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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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9th Oct 2018, 7:23 am | #1 |
Tetrode
Join Date: Sep 2017
Location: Dorchester, Dorset, UK.
Posts: 81
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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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9th Oct 2018, 8:50 am | #2 |
Banned
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
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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 |
9th Oct 2018, 10:09 am | #3 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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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 |
9th Oct 2018, 10:37 am | #4 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,163
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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. |
12th Oct 2018, 10:21 pm | #5 |
Tetrode
Join Date: Sep 2017
Location: Dorchester, Dorset, UK.
Posts: 81
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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. |
12th Oct 2018, 11:05 pm | #6 |
Banned
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
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Re: GEC BC5445 Potentiometer question
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 |
13th Oct 2018, 12:28 am | #7 |
Octode
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
Posts: 1,180
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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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13th Oct 2018, 10:24 am | #8 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,163
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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. |
13th Oct 2018, 10:43 am | #9 |
Hexode
Join Date: May 2003
Location: Hertfordshire, UK
Posts: 258
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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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13th Oct 2018, 10:47 am | #10 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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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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13th Oct 2018, 11:29 am | #11 | |
Octode
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
Posts: 1,180
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Re: GEC BC5445 Potentiometer question
Quote:
Andrew
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13th Oct 2018, 11:34 am | #12 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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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 |
13th Oct 2018, 11:38 am | #13 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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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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18th Oct 2018, 8:04 pm | #14 |
Tetrode
Join Date: Sep 2017
Location: Dorchester, Dorset, UK.
Posts: 81
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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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18th Oct 2018, 9:25 pm | #15 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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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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18th Oct 2018, 11:28 pm | #16 |
Tetrode
Join Date: Sep 2017
Location: Dorchester, Dorset, UK.
Posts: 81
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Re: GEC BC5445 Potentiometer question
Also replaced the 33k resistor on the 90v 3mA line just because....well why not
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21st Oct 2018, 10:53 pm | #17 |
Dekatron
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,748
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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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