Acoustic 370 Troubleshooting

[garytoosweet]

Hey,

Acoustic 370 is the amp. I have full basic test equipment setup. Getting the hang or troubleshooting tube stuff, but not quite there with solid state yet. I bought this amp in broken condition about a year ago, plugged it in and it had a weak and distorted output. It has sat on the shelf until now.

I have replaced ALL electrolytic's as well as a few out of spec resistors and caps.

Right now it has ~30VDC on the speaker outs.

Power supply is a stable 87.5VDC.

Feeding a ~56mv Sine into the low input, I have a 430mVAC signal on the monitor outputs (0VDC). Tone controls at approximately half and volume maxed.

At C404 (coupling cap to speakers) I have on positive side 37VAC 47VDC, on negative side 37VAC 28VDC.

I have a scope on the output and I can see that it is passing a nice sine wave until the volume is up to about 8 or 9 and then it clips symmetrically.

I removed the power transistors to test for shorts (ok) also tested on a cheapie LCR-T4 tester which showed OK, although I do not trust the tester.

Q307/308/309 is reading: C: 87VDC E: 47VDC B: 47VDC

Q312/313/314 is reading C: 47VDC E: 0VDC B: 0VDC

Any help with the amp and understanding solid state troubleshooting is appreciated.

Thank you in advance.

[The Dude]

The amp does not have the typical + & - rails. It has one supply and the other side (where there is typically a negative rail) goes to ground. So, the DC voltage at the output (before the cap) should be about midway between ground and supply. That is why there is a coupling cap- to keep DC from the speaker. You will have voltage on the speaker side of the cap until a load is supplied or speaker is hooked up because that side of the cap will float. The other possibility is that the cap is shorted, though not a common occurrence. Your voltage readings on the output transistors show no bias. I suspect that is because you were on a higher meter scale and didn't note the .6 or so volts that should be between base and emitter on the transistors. You say the amp clips at 8 or 9 on the volume knob, but that doesn't mean much since we don't know your input or output signal level. It's possible that the amp is working fine and you've just reached maximum output before clipping.

Measure base to emitter on the outputs on a lower meter scale and let us know what you have. Also, please measure the output just before clip into a load and post so that we can approximate your power out before clipping.

[garytoosweet]

Meter is auto ranging Fluke 179.

Confirmed there is no voltage on Emitter and Base of Q312/313/314.

I connected a 4.5 ohm load and brought the amp up on a current limited supply. Once there was enough voltage for the amp to turn around (around 85V or so) I was getting pulsating output, that is output for approx 1 second, then nothing for approx. 1 second, repetitively. I continued to up the voltage to 120V (current limited) to see if the issue remained. Every time there was output the 100W current limiting bulb would illuminate rather brightly.

Here's a short vid of the scope showing what I am trying to describe: https://imgur.com/a/TEgTzY0

There was no longer DC on the speaker outs with a load connected.

[g1]

No DC on output? Then take it off the current limiter. The limiter is creating the pulsing and you don't need it anymore.

[garytoosweet]

Right, the current limiter was creating the pulsing. Haven't seen that before and thought it was alarming. Going to think about why that occurred.

56mv sine into Low input.

Q307/308/309 is reading: C: 78VDC E: 43VDC B: 43VDC

Q312/313/314 is reading C: 42VDC E: .170VDC B: .545VDC

22.7VDC on speaker output just prior to distortion. Into 4.5ohm load if P = E2/R the it's outputting 114W. If we change R to 3.2 ohm as stated in the technical specs (attached here) then we get 161W which is just under the 205W stated. Mind you the 205W is at stated 5% distortion. Is this close enough?

There is the slightest bit of crossover distortion visible on the scope. I plugged in a speaker and guitar, there is good output but audible crossover distortion. Tomorrow I will trace from the output backwards with the scope and see if I can find when the crossover dist. occurs and find the cause.

[garytoosweet]

One thing to add "Standby Consumption" is listed as 50W in the tech sheet. I'm assuming this means amp on no input/volume at 0?

I'm drawing max .2A from 120V, putting me at 24W, half of the listed power consumption.

[The Dude]

Are you sure it's crossover distortion you're hearing and not asymmetrical clipping? I'd look at it on a scope to be sure. Or, maybe you already did? R304 is an adjustment for symmetrical clipping.

[J M Fahey]

No DC on output? Then take it off the current limiter. The limiter is creating the pulsing and you don't need it anymore.

DC on output is not relevant here since Acoustic 370 is capacitor output.

Instead, health comes from having positive leg of output capacitor "about" halfway between +V and ground.

Shorted transistors may cause +B or 0V there.

[garytoosweet]

I did see crossover distortion on the scope, and I had previously adjusted R304 according to the schematic.

Because of the low idle current (and due to another similar 370 thread on the forum) I suspected the bias resistor R319 680 ohm. It had actually been swapped out, probably by me when I was replacing out of spec resistors, but it had a 6.8k in there! Changed back to 680 and crossover distortion is gone.

Now getting around 25V on the output just before breakup on the scope, which puts me at just under 200W, very close to spec.

Output cap voltage is almost exactly half of V+, within a couple tenths of a volt.

So the amp seems to be working fine. There are a few other small issues to address like failed inductors for the graphic eq, but should be simple.

Thanks to all

[g1]

One thing I forgot to mention with regard to your post #5. Your power calculation must use the load resistance you are actually connecting to the amp. If you want to calculate with their 3.2ohm resistance from specs., you must use a 3.2 ohm load resistor in your test circuit.

Into 4.5ohm load if P = E2/R the it's outputting 114W. If we change R to 3.2 ohm as stated in the technical specs (attached here) then we get 161W which is just under the 205W stated.

[vintagekiki]

Acoustic 370 is the amp ...

Questions
- Where you measure "Right now it has ~30VDC on the speaker outs"
- Where you measure "on positive side 37VAC 47VDC"
- Where you measure "on negative side 37VAC 28VDC"

When measuring on speaker outputs (J404) load must be connected.
DC voltage at C404 is measured on its positive side relative to gnd.
The voltage is of the order UC404 = (1/2 UC403) = 45VDC and is adjusted with R304.


https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/50421-acoustic-370-repair

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/48742-acoustic-370-output-distortion-problem