Acoustic 370 Repair Procedure

[Phostenix]

I just finished going through 2 of these bass amps, so I want to document what I did with them (the internet is my notebook )

Here's my repair procedure:

Acoustic 370 Repair Procedure

1) Replace all output transistors Q306-Q309 and Q311-Q314
Match transistors for Q307-Q309 and Q312-Q314
RCA 2N3055 Hometaxial - 2N3055HOM - Transistor, NPN silicon. P/N: 2N3055.
RCA 2n3773 - 2N3773 HOUSE - Transistor, NPN. House No RCA1B01.
MJ15015- MJ15015G ON Semiconductor Bipolar Power

2) Replace driver transistors Q305 & Q310
RCA 40409 & RCA 404010 (integrated heatsinks) or 2N5320 & 2N5322 (add heatsinks)

3) Check all resistors & caps around the output & bias circuits.
With Q305-Q314 removed, it's easy to accurately check the values of most of the related output section components without having to lift them from the PCB.

4) Replace bias transistors Q303 & Q304
2N2222 or 2N2484 or MPSA09 for Q304
2N3053 or RCA 40408 for Q303

5) Replace diodes CR306-CR310
1N914 for CR306 & CR307
1N4731A for CR308
1N4933 for CR309
2 1N914 in series for CR310 or 1N3886 or MR2361

6) Replace R339 & R340
100 ohm 1 Watt Flame Proof - mount 1/4" up off the PCB (I've seen these burn)

7) Replace R319
680 ohm 1 Watt Flame Proof - mount 1/4" up off the PCB (I've seen this one burn, too)

8) Replace rectifier bridge (both amps I had with blown output transistors also had blown rectifier bridges)
BR401 or equivalent (like GBPC4002)

9) Replace all electrolytic caps in preamp & power amp
13 1uF 35V
3 2.2uF 35V
1 6.8uf 35V
3 10uF 50V
3 100uF 50V

10) Replace power supply caps
1 1000uF 100V Screw Terminal
1 3900uF 100V Screw Terminal
1 8200uF 100V Screw Terminal
New Caps are smaller diameter than the old ones, but the biggest clamp can be removed & replaced with a new 1 3/8" clamp (Mallory VR3A). Shift the 8200uF cap over to the clamp where the 3900uF cap was, put the 3900uF cap in the new 1 3/8" clamp.

11) Replace C401 & C401 EMI/noise suppression caps with current safety compliant X2/Y2 class caps.
C401 can be either X2 Class or Y2 Class. C402 MUST be a Y2 Class.
PME271Y547MR30
C401 can be mounted directly on the power switch, if preferred. If this cap is not present, you will get a loud pop when powering the amp off.

12) Check L301 & R328 for open

13) Check R131 100K (notes say a drift here will cause motorboating - change to metal film?)

14) Check R314 270K (I had one that read 410K instead of 270K)

Optional:
15) Replace C311 & C312
.002uF 1KV

16) Replace R104 10M carbon comp resistor with 10M ohm metal film & R125 8.2M resistor with 8.2M metal film

17) Replace R330 with 6.8M ohm metal film



Additional parts to have on hand:
1.0 ohm 1/2 Watt(3 total - R321, R324, R326)
2N4248 or 2N3906 (3 total - Q102, Q301, Q302)
2N2484 or 2N2222 (9 total)
2N4360 or 2N5462 P-Channel FET (1 - Q316)
1N914 (5 more total)
7A Slo-Blo Fuses
7A or 8A Slo-Blo Fuses w/pigtails for internal fuse

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I marked the components on the PCBs as I worked on them, so here are pics of the preamp & power amp boards, starting with the preamp (I have both versions of the PCBs in the 2 amps I have. C129 in the preamp is on the trace side of the PCB in the earlier rev):

[Phostenix]

More preamp pics:

[Phostenix]

Power Amp Pics

Power amp pics:

[Phostenix]

More power amp pics:

[Phostenix]

Other notes:

120-130mV of ac ripple on the main filter cap.

