Thomas Vox V1032 troubleshooting help?

[MWaldorf]

Hi all. I've got a Vox V1032 that I'm trying to restore. It had a number of modifications so I decided to update to one of R.G. Keen's replacement boards with all new components (except for the MRB). The amp is running, but there are a couple of likely related problems. The sound coming out of the speaker has a fair bit of distortion at all volumes. Oddly, with the volume all the way off there is sound, getting quieter till about 2, then getting louder until it makes crazy distortion/oscillation with the volume above 8 or so. To preface things, I am distinctly amateur with electronics. I know how to read schematics and layouts, but I don't understand the whys and hows of a circuit which makes troubleshooting very difficult. I've checked and re-checked my wiring, found a few mistakes I've fixed, but I'm stumped.

The main transformer seems to be working alright, though a bit low - thought the power supply the 33.5V source reads 31.5V, the 27V source reads 26.6V and the 17V source reads 16.6V.

The transistor voltages are within the range I'd expect on Q1, Q2, Q3, Q4, Q7, Q8, Q9 and Q10. I checked the transistors at the Collector, Base and Emitter where I could. Q5 reads 0.7C, 0.6B and 0.06E which is substantially low on the collector (should be in the 2.5-4V range). Q6 is wrong across the board at 12C, 5B and 4.5E where it should be 15-16C, 1.4-2.4B and 1-2E. Q11 reads 26.4C, 10B and 9.5E where it should read 27C, 4.5B and 3.8-3.9E. Also, the voltage on the emitter changes when I move the speed control, from 8.3V with the speed at 0 to 9.5 with the speed at 10.

The reverb pan has 72ohms at the input and 184 at the output. This is out of spec - according to the VoxShowroom website, it should be 1475 ohms at the input, and 2250 ohms at the output. I tried using the pan from my Buckingham, and while there was much much more reverb, the transistor voltages remain the same and there's no effect on the distortion or odd volume behavior.

The amp also has an "E-tuner" feature, which appears to be working and doesn't have any obvious distortion. The volume, treble and bass controls have no effect on this signal but the reverb and tremolo do. Turning up the reverb adds some of the distortion noise on the instrument inputs. Tremolo works as expected and doesn't otherwise affect the signal.

I got an oscilloscope and traced a generated signal through the preamp, images and description below. As I said, I don't know much about why electronics do what they do, just that they do. That's part of why I wanted to work on a solid state amp - less voltage! I'd love it if anyone has suggestions on further troubleshooting.

Thanks so much, Mel

Now, on to the scope views. First up is the signal itself. It's a sine wave with start frequency of 10kHz, amplitude 300 mV and offset of 0V. I'm not sure why the signal has spikes or isn't completely smooth; I assume I missed a setting somewhere.


Next I measured the signal at the input 1 jack. It looks pretty much the same, though the spikes are larger.


I was curious about the spikes, and measured across the jack with no signal and got this. I did note that the act of measuring itself added noise to the amp output, so perhaps this is something coming from my laptop? The level of noise coming out of the amp dropped significantly when I removed the testing lead ground from the chassis.


I reconnected the signal generator and had a look at the signal after R1. The wave seems to have dropped in amplitude and the noise spikes got larger.


Next I put the probe on the collector of Q2. The scale is in volts rather than milliVolts, but it still looks like a sine wave.


At the hot lead on the Treble pot, the sine wave is back on the milliVolt scale, but an order of magnitude higher than at R1.


Here's the signal on the hot lead of the Volume pot. Same basic waveform as on the hot Treble lead, but the noise spikes are bigger.

[MWaldorf]

Here are the final two oscilloscope views. These are odd, at least to me who doesn't really understand. Here's the signal on the cold lead of the Bass pot. There are huge noise spikes.


Finally, here's the signal on the Volume pot wiper. Again major noise spikes.

[The Dude]

A schematic of what you built would be helpful.

[Tom Phillips]

Also tell us the period of the spikes. From that you can calculate the frequency and that will give us a good clue to the possible origin.

[J M Fahey]

Wow, LOTS of questions.

Will answer a few:

I decided to update to one of R.G. Keen's replacement boards with all new components

Way to go.

I know how to read schematics and layouts,

We too, and it´s necessary to offer any kind of advice, so POST them here or we-don´t-know-what-are-you-talking-about.

The main transformer seems to be working alright, though a bit low - thought the power supply the 33.5V source reads 31.5V, the 27V source reads 26.6V and the 17V source reads 16.6V.

That´s no difference, voltages are *normally* within 10%, in some cases even 20% spread is acceptable.

The transistor voltages are within the range I'd expect on Q1, Q2, Q3, Q4, Q7, Q8, Q9 and Q10. I checked the transistors at the Collector, Base and Emitter where I could. Q5 reads 0.7C, 0.6B and 0.06E which is substantially low on the collector (should be in the 2.5-4V range). Q6 is wrong across the board at 12C, 5B and 4.5E where it should be 15-16C, 1.4-2.4B and 1-2E. Q11 reads 26.4C, 10B and 9.5E where it should read 27C, 4.5B and 3.8-3.9E. Also, the voltage on the emitter changes when I move the speed control, from 8.3V with the speed at 0 to 9.5 with the speed at 10.

Without a schematic, we don´t know what are you talking about.

