Vox Buckingham V112 rebuild (was Vox Buckingham questions)

[R.G.]

I did this as I didn't want to put in new transistors in the preamps, the mixer and the reverb recovery just to then have a cap or resistor go south. I do NOT want to go back into this preamp again if I can help it!

That's the voice of wisdom yelling at you. It is HARD to repair these things because of all the side effects of anything you do to them, including just looking at the bottom of the PCB. AACK!

Well, to be clear: the new Mod tank with the 600 ohm input impedance and input insulated did not work. My buddy's spare Mod tank with a 150 ohm input impedance and a grounded input DID work. Still trying to understand that because as Stevem says, a "4F" tank (1475 input) should work...? So why would a 150 ohm tank work and not a 600 ohm tank work if a 1475 ohm tank works? Do you think the tank's input needs to be grounded (despite the schem indicating insulated tank input)? Maybe that's why the 600 ohm tank doesn't work?

Tanks always have to have a ground reference for both input and output >>coils<<, otherwise, current won't flow, springs don't move, and no signal comes out the output. So both input and output coils have to connect to signal ground somewhere. This can be each coil connects to the shell of its RCA jack, or both coils connect to the shell of either the input RCA jack or the output RCA jack, leaving the opposite RCA jack shell not connected. Tanks have models with all the possible connections.
It's also possible to not wire the shield on either the input cable or output cable. Thomas Organ did this, not wiring the ground/shield on the drive/send side. The considerations of which cable(s) to leave ground/shield open on get into some subtleties. Thomas not running the signal ground along the send cable to the reverb tank input jack means that to get current to flow into the input/driven coil in the tank, the input coil >>must<< have its nominally grounded side connected to the shell of the return RCA jack or nothing will happen. This is along the lines of Martin's comment. If your tank does not have an internal connection between the ground side of the coils to the shell of the return RCA jack, no signal gets into the input/driven coil and therefore no signal comes out.

This sounds complicated, but in fact a few minutes with an ohmmeter to read resistance between the center and shell connections inside the tank itself will tell the entire story. You might flip the non-working tanks upside down and just measure the coil resistances, the resistance between the two RCA jack signal pins, and the resistance between the two RCA jack shell solder tabs. If there's no continuity between the two RCA jack shell connections, the Thomas single-wire send to the input coil will leave the coil open circuited, and nothing will happen. But if the resistance of either coil from RCA center pin to shell is open circuit, it's an open coil.

Another question: will I be damaging the reverb send circuit (specifically Q201 and Q202 transistors) using a lower input impedance tank like this 150 ohm one? As a reminder, this is an early Buckingham (V112).

I don't think so. Q202 would be the most vulnerable. But it's unlikely to hurt it.

I also noticed a hum when switching the brilliant or the normal channels to reverb. I presume this is normal? Could this hum be improved somehow? Maybe new transistors for Q201 and Q202?

It's not normal so much as there is a minimum level of hum possible given Thomas Organ's design of the grounding. Just so I'm clear - is this with the friend's reverb tank installed? Or do you also get it with the non-functional tank you bought installed? Or with no tank attached at all?

[martin manning]

the input coil >>must<< have its nominally grounded side connected to the shell of the return RCA jack or nothing will happen. This is along the lines of Martin's comment. If your tank does not have an internal connection between the ground side of the coils to the shell of the return RCA jack, no signal gets into the input/driven coil and therefore no signal comes out.

This was my thought exactly. I'll add that the cable shields and the pan must be grounded to reduce noise, but that must not result in a ground loop, so the shield continuity has to be broken somewhere, at one of the tank's jacks or at the send or return jack.

[seveneves]

OK... update.

The 150 ohm input impedance tank measures DC resistance of ~29 ohms. The 600 ohm input impedance tank measures ~80 ohms.

I guess I was tired last night. Upon closer inspection today, I missed that the 600 ohm tank in actuality has an insulated input. And the 150 ohm tank has a grounded input. Being that the 150 ohm tank worked... hmmm... I grounded the input on the 600 with a jumper on the sleeve of the output jack connected to the sleeve of the input jack. Guess what? It worked! FWIW between the two, I think I prefer ever so slightly the 150 ohm tank. Hard to describe but to me the 150 ohm tank sounds ever so slightly less washy/clangy/"trashy".

