Hi everyone,
Please do check the condition of the electrolytic caps on the Quad 303 driver boards every once in a blue moon, especially the condition of C101.
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| C101 as per high-lighted, on the Quad 303 driver board pix from the service manual |
I chance upon mine while investigating why only one channel was audible when using the Quad 33 from last month (since discovered had several problems).
Quad 303 #1
Please refer to the pix below.
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| C101 on each driver board - note the darken area towards bottom half of each capacitor |
Even though the power amplifier performed flawlessly, the visually impacted C101 could be sign of a latent issue at large as these FC(s) were purchased from Element-14.
I troll the DIY forums for information regarding any similar observations. While I did not manage to locate any, I did note there were many reported issues originating from the PSU board. Since C101 on both boards were impacted, this had to be the common source of the issue!
Thus I obtained an updated Quad 303 refresh PDF from DaDa Electronics and followed the bias-ing procedure.
Immediately discovered the issue - PSU was pumping out 74V and adjusting trimmer RV200 had no effect at all!!!
Testing TR201 using a el-cheapo tester from AliExpress shows that it was faulty and behaved as if it was 2 diode(s) which output to the 3rd pin - see following pix.
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| Result when testing TR201 on the transistor tester |
Ordered a replacement BC441 as I already have the appropriate heatsink. While awaiting for the BC441 to arrive, I changed RV101 from 2K2 to 22K on both driver boards (since the S/N of the unit was well below 11000) as per recommend by the PDF.
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| After repairs |
Since I have a few spare 470uF 35V lying around, will use them 1st to "test the waters" till I can purchase proper audio EC capacitors for C101.
Upon installing BC441 as TR201, the PSU was initially outputting 60V. I then followed the bias-ing procedure and obtained a result close to the recommendations.
Quad 303 #2
Checking #2, found the same C101 issues as well.
The PSU on this unit was outputting 78V+!! Adjusting RV200 could only bring down the voltage by decimal points! Hence power-ed down the unit immediately and re-position the trimmer to mid-point.
The cause for unit #2 was not TR201, as checking the transistor via the el-cheapo AliExpress tester indicates it's working fine.
Proceeded to test the various components on the PSU board via a Digital Multi-meter (aka DMM) which then reveal diode MR200 was in open status.
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| Tester confirm bad diode detected during DMM inspection of the PCB |
Replaced MR200 with a 1N4004 diode, as per suggested by the PDF.
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| The diode, MR200 on the Quad 303 PSU |
Took the opportunity to change RV101 from 2K2 to 22K on both driver boards since the S/N of the unit was well below 11000. And used the spare 470uF 35V until I can purchase proper audio EC capacitors for C101.
Upon power up, voltage was 76V+ but could be reduced via adjusting trimmer RV200. Again, followed the bias-ing procedure and obtained a result close to the recommendations.
Quad 303 #3
Will review unit#3 at a later date as it's a spare and in storage ... TBC
