Acoustic 370 resistor toaster.

[ManRay]

I have an Acoustic 370 which is powering on, but not passing any signal, not even a hum. I opened it up to find 2 blown 100 Ohm resistors (R339 and R320). I replaced them and the Transistor which was in series with them (Q305) and just to be safe I replaced its counterpart (Q310) in my amp these are a 40409 and 40410 respectively.

That said, I just powered everything up hoping to hear it sing again, and I blew the same 100 Ohm resistor (R339). I've been leaning pretty heavy on my local tube tech for guidance, but he doesn't really work on solid state stuff so I'm reaching out. Anybody have some advice on what I should try next? Below is a link to the schematic I've been working off.

Thanks,

http://mikesgig.com/wp-content/uploa...ual-410056.pdf

[DrGonz78]

Every time you replace blown parts and then just flip the power switch to a failure, you are asking for more problems. You need to research and build a light bulb limiter(aka. Current Limiter). The idea is that your amp circuit is drawing current in excess and that is a major reason why you need a current limiter. The light bulb limiter will soak up the current draw from your amp and save the freshly replaced parts from toasting up again. Then you can get the limping unit at least powered up and test voltages to find out where the real problem is located.

[ManRay]

Thanks for the tip DrGonz. Any guidance on how to solve the problem I'm having now? What voltages should I be checking once I've powered up my limping amp? the next transistor in line after Q305 is the start of the power amplifiers. Should I be looking to replace all of these?

Thanks again.

[52 Bill]

When you pulled Q305 did it test shorted? Have you tested any of the output transistors or any of the other parts in the output stage?

Playing parts roulette may work if you're lucky, but it would be much faster and safer to only replace parts that are bad.

[ManRay]

I did not test the old transistors that I swapped out, but the new ones test fine. What parts in the output should I be testing? I haven't tested the 2N3055 because the collector is the case and I'm not sure how to test in this situation. I would rather not play parts roulette, so I'm looking for some guidance here.

Thanks,

[J M Fahey]

Just for starters you very probably have shorted power transistors and open emitter ballast resistors , so the only remaining path for high currentn is through those poor 100 ohms resiostors.

Unless you replace *all* bad parts (and no, shotgunning is not a good method ) your amplifier will still be unrepaired and continue misbehaving.

Just as in the old Jewish joke there is not such a thing as a "slightly pregnant girl", there is not a "slightly working amp" , specially an SS one, because many parts are "direct coupled" so a problem in a small "unimportant" one will rise its head in a very damaging place.

So start by building the lamp limiter, get a place where you can leave the amp unmolested for days or weeks and we can probably help you.

A`lot of testing and measuring will go in between, so get ready for that .

and yes, it's a "repairable" amp.

[The Dude]

^^^^^^^^^ What he said. Don't just replace parts. Get out the meter and check them. Start with everything in the immediate electronic area. There is a group of diodes attached to the base of Q305 that would effect transistor bias. Any of them open or shorted? All the resistors off of the emitter good? It's a push/pull amp. Are the voltages on Q305 similar and opposite to those on Q340? R339 and R340 serve similar functions. Is there a similar voltage drop across them? Do as DrGonz says and build a light bulb limiter so that you can safely do some testing.

[The Dude]

Just for starters you very probably have shorted power transistors and open emitter ballast resistors , so the only remaining path for high currentn is through those poor 100 ohms resiostors.

Very good point Juan! Sometimes you miss the obvious. Yes, a good start would be to check the outputs and ballast resistors.

[g1]

Where did you get 40409 & 40410, or what did you use for substitutes?

[DrGonz78]

One point that I think was not mentioned is to not hook this amp up to a speaker or load until it is repaired. Not sure if you know this already so just thought to check.

[ManRay]

One point that I think was not mentioned is to not hook this amp up to a speaker or load until it is repaired. Not sure if you know this already so just thought to check.

I did not know this. I did have a speaker hooked up when I turned everything on.

As far as this lamp limiter goes, can I substitute a Variac instead and get the same results?

I got the 40409 and 40410 from a dealer on ebay. I have a pair of each. It seemed like a simple fix to just swap the transistor immediately in series with the burning resistor. Since it seemed like it was performing (what I assumed to be) a phase-inverter-type function I swapped its counterpart too. What parts should I be testing next?

Thanks,

[loudthud]

Q304 is real important.

[ManRay]

Q304 is real important.

How so?

[DrGonz78]

As far as this lamp limiter goes, can I substitute a Variac instead and get the same results?

A variac will work if you can monitor the current with an ammeter. If not the light bulb limiter is fast/cheap to make and it is a good tool to have around. Some people have expressed that they never use one and other's have said they always have used one. Try it out for less than $10 bucks.

[g1]

Q304 sets the bias for the output stage. A problem with it can make the drivers and output devices burn up.