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Old 9th Oct 2018, 7:23 am   #1
cernem1alt
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Default GEC BC5445 Potentiometer question

I think I need to replace the 1M volume potentiometer with d.p switch 1 inch spindle semi-log law, and I am not sure apart from the 1M bit what the modern equivalent is or where to look. The radio is a GEC BC5445 I have done all the caps and some of the resistors I have stations on all three bands but at a fixed low audio level with occasional crackle. I have the GEC service bulletin 167 and circuit diagrams, it had a good squirt with Servisol which has slightly improved things but the radio comes on at a set volume and cannot respond to the volume control so potentiometer or is it something else I have missed?
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Old 9th Oct 2018, 8:50 am   #2
Boater Sam
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Default Re: GEC Potentiometer question

Unless the "set volume" that it comes on at is full volume it is unlikely to be just a faulty pot, the only other possibility is that the wiper is not turning with the spindle which is very unlikely.
A 1M log pot with DP switch is easy to source, I doubt the semi log description is vital, its just a volume control.
Sam.

Last edited by Boater Sam; 9th Oct 2018 at 8:54 am. Reason: Added
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Old 9th Oct 2018, 10:09 am   #3
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Default Re: GEC Potentiometer question

Sounds like the problem is in the DH77 circuit. Worth trying the following if you haven't done so already:
1. Check the resistance from slider to both ends of the pot in turn using a multimeter.
2. If the pot is OK then check DH77 anode resistor R12 150k.
3. Check the grid resistor R13 (10M) they often go o/c.
4. Check the anode voltage of the DH77.
5. Clean the pins of the DH77 and the sockets and check for broken socket pins.
6. Try a replacement DH77
Cheers, Jerry
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Old 9th Oct 2018, 10:37 am   #4
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Default Re: GEC Potentiometer question

The slider may be open circuit with respect to the resistive track of the volume control.

However there may be a tone adjusting component such as a capacitor which is wired from the top of the volume control to the solder tag of the slider. This could give you a fixed volume.
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Old 12th Oct 2018, 10:21 pm   #5
cernem1alt
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Default Re: GEC BC5445 Potentiometer question

I checked the resistance from slider to both ends of the pot and it reads around 450ohms with all configurations turning the knob etc Anyhow as a matter of course I replaced R13 and R12.
DH77 is encased in a square can so will proceed with caution here so not tried anything yet.
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Old 12th Oct 2018, 11:05 pm   #6
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Default Re: GEC BC5445 Potentiometer question

Quote:
Originally Posted by cernem1alt View Post
I checked the resistance from slider to both ends of the pot and it reads around 450ohms with all configurations turning the knob etc
Something very wrong there, is something shunting the pot? Looking at the maker's sheet there is no reason obvious for this resistance reading.

Last edited by Boater Sam; 12th Oct 2018 at 11:26 pm. Reason: Added
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Old 13th Oct 2018, 12:28 am   #7
Andrewausfa
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Default Re: GEC BC5445 Potentiometer question

Just wondered if you had an replacement and/or different pot in there. This is one from one of my 5445s.

Andrew
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Old 13th Oct 2018, 10:24 am   #8
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Default Re: GEC BC5445 Potentiometer question

Some potentiometers have a fourth solder tab.

It is a fixed connection to the middle of the resistive track.
Mainly used for tone correction.

Can you confirm that the resistance was 450 Ohms and not 450,000 Ohms.
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Old 13th Oct 2018, 10:43 am   #9
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Default Re: GEC BC5445 Potentiometer question

"Tin whiskers" inside pots can cause odd readings such as these. Sometimes visible on the outside as well, but not always.
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Old 13th Oct 2018, 10:47 am   #10
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Default Re: GEC BC5445 Potentiometer question

I would look at the screened cable first. Disconnect the screened cable from the pot wiper and try measuring the pot resistance again.
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Old 13th Oct 2018, 11:29 am   #11
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Default Re: GEC BC5445 Potentiometer question

Quote:
Originally Posted by Silicon View Post
Some potentiometers have a fourth solder tab.It is a fixed connection to the middle of the resistive track.Mainly used for tone correction.
Correct, this one has four tabs, I wondered if that would be visible in my rather dark photo!

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Old 13th Oct 2018, 11:34 am   #12
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Default Re: GEC BC5445 Potentiometer question

Good suggestions particularly removing the wires from the pot before measuring its resistance. If the pot is genuinely at fault then you can buy a suitable replacement at RS Components here https://uk.rs-online.com/web/p/potentiometers/7456781/ for £6.25. Alternatively I have got a used one that measures correct resistance and has continuity across both switch contacts (see pic). Unfortunately the shaft length is only 5/8 inch so you would need to find a shaft extension and coupler. If you want it you can have it for £0.60 postage. Send me a PM.
Cheers, Jerry
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Old 13th Oct 2018, 11:38 am   #13
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Default Re: GEC BC5445 Potentiometer question

There is no evidence on the service of sheet of the volume control being tapped. The fourth connection is possibly only an earth tab connected to the pot shell.
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Old 18th Oct 2018, 8:04 pm   #14
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Default Re: GEC BC5445 Potentiometer question

Found the problem C5 47pF small brown bean shaped capacitor gone rogue at V1 and of course no spare in the box so on order
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Old 18th Oct 2018, 9:25 pm   #15
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Default Re: GEC BC5445 Potentiometer question

That's bizarre. C5 is the oscillator coupling capacitor and it might well have failed but I cannot see how it could be responsible for the reported symptom of fixed low audio level!
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Old 18th Oct 2018, 11:28 pm   #16
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Default Re: GEC BC5445 Potentiometer question

Also replaced the 33k resistor on the 90v 3mA line just because....well why not
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Old 21st Oct 2018, 10:53 pm   #17
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Default Re: GEC BC5445 Potentiometer question

Did you measure its resistance and find it was out of tolerance? If not, why did you replace it?
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