Quad 303 - Voltage calibration issue

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03-Mar-2020 02:55 PM

koozoop

koozoop

Posts: 19

Good evening. I have upgraded a Quad 303 but when I get to calibrate the voltages I get 67V on pins 1-9 (adjusted by RV200 but I can't adjust the 33.5V on each board. The readings on pins 1-5 are 65-67volts and can't be adjusted by RV100. I also get 0.00mV on pins 4-6. Where should I start looking?

Thank you in advance

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03-Mar-2020 11:29 PM

koozoop

koozoop

Posts: 19

Just an update: I have measured all transistors on driver boards and compared the values with my second quad 303 which has exactly the same components as the faulty one. I get similar values on all of them. I have an inconsistency when measuring R115: on the working one the value is fixed but on the faulty one varies. I have also checked TR1/2/3/4/5 and though I get similar values on the right channel, TR 4 on the left channel yields different values. I have disconnected the power from left channel to see if the right one is ok but I got the same results: 65-67v on pins 1-5. Could it be due to broken micas on TR1/2/3/4/5?
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04-Mar-2020 04:58 AM

EJP

EJP

Posts: 1742

I would be primarily suspecting TR100/1/2 and the resistors in the vertical chain to and from RV100. R130 in particular is a classic: it often drifts way up. If the output voltage is up the voltage out of Tr100 or Tr102 is up, or the voltage out of Tr101 is down. Check against the voltages shown on the schematic everywhere shown, starting from the input. I would also have a look at Mr103/4 if you have them, otherwise Tr107 and associated resistors. Maybe also R108 and the resistors around C104. And of course check Tr1/2/4/5 for shorts from case to ground, or c/e shorts. (Tr3 case shouldbe s/c to ground.)
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04-Mar-2020 01:46 PM

koozoop

koozoop

Posts: 19

Thank you for your response. I decided to start from transistors TR1/2/3/4/5 using a DMM on diode mode. All of them passed except TR4 where Collector - Emitter gave a value of 0.047v instead of OL. But even if this TR is gone, shouldn't I be still able to set the voltage on right channel? I'm going to order a new set of 2N3055H (homotaxial) for both channels. I will also check the boards now.
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05-Mar-2020 02:03 AM

EJP

EJP

Posts: 1742

Don't use 2N3055, they are marginal in this application. The originals were selected and are no longer available. Use MJ15003G, and MJ802 for Tr3 if necessary.
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05-Mar-2020 12:11 PM

koozoop

koozoop

Posts: 19

I will only use 2N3055 for testing purposes because they are cheap (2 euros). When I'm done I will either replace with MJ's or find a used one from ebay. I want my two 303's to be identical because they are to be used for bi-amping (vertical).
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