Acoustic Control Corporation 150b-Low steady crackle issue

[wardevans]

Hello! The Acoustic 150b was blowing fuses so I opened it up and I found two output transistors blown, replaced them, all good there. As well I changed out the thermistors because they were reading at 80 ohms when they should read 100 ohms cold. Fired the amp up and now there is a steady low crackling sound out of the speaker that does not change with the adjustments at any volume or tone. I have nothing plugged into the inputs and it does not change when I plug an instrument in. It sounds like bacon frying if I were standing right outside the kitchen. I checked the power amp board and found a few questionable connections there, I re-soldered a few connections at the 400 ohm resistors.

I have cleaned all pots , jacks, went through all resistors in the power board, replaced the output cap (crackling was there before the new cap but I figured it wouldnt hurt) I did check R404 and it was reading 30 ohms, so I replaced it, no change. C404 I jumpered in a new cap and that didnt work either. I checked Q301 and 302 out of circuit, they are both reading fine. As far as the power transistors, I originally did not replace as a set, I used NTE-130 subs for the two that were blown and decided to swap out the other two but I only had one more 130 so I used one 2n3055 and three 130's. The filter cap I could try swapping out but the smallest size I have is 20,000 mfd, the voltage is correct though.

[wardevans]

bump.

[Enzo]

If you have narrowed it down to the powr amp, then keep looking. Scope through it. get a can of freeze spray and individually chill each semiconductor and see if any reacts to the cold.

[wardevans]

Well Enzo, I tried to "freeze" the output transistors one at a time and the "frying bacon" is still there. I do not have a scope here so thats out of the question. A few things I did find: a 680 ohm resistor in the power board, that was reading at 770 ohms out of circuit, changed that out and the two 6.8 ohm resistors that are tied to the thermistors are reading 8.2 ohms each out of circuit, i have to get replacements for those. Just to summarize, when troubleshooting this from the start I made sure to isolate the boards: I completely disconnected the two pre-amp boards from the power board and the "frying bacon" sound increased in volume and opened up, as in sounded less compressed. The only thing I havent done is to change the smaller capacitors on the power amp board. What do you think?

[Enzo]

Isolate the problem.

This is a simple amp. The output side is transformer coupled. it will sit there all b y itself just fine. So looking onthe drawing, the drive side is the blue and red wires, driven by Q302, a 40410. Disconnect the red or blue wire to open the drive circuit. Now if there is still bacon, it is on the output side, and if the bacon is gone, it is on the drive side.

Your problem is noise, not loss of function, so the transistors will all test OK, since they work. Your tester won;t find noise.

C301 1uf/input cap to power amp. I'd change it. Does the bacon stay while it is removed?

[wardevans]

thanks enzo. i disconnected the red and blue wires, bacon gone. however, i removed the 1uf input cap from circuit, re-connected the red and blue wires, now bacon back and much louder than before. hopefully the q302 is not leaking, i understand these are difficult to find. what are your thoughts from here?
thanks again by the way.

[Enzo]

Disconnecting the preamps made the bacon louder. disconnecting that 1uf cap also did. Those things were parallel ground paths from the input stage of that amp. Disconnect it and the noise is less loaded.

Seems to me 40409 and 40410 were on sale at Magic Parts. Just google them and see what pops up. Do not use NTE. The NTE cross wiill not have the heat sink.

We have eliminated half the amp. Your noise is after the 1uf cap and before the splitter transformer. That leaves two whole transistor circuits. You sho0uld be able to determine the source from here.

[gbono]

Are you sure that your problem is not coming from the 17-10 pre-amp boards? I assume you have lifted one end of both R301/302 on the 17-12 board and you still have a noise issue?

Pop corn noise on these old ACC amps can be a range of things. You've checked all the bypass and decoupling caps? I usually find the pre-amp transistors are noisey - though I do not have access to a good curve tracer to verify them as such. Also check solder joints on R311 while you have access to the 17-12 board.

Here is my fix for Q302 - though many folks solder in a TO-220 or 125 I replace this driver transistor with a 2N4033 (or equivalent PNP). Take a punch and drive out the old transistor from the heatsink - you may need to carefully ream out the HS for the new transistor package.

BTW - where did you find thermisitors for R317/316 (GE- 2D304) - These usually do not fail - make sure you are at 25C when testing them.

[wardevans]

Well,um,well...update, checked everything after c301 (input cap) and before r310 (4.7k) and everything checks out. I pulled the mpsa09 and checked it out. The only thing I can think it to be at this point is the 40410...I checked with Scott at Magic and they do indeed stock them. One more question here, are there in circuit tests I can do to make absolute sure there is leakage? Thanks for all your help!

[gbono]

One more question here, are there in circuit tests I can do to make absolute sure there is leakage? Thanks for all your help!

You will need either a Tek 576 curve tracer or better yet an HP/Agilent 4145B (now discontinued) to ACCURATELY measure the leakage currents of the EB, CB and CE junctions.BTW did you freeze spray the 40410?

Buy the 40410 from Magic Parts (they're 2 miles from me) it will be faster.

BTW, wheredid you findthe thermistors?

[Enzo]

How did you check out those transistors for noise, especially by pulling them out? A hand meter will tell you nothing. Your amp works, it is just noisy. That may or may not be involved with some leakiness, but leaky and noisy are not the same thing. Noise like that is usually a semiconductor junction breaking down, but not so you could read it on a meter.

Unless of course you have that curve tracer, you could watch the breakdown spikes on the screen.

You have a MPSA09 and a 40410. You'd have to order the 40410, so swap out the MPSA09 with somethihg - most anything. it is a 50v NPN low noise. Slap some other NPN in there with enough voltage for the circuit. If the noise changes, then you found it. If the new part is noisy too, I'd bet the noise sounds different, so you could still tell that the part removed was indeed the problem. Now you'll know if you still need a new 40410 or not.

[wardevans]

Nailed it! It was the mpsa09. I replaced with a transistor I had from an Sunn Alpha power board, bacon gone! Enzo thank you for helping me troubleshoot this. I am glad I ran into this issue it is a new thing I hadn't experienced with these amps. Thanks again!

[wardevans]

gbono, I did use freeze spray on the 40410 and of course it changed nothing. The thermistors I found at an electronics surplus store, they didnt realize they even had them, I found them buried in a pile, so I couldn't source them, sorry. I bought them out of the 10 they had because I knew they are kinda hard to find at local stores. They may have more there, but who's to say? By the way thanks for your insight on this too. Im thankful we got rid of the bacon! I am considering ordering some 40410's to have on hand. Scott at Magic is a real good guy and I would feel good supporting their biz.

here is a link to the place I found the thermistors:

http://www.cascadesurplus.com/

They are real nice dudes, sometimes you are best off just calling them.
Again not sure if they have anymore in stock or not.

[Enzo]

Great!

Many different transistors would have worked fine there, if you have no MPSA09.

[gbono]

MPSA09 = PN2484

$.22 at Digikey - low noise transistor for audio application used all over Acoustic amps.