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Old 28th Nov 2020, 5:51 pm   #1
HowardB
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Default GECoPhone BC3235: Voltages

I am starting to work on a GECoPhone BC3235 (around 1930) - see attached photos.

It only receives one station, the wave change switch doesn't seem to do much and the volume is quite low even with a 100ft aerial.

I have measured the voltages throughout the set and recorded them as annotations to the attached PDF circuit. Could somebody please check these voltages and let me know if there are any concerns?

I'm also looking for general advice on this one.
The big capacitors and aerial coil assembly are locked away in metal boxes on the top of the chassis. I can't see any way into these.
The resistors are looking pretty good, all fairly close to what they should be.
The capacitors under the chassis are not of the waxy type, they are in sturdy metal cases,
but they have drifted, e.g. 0.25uF is now 0.9uF
Should I replace them?

Thanks for reading this.

Howard
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Last edited by Cobaltblue; 28th Nov 2020 at 7:03 pm. Reason: Circuits available up top cannot be shared forum rule B8
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Old 28th Nov 2020, 8:25 pm   #2
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Default Re: GECoPhone BC3235: Voltages

Hi Howard, please do not power up the set again until you have replaced several vital components or you may cause very expensive damage.
The fact that the 0.25 cap reads 0.9 uF is an indication of it being leaky.

It should be an easy set to work on but many of the components will be well out of spec.
If you want to preserve originality I may have some of the "dog bone " style of resistors if you need to replace any.

Ed
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Old 29th Nov 2020, 10:42 am   #3
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Default Re: GECoPhone BC3235: Voltages

I've uploaded the voltages as a spreadsheet.

Howard
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File Type: xlsx BC3235 Volts.xlsx (14.2 KB, 74 views)
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Old 29th Nov 2020, 11:30 am   #4
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Default Re: GECoPhone BC3235: Voltages

Hi Ed, thanks for your kind offer but I think the resistors are ok. I'll focus on the Caps.
Any comments on the voltages?

Howard
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Old 29th Nov 2020, 8:35 pm   #5
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Default Re: GECoPhone BC3235: Voltages

Hi Howard, I'm not familiar with that circuit, but a + voltage on the grid of V2 seems suspicious, others may differ

Ed
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Old 29th Nov 2020, 9:27 pm   #6
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Default Re: GECoPhone BC3235: Voltages

According to a schematic I'm looking at the grid of V2 should be +ve WRT chassis.

Lawrence.
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Old 29th Nov 2020, 10:28 pm   #7
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Default Re: GECoPhone BC3235: Voltages

Thanks Guys,

V2 Cathode is 2.9v wrt chassis.
V2 Grid is 2.0v wrt chassis.
V2 Grid is therefore 0.9v negative wrt cathode.
The grid should be negative wrt cathode - correct?
Is it negative enough?
Any other voltage concerns?

Thanks
Howard
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Old 29th Nov 2020, 10:44 pm   #8
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Default Re: GECoPhone BC3235: Voltages

Referring to the schematic I'm looking at, there's no cathode bias used for the detector function, it's only used when a gramophone pickup is connected, for the detector function grid leak bias is used (0.0002uF capacitor plus the 2meg resistor to ground via the cathode bias resistor)

The input resistance of your meter when measuring will reduce the actual voltage on the grid.

Lawrence.

Last edited by ms660; 29th Nov 2020 at 10:54 pm. Reason: extra info
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Old 30th Nov 2020, 12:17 am   #9
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Default Re: GECoPhone BC3235: Voltages

The total current is 33mA
The MH4 is drawing 4.6mA (650 ohm cathode bias)
The MS4 is drawing 4.4mA (650 ohm cathode bias)
The PT4 is drawing 24mA (400 ohm bias less other cathode currents)
These figures don't ring any alarm bells.

The wavechange switch shorts out the LW windings. Have you cleaned the switch properly as it might be that you are receiving LW only?

Also, as Ed says, the capacitors in the tin boxes (usually paper capacitors packed with wax) are very likely to be leaky/lossy and that will impact performance. If you want to check the capacitors you need to measure the leakage current when working voltage is applied as well as the capacitance itself.

The circuit on radiomuseum shows the cathode auto-bias is always there.

Lastly, this is a very basic set so don't expect too much from it.
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Old 30th Nov 2020, 12:31 am   #10
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Default Re: GECoPhone BC3235: Voltages

Hi Lawrence,

Pretty sure you have the right schematic. I've attached a small section to make sure.

Have I got this right - the grid become slightly negative due to electrons accumulating on the grid and having a hard time getting to ground through the 2M?

And you're saying the small bias (-0.9v) is actually larger, it's just that my multimeter (10M) is comparable to the 2M so it's affecting the measurement?

-H
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Old 30th Nov 2020, 1:07 am   #11
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Default Re: GECoPhone BC3235: Voltages

Hi PJL,

Thanks for those calculations - I've learnt a lot tonight. Ashamed to say that I didn't realise you could find the current flowing through the valve by knowing the cathode voltage and cathode resistor. I suppose obvious really. Anyway, it looks like the valves are operating correctly.

I think you are right about the switch, the set seems to be stuck on LW. I can only get Radio 4 Long Wave. Unfortunately, the wave-change switch and the smoothing caps are enclosed in the metal boxes. I'll have another go at opening them.

Thanks,
Howard
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Old 30th Nov 2020, 2:33 pm   #12
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Default Re: GECoPhone BC3235: Voltages

Quote:
Originally Posted by HowardB View Post
Pretty sure you have the right schematic. I've attached a small section to make sure.

Have I got this right - the grid become slightly negative due to electrons accumulating on the grid and having a hard time getting to ground through the 2M?

