Repairing old solid state bass amp

[[email protected]]

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:
http://img258.imageshack.us/img258/4232/ampkh8.png

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.

 

[mc]

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.

 

[Adrian]

Hi

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).

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

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.

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...

Here's an excerpt of the schematic:
http://img258.imageshack.us/img258/4232/ampkh8.png

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.

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

 

[[email protected]]

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.

 

[Adrian]

HI again

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.

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

 

[Franc Zabkar]

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:
http://img258.imageshack.us/img258/4232/ampkh8.png

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.

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