Friday, August 24, 2007

Quad 34 capacitor problems

Last week I visit our shop in Antwerp. Stefaan mentioned a problem with a Quad 34. It was virtually impossible to solder a electrolytic capacitor. After scraping the track with a hobby knife, a good solder joint was possible. On the way back to the Netherlands I remembered a remark Keith Snook made on his website about the placing of some capacitors, they where incorrect biased, with leakage of the elco’s as a result. I bought a batch of 34’s with different colours and serial numbers, so all the mod levels where present. The major conclusion after looking at the diagram’s and the real thing; the incorrect placing of the elco’s is not an incident but a structural problem in the 34. In the picture you can see the result of the leakage of the elco’s and the different phases of decay of the Pcb, which causes the solder problems. Because of the asymmetrical power supply the DC voltage level of the output of all Opamps, except IC25 and 27 in the model above serial 8000, will be negative, relative to ground. So the minus connection of the elco should be facing the output terminal of the Opamp. In the diagram and in real life most of the elco’s are inverted, except C30 and 31, check your 34, the status maybe vary from model to model. There is only one solution, replace all elco’s and solder them in the correct position concerning the bias level. Check the DC voltage across the elco’s with no signal connections made to the 34. The bias voltage should be positive, a few millivolts is enough. So the plus of the elco should be positive, relative to ground level when measuring the DC voltage.

Joost Plugge

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