[David G4EBT]

In post #13 I mentioned the series of articles in Radio Constructor, in 1970/’71, which included modifications to reduce hum. I stated: “I'm not suggesting that those modifications will cure an abnormally high level of hum in this particular set”. The reason I said that is that the articles were on how to improve the performance of a set which – back in 1970 - was almost new - not a set such as the one that is the subject of this post, now getting on for 50 years old.

In post #15, the O.P said "I'm out of my depth on electronics". When I see that, ordinarily, I cease to get further involved.

We've been here many times, (as often as not with Dansettes and the like), and we have to assume a minimum level of competence due to safety and competence concerns. I don't want to sound rude or unkind, but if someone doesn't know how to use a multi-meter to carry out live testing and to interpret the results, little progress can be made and the individual can be exposed to safety hazards. Reading the posts, it's still not evident to me whether voltage readings have been taken and compared with the circuit. In particular, with regard to the anode and cathode voltages on V8.

On a set of this age, the most likely cause of hum would I think be the three x 40uF reservoir/smoothing caps, but that would have to be determined not by speculation, but by a proper diagnosis, because in some instances, (notably the DAC90A), the hum can be from other causes associated with the output valve, and rarely from the reservoir/smoothing caps.

If this was my set, I'd start by checking if there is 160V AC at the input to D6 & D7 - the rectifier diodes? If yes, is there 200V DC at the output of the diodes? If not, the diodes are suspect. If there is 200V DC or thereabouts, is there 200V at the anode of V8, pin 5? and is there 6.5V at the cathode of V8, pin 2? Advice has been given to check those voltages, but it's not apparent to me with any certainty if the actual voltages have been established.

Is there any DC on the grid of V8 (pin1). If so, that is arriving via C34 - the audio coupling cap from the anode (pin 6) of V7 due to the cap being 'leaky' in the electrical sense. However, if C34 is a disc ceramic rather than paper capacitor, and there is no DC on pin 1 of V8, that's all to the good.

Unless the voltage conditions around V8 are close to the circuit, that would need to be resolved. I think it's unlikely that any of the valves are the source of hum. If V8 was removed and a signal tracer applied to pin 6 of V7 via a .01µF cap and hum was still present, that would tend to exonerate V8 and point to the reservoir/smoothing caps as the HT of both valves of course are via the common HT line.

Assuming that it's been confirmed there is 200V DC from the output of D6 & D7, my approach would be to temporarily substitute the reservoir/smoothing caps, of which there are three in a single can - all 40uF, the nearest modern value being 47uF caps. If that cured the fault, it would be up to the individual whether or not to leave C40, 41 and 42 disconnected and to fit replacements under the chassis, leaving the redundant can in place for appearance sake, or whether to disgorge the contents of the can and re-stuff it.

On the O.Ps own admission, he doesn't have the competence to carry out such tests and rectification work and I'm not suggesting that he does so as he is unable to locate components on the chassis by reference to the circuit diagram. I think it would be much better if some kind soul in his vicinity had a look at it with him. Clearly to entrust the work to someone on a commercial basis would cost way beyond what the set is worth.

These are just my own observations, others may take quite a different approach or may disagree. I really don't mind, and for the reasons stated, I'd prefer not to engage further in this thread.