Author Topic: Vox Buckingham Amp - Reverb Transformer Issue  (Read 171 times)

Frank_NH

Vox Buckingham Amp - Reverb Transformer Issue
« on: March 24, 2020, 07:59:26 PM »
Hey all,

Since I'm working from home :), I've had an opportunity to finish fixing up my Thomas Vox Buckingham project amp.  I've replaced all the electrolytic caps and it sounds pretty good (relatively noise free compared to its original state).  However, the reverb is not working correctly - I noticed that as I turn up the blend knob, the output gets noticeably lower.  I've checked all the reverb circuit board wiring and voltages and they seem to be OK. 

However, the reverb circuit uses a transformer, and based on my research, I think it's wired wrong (see image below).  I think the ground and reverb out wires should be on the left and the 25V supply connections on the right.  I didn't do anything to it and it's possible someone messed this up at the factory.  Also, I can't seem to find any information on this transformer - is there a replacement/equivalent if I need one?

Thanks in advance for any help!


duck_arse

Re: Vox Buckingham Amp - Reverb Transformer Issue
« Reply #1 on: March 25, 2020, 10:22:02 AM »
erm - the image you've linked to is residing in an outlook account somewhere - we can't see it unless we log into your account. so, what's the password?
ow zit goin, mate?
sweet az bro, sweet azz.

Mark Hammer

Re: Vox Buckingham Amp - Reverb Transformer Issue
« Reply #2 on: March 25, 2020, 10:29:13 AM »
Calling RG!  Calling RG!  That's the guy you want to speak to regarding anything pertaining to solid-state Vox amps.

R.G.

Re: Vox Buckingham Amp - Reverb Transformer Issue
« Reply #3 on: March 25, 2020, 11:14:49 AM »
mmm.... huh? uh.. I'm awake! Brazil!
No, wait...
OK, better now.  :icon_lol:

Frank: good on you for fixing the old Thomas Vox stuff! One of my several sub-missions in life is to help save these things.

You did the right thing replacing all the caps first. It's a huge first step, although a PITA one, as you know.  :icon_biggrin:
For some detailed answers:

However, the reverb is not working correctly - I noticed that as I turn up the blend knob, the output gets noticeably lower.  I've checked all the reverb circuit board wiring and voltages and they seem to be OK. 
Given that, the problem may be in the reverb recovery circuit. The reverb blend knob is a simple pot-in-the-middle fader between the buffered dry signal from the driver circuit to the output of the recovery circuit. If you're getting dry signal that gets quieter as you turn up the blend knob, the recovery circuit is not providing signal to the hot/reverb side of the blend knob.

This can be either passive (broken wire to the blend knob hot lug, busted blend pot) or active (the recovery circuit is actively providing zero signal). If it's passive, I'd expect there to be dry signal at all positions of the blend knob, just quieter at max reverb positions. If it's active, I'd expect the dry part of the signal to go nearly to zero at max reverb position. These are guesses based on looking at the circuit.

An active-type fault could be anything that fails to provide recovered reverb signal. This includes the recovery circuit itself before the blend pot, the reverb tank, the cables to/from the tank, the transformer, the transformer driver, etc.
Quote
However, the reverb circuit uses a transformer, and based on my research, I think it's wired wrong (see image below).  I think the ground and reverb out wires should be on the left and the 25V supply connections on the right.  I didn't do anything to it and it's possible someone messed this up at the factory.  Also, I can't seem to find any information on this transformer - is there a replacement/equivalent if I need one?
Let's start with hoping it's not the reverb transformer, because there are no replacements commercially available. The only direct replacement would be to cannibalize another amp (ugh!! sacrilege!) or winding/re-winding one. Rewinding is not impossible, just a worse and more painful PITA than cap replacement. But on the bright side, it would be incredibly rare for the reverb transformer to be dead. These are just not stressed in this circuit like power or output driver transformers are.

It is possible it was mis-wired at the factory. My amp tech friend once repaired a Fender that was having BAD intermittent pops, and it turned out to have the wires to the output tube socket lugs wound into the lugs, but not soldered. Worked that way for thirty years.

The reverb transformer has a high resistance winding and a low resistance winding. I measured those one time but I haven't turned up my notes yet. If I remember correctly, the primary was 125 ohms, the secondary much less. It's worth checking the resistances on the windings. It's also worth noting that as long as you're unsoldering wires, you can just try it both ways. This will not kill the transformer if you hook it up wrong for a while. They're tough.

I mention doing this by measuring the resistances on windings because Thomas transformers don't necessarily conform to a specific color scheme. I've seen a few counter examples on power amp driver transformers. Not many, but enough to make me trust, but verify.

