Input transistors burned/replacements

I have finished my Pass preamp, but was so stupid to forget ground connection from PSU to line stage before plugging in to my power amp. Drains on my FETS measured 118 volt, where they should have been 30 v.

See this thread:

Problem fixed, unfortunately my new pre was plugged into my power amp, Audio 60 (Doxa) from mid 80s, while I was testing.
My stupidity has blown away my Audio 60. When I turn it on, either using my my old pre-amp or my variable cd-direct, it get this VERY HIGH houling sound. Guess my power amp was getting 118 volt or so from the pre. My speakers are OK.

A friend of mine believes that my NEC transistors at the output are not damaged, only the small signal transistors in the input.
See photo:


Red arrows are two BC 557B and two B547C.
Yellow arrows are two BD 529 and two BD 530.
The black heat marks over the resitors are NOT from this mistake, since I saw it long time ago !

I have the following questions:

1. Is it likely that my NEC power transitors also are damaged. Would it be able to make make this howling sound then?

2. BC 557B and B547C are easy to find today. Are there any better ones I could use instead? Better quality, I mean. I can't find BD 529 and BD 530 anywhere on the Internet. Which other good types could replace them?

3. Would 118 volt at input normally burn off all eight input transistors, or could I hope for that only BC 557B and B547C are burned?

4. How do I measure using a multimeter if a transistor is intact?



2000-11-24 12:02 am
Your second option is correct. At least according to my
old transistor databook.

so 1 = E, 2 = B, 3 =C where pin 1 is the left pin looking
at the markings on the transistor and the leads down.

BC560B and BC550C are a little better than BC557B and BC547C
Other (better) selection could be small signal japanese
transistors. You will need to select for equal parameters though.

If you have an analog multimeter, you can use the following
Put the meter in the lowest resistance range x1

B to E must show slightly more than half.
+ to B, - to E
And B to E must be open
- to B, + to E
or the other way around

Same for B to C

C to E must be open (no zero resistance)
so + to C, - to E must be open
and - to C, + to E must be open

When the transistors are damaged, it is most likely that you
will measure a short from C to E.

These measurements are meaningfull only with the transistor
out of the circuit. Or disconnect at least two of the three pins from
the circuit.

If you have a digital multimeter and you have a diode
measurement range, select it, and follow the above procedure
Your readout should be something like 0,5 to 0,7 or open ( mostly your display will show a left most 1)

Do not forget to completly disconnect your amplifier, before you
do any of these measurements.
Also check to see if the powersupply capacitors are discharged.
If you omit this you will have more trouble!

You might also like to check(and or replace) the input capacitors.
I do not think they liked 100+ volts. (Check for short)

Hope this helps a little and please be very carefull.
All 16 transistors now changed, but more parts must be damaged, since it 's still whining like crazy.
I used the BC560B and BC550C as recommended.

Next thing I'll change are the film caps. Left side has two 1,5 uf 250 v MKP1841 (these must have been put in sometime during another service), right side has two 1,5uf 100v MKC1862.
Guess I'll try Mcap's, cause these I also use in my Nelson Pass pre.
Would damaged input caps cause this whining?

However, left channel with these 250v's also is smashed and might indicate that other parts besides transistors was wiped out...?

What a stupid mistake I did


2000-11-24 12:02 am
Were all transistors damaged? Or you just changed them.
MKP at 250V is able to withstand 100+ volts. So I do not think
they are damaged.
btw you are using old test speakers I hope.
Could you check for any DC at the output?
Do you have the schematic, and if so could you post it?
Did you measure any resistors on the input cicuit and check for correct value?

A couple of the BC 557B and B547C were intact. Alle BD 529 and BD 530 were damaged (almost certain).
DC-check: Would you please give me small guide, using my digital multimeter.
No schematics unfortunately, bought second hand. This amp is from mid 80's, and I've searched Internet unsuccesfully.
I will now check all resistors at the input, but I guess I will have to solder them out.
Here is a close up of the input part of the PCB.
Notice my bungling mounting of the BD243C and BD244C using wire to swap pins :(



2000-11-24 12:02 am
DC check
Power off the amp
Disconnect anything on the input of the amp.
Better would be a shorting plug on the input.
Take two old rca connectors and put a small wire from the pin to
the shield in each. Plug these to the input.
Connect a dummy load or an old speaker on the output.
Take your digital multimeter (dvm) and select the highest DC
voltage range (usually 500 or 1000V)
Connect the leads to the dummy or speaker. black to -, red to plus.
Check the readout. If you do not get a readout, disconnect one
lead select the next lower range and reconnect.
Their should be less than 100milliVolt on the output.

