Vox Beatle Super Reverb V1143 Restoration Project

[drewl]

I'm not ripping on you, your work and documentation is incredible and a benefit to all.
but you could have saved yourself a lot of time by just using a piece of wire to test functions instead of building a one off box, as all the TV Vox amps are different.
Keep up the excellent work!

I have built footswitches for these amps for customers, same for Peavey amps with din connectors.

Go see what people charge for these footswitches online.
You'd could make some extra $$$$ if you want.

[nevetslab]

This morning, after pondering the problems I'm having with the Reverb Drive amp loading down the output of either the Normal Ch or Brilliant Channel (3dB down on the Normal Ch, 6dB down on the Brilliant Ch, both with volume pots at Max), I discovered the amp oscillates badly. Further digging revealed the power amp by itself isn't oscillating....nor does it when driven thru the Normal Ch. The Brilliant Ch seems to be the culprit.

I disconnected the power amp, and made some basic measurements, driving 2 ohm resistive load. Overall gain of the power amp is 49dB, which really surprised me. The -3dB response corners are 120Hz and 60kHz. It's 10dB down at 36Hz, and 16dB down at 20Hz. So much for it being a bass amp.

I had noticed yesterday the Brilliant Ch seemed to have a bit of HF noise, when I was driving it using Burst Pink Noise. Driving it coupled with the power amp, using an oscillator signal, it breaks into oscillation very easily. I don't find that problem on the Normal Ch. Currently, there's 12dB gain difference between the Normal and Brilliant Ch...I think. I didn't find the power amp by itself having this oscillation problem. Need to do more measurements, this time disconnected from the power amp. And I thought I just had a bit of trouble with the Distortion Amp muting the channel, and NO Repeat Percussion effect.

Max output on the preamp is around 1V RMS.

Has anyone seen or have frequency response graphs of the Preamp on this amp? My plotting system is not here, and I keep finding oscillation problems trying to do this manually.

What was I thinking....sure, I can restore this amp.

[nevetslab]

I'm working on the Normal Ch preamp. I had replaced the N-Ch Switching Fet Q106 with a J112. There's positive voltage on the Gate by way of the 15k resistor R119, which is fed from +23VDC via R118 (12k). When I switch to DIST with the pedal box, that grounds the end of R118 and R119, and, also grounds the GATE of the P-Ch FET Q106 (now a J174). This FET isn't switching, and I suspect never will, as there is NO negative bias on it. There's a negative bias network on the N-Ch Gate from-31V using R121 22k and R122 33k from the -31VDC. When I switch to Dist, there's now neg bias on this N-Ch Gate. Is this circuit for Q105 wrong, since there's NO negative bias to make it switch? Q106 gate goes to -6.6V in that instance, insuring it's turned off.

VOX Beatle V1143 Preamp Channels Schematic.pdf

[nevetslab]

I was wrong. The J174 P-Ch FET IS switching as the circuit is drawn. I have the Distortion signal thru Q105, and the Clean signal thru Q106. Their Source terminals are connected to a 10uF NP cap and 100k resistor to ground. The outputs of the two caps (C112A and C112B ) are joined and feed the Tone Circuits and Volume pot. But, for some reason, I do NOT get the signal thru C112A. I get the Clean signal thru C112B just fine, but when I engage the pedal, which grounds the control line to both gates, I loose the Clean signal, though still have it at the output of Q105. I've been avoiding lifting up the PCB again, but, I guess I'll have to do so to replace those two caps. Sigh..........

I don't measure any Ground on the joined caps....and, that would have shown up in the first place, since I do get signal thru Q106 and C112B. No idea what's wrong, but, get NO signal at their junction when I switch in Distortion, that signal passing thru Q105.

[nevetslab]

OK....let me wipe the egg off my face. This was that dual non-polar 11uF capacitor that I had removed, but somehow got mixed up when I installed the pair of 10uF NP's to replace it, and I managed to miss one of the solder pads. This was one of the numerous pads that have two leads shoved into the same hole that we've heard about. Not sure what I WAS connected to by mistake, but it wasn't the Source lead of Q105. Now I have the replacement caps in place, and have the second half of the Normal Ch in place. There's quite a gain difference between overdrive and clean. Must be why you have three channels to work with. Hopefully I'll get a chance to hear this with a guitar tomorrow (I play bass only, sorry to say).

Now, the next mystery is WHY there is SO MUCH Loss of Level when engaging the Reverb. Maybe the first stage xstr Q201 is bad? Measured ok on the DMM, but, who knows.

[nevetslab]

This morning, pondering the oscillation issue I found yesterday, I found it wasn't happening in the preamp by itself. So, I re-connected the power amp, and connecting my balanced analyzer cable across the XLR output connector (short adapter to my 2 ohm dummy load), if I grounded the shield connection to the (-) input of the analyzer, I got the oscillation. Dropping the shield at the amp output, while the shield IS connected to ground at the analyzer, I no longer had the oscillation. So, a simple instrumentation problem, thankfully.

