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Resistor Getting Hot...

Suffolk Tony

Suffolk Tony

Aim low, achieve your goals, avoid disappointment.
Hi folks,

What with the bad weather and all, I've been messing about with some old DIY stuff again. Just picked up this abandoned project, an NCC-type(?) board that, for reasons I can't work out, burns out the R26/390R resistor marked in the diagram. I replaced the MJE15031 transistor because that appeared to be shot, but the main output transistors are OK. That's the limit of my diagnosing skills, so I'd be grateful if someone could point me at what to look at next.

jNFPyAC.jpg
 
Tony,
Looking at the schematic above R2, 1K reads "2R, K1"; R3, 22K reads "3R, K22" and numerous other resistors and values are written "back-to-front". It is possible you have some inappropriate resistor values around the driver and output transistors, e.g. R001, 82R and R001, 22R should be R22 and R28, both 100R value. I would find a copy of the NCC200 schematic and check your board for errors.
Best,
AP
 
I have never seen mirroring like this before. Q9 emitter is probably meant to be 68R, not 03R, Q8 emitter 620R
Every vertical resistor is wrong
 
All the vertical labels are mirrored.

Assuming it's built with the right values... the reason R26 is hot is because it is conducting too much current e.g. 50mA in 390R = 1W = 'ing hot!

So where is the current going?
It's either base-emitter current or base-collector current in Q7. Neither of which is "right".
Since the VAS can only supply 10mA, I suspect Q9 is shorted (to the neg rail via R30, 86R).

Anyway, enough thinking. Just measure the Vbe of every transistor in the amp and let us know which one(s) are not at about 0.65V.
 
So...I've desoldered and tested Q9, which seems OK. The resistors are all the correct values (tested with the meter), & the diodes are the right way round and intact. I don't want to blow this up again, so how can I test the board without connecting it to its proper supply? I do have a variable bench power supply.
 
You could run it off +15 / 0 / -15 which would put less stress on the 390R because as S-man said there is clearly too much current passing through it. You then need to measure some voltages across components in the vicinity of the 390R and compare to a good board. As a guess (can't measure my NCC200s at the mo) there is circa 15V across the 390R causing the heating where there should only be a tiny current and <1V across it (just guessing but not unreasonably so) so you need to find where this disparity is coming from as it indicates something burned out or shorted. Is the replacement MJE15031 the same pin-out as the original?
 
Suffolk Tony said:
So...I've desoldered and tested Q9, which seems OK. The resistors are all the correct values (tested with the meter), & the diodes are the right way round and intact. I don't want to blow this up again, so how can I test the board without connecting it to its proper supply? I do have a variable bench power supply.
Click to expand...

You would need a split rail bench power supply capable of say plus AND minus 30V and with current limiting.

If the resistor values are all OK and the diodes the right way round then it will be a blown transistor or possibly one fitted wrongly or with the wrong pin-out eg BC182 and BC182L are the same device but have different pinouts. Check that output devices are properly insulated from the heatsink also. Sometimes when a repair solution is giving you a hard time it can ultimately be quicker and easier to just replace all the transistors... other than the drivers and outputs the rest are going to be only like 10p - 60p each.
 
With these boards it is very easy to test them without the output devices. They should exhibit very little DC on the output and you can prod them to your hearts content, any fault or issue is goint to show up as a DC error on the output - the polarity helps narrow it down. Don't forget the 0v connection on the signal input. That catches many people unaware.
 
Many thanks again for all the great advice folks. I've not powered the board up since replacing MJE15031, but I think, before doing anything else, I'll take Mr Arkless's advice & replace all the other transistors. I'll need to leave this alone for a few days now.
 
Weird to see this because I'm in the process of trying to get a drawing rendering (computer) program to write text strings as a set of characters spread in a non left-to-right manner! Kind of "Oops!" I'd make along the way, but then fix! 8-]
 
Suffolk Tony said:
Many thanks again for all the great advice folks. I've not powered the board up since replacing MJE15031, but I think, before doing anything else, I'll take Mr Arkless's advice & replace all the other transistors. I'll need to leave this alone for a few days now.
Click to expand...
Bear in mind the chance of introducing more errors by doing this. Personally I would measure as much as possible before changing anything.
 
Suffolk Tony said:
Many thanks again for all the great advice folks. I've not powered the board up since replacing MJE15031, but I think, before doing anything else, I'll take Mr Arkless's advice & replace all the other transistors. I'll need to leave this alone for a few days now.
Click to expand...
Remember that he fixes equipment for a living, so his time is often more expensive than the parts.
Once output devices fail, they often all blow in a cascade.

99% of the time transistors fail with a hard short between pins. A meter in diode test mode will see this
 
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