Props to Martin Manning calling out the grounding of the input on the 600 ohm tank!

In his last reply, R.G. asked about the hum with the reverb channel select switch in either the brilliant or normal reverb channel select. Maybe it was due to my guitar's single coils or again maybe I was just tired but today I can't detect any real difference in hum regardless of where that switch is positioned. So that is good!

However, one step forward and one step back. I'm having a problem now with the tremolo. To be honest, I don't really know if the problem was there before, I'm only noticing it now spending more time playing the amp. More details in my next post...

[seveneves]

So while I was enjoying the amp earlier today with the reverb now working, I went a little crazy and cranked up the reverb blend for a real "soupy" type sound. I was just "exploring the reverb space", as it were. In a further fit of craziness, I decided to also include some trem along with it, to hear how wacky it would/could sound. Initially both sounds co-existed in a cachophony of sound and it was interesting but not very versatile/useful sonically, IMO.

After about 10 seconds, however, I started to hear a distinct difference in the sound that went beyond a Thomas Vox tremolo soaked in reverb. It was tremolo with a thumping, "motorboating" sound, awash with reverb. Its speed of which seemed to increase with the speed knob, strangely enough. And it got louder with the depth control turned up. As horrible as "ultra trem" with deep washy reverb sounds together, it sounds markedly worse when there's a thumping sound component thrown into that mix. Trust me on that!

So I turned off the reverb and to my disappointment, the thumping was still there as a part of the trem. I don't recall this thumping trem happening the other day when I trialed the amp after aforementioned replacing of nearly every component in the amp. However, I didn't spend much time demoing everything very thoroughly during that initial trial so maybe I missed it. I could be imagining it but the thumping seems a little inconsistent: I'll play a little bit, turn up the depth and then the thumping is apparent within about 5 seconds. I turn the depth back down and no thumping, play again for another 10 minutes, turn the depth back up and for the first 5 seconds there's no thumping and then, wham-o, the thumping kicks back in...

My first thoughts on this were naturally of disappointment: I did a LOT of work to get the components on the PCBs replaced. I actually desoldered every wire from the harness carefully from the wire wrap posts. I know, CRAZEE, right? In my mind, it was necessary to do this in order to carry out this nearly wholesale component replacement. In fact, now's a good time for me to share some pictures of the progress I made on this preamp to this point to illustrate the amount of work it took to do this. Just in case anyone is hankering to attempt to do something this insane in the future... you know what you're getting into up front. Lots of risk with breaking wires involved here! You've been warned!

With the main PCB out of the amp, swapping out the old components for new was SO much easier. Despite it being more work up front to label/identify/map the wiring from the harness and carefully desolder each connection to the board, it was kinda worth it in the long run. No "flexing" of the board or being some kind of impromptu contortionist to constantly get under the board to remove solder from the pads, etc. I still haven't figured how to do that upside down...

Here's how it looked with the normal preamp, mixer and limiter sections done:



And then the whole board completed:



Thomas also put components UNDER the board that needed to be replaced, mainly the electrolytics for the preamp. Here it is with the old caps removed:



And with them replaced:



Yeah. Check out all those wires. Ugh! Pretty intimidating! (R.G. is shaking his head here.) I had to connect all these back to the circuit board now! Remember that old Star Trek episode where Bones has to rewire Spock's brain?

But once I committed to it and just started it, it kind of came together pretty simply. One at a time. Like Bones said in that very same episode: "Of course; a child could do it."



So, short story long and going back to the tremolo thumping: I'm thinking that because I changed out everything in that circuit for new components (including the diodes), I can't help but wonder if this thumping sound in the trem could have been cured by just replacing those three transistors?

R.G.? Anyone else? Couldn't find anything specific in R.G.'s repair supplement re: this malady. Call me nutty but it would be kinda nice to have a non-thumpy sounding trem. It's kind of a unique sounding trem based on its design, but even better sounding when it's not thumping! Maybe I wouldn't use it all the time but I've gone this far...

[Stevem]

Not for nothing, but I did post to you that my original tank has the rca jacks grounded to each other thru the tank case

So your tank does not mount to the inside front lower wall as mine does?

In reference to your now tremolo thumping issue check this if you would.

