Acoustic 270 repair

[stosostu]

I have an Acoustic Control Corporation 270 in for repair and as I am really a valve amp technician I thought I'd run this past to see if I'm on the right track.

The amp turns on and initially there is sound but very distorted, after a few seconds this fades to no sound at all.

I have isolated the fault to the power amp board (which I believe is the same as the 370, which is the schematic I'm using, acoustic control corporation : les amplis basse et guitare and http://acoustic.control.free.fr/file...chematics2.pdf). The line outputs from the pre amp board work fine and the speakers (this is a 2x12 combo) work fine with a different amp.

I have measured the voltage at the + side of C404, at the output, and this increases at turn on to approximately 80 volts DC, in sync with the loss of sound so I am assuming it is related, instead of the 45 volts specified on the schematic.

My attempts to isolate the fault so far, are:

Lift the collectors of Q307, 308 and 309. No change to the voltage at C404.
Lift the collectors of Q312, 313 and 314. No change to the voltage at C404.
Lift the end of R322, where it joins L301. No change to the voltage at C404.

My next thoughts are C307, Q305 (I hope not as this is a 40409 and they are almost impossible to find) and Q306.

Initially I expected a shorted 2N3055 and I may yet pull them all and test them.

I feels to me as though a capacitor is charging up and cutting something off.

Any thoughts or prior experiences would be appreciated, thanks.

[Mars Amp Repair]

Hello Stosostu,
My initial reaction to the 90v no matter what you do is that C404 is charging up & never getting discharged...so you're really just reading the charge on the cap.
Were I you, I would first thing pull the power transistors & the drivers & check for shorts.
you should search the forum for other entries on the acoustic 370, as there is good info regarding replacement of the power transistors with newer versions & the problems that ensue with that.
glen

[Enzo]

This is a single supply amp, but otherwise it is the same as a split supply amp. In a push pull amp like this or any, the output bus should settle midway between the rails. In the common split supply this usually means zero since the supplies are to either side of zero. But here the two "rails" are zero and +90, so the output should sit at about +45.

Your amp is slammed over against the positive rail. Since output cap C404 is connected there, it will charge up to whatever is there. You could even disconnected C404 altogether and work on the amp until you find +45v at the output point.

And like Glen said, once the cap charges up, it has little reason to discharge real soon.

It is a simple matter to find if the output transistors are shorted. You don;t even have to remove them from the circuit, after all ther is nothing the circuit could do to mask a short circuit.

Lifting the collectors of the output transistors is not a good idea. You wouldn't lift the plate resistors to a 12AX7 and expect to get meaningful readings on the tube would you?

You could lift L301 and R328, leaving C404 floating, and it would probably hold its 90v a long time. Without the circuit to pull it back down, it will stay there.

It is a push pull output, so the xstrs on the two polarities take turns conducting. The same signal is applied to the bases of both sides, they are just spaced a couple volts apart by the bias network - Q304 and its friends.

Q305 is the positive predriver, driving driver Q306, which in turn drives the + outputs. And on the neg side, Q310 predriver drives Q311 driver, to the - outputs. Q305 tries to pull the output +, and Q310 tries to pull the output -. Pulling the base of Q305 towards + turns it on. R316 and R317 try to do just that, but Q304 pulls that base down to a rest voltage a little higher than the output.

The base of Q310 does the similar job on the other end. If Q303 turns on it pulls the base of Q310 towards the - rail (zero v in this amp). That trys to turn it on, which would pull the outputs more -. Once again Q304 is there to make sure the voktage at rest is a little under the output. Look at the schemo, the base of Q305 is +43.7 and the base of Q310 is +41.7. Q304 maintains this 2v space ALWAYS. In fact you could just short Q304 dead across, and the amp would work fine, just some crossover distortion would result.

SO no matter what the signal is, the base of Q305 will always be +2v higher than the base of Q310. Assuming the amp is working.

SO basically if you could pull Q304 up and down with the music, your output would follow. That is how the amp works. Without Q303 there, nothing would stop R316,317 from pulling Q304 up to +90. That would pull the bases of the predrivers up, so the output would follow.

That is what I see in your amp.

Q303 is what we call the voltage amp stage of the power amp - the VAS. Normally it is turned on enough to pull Q304 back down to about 45v.

AT this point go verify the voltages. FInd Q304, does it have about 80v on it? Then so will the output.

Q303 controls current through Q304 and the resistors I mentioned. If it opens, then it can;t do that and everything slams up to +90. I suspect it isn;t doing its job. SO is the part bad or is it simply being controlled to be off? Its emitter is grounded, that makes things simple. (On most amps that emitter would go to -45v) The collector SHOULD sit at about 41.7. Since it is wired to Q304 - we hope - the voltage should be the same there.

You know, I am not sure how 43.7v on the base of Q305 results in 45v on the output. Seems to me it would be more like 42v. But lets just call anything within a few volts of 45v just plain 45v, OK?

So we gotta get 45v on the collector of Q303. I bet you have 90 there now, or zero. If thet trace between collector of Q303 and the rest of the stuff is brokoen, then you get zero on Q303 and +90 everywhere else up there. That's a quick check.

If Q303 were shorted, then its collector would be at zero and so would the output.

So if there is 90v on the collector Q303, what is at its base? The voltage across R307. If the base of Q303 is not shorted to its emitter, then there should be a small voltage - maybe half a volt - at the base. If no voltage, then Q303 is simply not being turned on.

At that point we then would explore the differential pair Q301 Q302.

SO check Q303 with your meter, and check Q301,302 while you are at it. They are far more likely to be what's wrong than the resistors.

[stosostu]

Thanks for the replies and especially Enzo for all that detail.

I'll have it in the workshop this weekend so I'll follow your suggestons and get back to you.

Bob

[stosostu]

Well I think I've found the fault. Q305, the 40409, measures 78 ohms base to collector, both ways. I have found a replacement at Vibroworld for only US$9.50, however, the killer is that they want US$30.00 to post it to Australia!!!

[stosostu]

It was after midnight when I quit working on the 270, but tonight I think I'll check out Q303 and 304, and the associated circuitry, just to be sure.

The 40409 is definitely busted, but I'd like to see if there's anything else going on before I put a new one in.

[gbono]

I have used the 2N3440 as a substitute for the 40409 (NPN). I use a punch and hammer and drive out the defective BJT in the heatsink and ream out the hole (carefully) and press in the 2N3440.

[Enzo]

40409 has the integral heatsink. You can punch out the old part and press fit a sub into the sink. I have some 40409s in stock - and no, I don;t want to sell any - but if you want to sub one, the 40409 is rated for 700ma, 90v, 3 watts.

SO you favorite TO39 might work. gbono's 2N3440 should work fine. Just make sure to use the heat sink.

2N5320 ought to work too.

[stosostu]

I have sourced a 40409, just waiting for it to arrive. I've checked out all the other transistors and they look OK. I noticed that the 2N3055s have quite a variation in hfe, from around 10 to 25, is this an issue at all?

One other question. This amp is going to be used for bass guitar, and I notice that the centre frequencies of the graphic equaliser for the 270 are about an octave higher than the 370 bass amp. It looks like a big task to change them, do you think this will be a big problem? The bass, middle and treble pots are all the same in both amps.

[Enzo]

The outputs should be fine.

When you have the amp running, play through it. Then determine if you don;t like the EQ ranges.

[stosostu]

Well I put the new 40409 in on the weekend and no change, the problem remains the same, I hope I haven't damaged the new transistor. So, it's back to the drawing board to see if I can track down this fault.