Acoustic 370 Goes dead after 4 seconds

[ixe13]

Hi all,

I've got this Acoustic 370 head that I owned for the last 20 years, always worked perfect.

But now, this is what happening:

I turn it on, I hear the first thump on the speakers.
Than I can hear the hiss that quiet down in 2-3 second and while the hiss is present, I can play a few notes before what I'm playing fades out along with the hiss.
Then, dead, no more hiss, nothing...

I quickly mesure the voltage on the big electrolytics caps cause its fairly easy and fast), and I noticed the 3900mf one mesure 85 Volts when it should be around 45. Scary...

The 2 other one are fine. 85V on the7800mf and 57mf on the 1000mf one.

By the way I replaced the C304 (100mf), C305 (10mf), and C307 (10mf). Electrolytic caps on the power board, cause I could see that both the 10mf where leaking a bit. So I decide to do a maintenance job and replace them

The amp was working before I changes those caps but was kind of noisy lately: small scratchy pops. "Crackles" you know?

It's sad, it's been a friend for a long time and now it's sick

I would be very grateful for any help as to where to start to look for some troobleshooting. I suppose those 3 caps to start with?

By the way, I have some basic knowledge in electronics, I built 4 HI FI tube amps from scratchs, so I know how to work safely in an amp, but I'm a total noob when it come to work on a Transistor amp.

Thank in advance


Yves

[Enzo]

OK, so the powr amp is drifting over to V+. That 3900uf cap blocks that DC from your speakers.

SO when it does this, look at the schematic. See all those rectangle boxes with voltages? How do yours compare?

[ixe13]

OK, so the powr amp is drifting over to V+. That 3900uf cap blocks that DC from your speakers.

SO when it does this, look at the schematic. See all those rectangle boxes with voltages? How do yours compare?

Guy's, you wont believe this: I started to mesure the voltage as suggested by Enzo (thanks so much Enzo for taking time to guide me) and while doing so,
I noticed one of the cap I replace (C305) had the polarity reversed. Duh!



Now please dont laugh!

Not a great start.

I feel stupid, a real noob mistake. ...Ah well!



Hopefully I wont be back here with more goofy mistake like that. At least it provide good entertainment...

On the good side; the crackling sound is gone, yeah! and the voltage is back to normal on the 3900mf Cap. Well, almost: 42.5V. Should I replace it and the cap that was reversed since they might have been badly "beaten" with the mistake I did?

Thanx!

Yves Simson.

[52 Bill]

On the good side; the crackling sound is gone, yeah! and the voltage is back to normal on the 3900mf Cap. Well, almost: 42.5V. Should I replace it and the cap that was reversed since they might have been badly "beaten" with the mistake I did?

You probably should replace both of them, but certainly replace the one that was installed backwards, as it was damaged by the reverse voltage. The crackling and popping that you were hearing was probably the sound of the cap being damaged.

[ixe13]

You probably should replace both of them, but certainly replace the one that was installed backwards, as it was damaged by the reverse voltage. The crackling and popping that you were hearing was probably the sound of the cap being damaged.

Hi Bill,

Actually, the crackling was before I replaced these caps. That's what prompt me to have a look around inside and see those 2 caps that were leaking.

And after reading here on this forum and the unofficial Acoustic forum that these 30 years old electrolytic caps should be replace, I decided to do that "maintenance" bit.

Thanx for the advice, I just tried it with my P. Bass. The amp is its old self again. But I wont use it extensivly before replacing both caps.



Omer

[Enzo]

The backwards cap should be replaced as no longer reliable. The 3900 uf cap at the output would not have been damaged by the circuit failure. You might want to replace it because of age, but it did not suffer from the voltages that were present.

[tbreeden]

I just worked on a 370 with issues you first described. Sometimes the volume would just fade away and them come back another day. At first I suspected the muting FET Q313 or the delay capacitor C315, but I found the B+ for the muting circuit was low. It needs 4.5 volts to hit the threshold voltage of the muting FET and I measured an unstable 7.5 volts. The spec is 10.4 volts.
When I measured R330 in circuit, it was open. After I pulled it, it measured OK. But I replaced it with a carbon composition resistor and the muting B+ went up to 10.3VDC. No more problem. I have found other high value carbon film resistors to go intermittent or open after aging several years.
This muting circuit is for delayed signal turn on and protection for output transistor imbalance. It doesn't seem to have any AGC function.

Biasing the amp, I used their spec of 720MV at the base of Q310 and 160MV at the base of the output transistors Q312, 313,314. I changed R319 to a 1K VR to adjust the bias.
When I measure the emitter resistors, this translates to about 10MV drop across each resistor. These are the large black resistors at the end of the board, .1 ohm 10 watt. 10MV is a little high just for eliminating crossover distortion, but in reading some of these posts, higher bias cleans up the tone, so I'll go with their recommendations.
Bias the input stage as well. Adjust the VR on top, R304 for 11 volts at the base of Q301. It adjusts slowly while C304 100uF charges and discharges.