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Marshall SS hum......no sound

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3/27/2003 5:35 AM

Marshall SS hum......no sound

You know the problem. It's the typical shorted output transistor sound. Loud low frequency hum, no sound at all from guitar. But i cannot find any problems with the outputs as far as shorts or open conditions. This is a 1980something Marshall solid stater. I have checked every component in the thing. However, i am by no means a tech, and checking semiconductors for me is an exercize in looking for open or shorted components. All diodes, electrolitics, and transistors seem to read ok as far as shorts or open conditions, and all IC's are socket mounted, so i swapped them out with known good ones. I also checked all resistors and they're fine.

What would your best guess be? This is a small SS 1-10" combo. Thanks in advance.

3/27/2003 10:09 AM

OK, you have a loud hum coming from the speaker. Is there DC there or just a lot of AC? Watch the speaker cone when you power up. DOes the cone move to a new position and stay there? Or does it just vibrate. DC will move the cone. COnnect your meter to the speaker wires and look for DC that way if you like.

DC on the output means a blown output stage - shorted transistor or power mudule.

On the othre hand, if there is just a loud hum but no DC, then I suspect the main filter caps. Flip the board over and resolder the biggest caps. They could be bad, but probably just cracked free of their solder.

Check the power rails. The ICs probably run on +/-12VDC. ARe they both present and clean? Your output stage probably runs on something like +/-20-30VDC. Make sure both rails are OK. They should be clean and close to each other in reading.

3/27/2003 3:03 PM
Ralph	Thanks. I'll check for DC on the speaker when i get home from work. As for loose filters, i already checked those and all solder joints. The rails......well, other than being a power source i reallt don't know what it is. The outputs don't show any shorts. Thanks

3/28/2003 4:25 AM
Ralph	There IS DC......... Thanks, i got it. The cone does move and stay, and i measured 12 volts DC at the speaker out. Unfortunatly i haven't been able to find a source for the MJ3001 and MJ2501 outputs. So if anyone knows where i can get these i'd appriciate it. Thanks Enzo....

3/28/2003 6:59 AM

Are they shorted? CHeck with a meter - out of circuit now. Those MJ numbers are stocked at Mouser and I am sure ALlied as well. Mouser only wants like $1.75 each.

MAke sure some other parrts are not bad. This is a solid state amp, so you do not need to connect a speaker for testing. Save your speaker, use a scope or a meter to monitor the output. With only 12 V on the output instead of the full rail voltage, it may not be an output device but soimething further back.

Rails? Just another word for buss I suppose. There are High voltage rails in a solid state amp. High being relative. Maybe +/- 30 VDC. There are low voltage rails for the preamp - usually +/-12VDC or 15. To check them just means find the source of those voltages. The typical 8 pin dual op amp is powered at pins 4 and 8. So find a chip and check the voltages at the corners. We want them both the same more or less except for polarity. HV rails are usually on the output transistor collectors.

3/29/2003 12:40 AM

The outputs are not shorted. In fact, i have checked every component in the amp and nothing is shorted. Anything that read short i took out of circit and then did not. The outputs i checked in and out of circuit. They read fine as far as i can understand........no shorts, not open except when probes reversed. So it seems fine. All diodes read one way, not shorts. Caps seem fine as far as shorts.
So if like you say it doesn't seem like an output, i don't know where else to look except maybe the power supply, but then i've checked that too. And I did check all the IC'S first thing, and they are fine......the voltage is there (about 13v) and swapped em out with no change.

Well, thanks for trying....i'll keep messing with it to see what i may have missed. One thing tho....i can't help but think it is somehow an output because the way the cone moved and stayed as you said, and because i have heard this same sound a number of times and it was always the outputs.

Thanks...

3/29/2003 12:44 AM
Ralph	oh, by the way.... I forgot.......the IC's were different at the positive and negitive pins.....13 at the positive i think, and 15 plus change at the negitive. (may be 15 pos, 13 neg, but i think i was right the 1st time)

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