Guyatone PS-103/ Vox 1903 Compressor troubleshooting

[mckenzie]

Hi, was wondering if anyone could help with my attempt at troubleshooting my PS-103, schematic attached.

When it's on the pedal just outputs clean audio but with with no effect & the sensitivity knob just makes it louder or quieter. I think I get the general gist of how it works ie audio feedback to the bias input of CA3080 on pin 5 which controls it's volume. I've traced the audio signal & its clear up until the transistor in the middle next to C9. The signal at its base is good but at the collector it's very low & very distorted & hence then a bad signal on transistor above.

Voltages on the CA3080 all seem good. Pins 2,3 = 4v. Pin 4 = 0v. Pin 5 = 0.6 > 0.7 depending on pot position. Pin 6 = 2.8v. Pin 7 = 9v.

Some other voltages seem wrong though. Vbe on the middle transistor is 0v & same with the transistor on far right. All others are around 0.6v.

I've put 3 new transistors in (the ones on the right). Also new C9 cap. All diodes have tested good apart from the LED which I've taken out of the circuit for now. Still same problem though.

Im assuming the CA3080 is ok as voltages seem good, audio is passing through & also the sensitivity knob does still have some effect on the volume somewhat via pin 5 bias input (although the actual feedback/control signal doesn't appear to be there).

Maybe I'm not understanding how this should work Thanks for any help

[antonis]

What I can't get is that R11/R12/C16 configuration..
(it looks like a 100μA "bleeding" resistor for IC power supply regulation, but...)

[duck_arse]

check a dynacomp circuit diagram. it appears to be a Vb.

[edit :]

[antonis]

Shouldn't be connected to IC pin 6 /BJT Base through C5/R10..??

[mckenzie]

Sorry not sure what you mean about this, this is how its wired & it did used to work.

[antonis]

Sorry not sure what you mean about this, this is how its wired & it did used to work.

I was talking about the (probably wrong) schematic..
(there is a missing connection between R11/R12/C6 joint and C5/R10/R8 joint)

[mckenzie]

Sorry yes, I just checked the board & schematic is wrong, yours is right.

Can anyone confirm the voltages look ok on the CA3080 ? Also the 0v Im getting across those base/emitter's ? Sorry I'm a beginner so im prob asking stupid questions!

[antonis]

the 0v Im getting across those base/emitter's ? Sorry I'm a beginner so im prob asking stupid questions!

No such thing as a stupid question..!!

Now about those 0V B-E drops..
Did you take measurements between respective pins (i.e. DMM probes directly set on BJTs legs) or between each pin and GND..??

[duck_arse]

but Antonis - the transistors have grounded emitter, AC coupled base, and base pulldown resistors. no volts there till signal applied.

yes, 0V on the base of the transistors marked Q4 and Q5 on the R.G. circuit I posted is correct operation. please measure the voltages, with respect to ground, on each pin of each transistor. with no input signal.

[antonis]

but Antonis - the transistors have grounded emitter, AC coupled base, and base pulldown resistors.

but Stephen - without transistors marking, I couldn't say anything else..

[anotherjim]

Seems to me you need a signal going in then you should see DC voltage on C9 rise according to an increasing input signal.

It would help if the transistors are numbered. Let's say Q1 left most thru to Q5 with Q4 the top one.

So audio probe for similar signals at Q2 emitter & collector and clipped signal at Q3 and Q5 bases.
R19 needs to be at minimum resistance so the compression doesn't fool you by reducing the signal too much out of the 3080 when signal is what you want to hear.

[mckenzie]

Ok so I am an idiot & I think the pedal is working. Since I replaced the components I mentioned before I didn't check the actual output & was just still checking with the audio probe & hearing the distorted signal on base of Q4 & assuming a problem was still there! So either the new cap, a transistor or resoldering sorted the problem.

I was expecting that Q4's base needed a clean signal, but that's not how this works so it seems ? Would be interesting to understand what's going on in the circuit ?

Pedal does seem a bit low on volume though as you need vol knob at about 3 o'clock to match the bypassed signal. I cant remember if this is normal, I don't think it is. What is R4 doing ? Changing its value doesn't seem to do anything.

[ElectricDruid]

Would be interesting to understand what's going on in the circuit ?

Ok, here's the executive summary version, referring to Duck's Dynacomp schematic, since it has transistor numbers on it.

Q1 is an input buffer.
CA3080 is an OTA, a variable-gain amplifier. Gain is controlled by the current into Pin 5. Output is a current from pin 6.
Pin 6 current is converted to a voltage across 150K to Vb.
Q2 is a buffer, acting as a phase splitter. There's an inverted and a non-inverted version of the signal on the collector and emitter.
These two signals are taken to Q3 and Q4, but the diodes to ground on the way there chop half the signal off (half-wave rectified). Since that happens to *opposite* halves in the two transistors, the net result is that it acts like a full-wave rectifier, with one transistor being on when the other is off, push-pull style. The output from this is smoothed by 10uF, buffered by Q5, and then provided to pin5 of the CA3080 via a pot and a 27K resistor.


HTH!

[mckenzie]

Thanks a lot for this, of course it makes a lot more sense now. After the signal gets rectified to DC is the 10uf kind of setting the release time ? Also what is the 2k variable resistor doing after Q1 ?

thanks

[duck_arse]

the 10uF is charged up by the 150k and turns on Q5 [RG's numbers]. when the signal turns on one or other of Q3 or Q4, they try and discharge the cap to ground, so turning off/lagging that other transistor and messing about with the current fed to the Iabc pin. the trimpot is for balancing the inputs of the OTA.

I thought RG had a dissection of the Dyna/Ross circuit at geofex. might be worth a search.

[antonis]

the 10uF is charged up by the 150k and turns on Q5 [RG's numbers]. when the signal turns on one or other of Q3 or Q4, they try and discharge the cap to ground, so turning off/lagging that other transistor and messing about with the current fed to the Iabc pin.

Another way to see it is:
A very good power supply LPF (0.1Hz -3dB corner frequency), hence a very stable Q5 Base bias, which is brutally violated by Q3/Q4  heavy loading..
Or a 10μF capacitance multiplier turned to capacitance divisor..

[mckenzie]

Thanks for all the explanations, it really didn't work the way I thought it did.

[duck_arse]

Thanks for all the explanations, it really didn't work the way I thought it did.

tell.

[antonis]

Thanks for all the explanations, it really didn't work the way I thought it did.

tell.

Scientific or gossipy concern..??

[duck_arse]

Thanks for all the explanations, it really didn't work the way I thought it did.

tell.

Scientific or gossipy concern..??

you know I never gossip. or argue.