wire is the output wire from the preamp side connector harness. That wire color made it's way thru the power switch Standby function, and on out to the output connector. But, there wasn't a BRN/WHT wire at the output node on the circuit board...only a GRY wire, which I had selected to place the Output Fuse Holder in series with. Thinking I would find a GRY wire at the power amp's 9-pin connector, I found a BRN/WHT wire there. Where is it's origin? Now, hunting thru the chassis and the transistor sockets, I found a BRN/WHT wire off the emitter lead of Q306, and the other end of it on R317. That can't be it, but, that's the right color. I then found the source of the connector's output wire soldered together with a RED wire at Q306 collector. Other end of that RED wire soldered to Q304 Collector, where the end of that GRY wire I had placed the fuse holder in series with.

I guess I’m used to having output current flowing from the output node of a solid state power amp circuit rather than from a branch node at one end, where that could be the nearest wire length to the connector.

So, I unsoldered the BRN/WHT wire from Q306 emitter, cut the BRN/WHT wire from the collector of Q306, and to unravel the wires, unsoldered the base wire from Q306, lifting that heat sink assy from the chassis. I joined the Output BRN/WHT wire with that from Q306 emitter, as there’s enough wire length to reach the fuse holder. I’ll move that GRY wire at the fuse holder back to the output node, so it’s connected to the collectors of Q304 & Q306. Then, once the BRN/WHT wire is on the fuse holder, I’ll send another replacement wire between Q306 emitter terminal and R317 where the BRN/WHT wire was attached.

I guess a lot can happen in 55 years since this amp left the factory. Or, maybe, since everything did match the schematic, it all worked and don’t bother with details. The amp did, after all, work.

[nevetslab]

OK, got that sorted out and restored. Now the fuse IS in the path from the Output node to the Amp Output Connector, by way of the S/B Switch.





If it wasn't for how fragile these wire posts are that are pressed into the phenolic circuit board, I would have unsoldered the cluster of wires and three resistor leads on that output node. That's where R310, R314 & R316 join, with a BRN wire and the GRY wire at the base of the pin. I think it's phenolic, as when I bored the 1/2" dia hole with a brad-point drill bit, getting thru the other side, as the edges of the brad point scribes the hole on the way thru, it broke into a couple pieces. Not used to seeing fiberglass do that. I really dislike having those terminal posts get unstable while making wiring changes! As such, I ended up having to add more solder in adding those two GRY wires to the output node. Sigh.........

[nevetslab]

When I had posted images during the removal and replacement of all those black Temple plastic-encased electrolytic caps in the Preamp, the page hickup'd on me and froze, disallowing going forward. I had to exit the internet connection, then got back on, logged back in, and went to start over, but got that option to restore what I had done, and took that option. I had three more photos to add, but, while there were the photos I had posted present, the 'Attached Files' section was missing. Days later, all those photos vanished, except for the three I had added. The text remained in place.

I also forgot to post the images down on the floor of the preamp 'basement' where there are eight power supply filter caps parked down there.



The following images is after re-populating the preamp board with new electrolytic caps:



Then, finally removal and replacement of the power supply caps on the floor of the preamp 'basement':



After I had installed those power supply caps, I resumed cleaning the pots on the rear panel....those just forward of these supply caps. While I was able to remove the bushing assembly and the rotor of the first deck of the dual gang CTS Speed Control, the mechanics of the housing prevents going any further, so I had to resort to brushing in Caig DeOxit into the opening of the second deck of the pot. Removal, cleaning and re-installing the remaining 3 pots was straight-forward, apart from working in very cramped corners and occassionally burning my fingers. I was able to brush DeOxit onto the upper deck of the 2-deck rotary switch, but could only resort to using a bent-nozzle spray to get at the remaining rotary switches.

[nevetslab]

Yesterday I put together the small Foot Pedal Substitution box, so I could turn on and off the five functions....three of which apparently are only available with the 5-button pedal. I didn't bother with the lamps, other than providing a current-limited LED to verify the -31VDC is present. The Tremolo and Reverb are always on with no pedal, just no way to turn them off without changing settings.

