Acoustic 150 Tremolo

[gbono]

Hello,

  I am repairing a ACC model 150 amplifier for a struggling student who is a very good guitarist and drummer but he is very short on funds (I'm sure we can all relate ). The amplifier is one of the later versions (>1973) and after rebuilding the power supply, new power cord, replacing the output transistors, bypass capacitors, etc I have run into a problem with the tremolo circuit.

The tremolo will work for about 5 min.s and then stop. I have replaced the FET (2N4360) which ACC replaced the LDR with on this later version of the amplifier. This portion of the circuit also has a trimmer pot which appears to adjust the mix of the tremolo output t on the output stage of the pre-amp (17-11E). Is there a procedure for adjusting this level?

I have also noticed that there is a transistor across this trimmer (2N2926) which is shown to have its emitter and collector connected across the trimmer in the schematic with the base floating. This makes no sense to me and I assume it's a typo in the schematic.

Well thanks in advance for any help or comments.   

[tonyharker]

Difficult to diagnose without the schematic.  Can you post it?

[teemuk]

http://acc.homeunix.net/schematics.html

According to my notes the schematic for model 134 is pretty close to the circuit of newer 150, if not identical.

The transistor is actually an 8V "Zener", that the 2N2926 transistor forms (or is supposed to form) when it is connected that way. It's one of those weird things of Acoustic Control amps. (This is not the only circuit where they use it). I suppose it would be more reliable to replace that with a real Zener and I really don't know why they didn't do so in the first place.

The trimmer doesn't adjust "mix". It adjusts the DC voltage level at the FET's gate, which determines when the FET begins to conduct. It's sort of like a "sensitivity" control for the circuit and it also allows to trim the varying FETs to perform properly in the circuit. I'm pretty sure that the principle is to tweak the trimmer so that you get a nice looking tremolo "shape" at a certain amplitude of the tremolo oscillator's signal. The idea is to get the tremolo to shift the volume level "smoothly" instead of abruptly switching between loud and quiet levels. Unfortunately I don't know how to set the circuit up properly, service manual would likely be very helpful there.

Since the transistor "zener" is important for the correct DC voltage range you should check that it works like it's supposed to.

And always when you troubleshoot, divide an conquer: When the tremolo stops working does the oscillator seize its oscillation or does the modulator just seize to act as a voltage-controlled resistor? These two circuits operate pretty independently. Oscillator is just a signal generator and its output signal controls the modulator. If the oscillator part has issues then there is pretty likely a component value drift that stops the circuit from functioning (its actually a quite delicate circuit). If so, my first suspect would be the old electrolytic capacitors. If the oscillator is fine but the modulator has issues my first suspect would be the 2N2926 "Zener".

[gbono]

Teemuk,

   Thanks for the comments and ideas. The idea of using the bipolar as a zener is probably based on the fact that if you can reverse bias the EB junction and forward bias the CB junction you will have a zero TC zener diode (the forward and reverse junction TC's cancel in this case).

   I'll get the amplifier back and scope out the tremolo circuit - the service manual that is on the Acoustic wiki is for the earlier version of the 150 tremolo circuit (uses a LDR). I'll also look into any resistors that may have drifted.

   BTW, your solid state amplifier book is a master piece. I have started reading it and would only comment on the section regarding amplifier output ratings. I would suggest a burst power (gated sinusoid) rating in lieu of average. This test may not be known to many designers or users of MI amplifiers. Have you read any works by Douglas Self?

Thanks again.

[teemuk]

I have his "Audio Power Amplifier Design Handbook", third edition. It's an awesome book but I wish he had discussed other amplifier topologies than the basic "Blameless" as well. Symmetric designs and those driven by OpAmps were completely omitted as if they didn't even exist. He is also pretty obsessed with the superiority of some topologies over the others, which is sort of disturbing. ...But at least he explains the reasons behind those preferences quite validly. Anyway, it's nevertheless an extremely great book. Truly the best one discussing SS power amps I've read so far. I wish there were more like it. Was the "gated sinusoid" rating discussed somewhere in that book? If it was then it must have slipped past my eyes.

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The Acoustic Wiki has schematics for the earlier 150 as well as for the 134, which I believe was a combo version of the revised 150. That's at least what my schematic notes tell. The tremolo circuit in it fits your description perfectly as well.