With it thumping but with no input signal to the amp do you see any of your power supply voltages changing in sync with the thrumping?

If so, and especially if you see such all the way back to the first filter node then maybe one of those Trem Transistors that you did not replace is breaking down and loading the PS way more then it should.

A thermal gun might help you see if one of them is getting hot once the thumping kicks in.

I have found bad semiconductors in this way before, might work for you?

I can't believe that with all the wiring in these bigger size TV amps like yours that they just plain don't oscillate like mad!

Anyway, real nice rebuild job!

PS.
My amp has more reverb then I could use ( and I have been known to like The Beach Boys) but it is bogey and kinda unfocused , like it's getting hit with too much bottom.

Maybe since I still have it open I will investigate limiting the lows going into the tank and report back over the weekend.

[seveneves]

Not for nothing, but I did post to you that my original tank has the rca jacks grounded to each other thru the tank case

So your tank does not mount to the inside front lower wall as mine does?

No. I have a Buckingham, not a Berkeley. This is the "big head" (as R.G. calls it) Thomas Vox. Same as the Royal Guardsman and Beatle. The reverb tank is a horizontal mount. It lays flat, behind the power amp and underneath the preamp.

In reference to your now tremolo thumping issue check this if you would.

With it thumping but with no input signal to the amp do you see any of your power supply voltages changing in sync with the thrumping?

If so, and especially if you see such all the way back to the first filter node then maybe one of those Trem Transistors that you did not replace is breaking down and loading the PS way more then it should.

A thermal gun might help you see if one of them is getting hot once the thumping kicks in.

I have found bad semiconductors in this way before, might work for you?

Yes. Anything is worth trying at this point. I'm so close to wrapping this amp up.

Anyway, real nice rebuild job!

Thank you.

Maybe since I still have it open I will investigate limiting the lows going into the tank and report back over the weekend.

Cool. Would be interesting to know.

[R.G.]

Something tickled my mind on this one. The tremolo relies on a diode modulator for thump rejection. I've never seen this, but I'm speculating that maybe the diode "bridge" has come unbalanced in some way. A leaky diode might do this, or a failing cap across the modulator, something like that. The diodes and cap also might hit on the 2-10% of the parts you didn't replace yet. I suspect that there is a wobbling DC level on the clockwise lug of the trem pot and a bum diode or cap in the modulator. Just a guess.

[seveneves]

Something tickled my mind on this one. The tremolo relies on a diode modulator for thump rejection. I've never seen this, but I'm speculating that maybe the diode "bridge" has come unbalanced in some way. A leaky diode might do this, or a failing cap across the modulator, something like that. The diodes and cap also might hit on the 2-10% of the parts you didn't replace yet. I suspect that there is a wobbling DC level on the clockwise lug of the trem pot and a bum diode or cap in the modulator. Just a guess.

Hi R.G.,

Thanks for your reply and giving my issue some thought.

FWIW, the diodes and the caps in the tremolo have been replaced. They appear to be properly oriented. The only components that were not replaced were the transistors (Q204, Q205, Q206).

I did some googling and saw something similar from about 4 years ago. Funnily enough, with a V114 (early production, Beatle):

https://music-electronics-forum.com/for ... air/49816-

(note: I can't access the site via Chrome, but can via Edge - I think that forum/site has an expired security certificate.)

In that post, you have provided a sort of theory of operation, which I am very slowly starting to understand (on some level). You then go on to request voltages around the transistors. There's something not quite right with Q206 and you have the poster disable the LFO by shorting C217 to better validate.

I have the same situation with the emitter on Q206 measuring at 65.4mV. Collector at 11.75V and base at 0.647V. Seems that the emitter needs to be somewhere around 4-7V? Yes?

So, you then go on to have the poster check resistance of Q206 emitter to ground. You're expecting 10k-15k ohms, due to R229 and R230 (mod. bal. pot).

I initially measured 2.5k. (The poster in that thread measures an order of magnitude even lower than that...)

You then suspect there's some extraneous/erroneous connection going on.

I then go back to the circuit board and do a measurement from underneath, because I can get a better connection with my DMM's probes on the solder joints than topside. This also gives me an opportunity to double-check my rebuild and see that everything is connected where it should be (i.e., no extraneous/erroneous connection(s)).