Tremolo, Distortion, Reverb, Midrange Boost, Repeat Percussion

I have a small stash of Arrow-Hart GOLD Plated SPST toggle switches that I've always liked...Military made liberal use of them in those days before the digital age.

[nevetslab]

I"m having trouble importing images into this thread, so editing is in progress. I did load a third image in the first set, but it didn't arrive in the thread, though is in the Attached Files. I dunno how to make it appear.....multiple attempts, gave up. Sigh........

Yesterday afternoon, after first correcting some of the terminals in the 9-pin connector shells of both the Preamp and Power amp chassis, I moved them onto a foam rubber pad on a road case, parked the Reverb tank underneath the relief in the power amp chassis (as it sits in the cabinet), and the preamp chassis placed up front. Connected the AC Mains and Output Load cables to the rear panel connections and connected DMM's to monitor the upper and lower output xstr emitter resistors, the Driver emitter resistor and the DC output level of the power amp. Switched on to S/B, and brought up the variac, watching signs of life or disaster. All came up ok. I was getting millivolt readings on all four output xstrs, ranging from 11.8mV to 14.3mV, with the emitter resistors ranging from 0.25 ohms to 0.34 ohms for currents averaging 48mA on upper and lower pairs. I had 2.2VDC on the driver emitter, with 10.5 ohm emitter resistor. AC Mains current draw 0.73A/37W @ 120VAC. But, I had -2.21VDC on the output.

Nothing changed switching out of S/B. Then, I remembered I didn't have a DC load, so switched back to S/B, and connected a 2 ohm load, and switched back out of S/B. The DC output level dropped to 0.3VDC, and the emitter voltages dropped a bit as well. I hadn’t yet connected the scope/audio analyzer nor speaker up yet.

In the test set-up, I'm using the two old Simpson 464 DMM's to read the pwr xstr emitter resistor voltages, the Tek DM501A to read the Driver xstr emitter resistor voltage, and the left-hand Fluke 8060A to read the DC Output level.



This morning I began by first adjusting the one emitter resistor that was reading 0.34 ohms down to 0.25 ohms (took adding 1 ohm/3W in parallel with it). Looking at the bias resistor string on each pair (220 ohms collector to base, then 3.9 ohms from base to junction with the emitter resistors), the Negative half 3.9 ohm resistor was reading 4.45 ohms, while the upper one was 3.94 ohms. I found I had 3.9 ohm/3W MO on hand, and swapped out the original reading high.



Started over, and now I had lowered the DC output level, though the emitter voltages were now a bit lower....in the 4-5mV range, and DC output level now -0.043V with a 2 ohm load connected. Current level now lower in the 15mA range, though one was reading almost twice that, having nearly 8mV across it’s emitter resistor. The emitter resistors now reading between 0.25 ohms and 0.28 ohms. I still have the original RCA power xstrs in place, along with the original Driver xstr.



I connected a speaker and the audio analyzer/scope. I didn’t have a 2 ohm speaker load here, so I placed an 8 ohm cabinet across a 4 ohm DC load to get me in the ballpark.

Started with no signal, just looking to see if I had residual noise present. I had a very little on the Normal Ch, the Brilliant Ch seemed to come to life, but the Bass Ch has lots of hum and the ToneX control makes too much noise when adjusted.

I applied burst pink noise, and found the Brilliant Ch appearing to be the one with respectable gain. Tone controls worked, MRB functions worked, but no Reverb. The Normal Ch worked, but I had to increase my input signal 20dB to get close to where the Brilliant Ch was. Also no Reverb on this channel.

So, I may have fresh caps in the system and disregarding the ToneX pot of the Bass Channel, the amp is quiet. But, I have NO Distortion function, No Reverb Function, No Repeat Percussion function, a gain issue with the Normal Ch, using the Brilliant Ch as a frame of reference. The Bass Ch’s gain is about 10dB lower than that of the Brilliant Ch, and has hum, though the ToneX control does work….it just makes a lot of noise while adjusting it, but is a cool sounding control.

So, the project is far from complete. Turns on without burning up.

[drewl]

I usually have to replace some preamp transistors in these, the originals are usually bad or weak after fifty years.

[52 Bill]

Do you have the e-tuner board plugged in? The Bass channel will not work without it.

