Shaftesbury 519 amplifier

[Station X]

It takes a long time to read through this thread!

Originally Posted by Guitarist28

shall I try the resistor/diode again?

I don't see why not. With 285V from half the secondary winding and 100k in series the reforming current will be limited to about 3mA.

Put your meter on DC Volts across the reservoir capacitor before you apply mains. Once the voltage has stopped rising, remove the mains. If the reservoir cap is OK the voltage will fall SLOWLY.

[Refugee]

The voltage may take an hour or so to fully rise for the first time if the capacitors need a lot of reforming.
After this it will take a minute or so to rise.
Once they are fully reformed the voltage will take about 10 times as long to fall with just the DMM connected. The diode will not make any difference when you turn the power off.

[Guitarist28]

Many thanks
I can't beleive I didn't tighten up the shrouds!

[Refugee]

It scared everyone.

[Guitarist28]

I won't mention that I had I minor jolt when trying to take some measurements. Lesson learnt very quickly!!

[Refugee]

Ouch

[Guitarist28]

It's the first chance I've had to reform the caps. One the first attempt, the voltage rose to 157v, held very steady and then started to drop. I've switched off, and the voltage has been dropping steadily, down to 15v in just over a minute or so. What am I seeking to indicate all is OK?

Many thanks

[Station X]

Sounds good. What's the voltage read with power applied?

There'll be some volt drop in the series resistor.

[Guitarist28]

Hi Graham
As indicated 157volts across one section of the smoothing cap....no higher. I was planning to switch on again to remeasure.

Best regards

Rob

[Station X]

You said that the voltage rose to 157V and then dropped, presumably with power still applied?

[Guitarist28]

Yes indeed I did. I was expecting it to be higher than this?

[Station X]

An electrolytic capacitor will inevitably pass some leakage current. That current passes through the series resistor causing a volt drop across it. With a 200k resistor a current of 1mA will drop 200 volts.

If you reduce the value of the series resistor to say 100k the voltage across the capacitor will be higher. I generally reform via 10k.

[julie_m]

That initial voltage drop does not sound right. Does it happen every time you measure the voltage? What happens if you swap the anode of the temporary diode to the other end of the secondary?

It's possible the capacitor is becoming leaky at a certain applied voltage, then drawing current through its own leakage resistance and pulling the voltage down below where it first started conducting. Have you any high-voltage electrolytics you could tack in temporarily?

[Guitarist28]

Ok, will do.
I'm measuring across the other half of the smoothing cap and that holding steady at 155.6volts.

Best regards

[Guitarist28]

I do have a replacement JJ Tesla cap.... I was wondering if I could use the original in the first instance.

[Station X]

This is getting confusing. What did the voltage start at before it dropped to c 155V?

Do the caps get warm?

[julie_m]

That means the second (smoothing) capacitor can't be bad, because there is not much voltage drop across the resistor between the two capacitors and therefore not much current flowing through it. If nothing else is connected to that capacitor, then that current is just the leakage current.

If the voltage goes up to a sensible amount as the resistance goes down, then the reservoir leakage current might be reasonable. The voltage readings using the two ends of the transformer should be similar to each other.

[Guitarist28]

The voltage was in the mV range (negligible) before I powered up. The voltage then steadily rose to 156/7v or so mark and then held steady for a while and then started to drop by a few volts.
I understood what you had indictated and will reduced the resistor in the first instance and remeasure.

Best regards

[Station X]

The AC voltage across both halves of the HT secondary was measured at 570VAC in post #71, so approximately 285-155=130V is being dropped in the 200k series resistor representing 0.65mA leakage current, which is fine.

[Guitarist28]

Excellent and thank you. I will lower the resistor just to double check.