Do you have access to an oscilloscope? The simple thing would be to look for signal on both sides of the blend pot and then track it back through the circuit.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

Frank_NH

Re: Vox Buckingham Amp - Reverb Transformer Issue
« Reply #4 on: March 25, 2020, 02:16:52 PM »
erm - the image you've linked to is residing in an outlook account somewhere - we can't see it unless we log into your account. so, what's the password?

Hmmm...posting these images are always problematic for me.  Here's a link - let me know if you can access it now.

https://1drv.ms/u/s!ApPayh2_367xiCFC8AU-FWHTRg5-

Frank_NH

Re: Vox Buckingham Amp - Reverb Transformer Issue
« Reply #5 on: March 25, 2020, 02:46:26 PM »
R.G.  Thanks much!  I'll check the blend pot for problems.  I'm also planning to remove the transformer and reversing the wires.  If the transformer is bad, well I'll have to find an old junker that has an intact transformer.  Or maybe learn to rewind my own  :(.

For reference, the transformer is shown in this link on your geofex site:

http://geofex.com/Article_Folders/Repair%20Boards%20for%20US%20Vox%20Amps.htm

You can see that your ground/reverb tank inputs are on the left (with the 1 nF ceramic cap to ground) and the power connections on the right.  Mine are reversed.  The cap on mine however is in the correct spot on the left!  So obviously that was wrong and made me suspect it was a factory problem.

What may have happened is that this amp was upgraded at some point, as one thing I noticed is that the input jack is isolated from the chassis by nylon washers which probably helps with noise.  I think this was recommended by Thomas Vox after the release of the V1121 version of the amp along with other changes. When I first got the amp, it was quite noisy but changing out all the electros helped that a lot! 

Another interesting anomaly - the peak limiter resistors connected to the two transistors did not entirely match the schematic.  On the first transistor, Q210, they were 10K collector (correct) and 2.7K emitter (should be 4.7K).  For the second transistor, they were 680 ohms collector (should be 2.2K) and 2.2K emitter (correct).  Maybe they wanted to change that second transistor resistors to 680/560, and I tried that but the emitter voltage seemed too low (0.2V).  So I'm going back to the original values (2.2K/2.2K) and hope that is correct for the transistors I have.  Adjusting the peak limiter pot didn't seem to do a whole lot to the sound - maybe this is the problem?

At any rate, I know just about every wire and circuit component by heart now in that amp.  It's a pain to work on for obvious reasons but rewarding nonetheless.  And for those who are wondering about the tone (I have the 2x12 Vox cab) -  by themselves, the normal and brilliant channels are mostly clean with a little hair, so don't expect tube amp tone.  But running an overdrive pedal up front sounds fantastic!  And the tremolo sounds great as well.  I would comment about the fuzz but that needs to be fixed (the relay which switches the fuzz on wasn't working for some reason).
« Last Edit: March 25, 2020, 02:52:03 PM by Frank_NH »

R.G.

Re: Vox Buckingham Amp - Reverb Transformer Issue
« Reply #6 on: March 25, 2020, 06:39:37 PM »
You sound like you're already past needing it, but there's a Buckingham repair manual in the works for thomas-vox-repair.com. The Buck differs only in the power amp from the Guardsman and  Beatle of the same series. The preamp board at thomas-vox-repair works in the Buckingham just like the Guardsman and Beatle as well.

If you have isolating washers on the input jacks, it's definitely been repaired at. The grounding system in all of these amps make for a little hum that's impossible to remove without some special tricks. One of them is isolating the input jack bushings and grounding those only to the preamp board. I use nylon-nose jacks when I do this, but iso bushings work too. The other tricks are to break the preamp-output jack ground loose from chassis and ground it to the PCB and to the chassis through a 10 ohm resistor, and adding a return wire for the on and standby lights.

The preamp chassis can then be tied to the power supply ground with one wire, and perform it's main duty as a shield. The return wire for the panel lights runs AC, and if the preamp signal ground is attached to the chassis, the light's 120Hz current rides on the same power return as the chassis and inserts the wire resistance times the current as a ground offset of the signal out to the power amp. This took me a long time to figure out.  :icon_lol:

When you get the reverb working, here are some sprinkles to put on the icing. You can replace the input transistors with 2SC1815 or 2N5088 (different pinouts! watch that!) and get lower input noise. You can keep that lower input noise by soldering a silicon diode reverse biased across the base emitter. This prevents input transients from reverse breaking the input transistor base-emitter, and making it noisier. Other hiss points are the reverb input, and the three transistors in the mixer limiter.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

duck_arse

Re: Vox Buckingham Amp - Reverb Transformer Issue
« Reply #7 on: March 26, 2020, 09:57:34 AM »
image is visible here now - which usually means everyone can see it.
ow zit goin, mate?
sweet az bro, sweet azz.