The reverb pan has 72ohms at the input and 184 at the output. This is out of spec - according to the VoxShowroom website, it should be 1475 ohms at the input, and 2250 ohms at the output.

What you measured with your multimeter is the drive nd recovery coils *resistance* , official spec is *impedance* measured at 1 kHz , not the same , specially because both coils are inductors, so resistance and impedance are very different.
Your tank is probably fine, those DC resistance values sound most reasonable.

the signal itself. It's a sine wave with start frequency of 10kHz,

Unless needed for some specific test, 10kHz signal is way to high for audio testing and absolutely unrealistic in a Guitar amplifier, where speaker response drops like a brick (seriously) above 2000 to 4000Hz
1kHz is a standard test frequency, or something like 400/440 Hz, simply because multimeters read them well.
Download from: Download Audio Tone Files
At least the 440 and 1000 Hz 30 second MP3 , and set player to loop/repeat so you have continuous tone available.
The spikes you show look very much like Laptop Power brick switching/digital noise, move it away from the amp, be careful with audio wires orientation, etc.
As of signal feedthrough, oscillation, noise, etc, in principle sound like ground problems ... which in itself is a sort of black art, only practice makes perfect (or at least acceptable).
Post a couple amp pictures, it´s a lovely amp, WELL worth restoring.

[MWaldorf]

Sorry guys, I was so involved in including as much data as possible, I left out the schematic. Duh, that's a rather obvious important thing!


As for the spikes, the images are 200mS, and there seem to be between 28 and 29 spikes per image, so about 7mS each.

Thanks so much, guys!
Mel

[The Dude]

I'm not sure if it will help or not. I found the schematic hard to read. Text is quite small. I resized it and enhanced it best I could for easier reading. It will require scrolling, but at least it's easier to read the text.

[The Dude]

Of your voltage readings in post #1, Q5 seems most out of whack to me. B/E reading indicates it's not biased on, yet collector voltage is low as if it's conducting hard or missing/low supply. Is transistor shorted? Is it installed correctly (basing diagram). Are R21&R22 good (collector supply)?

[MWaldorf]

All the components are new. R21 and R22 read the correct resistance. I just swapped in a new transistor for Q5 (type 2N5088, as spec'ed by R.G.) and the voltage readings remain the same.

[MWaldorf]

Juan,
Thanks so much for the response. The schematic for the V1032 is here:


I'll try moving the laptop away from the amp. I'm using a USB Picoscope, so there's only so far away it can be, but the power brick was on the floor under my workbench. The picoscope software has a signal generator, so I can adjust it to 1kHz or 440Hz. I guess I'll try 440Hz first if it's easier on the multimeter.

The amp itself is pretty clean, electronics excepted currently

[MWaldorf]

Based on J M Fahey's advice, I rescoped with my laptop farther from the amp and used a different signal - 440Hz. I changed the time scale to 10ms to capture more of the waveform.

Here's the signal at input 1:


Here post R1:


Here's the hot lug of the treble pot:


Here's the wiper of the volume pot with the volume turned way up. I don't think the waveform is supposed to be that noisy.


Also, here's the signal before R10, i.e. the leg closer to the Q2 collector. I'm not sure what the big blobs are, but they're also on the hot wiper of the treble pot. Weird.

[MWaldorf]

Here are some pictures of the actual amp. First, a full view of chassis. Ignore the push/pull on the back of the volume pot; before the overhaul, I'd had an extra 1uF cap that I could add in parallel to C4, the cap in the RC circuit on Q2 transmitter, to get a sound more like that of the brillant channel of the Vox Buckingham, etc. Until I get the current circuit figured out, it's not in use.


Left side:


Right side:


Underneath. You can see the wires where I had to splice on extra wire to reach the PCB. All wire I used is 22 awg.


Again, I appreciate everyone's help!
Mel

[Tom Phillips]

Mel,
Your posted photos do not expand when clicked. (At least for me) The last set of chassis shots would be helpful to see in larger size & resolution. Please consider uploading your photos directly to the MEF server. This will prevent the information from being lost to dead links in the future and make it easier for forum members to help you. When members contribute to a discussion part of the incentive is that we are building a data base that will be useful for many years to come. With supporting information attached to the thread you will get the best overall support. The forum FAQ discusses the attachment uploading process at Reading and Posting Messages if you need specific information.
Cheers,
Tom

[MWaldorf]

Thanks Tom, I've updated my previous post with photos on the MEF server.

Everyone, I really appreciate your help. Thanks again, Mel

[MWaldorf]

Not sure if this is a clue or not, but I plugged in the footpedal, and while the MRB and Reverb switches behaved as expected, the Tremolo had a very fast oscillation when switched off. Switched on, the speed and trem function correctly. Could this be related to the higher than spec voltages on the base and emitter of Q11? Base reads 10V instead of 4.5V, and the emitter ready 9.5 instead of 3.8-3.9V. The tremolo circuit is tied to the emitter of Q6, which, along with Q5, has incorrect voltages (see my first post). I suspect these are all messing each other up.

Also, I accidentally jostled the reverb pan and the crashing sound was enormous, loud and not distorted. Very startling at 11PM!

Any suggestions on what the next avenues for debugging? Would it make sense to try to inject a test signal someplace after the volume control and see if it's distorted? Or use a jumper to connect the volume wiper to the Q6 collector? Maybe I could isolate the source of the distortion that way.

Thanks again for all the help,
Mel