And you're saying the small bias (-0.9v) is actually larger, it's just that my multimeter (10M) is comparable to the 2M so it's affecting the measurement?
Yes, we're both looking at the same schematic.

The ā€œ-0.9vā€ isn't the bias voltage (see grid bias explanations below) but yes the meter will affect the voltage on the grid WRT (With Respect To) chassis/ground, the input resistance of the meter forms a potential divider with the 2meg resistor and the voltage indicated under normal working conditions with no signal received will be the cathode voltage minus the voltage developed across the 2meg resistor.

V2 operating as a detector (no pickup connected):

Under no signal conditions there is in effect no grid bias voltage because the grid is connected to the cathode via the 2meg resistor alone, when a received signal is applied the grid acts like the anode of a diode and current flows from the cathode to the grid during the +ve half of each cycle, the 0.0002uF capacitor charges which makes the grid -ve WRT the cathode, the value of the 0.0002uF capacitor and the 2meg grid resistor are such that the -ve voltage on the grid follows the audio modulation, in other words there's now a varying -ve voltage on the grid at audio frequency, that means that the voltage developed across V2's anode load resistor will be an amplified version of that varying voltage on the grid.

V2 operating as a gramophone amplifier (pickup connected):

Connecting a pickup that has a DC path such as a magnetic pickup means that in DC terms the grid is now connected to chassis/ground, that means that the voltage developed across the cathode resistor will be applied to the grid making the grid -ve WRT the cathode, that bias voltage will be equal to the voltage across the cathode resistor and the valve will now be biased for Class A operation.

Lawrence.
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Old 30th Nov 2020, 2:58 pm   #13
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Default Re: GECoPhone BC3235: Voltages

Lawrence, my mistake, I agree that the circuit shows the bias without the pickup should be close to zero and therefore the grid should measure near the cathode voltage.

It would be a good idea to check the 2M resistor value. Measuring the grid voltage may be difficult as any signal pickup (i.e. 50Hz) will be rectified on the grid causing it to be driven -ve.
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Old 30th Nov 2020, 4:18 pm   #14
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Default Re: GECoPhone BC3235: Voltages

Hi PJL,

I just measured the resistor at 2.9M, so it's +45%.
I guess this needs to be replaced.
By the way, it doesn't look like a resistor at all - see photo.

Howard
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Old 30th Nov 2020, 4:23 pm   #15
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Default Re: GECoPhone BC3235: Voltages

Hi Lawrence,

Thanks so much for the explanations. I didn't realise that the 2M and 0.0002uF on the grid had a role in the detection. I thought the valve acted as a diode and the following R and C were responsible for detection. I'm learning something here.

Unfortunately, I'm stuck on your last section. Have read it many times but cannot grasp it.
Quote:
that means that the voltage developed across the cathode resistor will be applied to the grid making the grid -ve WRT the cathode,
"the voltage developed across the cathode resistor" are you still talking DC or is that voltage due to the signal from the pickup?

Howard
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Old 30th Nov 2020, 5:54 pm   #16
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Default Re: GECoPhone BC3235: Voltages

Quote:
Originally Posted by HowardB View Post
"the voltage developed across the cathode resistor" are you still talking DC or is that voltage due to the signal from the pickup?
Yes, it's the DC voltage that's developed across the cathode resistor, the 0.25uF capacitor connected across it in effect evens out the voltage variations caused by the signal to a certain extent, without it the gain of that stage would be reduced.

Lawrence.
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Old 30th Nov 2020, 8:29 pm   #17
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Default Re: GECoPhone BC3235: Voltages

OK, I've removed the box that covers the 'aerial coil assembly', so I can now access the LW/MW switches for cleaning.

I'm still trying to open the metal box that contains the 'condenser pack'.
I don't think it was designed to be opened but I've managed to prise one corner of the top lid off to allow a crafty peek inside.
It seems to be full of black bitumen like material, I can't see any discreet components.
I expect that under the black stuff it contains the 2uF, 4uF, and 2 x 1uF caps.
It has a terminal block with 7 connections.
I'm starting to think this is a step too far and I should leave well alone

Does anybody have one of these sets and have experience of opening the box and checking / replacing the contents?

Howard
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Old 30th Nov 2020, 8:36 pm   #18
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Default Re: GECoPhone BC3235: Voltages

Hi Howard, capacitor box usually has the lid soldered in place. Once removed you will see thE caps buried in pitch (cheap insulation). This can be melted out, new caps assembled and the box refilled with pitch (messy business) or wax can be used (easier) or leave it unfilled.

Then resolder the lid and respray an appropriate colour.

Ed
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Old 30th Nov 2020, 9:24 pm   #19
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Default Re: GECoPhone BC3235: Voltages

Hi Ed,

Yes, I thought it might be soldered so I put my iron onto it, but it didn't do much - I'll have another go.
I've got a powerful hot air gun for stripping paint, maybe that could be used to melt the pitch.
It sounds like an absolutely horrible job but it might just be possible.
I need to decide whether to leave it alone or go for it and replace the caps!

Howard
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Old 30th Nov 2020, 9:41 pm   #20
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Default Re: GECoPhone BC3235: Voltages

Unsoldering seams requires a great deal of heat as you have to melt all the solder simultaneously, whereas when you solder seams you can work progressively. A blow torch may be more effective than a soldering iron.

Often the pitch can be pulled out all in one lump using wood screws as corkscrews.

I don't bother putting pitch or wax into restuffed boxes and if the lid is underneath like in AR88's I don't bother to replace it.

It would be nice to see a picture of the capacitor box.
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