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Old 18th May 2024, 12:04 pm   #61
Dazz100
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Default Re: GEC Miniscope model BW 464

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Originally Posted by GMB View Post
I have to say that I think your calculations for the HT just look wrong.
The HV supplies are using voltage doublers so this gives intrinsicc current limiting which is a useful feature, and it also means that the voltage will come down a lot as the load comes on.

Your plots suggest that your selenium stacks needed a bit of action to see if they would "reform" as has been reported. But now that you have dismantled them they are unlikely to work well again I suspect.

I have protected selenium stacks with a modern diode but left them otherwise in place because it looks like excessive reverse leakage may be the road to overheat and hence failure. I think that as designed there would not be much wasted reverse flow so if you are not seeing an obvious reverse voltage offset then that is their age showing through.

When I get some time I will continue the fire-up of my example and see if I can get better info on the supplies. Have you identified the values of the HV capacitors?
Hi
When I look at the circuit diagram, I am seeing full wave rectifiers, no voltage doubling. The voltages specified in the manual, the open circuit transformer voltages and the valve datasheets are consistent with a rectified peak voltage. Measurements on your unit will be really useful info.

I was really surprised when I dismantled the selenium diode. I thought the disks would be potted or similar. They just fell out. These diodes were reformed a few weeks ago with a variac. The 10 cell test rectifier showed good performance compared to the generic data. I don't think they have suffered at all for being liberated.

If selenium needs reverse voltage to reform, then adding silicon in series would deprive them of a reforming voltage.

The HT supply has 2x diodes in series (4x diodes total). I suspect this is to reduce the reverse leakage current at the expense of very high forward voltage drop but it could be for other reasons. Maybe they needed to save space by, in effect, folding the diodes in half. Maybe they used the selenium diodes forward voltage like a HV zener to provide some voltage regulation. They must have had a good reason. Simply replacing selenium with silicon would over-volt the valves.
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Old 18th May 2024, 5:46 pm   #62
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Default Re: GEC Miniscope model BW 464

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When I look at the circuit diagram, I am seeing full wave rectifiers, no voltage doubling
Then you must have a different circuit or you do not understand it.

I am seeing only 2 diodes (or 2 pairs) from a single winding (no centre tap). This makes two half wave supplies in series, the simple voltage doubler. Think about where the current comes from and you will see no full-on connection to the input. The resovoirs effectively act as current limiters. I use circuits like this for psus as I like the limiting action but the ripple is not so good and the output voltage can be much lower than you expect if they are working hard.
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Old 19th May 2024, 11:04 am   #63
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Default Re: GEC Miniscope model BW 464

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Quote:
Originally Posted by Dazz100 View Post
When I look at the circuit diagram, I am seeing full wave rectifiers, no voltage doubling
Then you must have a different circuit or you do not understand it.

I am seeing only 2 diodes (or 2 pairs) from a single winding (no centre tap). This makes two half wave supplies in series, the simple voltage doubler. Think about where the current comes from and you will see no full-on connection to the input. The resovoirs effectively act as current limiters. I use circuits like this for psus as I like the limiting action but the ripple is not so good and the output voltage can be much lower than you expect if they are working hard.
Hi
In post 50, I stated "The open circuit HT measures 385VAC so with no losses, that would give an EHT voltage 1044VDC if I am reading the circuit correctly. It looks to me that the circuit is a half-wave rectifier on the +ve and -ve side, to end up with 1044V difference between the +ve and -ve sides, no load, no losses."

The circuit is also a full wave rectifier because the output is driven by both +ve and -ve sides of the waveform.

Between the leakage across the selenium diodes and the now ancient electrolytics, yes I can see that the loaded voltage is going to be a lot less.

I anticipate replacing the electrolytics with film caps and selenium with silicon is likely to exceed the rated voltage for the Z77 valves as a result of reducing the leakage currents from high to practically zero. The CRT has a fair bit of head room so more tolerant to voltage rises.

It would be really useful to measure the voltages on an original, unmodified Miniscope to get the loaded values.
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Old 22nd May 2024, 9:42 pm   #64
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Default Re: GEC Miniscope model BW 464

I have only just noticed that my HT rectifiers have been replaced by BYX45-1000R diodes (no resistors). I presume this had been what I would recognise as an HT selenium rectifier in a flat aluminium package.

I think I still have the original EHT stacks but you never know what might be hidden inside the tubes.

It looks like the HT electrolytics were replaced. They are probably all 4.7?F 350V although they measure a little high.

It looks like my non-electrolytic capacitors are not great, showing about 30?A leakage (but I have seen worse!).

So a lot of work to make it go again I think.
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Old 23rd May 2024, 11:16 pm   #65
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Default Re: GEC Miniscope model BW 464

Hi

I am currently waiting for delivery of a bundle of film capacitors to replace all of the electrolytics. They cost twice what I paid for the scope, but being dry, they should last forever, and leakage current should be almost zero.

After a lot of work, the first seized potentiometer has freed up. I will need to work on the 2nd.

I think the risk of running a silicon diode without a resistor is:
  • The peak current is likely to saturate, and then fry, the transformer.
  • The rectifier VDC outputs will be too high for the Z77 tubes. Probably OK for the CRT.
  • Voltage regulation will be poor.

It is clear to me that the designers knew what they were doing and factored in the properties of the selenium diodes and leaky caps.

I think the ideal drop-in replacement for selenium would be:
  • a silicon diode for low leakage, low fwd voltage loss,
  • high resistance to limit peak transformer current,
  • discrete zener diode (eg. HV emitter follower) for voltage regulation.

My current plan is to replace all the caps, replace selected dodgy resistors, refit the selenium diodes. If the scope works, I may then look at silicon based rectifiers.
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