Acoustic 370 troubleshooting

[Grant]

i reposted this since i may have put it in the wrong section...

i recently acquired an old 370 head in an alley and assumed it would be pretty bad off, but it sounds alright, just much quieter than it should. i love 370s and would like to get this freebie running, but need some pointers in the right direction.

the 300 hz fader does nothing, but i think that the problem is in the output. tone and dynamics sound alright untill the volume pot is at about 10 oclock (but the amp still should be louder from the get go). once at 12 oclock and higher, the output "ducks" down when a loud note is played. it sounds like there could be a bad connection somewhere because the sound changes once in a while, but still it should be alot louder. i resoldered a few things, and didnt see anything obviously wrong.

glad to answer any questions. any help would be greatly appreciated!

[olddawg]

Sounds like a great score. Right off, with no troubleshooting and a big guess, I would say shotgun every electrolytic cap in it. Wouldn't cost a lot. Also meter out those pots and check the grounds. Make sure all of your power rails are correct and symmetrical. I would use a scope, but if your bench isn't that sophisticated and you have to shotgun a guess, that's a good inexpensive start. Get your polarities right when replacing the caps.

[Enzo]

I'll wait for the dust to settle, I just responded in the other section. See that.

[technicalxpectations]

370 eats outputs and draws over 3amps idle

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... 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 and climbing as soon as I tie in the power amp module onto 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!

[gbono]

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... 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 and climbing as soon as I tie in the power amp module onto 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!

There are multiple topics/discussions about the 370 over here http://unofficialacousticcontrolcorp...yuku.com/topic

This post came from the original designer regarding substitute output BJTs:

270/300/370 output transistors.

Posts: 7
Dec 27 10 12:05 PM
Reply Quote MoreMy Recent Posts The RCA 2N3055 (48-15, 480015 Homotaxial devices) were 100% tested for SOA (80V @ 1.25A to 80-100ºC). Not matched. The copper wiring of the modules along with the Vbe's of the RCA devices acted as the Emitter Resistors. If some one rewires and or changes the wire size as some repair tech have done, then FLAMES might be seen or the top copper trace on the pcb will open. (Thermal runaway after a short period of time,)

I do remember that both RCA and MOT changed the Mfg process in mid 70's and the parts were still "2N3055" however the stamped flange ID stamps were changed.

New output devices may need some additional emitter resistance; however the real problem will be stability as the outputs today have much greater Ft's. The Bandwidth of the Homotaxial devices were 600-800 KHz the Epitaxial-base are 2-4 MHz. Some of the common output devices today have Ft's to 10-15 MHz. Any one who uses the newer devices will have INSTANT FAILURES and SHORTED OUTPUTS unless you redesign the amplifier. You need to run a Bode plot and verify the phase margin.

Remember to verify any repair and always use an AC INPUT POWER WATTMETER. All of the benches had a variac and wattmeter as part of the basic test equipment. For the turn on test of the repaired amplifier use a LIGHT BULB (100w) in series. This will light bright upon turn on and darken after caps charge. No load on the outputs, then apply signal and the light shall remain dim unless bias is running away or output instability.

During my years @ Acoustic, BGW, JBL, Mackie and Audio Precision I have many short cuts tryed by technicans to repair products. Use yoy Brain.

I hope this helps. Best Regards
Duke.