wrote in message
ps.com...
I succeeded to repair my old Traynor 25B bass amp (which I asked and
received help for in this group a few months ago), however I now have
another problem with it.
Background: amp was producing a loud hum, originating in power stage. I
was recommended to replace some of the transistors, so I changed them
all. Problem solved. Here's the schematic btw:
http://img258.imageshack.us/img258/4232/ampkh8.png
Now at least it sounds as it should, but it quickly becomes devilishly
hot. It features two power transistors (BD911/912, formerly TIP100/105)
screwed on to an aluminium plate which is screwed on to the steel
chassis (applied thermal paste as well). I attached insulating silicone
washers to the transistors, and use nylon bushes to electrically
isolate the screws and nuts from both the chassis and the transistors,
and the ohm meter assures me that the plates of the transistors are not
in contact with the chassis (which is grounded) and thus not leaking.
After only 1½ minutes of operation, the chassis around the BD's are
hot enough to burn yourself on. Ten minutes later, the whole back and
bottom of the steel chassis is scorching hot, as well as the
transformer core! Something is clearly consuming a lot of power, but
the fuses do not blow. There is one 800mA slow blow fuse before the
transformer, and two 1.6A slow blow fuses after, in parallel. Just as
it's always been, as well as according to the schematic.
Voltage levels are fine, at least before and after the rectifier
diodes. The amp did not get near as hot as this when it was in working
condition last time.
What do I do?
When you say that you changed all the transistors, do you mean that all 8
transistors were replaced, or only those that were recommended to be changed?
The 0.5V across each of the 0.5ohm resistors means that the output transistors
are idling at 1 amp, which is WAAAYY too high. The idle current should be less
than 0.1 amp.
The BD911/912 transistors are not good substitutes for the TIP100/105
darlingtons. The BD911/912 units are not darlingtons. You need to get the
correct transistors for the amp. Since the DC voltage at the output is low, the
rest of the circuitry is probably OK.
Get the correct transistors and put them in. That will probably solve the
problem. Be sure to get a matched pair!!!! If you use unmatched transistors,
they will likely be unbalanced enough to give substantial DC at the speaker.
You don't want that to happen. The DC voltage at the speaker should be within a
few millivolts of 0V.
If you can't find a matched pair, you might buy a few of each type and match
them yourself. If you have to do that, ask here again and I can give you
directions or a link that will show you how. It only takes a DMM, a small power
supply or a few batteries, and a few resistors to do the matching.
Cheers!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)
Some days you're the dog, some days the hydrant.