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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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11th Mar 2023, 12:44 am | #21 |
Tetrode
Join Date: Mar 2022
Location: London, UK.
Posts: 75
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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 |
11th Mar 2023, 1:11 am | #22 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,315
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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! |
11th Mar 2023, 9:58 am | #23 |
Octode
Join Date: Aug 2003
Location: Welwyn Garden City, Herts. UK.
Posts: 1,906
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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. |
11th Mar 2023, 10:22 am | #24 |
Heptode
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 665
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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 |
11th Mar 2023, 12:40 pm | #25 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Resistor Value Issue
Quote:
Lawrence. |
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11th Mar 2023, 11:33 pm | #26 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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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. |
12th Mar 2023, 9:21 pm | #27 |
Tetrode
Join Date: Mar 2022
Location: London, UK.
Posts: 75
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Re: Resistor Value Issue
Thanks Everyone. Diodes orderd. Will report back.
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13th Mar 2023, 5:17 am | #28 |
Heptode
Join Date: Oct 2005
Location: Olympia, Washington, USA.
Posts: 663
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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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13th Mar 2023, 9:48 am | #29 |
Heptode
Join Date: Sep 2010
Location: Hythe, Hampshire, UK.
Posts: 626
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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. |
24th Mar 2023, 9:54 pm | #30 |
Tetrode
Join Date: Mar 2022
Location: London, UK.
Posts: 75
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Re: Resistor Value Issue
!!LEVEL DEVIL WORKING!!
Looks Like Diode CR3 on the bridge rectifier was the problem. Thanks so much everyone. |