So from that perspective, I can't find anything out of place.

However, interestingly, I find it really very difficult to get a good reading. There's times when I get ~62k ohms, there's other times I'm getting anywhere from ~4k to 12k.

I wonder if that's the cause for what's pulling the Q206 emitter voltage down? It's inconsistency (i.e., trem doesn't always thump, but moreso than not) kinda coincides with the inconsistency of the resistance readings of Q206's emitter to ground...

Cold solder joint?

[R.G.]

I have the same situation with the emitter on Q206 measuring at 65.4mV. Collector at 11.75V and base at 0.647V. Seems that the emitter needs to be somewhere around 4-7V? Yes?

So, you then go on to have the poster check resistance of Q206 emitter to ground. You're expecting 10k-15k ohms, due to R229 and R230 (mod. bal. pot).

I initially measured 2.5k. (The poster in that thread measures an order of magnitude even lower than that

It would be diagnostic if the value of that resistance varied as you tweaked the modulator balance pot. If the 10k resistor really is 10K, then the resistance to ground just can't be less than 10K unless (1) the "10K" is really some other value or (2) some other situation is paralleling the 10K, like a solder thread, solder ball, etc. If the resistance changes between nearly zero to nearly 5k, the supposed "10K" is some other value. I have very mild red-green color blindness, so the difference between brown-black-red (1K) and brown-black-orange (10K) trips me up regularly. Might be happening here. But if the soldering and parts values are correct, there's no good way for that point to measure less than 10K at any setting of the balance pot.

You then suspect there's some extraneous/erroneous connection going on.
[...]
Cold solder joint?

Always good to suspect. I like to dab suspect joints (with my soldering skills, generally all of them ) with a Q-tip dipped in liquid flux, then remelt. It saves me a lot of time. Give the joints a remelt with more flux if the resistor test above doesn't help.

[seveneves]

R.G.,

I've done some measuring and enough double-checking that I'm thinking there's a point of diminishing returns here: my head is hurting a little trying to understand what is going on as in my mind it doesn't make sense.

First of all, measuring to resistance of Q206's emitter to ground is really the ground in the power amp, right? The middle terminal of the mod. bal. pot goes to the node/terminal strip connection where the black wire comes into the preamp from the power amp (#5 from the 9-pin connector) and the negative terminal of C310 (2000uF).

I pulled R229 and it measured 9.9k. OK. I soldered it back in.

I'm still getting all kinds of varying resistances here... it's slightly annoying/frustrating as I can't remember the various resistances I've been reading... everything from ~7-8k ohms to mid-60k's ohms...

Something is keeping this Q206 emitter to ground resistance from being 10k-15k. But what?

I disconnected this point here (red 'x', this is the pink wire coming from the mod bal pot, R230):





and I measure R229 (again): one probe on the Q206 emitter/R229 junction and the other probe to the other side of R229 (now disconnected from R230) and get 9.9k ohms. OK.

I then measure R230: one probe on the terminal that's normally connected to the side of R229 not connected to Q206's emitter (now disconnected for this measurement) and the other probe on the middle terminal that ends up connect to the power amp ground (via 9-pin center connector #5) and get anything from 12 ohms to 4.9k ohms (as one would expect), depending on pot rotation.

But together, connected as per the schematic/layout, I get, with each measurement, varying resistances. Usually something around ~7-8k or 60k-something-ish ohms... seemingly randomly.

Checked the behavior/symptoms again, after all of these measurements: voltages on Q206 are pretty much the same as before. Trem still thumps.

Can't help but wonder if there's something about the V11x trem that was the reason for the V11x1 revision? I mean, that other forum's thread I put a link to in my last post was seemingly a similar problem, and it was a V114 Super Beatle.

[R.G.]

I've done some measuring and enough double-checking that I'm thinking there's a point of diminishing returns here: my head is hurting a little trying to understand what is going on as in my mind it doesn't make sense.

In a nutshell, Q206 is a linear amplifier arranged with a resistor-capacitor feedback network that makes the amplifier oscillate. For this to happen, the amplifier has to be biased properly so there is the right amount of gain for it to oscillate.
In addition, the amplifier is set up oddly for a phase shift oscillator stage (it is one of those) in that it makes a sine wave(ish) waveform at the collector, and also 180 degrees out of phase at the emitter; in this, it’s a bit like a tube phase splitter with equal plate and cathode resistors.
The two out-of-phase sines drive the diodes through 1M resistors, and if everything is ideal, cancel at the middles of the diodes.