I will check for DC asap. Need to check up on a few things first, cause I'm afraid to shorten the amplifier, it has no fuses or protection for the output.

These diodes (?) at the input with "220 H" on them wouldn't they have been blown along with the transistors?


I hope I'm going to use this great amp again. The mid and top are really superb in my opinion.
However if NECs at the output are gone, I maybe have to give up. I read somewhere that Doxa used expensive, liniear and fast NEC's, propably not made today.


2000-11-24 12:02 am
220 H is a capacitor 220pF. They look like the styroflex kind.
I guess they form an input filter. Check to see if one side of this
cap is connected to ground.
If it is, you could temporarly disconnect it, and do a quick test on an
speaker. If they are damaged, you will know.
jubii, it plays after having disconnected the top cap 220 H
It plays really like nothing is wrong with it even without the cab. Sweet music.
I'm flying :)

Which replacements should I look for?
Is 220 H a TCP code?

There are three of these on each side, so I guess I should replace all of 'em.

The reason I didn't perform the DC-check is that I'm afraid to shorten my amp with my multimeter.

I would do this, like you described:

1. make the shortened RCAs and plug them in at input.
2. Connect my test speaker to terminals.
3. power on amp
4. put red multimeter pin on amps red loudspeaker terminal with speaker connected, and the black multimeter pin on amps red loudspeaker terminal with speaker connected
5. measure dc-voltage

is this correct?

Thanks again.


2000-11-24 12:02 am
I am happy to see the problem is located.:)
I do not know the exact meaning of the H on the styroflex cap.
My styro's all have a J marking. I suspect it indicates a
voltage range.

So, replacements should be 220pF either styroflex, (I do not like
them, to be honest) or Silver Mica.
Please do not use any ceramic disc type.

Also do not use the amp without them for a long time. Replace
them first.

DC measurement as you describe is OK.

Found some Silva Micas at RS-Component's website I will order...

While I'm doing some soldering on the amp, I'll see if I can find other potential places to change parts hopefully making it sound better. Would like a little more bass control...

For example my four capacitors in the PSU, 10000uf, 50 v could possibly be replaced by new ones. The amp is nearly 20 years old, so they must be tired.
Also I could replace the film caps 1,5 uF for better typer, eg. Mcaps.


2000-11-24 12:02 am
If you like to change the input capacitors 1.5uF, you might
consider some nice Jensen Paper in Oil. Or Siemens MKV.

Also, if you plan to replace the PSU caps do not simply put more
capacitance in. The stress (charge peaks) could ruin the rectifier
bridge. And the whole amplfier and speakers for that matter.
Be sure you verify the max current for the bridge, before
you change the caps.

You could also decide on adding some nice bypass caps for the

I plan stick to 10.000 uFs, 60v, not make it bigger.
Which bypass caps would you normally use for psu electrolyts? What would be the capacitance and voltage of a bypass cap for a 10.000uF?
For my Nelson Pass preamp I found some expensive Caddock resistors, for this power amp I don't believe I can find Caddocks. Any alternative if I wanted to change the power resistors?
Would replacing power resistors from mid 80's make any sence at all?
20 years ago the components of this amp were super, I'm not sure they are anymore...


2000-11-24 12:02 am
Bypass caps from 2.2uF to 10uF. I would use MKV. Only because
I have quite a few of them. When space is an issue, I would use
siemens MKP or FKP. The nice red ones.

I would not replace the resistors. Remember, you are very happy
the way the amp sounds right now. Especially using expensive
caddock's, holco's or allen bradley's. Lot of people rave about
old (+30 years) allen bradley's. I am not one of them.
I would however check or redo all the solder connections. Also
a carefull inspection of the PCB would not hurt.

If you are looking for a serious upgrade of your set, why not
replace the amp with a nice ALEPH?
You could go really overboard with such an amp. (Component wise, of course)
I was planning to build an Aleph 3 to go with my new BoSoZ but I need to investigate more on Aleph 3 costs, pcb's, heatsinks etc before I proceed.
If I go for the Aleph, I will not upgrade this amp, except for these Silver Micas...or maybe even cheaper polystyrenes....

In fact I didn't listen to my BoSoZ yet because of my wiring mistake ;-)