Quad 606 bang 😞

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19-Dec-2019 01:25 AM

Mark B

Mark B

Posts: 12

Hi

I’m on my way to building another quad setup and I have managed to blow up one of my quad 606 boards. During testing after upgrading the caps and changing R11 to 12R, one channel tested fine as did the psu having correct voltages, however the other board, which I didn’t Earth correctly between the chassis and the board (ie the screws were not in tight) showed a D.C. on the output of 3.4 volts approx. I suspect this was just the poor earth, however after checking the circuit I tried again and forgot to connect the chassis to board earth.. this time I got smoke and R40 went up in flames. I replaced this and reconnected but unfortunately I’m now getting 17v D.C. on the outputs and r16 and r17 are getting nastily hot.

I’d be very grateful for any suggestions as to where to start looking for the blown components so I can start the repair!

cheers

mark 

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02-Jan-2020 12:47 PM

DavidA-C

DavidA-C

Posts: 3

Take a resistance reading on R12 (3k3) and R41 (10k) - in circuit you should get a resistance reading at or below their values; out of circuit, check their values and look for any "pin hole burns".  Too high a (current) value on these two could place a large DC error on the o/p, just as in the case of the 405 (? - a different configuration, here).  Resistors can go high in value if there is large voltage drop across them and are not high enough wattage-rated (as there should be with R12; I note the voltage "clamps" D2 and D12).

I would be curious to know what the o/p voltage from the TLC271 is (-115mV ? - if you could avoid the "fire risk" for a moment - !).  Certainly from my 405, a large error from the front-end opamp caused DC on the o/p, which in turn blew the fuses (to avoid DC to the loudspeakers).  The issue with my 405 was a "high" R8 (should have read 3k3).  Carbon/metal film resistors have a ceramic base and would crack after heating and cooling over a number of years, just like a rock in the desert.  That took me while to find ....

David

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02-Jan-2020 08:32 PM

Mark B

Mark B

Posts: 12

Thanks so much David. I have now managed to repair it.. I actually stripped the board right down to component level and checked every one... it was indeed a couple of resistors and Tr5. Having replaced these the amp is now working very nicely and both channels sound great.

I appreciate the help very much

hapoy new year

 mark 

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