I still have the loading of the preamp volume controls by switching in Reverb to either the Normal Ch or Brilliant Ch to deal with.

I decided to look into Repeat Percussion circuit. I found the Unijunction Ramp Oscillator IS indeed running, so that's good news. I had to replace the N-Ch FET Q111 (used a J112) that sits across the input circuit of the Brilliant Ch, and RG had advised that if that FET got replaced, it would have to be re-biased. There's a 330 resistor R151 from -31V to a 1M bias resistor R152, with C131 470uF cap filtering the supply at that R151/R152 junction. I placed a decade box between the -31V and the 1M resistor to see if that had any affect. With that open, and the Repeat Percussion was switched on (pedal box), I had no signal....near-full attenuation. With the decade box in place (having a max range of 1.111111M), I had full signal, so that was too much in parallel with R152. After playing with some added Meg-range resistors in series with the decade box, I found 1.8M across R152 gave the variable attenuation needed. The Speed Range Control trim pot R153 allows trimming of the depth of attenuation. The panel control appears to adjust the rate of this variable attenuation, so I'm now in the ballpark. I haven't heard this effect yet, but got it working.

VOX Beatle V1143 Preamp Channels Schematic.pdf

Now, onto the Loading issue of the Reverb source switch.

[nevetslab]

Now working on the Reverb Tank Drive circuit, where I'm getting about 6dB loss when I switch in the Reverb.

I’ve replaced the input cap C201 (10uF/50V NP), Q201 with a 2N5088, pinned correctly, C202 100pF NPO, C203 100nF/100V Film, C204 22uF/50V UPW Nichicon, correct polarity, measured R201 470k as 510k, R204 as 110 ohm, R203 as 16.6k, R202 330 ohm. From the schematic, with 50mV across R204, there would be 500uA current flowing thru Q201. That would yield a voltage drop of 7.5V across R203 and 165mV across R202, and with 24V source, so there should be 16.3VDC at the collector of Q201. Instead, there’s 1.7V at the collector. What is the voltage at R202/R203? 21.77VDC. What is the source voltage? 22.1VDC So, about 1mA flowing thru R202. R203 measures 16.6k, so there would be 16.6V drop across it, and the collector should then be 21.77 – 16.6V or 5.17V. But, there’s 1.7V instead. Where is the excess current going? At the emitter, I see 1.12V, so that jives with the 1mA current calculated thru R202.

I’m still getting about 6dB loss when I switch in the Reverb.



VOX Beatle V1143 Effects Schematic.pdf

While hoping somebody will have a clue on this issue above, and hopefully it's NOT something like the 55 year old phenolic circuit board absorbing current....I wheeled the rig over to our Guitar Dept, and connected it to a 4-12 Mesa 4 ohm guitar cabinet, and one of our guitarists plugged into it. Nice,quiet loud balsy sounding AMP!! After listening to the three discrete channels, we daisy-chained the channels together so you could dial in all three together.

Sounds like a winner.....hopefully I can solve this loss problem when switching in the reverb. As I expected, with the Distortion engaged, you gotta mess with the amp volume, guitar volume and tone controls to get what you want,since there's now SO MUCH EXTRA GAIN to get that distortion. Nearly ready to go back into the cabinet. Last adjustment I made was the Watchdog Limiter. Though I haven't done anything with the Tremolo Modulator Adjustment either. I'll have to read up on what RG had to say about that.

[The Dude]

Try unsoldering the collector of Q201. Measure the voltage on the pad to see if the voltage comes up. At least you'll know if the drop is because of transistor conduction or something else connected.

[nevetslab]

Try unsoldering the collector of Q201. Measure the voltage on the pad to see if the voltage comes up. At least you'll know if the drop is because of transistor conduction or something else connected.

With the collector unsoldered from the pad, and applying 20.0VDC at the top of the 330 ohm resistor (where voltage should be 24V), I get 19.3VDC at the junction of the 15k resistor, 470k resistor, 100pF cap and 0.1uF cap, and 0.7V at the base. So, that seems reasonable. No sneak path.

I had replaced the 15k and 470k, as both were measuring high, and I wondered if that 15k resistor was squirrelly anyhow. Soldered the collector back in, the voltage was now 1.58VDC. I measured 19.65V at the 330 ohm/15k junction,1.58V at the collector, 0.74V at the base, and 0.11V at the emitter. So, the problem appears to be too much current is flowing. Schematic shows 50mV at the emitter, for 500uA current. So much for trusting published voltages.

Sigh...... Measured the 330 ohm resistor as 303 ohms, the 100 ohm emitter resistor measured as 93.3 ohms. Applying 20.0V, I got 19.65V at the 330 and 15k junction, so 1.16mA is flowing. The 15k I installed is a 1% part, measuring 14.88k. So, 17.2V would be dropped across it, making the collector 2.46V. Only I measure 1.58V.

And, I still have a 6db or more drop in level when I engage the Reverb circuit.

