problems with 15 volt power supply in Eden amp

[bnwitt]

Ok, I've got an Eden bass amp that was brought in by the owner saying it smoked and smelled but then kept working. I put it on the bench and injected a signal which came out just fine and with plenty of power. After a close inspection I couldn't find any burned components.

After a few minutes of operation a 22 ohm 2 watt resistor (R101 on the schematic) in the 15 volt power supply smoked. Upon inspection, I noticed that the nylon screw holding down the heat sink of the TIP42C power switching transistor was broken allowing the heatsink to rise above the chassis. I figured that overheated the transistor and burned the resistor. I don't know if this was related to the previous smoke incident.

I replaced the 22 ohm resistor with a same rated resistor, replaced the TIP42C (shown as TIP32C on the schematic) and the nylon screw. I turned on the amp and the same resistor smoked again. I've checked all of the diodes and resistors in both the negative and positive sections of this stage of the power supply and they are good.

Attached is the schematic of the power supply. Any ideas from anyone what would blow this 22 ohm resistor over and over? I'm going to replace it again and disconnect the connector going to the preamp to rule out any downstream problem.

Any advice is appreciated.

Barry

[Jazz P Bass]

Yeah, I would disconnect P7.
Power it up & see what the +15 & -15 rails look like.
With P7 disconnected, take a resistance reading at the header from pin 1 to 2. That would be the +15V down stream.
Then check pin 3 to 4. See if they are close.

[Enzo]

R101 and the TIP are pass elements. If the resistor burns up, too much current passes through it. TIP 42 is just a higher current transistor, that should be fine in place of the 32.

I agree, disconnect the preamp. I have no idea where else the +/-15v go one the same board though. Monitor voltage across that 22 ohm resistor, that will tell you the current flowing. Is it high but stable? Or does it slowly ramp up? For that matter, is that 15v rail steady or does it start ramping up?

Possibly concerns within view. leaky C50? That 1N4746 D101 is protection. It is an 18v zener across the 15v rail. If its zener point has dropped, it may be trying to shunt the supply. Or if the 15v rail has climbed over 18v it is just doing its job. testing the diode with a meter will not reveal that. Likewise, a bad D56 could look OK on a meter but have its zener point up to 20v, which the 15v rail would try to become as well.

And if the thing works OK without the preamp, monitor that current again and plug the preamp in. Be prepared to unplug it fast. If the preamp is drawing excess on the -15, then we find out why.

[bnwitt]

Thanks JPB and Enzo I'll follow the steps mentioned.

[Billy R aka DynaFreak]

I am working on the same amp with the same fault, in my case caused by AC mains problems at the venue. R101 smoked. In addition to Q55, Q54 and D56 were bad. Since I had the board out I pretty much rebuilt the circuits on both rails. I am working on the outputs now. All were blown.

[bnwitt]

Billy,
Do you have any other section schematics for the amp? All I have is the one posted. If you do, I'd love to get them.
Barry

[Billy R aka DynaFreak]

Kevin at Eden (US Music Corp) was very helpful, even sent some parts I had a hard time finding, I would get in touch with them.