Shorting C217 kills the ability of the feedback to make the thing oscillate, so it settles to its DC idling point and we can measure voltages without several volts of sine wave interfering. The collector has to be up at 12-17V or so to have enough headroom for the sine wave at the collector, the emitter has to have 4V-7V dc so it has enough room to wobble opposite from the collector.

R234 is 12K, so in an ideal world, the emitter resistor R229 + R230 would be 12K. This isn’t an ideal world, so R230 is set up to make R229+R230 vary from 10K to 15K to get cancellation of the LFO in the diodes. If the emitter is only a fraction of a volt, the diodes are all pulled around by the collector and you get bad thumping.

First of all, measuring to resistance of Q206's emitter to ground is really the ground in the power amp, right?

Well, yes, all the grounds eventually resolve to the power ground at the power amp/chassis. But the more wires and connections and such between R229 and ground, the more opportunities for something to not quite connect right.

I pulled R229 and it measured 9.9k. OK. I soldered it back in.

I'm still getting all kinds of varying resistances here... it's slightly annoying/frustrating as I can't remember the various resistances I've been reading... everything from ~7-8k ohms to mid-60k's ohms…

I ...hate… intermittents. You just have to follow the path from wiper of R230 to ground, and the top end of R230 back to R229. Every wire and connection is suspect. Including a possible breaking/cracked wire just inside the insulation.

I then measure R230: one probe on the terminal that's normally connected to the side of R229 not connected to Q206's emitter (now disconnected for this measurement) and the other probe on the middle terminal that ends up connect to the power amp ground (via 9-pin center connector #5) and get anything from 12 ohms to 4.9k ohms (as one would expect), depending on pot rotation.

But together, connected as per the schematic/layout, I get, with each measurement, varying resistances. Usually something around ~7-8k or 60k-something-ish ohms... seemingly randomly.

Could be something as weird as the wiper inside the pot being intermittent or high resistance. Can you try patching in an different pot on flying wires?

Can't help but wonder if there's something about the V11x trem that was the reason for the V11x1 revision? I mean, that other forum's thread I put a link to in my last post was seemingly a similar problem, and it was a V114 Super Beatle.

Good quess based on the data, but no, that’s not the reason. The x1 models added distortion. The tremolo is identical.

[Stevem]

I know it’s a full on pain in the Butt, but I would put in the time to lift one end of each wire ( if you have to ) in this circuit and then continuity test it while wiggling it around a good amount.

On one of my amps I was working on some years ago I was installing a new length of wire from a new spool and 5 inches down its length it was open!

It took me hours to get back to the basics of checking such a simple yet rare thing!

[Stevem]

RG, thanks for your detailed help on these things!

[seveneves]

Well, I eliminated the mod balance pot with a known good/tested 2.5k ohm pot (was all I had on hand). I guess I could have subbed in a 2.5k - 5k resistor but figured the 2.5k pot should be good enough...

Checked the voltages on Q206 (with C217 shorted) and I get: E = 6V, C = 14V and B = 7V.

BUT... I remove the shunt on C217 and I'm still getting thumping.

I double-checked the wiring and the components on the board (that took a while with it already installed in the chassis). Also checked the wiring to the speed and depth pots and everything ohmed/beeped accordingly with the continuity test. I even tried chopsticking the wires a little but couldn't do so a whole lot as the wires are pretty rigid (there's not a lot of slack).

Not sure what else I can eliminate/check? I'm baffled.

[R.G.]

It's possible that your particular circuit needs more than 2.5K to tune out the offsets. You now know that 0 to 2.5K doesn't balance it. Maybe sticking a 2.2K resistor in series with the 2.5K pot would get it there. Maybe not.

Good that you're getting the correct DC voltages now. That removes some possible problems.

This circuit depends heavily on balance - the balance of opposing signals at collector and emitter, the balanced pair of 1M resistors, the balance/matching of four diodes. It also needs the capacitors feeding into and out of the diodes and across the diode quad to all be in good shape, not shorted, not leaky, not solder-depraved. I would check those next.