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Old 11th Mar 2023, 12:44 am   #21
EdGizzard
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Default Re: Resistor Value Issue

Thanks everyone.

So I can confirm continuity on the heaters for tubes XV1 and 2. I used this

https://frank.pocnet.net/sheets/127/5/5749.pdf

to find the correct pins. 3 and 4.

R65 reads 2.3 ohms (tested in circuit) so i assume that's close enough. See picture vertical beige resistor.

I've tested the diodes out of circuit and found CR1 to be open circuit both ways. - to + and + to -. All the others seem ok.

Should I change all the diodes while I'm at it? I'll probably just order the RS diodes Peter recommended

Anything else I should test?

Thanks everyone. Ed
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Old 11th Mar 2023, 1:11 am   #22
Uncle Bulgaria
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Default Re: Resistor Value Issue

That's hopeful for the valves then, and the resistor looks fine. It could be a 'high stability' type from the photo. Before removing the diodes I would have checked the heater voltage as I suggested earlier. It would be handy to know whether the bridge rectifier gives a voltage reading at input and output. Diodes are usually extremely reliable so it would not be the first thing I'd assume faulty, especially without checking the voltage conditions, or if there's been a known overload causing faults elsewhere.

Without testing a known good diode you also can't be sure that your measurement equipment is suitable. I have had problems testing early solid state stuff with a digital multimeter that then turned out to be OK when tested with an AVO meter. I think the DMM couldn't properly bias the devices of that era to get a reasonable reading.

Most problems show up with Ohm's Law, and the easiest way to verify that is by checking the voltage conditions at known points. If there's an AC voltage at the input to the bridge rectifier, but there is no or low DC output then the rectifier is suspect. If the valve heaters are not lit they're either not being supplied with current or the valves are faulty. It seems the latter is unlikely to be true based on your continuity test. A voltage test is the easiest way to trace where a component or wiring failure is likely to be, rather than removing components and testing individually before pinpointing the failure.

If you haven't enough in your mind's shopping basket to make up the fee-less RS minimum order (which unless you have a lot to buy at once can end up adding tens of pounds to a few pence of components) ESR have a variety of rectifier diodes. I expect I could find something suitable to send you FoC if you can wait a bit. I wouldn't bother to change things you don't know are broken. That road can lead to many head scratchings as you've introduced further variables to the mix!
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Old 11th Mar 2023, 9:58 am   #23
peter_sol
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Default Re: Resistor Value Issue

Those diodes in the bridge feeding the heaters are ordinarily diodes not schott diodes.
Fitting schott diodes may result in the heater voltage being too high.
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Old 11th Mar 2023, 10:22 am   #24
stuarth
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Default Re: Resistor Value Issue

Could it be that those two valves have their heaters deliberately under run to the point where they do not visibly light up?

Stuart
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Old 11th Mar 2023, 12:40 pm   #25
ms660
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Default Re: Resistor Value Issue

Quote:
Originally Posted by stuarth View Post
Could it be that those two valves have their heaters deliberately under run to the point where they do not visibly light up?
According to the voltages in the schematic, the voltage from the power supply for the 5749 heaters is 5.37 Volts, the output from the bridge rectifier being 6.6 Volts.

Lawrence.
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Old 11th Mar 2023, 11:33 pm   #26
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Default Re: Resistor Value Issue

In response to the OP's question re replacing all the diodes - yes replace all of them.

One going short circuit could have stressed the rest of them.
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Old 12th Mar 2023, 9:21 pm   #27
EdGizzard
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Default Re: Resistor Value Issue

Thanks Everyone. Diodes orderd. Will report back.
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Old 13th Mar 2023, 5:17 am   #28
FrankB
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Default Re: Resistor Value Issue

Not being familiar with this unit, I would ask: Is there a "ring crack" tot he filaments of the tubes? also Is there possibly a thin copper wire or trace in that DC fil ckt to the tubes? If so, it might have opened. Some sets used this thin wire/ trace as a "fuse" for the filaments, and didn't necessarily mark it on the schematic. .
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Old 13th Mar 2023, 9:48 am   #29
PJC58-Hythe
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Default Re: Resistor Value Issue

I'm guessing that the picture I have found shows the complete underside of the unit?

If so, and before you fit the diodes, you should follow FrankB's advice and check the continuity of the PCB tracks and wiring from the output of the DC supply to each valve heater to make sure they are intact and there aren't any hairline cracks or disconnection due to bad solder joints etc.
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Old 24th Mar 2023, 9:54 pm   #30
EdGizzard
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Default Re: Resistor Value Issue

!!LEVEL DEVIL WORKING!!

Looks Like Diode CR3 on the bridge rectifier was the problem.

Thanks so much everyone.
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