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Repairing old solid state bass amp

I have an old Traynor 25B that has performed perfectly for many years (around 20 I'd say) until now. When I switch it on, there's this loud humming reminiscent of a mains hum, only much much louder. Playing with the dials does absolute nothing, and feeding it a signal mixes it in with the hum. After some troubleshooting I think I have narrowed the fault down to the power amplifier circuitry. The preamp and power supply is not to blame, I've tested them both. There are three diodes that are destroyed as soon as I switch on the amp, and I can't figure out what's causing them to blow (there are two 1.6A fuses in parallel right after the transformer, but they don't blow). As far as I know, the only way the diodes could be destroyed in this application is through exceeding their maximum forward current rating (200mA). Maximum reverse voltage is 100V, and I'm only getting 50 from the power supply.

Here's an excerpt of the schematic:

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The three diodes in question are the three 1N4148 placed in series in the middle of the image.

I would be very grateful for any help you could give me. My next course of action would be hauling the beast to a repair shop, money I'm hoping I could save by repairing it myself.

Reply to
thegreatpain
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Thanks very much for posting a schematic. This particular problem doesn't ring any bells, but the next thing I'd check is the two transistors (MPSA05 and MPSA55) in series with the diodes. Maybe one of them is shorted.

Reply to
mc

Often the way with fuses around solid-state power amps - the semiconductors tend to protect the fuses, rather than then the other way round....

So one end or the other of this diode string has ended up 'tied' to one of the power rails ? - possibly by the demise of one of the associated transistors... You might be able to find which one has blown by checking continuity from the power rails to the collectors of the transistors - with the power off...

Another (probably cheaper) way to get the thing up & running might be to simply replace the power stages with (for instance) an IC power module - probably under £15 - and you already have the +/- power rails...

Bearing in mind that any decent tech is going to want £25-£50 just to take the back off the thing - then the 'drop-in' replacement looks more attractive....

Regards Adrian

Reply to
Adrian

Thanx for the quick replies! I just wanted to add that one of the TIP's heat up immensely in just half a minute, while the other remains barely luke warm. I do not recall which is which, but surely they should both heat up equally (indicating an equal load)? In that case, I guess I should probably be looking at the cold TIP and its MPS transistors.

Reply to
thegreatpain

As the whole thing is DC-coupled, a hot output transistor could well be caused by problems in the biassing. Try replacing the diodes (?again!) and put a car light bulb (as in stop-light bulb) in-line with both power rails. This will limit the current, and, with a bit of luck, will give you time to do some fault-finding. At a rough guess, the bulb in the line that lights up is associated with the transistor that's failed.....

Give it a try - but be careful - 50V dc = a fair bit of 'energy'...

Regards Adrian

Reply to
Adrian

On 19 Jul 2006 12:36:33 -0700, " snipped-for-privacy@gmail.com" put finger to keyboard and composed:

Transistor MPSA55 is operating as a 6mA current source. That's the maximum current that the diode string should ever see. Other than the MPSA55, I would suspect a problem with either of the TIPs, in which case I'd change them both, as well as the MPSA18, MPS8599, and MPSA05. I'd also check both 0R51 5W resistors.

- Franc Zabkar

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Reply to
Franc Zabkar

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