Clean output (onset of clipping) of both amps into resistive loads:

8 ohm load ~ 85 watts
4 ohm load ~ 150 watts
2 ohm load ~ 250 watts

[Phostenix]

Here's the full schematic for the 370 on one page. The package from Music Parts has some good info & it's nice to have the parts list, but their schematic is missing a couple of parts in the power amp. This one will print out nicely on an 11X17 page.

[burymyteeth]

hey, thanks for your post, extremely helpful. i'm working on a 370 right now and I'm having some trouble understanding this circuit. I'm losing signal, there is no signal on the base of q302. Therefore no signal on the base of q303 or q304. q302 is where i run into the problem. I've replaced it. My voltages are a bit high, 16V on the base of q302, about 50V on the collector of q304 and q303. This amp had been messed with pretty bad by a tech before me, had cut traces and moved things around. i have another 370 thats functioning and I restored the board to look just like the one thats working. That power amp board is 17-19E. The one I'm working on is 17-19C. Just wondering if you are aware of any differences between the two, I can't find a schematic that says it is specifically one or the other. Not sure if I've given you enough information about what my problems are but figured it was worth a shot. Thanks again for the photos and everything they are very helpful.

[Enzo]

The base of Q302 is a sample of the output through R314. If there is nothing on the output, there will be no signal on that base either.

Q304 is the bias transistor. Q303 is the voltage amp that drives the output with signal. Q301 is what drives Q303, not Q302. You need to find signal at the collector of Q301.


You didn't give us a symptom, may I assume it is no signal at the speaker jacks? APply a signal to the amplifier input and set the preamp controls as you normally would. Is ther signal coming out the Line Output jacks? Is ther signal at the power amp input, the left end of R301? How about the right end of R301? And does the signal make it through C301? If it doesn't get that far, then it sure won;t get amplified.

Look at power amp schematic, lower left. See JFET Q313? That is a mute. If it is shorted or not turned off, it will shunt your signal to ground. JFETs are on until turned off at their gate. Leave power turned on and measure resistance from the junction of R301 and C301 to ground. If you get a low resistance, then it is muting. Measure the same place with power off. NOW the JFET is conducting. Whatever resistance you get with power off should be a LOT higher with power on. If it is about the same, you are stuck.

[burymyteeth]

enzo, thanks for your quick reply. and yes, no output is my problem, could have stated that a bit more obviously. I have signal on the collector of q301. I lose it at q303, nothing coming out. q303 is testing fine, i've replaced it already. something must be sinking it after that right? any ideas? much appreciated! I checked the JFET as recommended and as long as I have this straight its working right: high resistance with power on (about 100K) low resistance with power off (about 300 ohms). Just to restate my voltages are a bit funky, about 5 or 6 volts high, I turned down my AC so my rails were looking about where they should be with no change. Just giving you more info to go off of, but I don't know if this is my problem or caused my problem.

[Enzo]

I'm not worried if the voltages are all a bit elevated. This is a single supply amp. The output should sit at about half that rail voltage - before the speaker cap. So if the rail climbs, so will the voltage things settle at.

If the bases of Q301,302 say 11v on each and they are both 14, I probably don;t care, they are both the same. If I find it unbalanced, like 8 and 14, then I see a problem.

Look at Q304, the bias transistor, see it says 41.7 and 43.7v on its ends? I don't care about the 40 something point 7, but what DOES matter a lot is that 2 volt space the transistor makes. 41-43, or 50-52, close enough for me. It keeps the 2v space. And no matter what Q303 does to it, yanking it up and down to the music, that space is always there. It is your bias.

SO, we inject a strong signal at the collector of Q303, it should then appear at the driver bases and make signal out the speaker. Does it? If so, then everything from Q304 to the right is probably OK.

Yes, your JFET is working right.

"Losing it at Q303." What? Got it as base but not collector? Not at base? SO signal at the top end of R308 but not the bottom end? Or it gets through R308, but won;t drive Q303?

Bang on the power stages: Use a cap - I don;t care, I grab 0.047uf caps because I have bags of them handy - to isolate from the DC, and couple a signal into the collector of Q303.

If I get signal to the base of Q303 and it isn;t on the collector, if my DC voltages are even remotely close, I'd replace that transistor. Just because it meters out OK doesn;t mean it amplifies in a circuit. That is how i will know if it is that or something else.