[The Dude]

I'd next try jumping tank in to out. See if the problem is still there. It's an old trick some used to convert reverb to boost. Of course you won't have reverb, but you can verify if the problem is the tank or the circuit by monitoring output level increase or decrease when you turn the reverb control up.

[nevetslab]

I'd next try jumping tank in to out. See if the problem is still there. It's an old trick some used to convert reverb to boost. Of course you won't have reverb, but you can verify if the problem is the tank or the circuit by monitoring output level increase or decrease when you turn the reverb control up.

I haven't tried that. The tank does work, and after setting the Tremolo Modulator Adjustment, I put the amp back into the cabinet, calling the project done at this point. It's still here in the shop, so I can try connecting the Send/Return to see if the loss is still there. The loss is present regardless of the Return level. I assumed this was a front-end problem with the Drive amp, and vast differences in what the voltage readings were for the first stage.

After I posted the Restored and Reassembled thread, I moved the amp back to the test bench and unplugged the tank wires, joined them with an RCA to RCA adapter. Left the Reverb Return level CCW. Switching in the Reverb on either Normal or Brilliant Channel, there was NO loss, and unplugging the cables, I had the loss again. The amount of loss depends on the setting of the Volume pot. In some settings, there was only 3dB loss. In use, I doubt if it will be noticed.

[nevetslab]

Vox Beatle Super Reverb....restored and reassembled:



The images above are shown with the Foot Pedal Substitution box plugged in, allowing the additional functions to work. On both sides of the top of the cabinet, just beyond the mounting screw of the top corner guard is a cupped flat head screw. Apparently the original preamp mounting screws for the chassis had pulled thru the particle board, and the left side had a filister head screw installed. I removed it, and fitted #10-32 BLK machine screw, and drilled the thru hole for this right corner screw to secure the forward panel of the preamp. Those are the screws that need the long extension shaft with nut driver to remove the nuts to extract the preamp chassis.



The next images show the added Reverb Tank Lock-down damper which the lock-down screw engages for transit.



Was it worth all the effort put into this project? Absolutely.....first time I got to hear and see one of these amps. I can certainly think of more enjoyable tasks than what it took to re-cap the amp and clean all the pots, but sure is a nice sounding amp after all of that. Would I do one again? If the price is right, probably. This took about a month....started into it on Memorial Day weekend, though that was to just open it up, take photos, make a list and order parts. The real work didn't begin until June 6. I just tallied up the hours on this project. 128 hrs....20 days, with a couple days off.

[nevetslab]

I managed to miss the Official Mike Campbell Reverb Shop /Reverb.com sale....I thought I saw it happening today July 27, but learned from our General Mgr it was on July 21. The Results are still up here: https://reverb.com/shop/the-official...ll-reverb-shop. I looked at it last night after hearing about the highlights. The Vox Beatle (stand, Super Beatle Spkr & amp head I restored) sold for $6500! https://reverb.com/item/42221523-vox...-mike-campbell

\\

From this auction page:

About This Listing



This Vox Beatle Head and Super Beatle Cabinet With Heartbreakers Logo is owned by Mike Campbell

The Beatle head (Not Super Beatle) is in overall pretty good shape other than some minor scuffs and small tolex tears. The Super Beatle cabinet also has a decent amount of small rips here and there but is in overall pretty good shape. Some of the screws that hold the back panels in place are stripped and the wood doesn't catch screws anymore.

Hearbreakers and Heartbreakers logo are stenciled on the cab itself!

This amp is fresh out of the shop. We have been informed that this amp has spent close to 100 hours in the shop being well maintained. There is a custom hand switching device that has been installed in which you can control the amp settings with.

Unfortunately this amp and cabinet connect using an "XLR" type cable which we do not have a spare of. A separate speaker cable will have to BE PURCHASED SEPARATELY.


All the other gear I had repaired and serviced sold, several fetching some amazing prices. The Fender Blonde Showman Amp & it's single 15" spkr cabinet sold for $7200! The Magnatone Custom 260 sold for $2400. Even the Heathkit TA-16 sold for $2000! Amazing! The other two Magnatone amps M10 & M10A sold for $1400 & $1200.

I only replaced tubes on a couple of the amps, and on the gear that was using cap cans, such as the Gretsch, Guild, Magnatones, Heathkit, replacing those wasn't an option, nor was spending the money on replacing tubes, nor buying/building foot pedals or cables on the project. They just wanted the pots cleaned, which I did on all of the gear. Everything was quiet when it left the shop, but.....most of the gear WAS 40-60 yrs old to start with. So, as whoever did the write-up on the auctions, spending the time to do full restoration was in order. Mike's guitars definitely sold well, as can be seen on that website!

[drewl]

You gave them your switch box?

You should have used foot switches like I said.

Great job!

[nevetslab]

You gave them your switch box?

You should have used foot switches like I said.

Great job!

I charged him for that. The spacing on those was like 0.60", so hardly suitable for foot switch use. As I had stated, it was a footswitch substitution box, used for test purposes. Our general manager asked if I wanted to part with it, and gave him a price which was accepted, so it went with the amp.