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Old 21st August 2002, 05:31 PM   #1
2Bak is offline 2Bak  Denmark
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Join Date: Apr 2002
Default 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:
BosoZ - damaged FETS ?

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:

Click the image to open in full size.

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?

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Old 21st August 2002, 05:39 PM   #2
2Bak is offline 2Bak  Denmark
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Can't resist posting a pic of this very good sounding amp.
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Old 22nd August 2002, 07:29 PM   #3
2Bak is offline 2Bak  Denmark
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Does anyone know the pin layout of BD529 og BD530 ?

1: B
2: C
3: E


1: E
2: B
3: C


The case is TO-202 type.

Found some BD243C and BD244C to replace them, but not certain of the pins of these old ones.
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Old 22nd August 2002, 08:25 PM   #4
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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.
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Old 22nd August 2002, 08:52 PM   #5
2Bak is offline 2Bak  Denmark
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Old 23rd August 2002, 06:55 PM   #6
2Bak is offline 2Bak  Denmark
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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
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Old 23rd August 2002, 07:03 PM   #7
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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?

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Old 23rd August 2002, 07:25 PM   #8
2Bak is offline 2Bak  Denmark
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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

Click the image to open in full size.
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Old 23rd August 2002, 07:45 PM   #9
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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.

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Old 24th August 2002, 09:41 AM   #10
2Bak is offline 2Bak  Denmark
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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?

Click the image to open in full size.

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.
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