SS tremolo

[gbono]

I have an Acoustic 150 which is going to be a X-mass present - if I can fix the tremolo circuit. The amp came to me in semi-working order with many interesting "modifications". I rebuilt the PA board and the pre-amp board (17-11). The preamp board works but I'm having issues with the tremolo ckt.

I had the tremolo ckt working after I replaced almost all the BJTs except Q202/203, but after I re-attached the board to the chassis front panel the tremolo stopped working. Checked all connections and found nothing obvious. The osc that Q202/203 create appears to be working but I have almost no signal at the darlington pair (Q204/5). In desperation I replaced the LDR but no luck.

I'll start looking at the various nodal voltages (again) tonight but wouldn't refuse any help or suggestions.

[Enzo]

I'd be more inclined to think the problem was those 1uf electrolytics than the transistors, but whatever.

So signal trace. You got oscillation? Is it at the top of the intensity pot? Turn that up, is at at the wiper? At the base of Q204? If it isn;t there, it won;t be further along either.

Any chance the footswitch connection has gotten grounded somehow?

Did you replace those caps? I would.

[Gtr_tech]

+1 on the lytics.

[gbono]

I replaced all the 1uF tantalums and no luck.

I measure the collector voltage a Q201 and it "looks" like a square wave (40V) and I can vary it's frequency with the speed knob from 2 to 8 Hz.

Q202/203 also have non sinusoidal wave forms on their bases but 1) they are not the same waveform (i.e. different waveform on each base 2) the waveforms have at least 15-20 V dc offset.

The other issue is that the intensity knob has no effect on the output of the tremolo ckt - I desoldered the pot and it checked out okay.

Needless to say ther is basically no signal at the base of the darlington pair Q204/205.

Is it the osc ckt working correctly?

[gbono]

bump.........

I found the problem.

There was a micro crack in the PCB trace to the wiper of R213

This may be a X-mass present after all.

Thanks Enzo for the usual "hand holding" and coaching. I sometimes find myself traveling to MI for work (Wixom/Novi). I need to look you up and buy you a beer.

THX

[Enzo]

OK, so it oscillates.

The waveforms are offset with DC, as expected. Just like the plate signal of a 12AX7 is offset by 150v. This transistor circuit runs on 40v so the signal should sit somewhere near half that.

The other issue is that the intensity knob has no effect on the output of the tremolo ckt - I desoldered the pot and it checked out okay.

Wait, you got to signal on Q203 then skipped right over the rest to report the pot has no effect on the output. We are signal tracing. You have a signal at Q203. From there it flows through that 1uf cap then the 68k resistor. is ther the signal at both ends of each of those?

I asked before, is the signal at the top of the pot? If it doesn;t get that far, then it can't get any farther than that either. We are not just looking for a bad part, we are troubleshooting a whole circuit. What if the pc board trace is cracked between the 68k resistor and the pot, or at the other end? Or if the pad the pot solders to is cracked away from the rest of the trace? it isn't always bad parts that bring amps down.

If the signal is there atop the pot, does it come out the wiper? Max the thing. From the wiper it flows through another cap on its way to Q204. We follow inch by inch the path of this signal through the circuit. If it is here, but not there, somewhere between here and there the path is broken. COUld be a bad part, a grounded signal path, a broken signal path, whatever. Our job is to find out where along the path the signal disppears.

Your pot is OK out of circuit, but when in circuit and turned all the way up, what resistance is there from wiper to ground? We would expect about 1M wouldn't we? But what do we get? That is why I mentioned earlier the possibility that the trace leading off to the connector on the right - where it says tremolo footswitch - could be grounded. If the pot is fine, but the stuff it connects to is grounded, then it can't push the signal past that.

DOn't worry what the waveform looks like. It is just a voltage that goes up and down. We need to follow that varying voltage through the whole circuit.

[Enzo]

OH well gee, I write all that and you were already announcing success.

How did I do?

If you are in Lansing, feel free to drop in and say hello - we work 6:30Pm until dawn.

[gbono]

Or if the pad the pot solders to is cracked away from the rest of the trace? it isn't always bad parts that bring amps down.

Like I said, I owe you a beer

[gbono]

Reverb

Okay just when I thought I could button this baby up; a check the reverb circuit finds it isn't working.

Here's my diagnostics
Signal flow to Q209 yields 10Vpp at negative side of C209 with 350mVpp input to amplifier - this seems reasonable - Q210 has very weak signal at it's base (maybe 1-2 millivolts). What should the tank output voltage(?) measure. Also replaced C212 for good measure.

Reverb tank - input and output resistance of tank measure 170 ohms (bobbin color is red) - also seems normal.

Resoldered R234

Could the problem be that Q209 isn't supplying enough drive current to the reverb tank? How does one test a reverb tank?

[Enzo]

If the transducers at each end of the reverb pan are intact, they are probably OK. Sure once in a whil it can fool you, but if it isn't open, it likely is fine. Think of them as tiny transformers.

Now isolate the problem, is it drive or recovery (return)? Pull the cables off the reverb pan jacks. Power up the amp and set the reverb control in the middle somewhere. Touch the tips of the cable plugs with your finger. The return cable should hum loudly. Does it? The one that hums is the return to the recovery circuit. Were they in the right jacks? The hummer plugs into the OUTPUT jack on the pan.

The drive end - connect it to some little amp for a listen. ANything there.

Oh lookie - C209 is one of those 1uf e-caps we were going to get rid of. Did you?

Q209 doesn't supply current for the pan transducer. C209 does. When signal at the base of Q209 rises, the collector drops and vice versa. When Q209 turns off, the voltage rises at its collector pulling C209 up. The charging current of C209 flows through the transducer. When Q209 turns on hard, the collector voltage drops and that discharges C209. That discharge current also flows thruogh the transducer. SO you see how that cap can kill the reverb.

Look at the top of C211, the decoupler. There should not be any signal there. IS there? If that cap is dried out, then the support for the reverb drive gets soggy.

I really suspect ALL these little lytics strewn throughout this amp.

I have no idea what the tank output voltage is. Bounce it, do you get spring noise out the speaker? The finger touch test told us if the return worked. If it doesn't. The circuit is not large, but it is larger than many.

If no return buzz when touched. The recovery stage is simple. Using my finger for a signal, is the hum signal at Q210 collector larger than at the base? through C214 past the footswitch JFET. That shorts the siognal to ground if not turned off. If in doubt remove it and see. Or while the circuit is live, measure resistance across that FET or if more convenient, across that 6.8k. If ti measures low - like under 100 ohms - the FET is shorted or not turned off. Just looking, seems to me that gate w\should have about 10v on it to turn it off.

You get past that, the reverb signal mixes through the reverb pot with the dry signal.

ANd if the amp is working?'

Turn things on and shake the pan. If the output end works, you hear the loud SPANG noises. If finger touch hums but no SPANG in hte pan, then the pan sounds bad.

Once you know that end works, if the drive circuit works, then plug it into the INPUT jack on the pan and you should have reverb.

ANd I am assuming this is a high Z pan, like a 4EB2C1B or even a 4FB2C1B, so if in doubt, yank a pan from some other amp for a try.

[gbono]

Enzo,

THX for the diagnostics. I spent several hours chasing my tail - it turned out the one end of C214 was broken The good news is this amp will now make it as a Christmas present.