303 calibration, 67v on pin 1-5

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21-Nov-2019 04:28 PM

Belgian Quad

Belgian Quad

Posts: 4

Dear Quad fans. 

One year ago I installed a Quad Revision set one one of my 303s. I truly enjoyed the fantastic sound last year. 

Now the left channel stopped working. I measure 67 volts on pin 1-5 on the left channel and cannot adjust it. The right channel works perfectly. 

I measured replaced RV100 for a 5k, tested R110, 111 all fine. On both boards R130 measures 15k (instead of 150k?) and all resistors are still original. I hope someone could give me advise. Thanks in advance! 

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25-Nov-2019 01:15 AM

EJP

EJP

Posts: 1676

If you didn't change C100, change it now. Otherwise I would investigate Tr100/101/102, with Tr102 being the most likely.
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26-Nov-2019 11:56 AM

Belgian Quad

Belgian Quad

Posts: 4

Thanks for your help EJP!

I changed C100 last year and it still is fine. I swapped Tr100/101/102 with the working (right) channel, one by one. The left channel now still gives 66 volts between 1 and 5, the right channel still works good and can be adjusted to a perfect 33,5v. 

Do you know of any other possible problems? 

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26-Nov-2019 12:31 PM

Belgian Quad

Belgian Quad

Posts: 4

I have been reading some other posts on this forum. I measure 0 volts between pin 6 and 7 on the left (faulty) channel and 32,7 volts on the working channel. 

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27-Nov-2019 05:01 AM

EJP

EJP

Posts: 1676

That sounds like Tr3, the lower power transistor, isn't even connected. Check the connections at the 9 pins and all the way through to the power transistor. Or just put your DMM onto diode range, red lead to pin 7, and black lead to pins 6 and then 8. You should get about 0.45V to pin 6 and 0.06V to pin 8 (because of the 68R resistor across them).
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27-Nov-2019 06:32 PM

Belgian Quad

Belgian Quad

Posts: 4

Thanks again EJP! I swapped the boards and the problem is not within TR3, since the left channel is now working fine and vice versa. Tonight I will double check all the resistors around RV100 to see if they are all OK, I think the problem must lie somewhere around there. Thanks for your help, again!  

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28-Nov-2019 01:58 AM

EJP

EJP

Posts: 1676

Hang on. Pin 6 is the collector of Tr7, which is connected via 0.3R to the centre voltage which you state is 67V. Pin 7 is the base, which is at most 0.6V above the emitter, which is grounded. So the only way you can be reading 0V from 6 to 7 is if 6 isn't connected or there is a c/b short, which is rare: mostly they fail c/e short.

Or are you numbering the pins left to right? They are numbered right to left. If so, certainly check the 150K resistor, it often drifts high. You will have to remove one leg to measure it.

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