[nevetslab]

Do you have the e-tuner board plugged in? The Bass channel will not work without it.

That's interesting. There wasn't one in the amp, so nothing is plugged into that harness. Looking at the schematic, I see the source leg of the input FET doesn't get to ground by way of that 2.7 ohm resistor in the module assy without it being plugged in. Sounds like a simple fix there....just grounding that 150 ohm resistor should be sufficient. Thanks!

[nevetslab]

I usually have to replace some preamp transistors in these, the originals are usually bad or weak after fifty years.

I suspect that may be the case. My first task, however, is how to deal with the lack of a harness extension cable. I've no idea if the two ends of the harness cable between the power amp and preamp were longer originally, but getting what I have to re-position in order to open up the preamp to start digging into it powered up is going to be a real thrill. RG had suggested in his book replacing the original plug/sockets with current hardware. I thought about that, but figured I'd leave that alone unless I find myself in this position.

And, of course, now cursing myself for not first powering this amp up to see what worked before digging into the restoration efforts! Not sure why I didn't at least do that. Sigh..........

[drewl]

Do you have the e-tuner board plugged in? The Bass channel will not work without it.

Ha!
That confused me for a few minutes the first one I worked on!

[nevetslab]

I pulled the preamp out of the set-up, opened the cover, and guided by the hard-to-read component placement guide that has half-tone circuit traces present (messed with the image to get it a bit better), I found the two terminals where the connector to the E-Tuner module was, and jumpered the 150 ohm source resistor to ground. That cured the Bass Channel preamp right up...no more hum, plenty of gain and the noisy pot nonsense also gone, so good call there!

In looking at the Normal Ch, I found J106 N-Ch Switching FET didn't measure right, Marking on the TO-92 body was 5095. I replaced it with a J112 Switching FET, and that restored the gain I was missing. The Distortion Stage that follows, which gets switched in by a P-Ch Switching FET...every time I switch the Distortion circuit in, I loose the signal. The FET measures ok, but I guess I'll replace it just to see if that was the issue.

The 5095 Switching FET that's across the input stage of the Brilliant Ch....when I switch it in, it mutes the channel, though I can faintly hear the thumping of the Repeat Percussion circuit that's not yet working. Not sure if that Unijunction xstr is bad. I looked thru my old semiconductor parts drawer to see if I had any 2N4871 Unijuncton xstrs that BGW used on the crowbar circuit in their original 250 power amp. Didn't find any. Central Semi still makes them, Mouser sells them. Not sure if that's a good replacement for the circuit yet.

The Reverb is now working. I checked the cable, no issues there, and I didn't have the cables swapped. I didn't do anything to the Reverb circuit....just now working, But, switching in the Reverb, there's a substantial drop in signal level...a good 10dB drop that I can't account for thus far.

On with the hunt.

[drewl]

You went to all the trouble to build that beautiful foot switch test box, if you had used actual footswitches you could have sold that beauty for $100 or more.

I just jumper some wire to test the functions.

[nevetslab]

You went to all the trouble to build that beautiful foot switch test box, if you had used actual footswitches you could have sold that beauty for $100 or more.

I just jumper some wire to test the functions.

Many thanks for the praise on the box. Typical mechanical methodology of mine on such tasks, armed with calipers and layout tools, along with punching tools.

I wasn't interested in building foot switches at this time. I mostly needed a simple way to check the functions that aren't accessible any other way, with spending as little $$ as possible. I had to buy 5 feet of 8-cond 22AWG cable for a touch under $10. Had everything else. As it is, it's a nice box to turn the amp off by selecting Distortion or Repeat Percussion. Select those functions, no output from the amp. The hardest part about that box was using the diecast Bud Box, which had very thick walls, and I had to hand-machine the outside of the box wall to shave down the thickness so I could use a Heyco Strain Relief. I am aware of there being a vast shortage of five-function lighted foot pedals for these amps. Right now is NOT the time to add yet another project that I can't get to, as the demands on my time is being pounded on by this amp that's not yet working.

I don't have a clue how to solve the Repeat Percussion problem, and mostly concentrating on finding what is causing the muting of the Normal Ch output when engaging the Distortion function. Hopefully tomorrow. Lack of a harness extender cable is a MAJOR problem.