Let the smoke out of a Concert Bass

[Frank9]

Or the guy who owned it before me did. Right now, it powers up and music will come out if you plug an instrument in. It hums a lot though and is very quiet. I popped it open and there's a couple resistors in the power section that are obviously burned out. Everything else looks fine but I haven't done any further testing. Any insights or advice? I am pretty handy w/ a soldering iron but new to the whole amp repair bit. Read up and know all about safety and so forth and just trying to diagnose this problem, I hear it's fairly common. It can't be as simple as the resistors going bad and simply failing, burning up and needing replaced can it?

Thanks,
Frank

[mckinnon audio]

  Hi there,usually when resistors burn up or fail,it's because to much current is being drawn through them.When you say power section do you mean power supply or output stage ?If it's power supply,you might have a bad cap. or diode.If it's output stage,it's probably a bad output transistor.Hope this is of some help,good luck Mel.

[Frank9]

Thanks for the info. I will try to post a picture of it once I can find the cable to connect my camera to my computer. This is the only schematic I can find and I'm not sure it's the same. The resistors that are burned out are 22 ohms. I am going to have to have a better look at the amp but if you put a gun to my head, I'd guess they were either r302 and r305 or r322 and r306.

[mckinnon audio]

  Hi there,check those resistors,they're probably .22 ohms @ 5 watts or more,not 22 ohms.I believe they're the emitter resistors in the output stage.If one or more of those is burnt,you've got a (some) bad outputs.Good luck,Mel.

[Frank9]

I double checked and they are definitely not the emitter resistors. Those are actually .27 ohm (hard to read on the schematic) and ten watts. There is something funny w/ one of those though. It's reading .4 ohms so maybe that is the cause of my problems right there.

Is there a way to look at the bottom of the board w/o having to take the whole thing apart?

[george]

The Concert series amplifiers use crimped pins to make connection with the test point like pins on the various PCBs. I borrowed a $85 crimping tool from a friend when working on these amps since I usuall break several connections when working on the PCBs. I have also experienced the handwork of previous owners (techs) who solder the broken connectors instead of repairing them.

You can get the PA board to tip up so you can remove components but I bet you'll break some of the 35 year old contacts in the process. Good Luck.

[Frank9]

Yeah, about half the connections on mine were soldered already. I was just really, really careful. Anyway, I think I got it worked out. I replaced the fried resistors. Only one actually fried of its own accord and the other was just fried by being next to the other one. Three of the five transistors were fried so I replaced those as well. Has a real righteous clean sound (at least at lower volume at home, can't crank it yet) but the distortion doesn't work. Anyone know if that's just because I don't have a footswitch and if there's some way around it?

Thanks again to those who helped. Never would have figured it out on my own.

[george]

Glad to hear its alive again. Do you know what year it is? It sounds like a silver/black face from the 70's. If it is, any chance that you could send a picture or two of the chassis - I'm working on a 1972 version of the same amp and I'm not sure how the +25V supply is connected to the PA board. The schematic on this site is for a slightly later version and this amplifier has several stock resistors not shown on the new schematic.   THX

[Frank9]

Mine is a 1973 or so the little sticker inside says. Knobs for volume, bass, mids, treble and distortion if that helps you date it. I think I might have some suitable pics already but I don't have the cable to get them off my camera. Once I get that, I'll try to post whatever you need.

[george]

Thanks - gbonosurf@yahoo.com

[Frank9]

Finally broke down and bought a new chord for my camera. Hopefully this is better late than never.

These are the best I have:





I can get some better ones if you still need them.

[mckinnon audio]

  Hi there,I couldn't help but notice the cap. in the first pic.,I believe it's a 1000mfd electrolytic,and it's shot,you can see the dialectric oozing out under the lead.It's off to the right of the resistor line up.Check your pc board traces,maybe this cap.had something to do with your res.'s burning up,good luck Mel.

[Frank9]

Thanks for the tip. Are you talking about that sort of beige colored goop stuff? I didn't know what that was. There's actually a couple like that. You can see one in the preamp too. I got it working and have ran it hard four or five times now and it seems to run just fine. Still, I can't imagine having busted caps on there is doing me any good.

[mckinnon audio]

  Hi there,yes it's the beige coloured stuff,not to be confused with the orangey coloured stuff,that's silicone to hold the caps in place.There's a little "blow hole" on the positive end of an electro. cap.When you see the dialectric has oozed out,that means the cap. has been heated up and will eventually short out,could happen at any time. I would replace any cap. that looks like that or is swollen a bit by the "blow hole".Good luck Mel.