Author Topic: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues  (Read 842 times)

rileyismycopilot

(AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« on: January 22, 2020, 08:20:19 PM »
Hello,

I just got this Vox Pathfinder 15R and the tremolo does not work (tried a footswitch too - nothing).
My version is NOT the optocoupler one. Schematic is below (trem ckt is at the bottom. VR5 is Speed and VR6 is Depth).

Symptoms are : ZERO trem sound. Adjusting the Depth or Speed pots produces zero effect.
Rest of the amp is fine but probably distorts quicker than it should (not sure about that though).

I would like to repair it and not just swap components mindlessly hoping for the best.

One thing I can tell you as a hint: when the amp is on and I touch the Speed pot solder joints or traces next to it with my fingers, I can hear a ticking sound through the speaker, that goes slower / faster as the Speed pot is adjusted. That leads me to think the oscillator part of the circuit does work (the IC3 NJM062D)... the problem would then lie around the Depth pot.
But What do I know really? This is just a guess.

Please let me know if you need voltage measurements and at which points, and if anything in this circuit is more likely to fail.

Any help is greatly appreciated, thanks so much in advance!




j_flanders

Re: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« Reply #1 on: January 23, 2020, 11:22:06 AM »
Since the LFO seems to be fine I would check its output and the path towards pin 5 of the CA3080(IC2).
Set trem speed low so your multimeter (in voltage setting) can follow and then start at output pin 7. You should see a varying voltage there.
Then follow and measure it along its path to the CA3080. The middle lug and the lug of the depth pot that is not connected to ground as well as the collector of Q5 are obvious places to check first.
Eventually the varying voltage (and current) has to end up at pin5 of the CA3080 and that is what will modulate the gain of that chip, right before it is mixed with reverb and then sent to the power amp section.

rileyismycopilot

Re: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« Reply #2 on: January 24, 2020, 09:38:20 AM »
Thanks very much for your help, I appreciate it.
I've taken voltage readings along the LFO path.

1/ First, I took my measurmeents with my multimeter black wire connected to the chassis (as I always do with tube amps). All I got was wacky readings. I then realised the circuit ground was not chassis connected. Is it normal for a transistor amp?

2/ I then took my measurements with a random circuit ground as a reference (a big solder spot next to the Reverb pot ground). Speed pot to 0 and Depth to the max.
Here's what I got, starting at the Speed pot.

- Speed pot = 8V oscillating
- IC3B input (pin 6) = 3mV oscillating
- IC3B output (pin 7) = 2V oscillating
- Diode D8 input = 180mV oscillating
- Diode D8 output = 30mV non oscillating
(note: these last two measurments led me to think D8 was shot but I switched off the amp and used my meter in continuity / diode testermode  and read .650, which is fine I guess??? - I know very little about solid state stuff)
- Q5 base = 30mV non oscillating (same as diode D8 output)
- Q5 emiter = 600mV non oscillating
- Q5 collector / CA3080 input pin5 = 14,7V non oscillating

Is D8 shot?
Is Q5 (KTA1266GR) shot? What may I replace it with ? Would a general purpose PNP like a 2N5087 work (different pinout though). I also have a ton of germanium PNPs, would they work?

Thanks very much!
« Last Edit: January 24, 2020, 09:49:09 AM by rileyismycopilot »

rileyismycopilot

Re: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« Reply #3 on: January 24, 2020, 12:09:20 PM »
(deleted)
« Last Edit: January 24, 2020, 01:42:15 PM by rileyismycopilot »

rileyismycopilot

Re: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« Reply #4 on: January 24, 2020, 01:48:02 PM »
Just a little update.
I (re)checked the D8 and Q5 semi conductors, amp switched off and components in circuit, with my diode continuity tester.

D8 seems fine as I have .650 in one way and infinite in the other.

Q5 is weird. C-E test reads 1.500 in one direction and infinite in the other.
B-C and B-E tests both read.750 in one direction and infinite in the other.
>>> seems like the 2 diodes work on their own, but are pointed in the same direction, NOT opposite.

How strange is that? Is it a sign of a failed transistor?
« Last Edit: January 24, 2020, 02:25:15 PM by rileyismycopilot »

j_flanders

Re: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« Reply #5 on: January 24, 2020, 07:48:05 PM »
I'm hoping for you some more experienced people will jump in as I don't an idea how to proceed further.
Q5 is PNP (instead of NPN) btw.

I don't understand how the footswitch (through Q3 and Q4) works.

Here's the LFO section of the circuit in an online simulator: https://tinyurl.com/v8ce2jo

PRR

Re: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« Reply #6 on: January 25, 2020, 11:21:22 PM »
Lift C29. Any change?

rileyismycopilot

Re: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« Reply #7 on: January 26, 2020, 05:00:45 PM »
Thanks so much for chiming in PRR.
I'm away until next weekend and can not try the C29 lift.
How this cap could be the culprit?
My readings show the LFO is still wiggling right at the input of the D8 diode, but stops right after it.
Or is it the wiggling amplitude that is too small before the diode and then gets completely cut off by the diode? I. E. Could it be C29 sucking out the lfo to the ground?
Thanks a lot for your explanation and guidance!

rileyismycopilot

Re: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« Reply #8 on: February 26, 2020, 07:51:58 AM »
UPDATE

So I lifted c29 and nothing happened
I am at a total loss on this one. Any tip?

R.G.

Re: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« Reply #9 on: February 26, 2020, 11:29:01 AM »
CA3080s are fragile and fussy about the current fed into their Iabc pin (modulation pin, pin 5). If that gets over about 1ma, they die. This circuit seems to be relying on using PNP Q5 to limit that current and modulate it with a variable amount of voltage fed into the depth pot on Q5's base. If some situation fed current into the control or Q5, it could kill the 3080.

To test, turn the amp on, no signal. Measure the voltage between pin 4 (-15V) and pin 5. If that's zero or more than about 0.7V, the 3080 is dead. Internal to the chip there is effectively a transistor base-emitter between pin 5 and pin 4. If it doesn't measure like a single base-emitter when powered, the 3080 can't work.

Ordinarily I'd expect the 100K in the emitter circuit of Q5 to limit current to the 3080, but I can see some situations that would feed too much current in there.

If the 3080 pin 5 measures OK, there may be something else messed up with the bias circuit  for the 3080. Test it by lifting the collector wire of Q5 to the 3080 pin 5, and subbing in an ordinary diode from the lifted collector wire to -15. Now measure the voltage across the diode that is faking the pin 5 to pin 4 diode. This should stay between zero and 0.7V for all possible settings of speed and depth pots. You should be able to see a variable LFO signal across the wiper to ground of the depth pot, and also from +15V to the emitter of Q5. If that's not true, there is a fault in the circuitry between the LFO opamp and the depth pot.

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?

rileyismycopilot

Re: (AMP REPAIR) Vox Pathfinder 15R Tremolo issues
« Reply #10 on: February 26, 2020, 06:01:44 PM »
Thanks so much for chiming in with this comprehensive description, RG. That's *greatly* appreciated.
I'll get back to the amp next week and will post results.
Cheers!