Acoustic 136 distortion

[BStone63]

Hello, I am new here.

Just bought an Acoustic 136. Sounds like crap. The 5W 560 ohm resistor has been hot, but still measures 560. The power cord, at the plug, looks like it may have been hot.

I am going to check the rectifier and power transistors, plus a couple of caps that look like they were warm.

Any other advice would be greatly appreciated.

[Jon Snell]

Are you asking about a Carvin Acoustic or a different make?
Please establish maker.

[Marshtech]

Healthy caps are NEVER warm. In a Champ, I've seen a white electrolytic baked 'to its death' from the heat of the cathode resistor that it was too close to. Other than looking hot, caps are a 'watt-less' component and never generate heat.

[Helmholtz]

.... caps are a 'watt-less' component and never generate heat.

That's only true for ideal capacitors.

Ecaps have losses caused by leakage current and the ESR, which generate some heat.
With a good cap the temperature rise should only be a few degrees.
But with age, leakage and ESR can considerably increase.

In a SS power amp the large ecaps typically carry large AC currents (power supply ripple and/or output signal) and these generate I²R losses (R being the ESR).
With increased ESR and high AC currents the self-heating can get substantial.
Increased ecap temperature will cause the ESR to increase even more and eventually the internal gas pressure may break the seal and cause blow-out.

[TomCarlos]

For reference, I am attaching the schematic.

Acoustic 136 140 Manual Schematic - acoustic_136_manual.pdf

I am also attaching photos of the preamp boards, power module, and the Power Amp section of the schematic. These are common to the ACC 1XX amps.

[BStone63]

Tom, that's the amp I have. I also have the schematic as well. R311 has been very hot, but still measures 560 ohms at room temperature. With my Fluke 87 set to diode, I get readings across every combination of pins on Q302, regardless of polarity. Do you have any ideas what might be the root cause of this?

[TomCarlos]

Are you testing Q302 in or out of circuit?

If you are testing in circuit, you have parallel devices that affect the readings. And what about DC voltages? Have you checked those?

Make sure you disconnect the speaker until you resolve the amp issues!!

[J M Fahey]

1) the golden rule is: measure voltages first, period.

More than randomly pulling parts and measuring outside .... and praying you find a "bad part".

2) you have lots of important voltages shown on schematic, use them.

3) both your hot resistor and "diodes everywhere" transistor measurements make me suspect a very shorted Q302

Which would be shown by "40V" shown at left of R560 being actually much lower.

So please confirm voltages shown, reposting those varying by more than 20% and pull Q302 for better checking it.

You can use TIP32C or any mid power PNP driver, TO220 or SOT 32 case. (MJE340/350 size)

Add a small heat sink, it´s dissipating a watt or two, 40410 already has some built in fins.

Beware those fins have 2 PCB mounting pins and sometimes casing is used as a jumper between 2 tracks, you replace transistor with a modern one and amp stops working, just in case bridge mounting pin pads with some wire.

Worry about caps later, when you have it functioning, and speakers "should" be reasonably safe, since it´s output cap coupled.
Nice amp when it works properly.

[Enzo]

I'd expect the 560 ohm resistor to get hot, it is dropping 35v? And thus dissipates over 2 watts.

[J M Fahey]

Yes, 2W is the normal rating, which is already hot but bearable.

But I suspect shorted Q302 in which it will drop and dissipate way more.

But let´s wait for measurements.

[nevetslab]

For reference, I am attaching the schematic.

Acoustic 136 140 Manual Schematic - [ATTACH]n941162[/ATTACH]

I am also attaching photos of the preamp boards, power module, and the Power Amp section of the schematic. These are common to the ACC 1XX amps.

Looking that schematic takes me back to the days of borrowing Acoustic Amps from the Doors or Sweetwater for gigs my band was playing at the time. And, seeing the signature of who approved that schematic...Duke Aguiar, Chief Engineer at Acoustic at that time. He was later the Chief Engineer for Brian Wachner/BGW Systems in 1978 and on for a few years while I was working for him. Best one Brian had, and one of my mentors.

[Leo_Gnardo]

Some very good suggestions here. BUT first let's make sure the amp is not being played through a defective speaker. If the speaker is an Acoustic from the time period in which the amp was made, those speakers are notoriously awful. Please DO test the amp through a known good speaker before any changes to the amp itself.

Next, this amp is run from a single ended power supply. There is a DC blocking capacitor on the output of the amp, in series with the speaker, so that direct current does not pass through the speaker. If this cap is defective the sound will suffer. Cheap and easy to install a new one. Considering electrolytic caps go bad with age, and yours must be 40 maybe nearly 50 years old, I would not take the chance. Fresh cap here and who knows maybe the problem will clear up. A leaky cap will unbalance the power supply for the amp, also pass DC through the speaker's voice coil, both conditions bad for good sound. A cap that has lost its capacitance will result in a thin and dissatisfying tone, not much bass if any at all. I have run across Acoustic amps where one or the other failure mode has existed.

Those items out of the way, check and make sure the voltage on the amp side of the blocking cap is exactly, or near as dammit exact to half the power supply voltage. If not, that would indicate immediately something is amiss with the power amp and then the game is on - start doing those things suggested by our fellow MEFsters.

[BStone63]

Thank you Leo. The speaker is very fresh. I bought the amp with no speaker because the guy said the original speaker was no good and he didn't have it. I am used to tube amps, can this be powered up with no load on the output?

[The Dude]

Yes, in fact it's preferred that you run it with no load until you have it repaired and it's confirmed that there is no DC voltage at the speaker output.

Edit: FWIW, that is a generalization for MOST solid state amps. This amp has a single rail output section and a blocking cap, so it's unlikely there will be DC on the output.

[BStone63]

Thanks.