[Enzo]

I'm not worried if the voltages are all a bit elevated. This is a single supply amp. The output should sit at about half that rail voltage - before the speaker cap. So if the rail climbs, so will the voltage things settle at.

If the bases of Q301,302 say 11v on each and they are both 14, I probably don;t care, they are both the same. If I find it unbalanced, like 8 and 14, then I see a problem.

Look at Q304, the bias transistor, see it says 41.7 and 43.7v on its ends? I don't care about the 40 something point 7, but what DOES matter a lot is that 2 volt space the transistor makes. 41-43, or 50-52, close enough for me. It keeps the 2v space. And no matter what Q303 does to it, yanking it up and down to the music, that space is always there. It is your bias.

SO, we inject a strong signal at the collector of Q303, it should then appear at the driver bases and make signal out the speaker. Does it? If so, then everything from Q304 to the right is probably OK.

Yes, your JFET is working right.

"Losing it at Q303." What? Got it as base but not collector? Not at base? SO signal at the top end of R308 but not the bottom end? Or it gets through R308, but won;t drive Q303?

Bang on the power stages: Use a cap - I don;t care, I grab 0.047uf caps because I have bags of them handy - to isolate from the DC, and couple a signal into the collector of Q303.

If I get signal to the base of Q303 and it isn;t on the collector, if my DC voltages are even remotely close, I'd replace that transistor. Just because it meters out OK doesn;t mean it amplifies in a circuit. That is how i will know if it is that or something else.

[technicalxpectations]

370 rebuild keeps going up in smoke

I took in a 370 and a 320 for repair last december. The 320 was not too bad to get going, but I'm gonna make a career out of the 370 pretty soon! The amp came from another tech who had replaced the outputs and drivers, but didn't have time to finish it. I put it back together, put it on the variac and a few minutes after getting to full ine voltage R326 and R324 burned open. I replaced those and using your post as a guide I went about troubleshooting and rebuilding. Well it's almost 2 mos later and I've fried 2 sets of outputs and replaced the 3900uf on the output (it was leaky) and almost everything in the bias circuit, it still draws 3amps (I only get to about 30 or 40 volts on the variac) with the power amp module connected to the B+. I MUST be missing something obvious here! The power supply puts out about 88 VDC unloaded (i've replaced the bridge rect).

I've worked on several Acoustic Control amps over the years (I've been a tech for about 25 yrs) but this one is "challenging". I've learned much from reading everyone's posts and would appreciate any help.

Thanks!

[waspclothes]

Just wanted to add to this thread -

I recently fixed an Acoustic 370. The output was heavily distorted. The output was only half of the wave, and this output was the same on the "line out" - so the problem was in the pre-amp section.

After shotgun replacing all of the electrolytic caps and transistors, the problem was pinned down to R136 being OPEN. I'm not sure if that's something that happens often with these amps or if it's just a fluke failure, but I thought I'd add it here.

[Sinclairx]

Acoustic 370 blowing and burning

I'm having nearly the same problem with an acoustic 370. It keeps blowing Output transistors and burning the 100 ohm resistor off the driver transistor. R339... How did you get yours resolved? Thanks
Steve

[Sinclairx]

Acoustic 370 smoke

I took in a 370 and a 320 for repair last december. The 320 was not too bad to get going, but I'm gonna make a career out of the 370 pretty soon! The amp came from another tech who had replaced the outputs and drivers, but didn't have time to finish it. I put it back together, put it on the variac and a few minutes after getting to full ine voltage R326 and R324 burned open. I replaced those and using your post as a guide I went about troubleshooting and rebuilding. Well it's almost 2 mos later and I've fried 2 sets of outputs and replaced the 3900uf on the output (it was leaky) and almost everything in the bias circuit, it still draws 3amps (I only get to about 30 or 40 volts on the variac) with the power amp module connected to the B+. I MUST be missing something obvious here! The power supply puts out about 88 VDC unloaded (i've replaced the bridge rect).

I've worked on several Acoustic Control amps over the years (I've been a tech for about 25 yrs) but this one is "challenging". I've learned much from reading everyone's posts and would appreciate any help.

Thanks!

Did you ever figure out the problem? I'm having a similar problem. Thanks
Steve