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Old 19th Jun 2022, 10:46 pm   #34
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
Default Re: Dansette HiFi Mk2 smoking amplifier

As a couple of people have mentioned, including myself in post 20, one needs DC voltage readings. They are the single most important diagnostic for most valve circuits and take precedence over most other tests as they will locate most problems in minutes or seconds. In particular, they will locate problems that are likely to cause further damage, such as a valve or transformer that is being stressed by overcurrent due to some other cause.

Once anything that is upsetting the DC conditions has been found and dealt with, many circuits will already be in working order, without smoking parts, hum or distortion. If you are unlucky and there is still a fault that is breaking or affecting the audio signal path, then at least the fact that the DC conditions are OK lets you know that it is probably safe to keep the thing powered on as long as you like, while you trace the signal fault.

To put this into a running-order for the repair I would have gone along something like this:
1) Replace the valve that was visibly down to air
2) Test / reform the main electrolytics and replace if suspect
3) Replace coupling capacitors unless of a known dependable type such as Mullard mustards.
4) Test the rectifier for major, obvious faults (minor loss of performance can be inferred from the voltage readings)
5) Power up, take DC voltage readings starting with main HT and output valve cathodes as these are the most important for stable operation
6) If all within reasonable bounds, it will probably produce sound. If voltages indicate faults, deal with those.
7) Once voltages all good, if still no sound, trace signal path.

Note that with the exception of known trouble-makers and those that should be replaced on sight before power-up, I would not tend to test or replace components or valves until circuit conditions indicate them to be at least suspect. Testing individual resistors etc. is not normally necessary.

So from where you are now, I would power it up and, taking the necessary precautions against shock, measure with respect to the DC negative rail, the voltages on all the valve electrodes except the heaters E.g. V1 triode a=100V, g1=0V, k=2.5V. Also the voltages at the reservoir and smoother and any other HT decouplers. Post your findings.

Last edited by Lucien Nunes; 19th Jun 2022 at 11:09 pm.
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