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Old 7th May 2022, 1:50 pm   #1
DonaldStott
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Default GEC BC5050 receiver.

Following a couple of pre-restoration Threads regarding failed rectifier valves and stuck knobs it's time to move on and start work on this late 1940s set which has been 'got at' previously - so be warned!

One of the first checks I always do is from Paul Stenning's excellent 'Repair and Restoration Information Index' namely 'With all the valves in place, connect the test meter between the live and neutral wires of the mains flex. Operate the power switch on the set and note the resistance reading.'

I am getting a dead short and a DMM reading of 35 Ohms - not a good start?

According to Trader Service Sheet 925 one of the main smoothing caps (C24) should be 16uF but someone has stuck in an 8uF and a 16uF in parallel giving a total capacitance of 24uF. Part of the problem with this change is that one of the original tags on the large C24 can has been used as a convenient anchor point for a number of wires?

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The other dual can smoothing cap contains C17 (4uF) and C25 (20uF) and has me bewildered. One of the three tags on the base has been cut off while the capacitance across the other two is reading 141uF !!!

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There is also another large 32uF cap that has seen better days not on the schematic that connects Tag 2 of the output transformer to the HT smoothing resistor R25?

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That's probably enough for now ... and help would be appreciated?
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Old 7th May 2022, 2:02 pm   #2
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Default Re: Gec bc5050

Forgot to mention that all the valves and dial lamps have been removed temporarily for safety reasons and to enable testing of the mains transformer outputs.

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Old 7th May 2022, 2:15 pm   #3
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Default Re: Gec bc5050

35 ohms DC resistance sounds fine. Why do you refer to that as a dead short?
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Old 7th May 2022, 2:18 pm   #4
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Default Re: GEC BC5050 receiver.

The dual cap with the cropped third terminal: this is likely to be the common negative connection and will also be connected to the can itself. So if the can is clamped to the chassis, the clamp makes the electrical connection and thus the terminal isn’t needed.
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Old 7th May 2022, 2:19 pm   #5
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Default Re: GEC BC5050 receiver.

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35 ohms DC resistance sounds fine. Why do you refer to that as a dead short?
Agreed. This is an AC set and it's the inductance of the mains transformer that limits the current drawn from the mains, not its resistance.
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Old 7th May 2022, 2:55 pm   #6
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Default Re: GEC BC5050 receiver.

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Originally Posted by DonaldStott View Post
I am getting a dead short and a DMM reading of 35 Ohms - not a good start?
Which is it then?

The manufactures service info states 34 Ohms resistance for a 250 Volt primary.

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Old 7th May 2022, 3:34 pm   #7
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Default Re: GEC BC5050 receiver.

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There is also another large 32uF cap that has seen better days not on the schematic that connects Tag 2 of the output transformer to the HT smoothing resistor R25?
The second tag up on the transformer in the photo is No: 1 on the Trader sheet and is connected to ground, the capacitor is likely to be a replacement HT filter capacitor, the fact that the original was 20uF and the replacement was 32uF is not important.

Note that the reservoir and the HT filter capacitors have different -ve connections to the circuit, this is due to the fact that the grid bias voltage for the Mixer and IF valves is obtained from the voltage developed across R24 (Trader Ref.) Hence the reservoir capacitors -ve connection is to the center tap of the HT secondary winding.

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Old 7th May 2022, 3:47 pm   #8
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Default Re: GEC BC5050 receiver.

Looks like I misunderstood this (my emdoldening) : -

"With all the valves in place, connect the test meter between the live and neutral wires of the mains flex. Operate the power switch on the set and note the resistance reading ... For an AC only set using a mains transformer it will be a couple of hundred ohms (the exact figure will be on the service sheet). Anywhere around this area is OK for now. If it is very low (tens of ohms) or short circuit, you have a problem that needs further investigation and correcting before power is applied. "
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Old 7th May 2022, 3:54 pm   #9
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Default Re: GEC BC5050 receiver.

Note the words "(the exact figure will be in the service sheet)".

As Lawrence says it's 34 Ohms.
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Old 7th May 2022, 5:48 pm   #10
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Default Re: GEC BC5050 receiver.

I'm being ultra-cautious as this GEC set arrived with an internal short between one of the anodes and the filament in the UL50 rectifier valve.

A replacement 5Y3GT for the UL50 has been very kindly supplied by Alan (snowman_al) and I am paranoid about fitting this in the set and powering up just in case there is a fault elsewhere that caused the internal short in the UL50!
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Old 7th May 2022, 5:58 pm   #11
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Default Re: GEC BC5050 receiver.

Most valve radios that work on AC with an isolation transformer will have a transformer primary resistance of below 100 Ohms.

Those that work on mains/battery with an isolation transformer such as 1.4 volt filament valves are likely to have a primary resistance of several hundred Ohms.

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Old 8th May 2022, 11:25 am   #12
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Default Re: GEC BC5050 receiver.

Moving on quickly from a shaky start, I've checked the output voltages from the mains transformer (no valves or bulbs) and they are broadly in line with those shown on the Manufacturers Service Sheet No. 116:-

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U50 Anodes: 281V
U50 Filament: 5.2V
Scale Lamps: 6.9V

Anywhere else I should check at this time?

I'm going to replace the smoothing caps and other electrolytics if only to try and understand the work that has already been carried out. On the Trader Sheet these will include C14 (25uF), C17 (4uF), C24 (16uF) and C25 (20uF). These either measure very high or no reading whatsoever?

I appreciate that it's not good practice to connect new components across the terminals of a duff capacitor but is it ok to use one terminal only of the C24 electrolytic as an anchor point?

I still haven't worked out what the electrolytic capacitor to the left of the output transformer is doing: -

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Normally with caps I would replace then test one at a time but as we had an internal short between one of the anodes and the filament in the UL50 rectifier valve I'm not taking any chances! Unless of course anyone can suggest where the problem may have been - my money is on the smoothing caps?
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Old 8th May 2022, 12:03 pm   #13
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Default Re: GEC BC5050 receiver.

Quote:
Originally Posted by DonaldStott View Post
I appreciate that it's not good practice to connect new components across the terminals of a duff capacitor but is it ok to use one terminal only of the C24 electrolytic as an anchor point?
I wouldn't unless C24's can was isolated from the chassis given the hammering the rectifier circuit appears to have received.

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Originally Posted by DonaldStott View Post
I still haven't worked out what the electrolytic capacitor to the left of the output transformer is doing
Looks like the common -ve of C17, C25 has been disconnected, so see if it's anything to do with Post#7

Lawrence.

Last edited by ms660; 8th May 2022 at 12:27 pm. Reason: Extra info
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Old 8th May 2022, 1:48 pm   #14
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Default Re: GEC BC5050 receiver.

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I wouldn't unless C24's can was isolated from the chassis given the hammering the rectifier circuit appears to have received.
Noted, thanks - I'll fit a new chassis tag here and a new 16uF 450V electrolytic.

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Looks like the common -ve of C17, C25 has been disconnected, so see if it's anything to do with Post#7.
The existing dual can should be 4uF and 20uF. Another two electrolytics to replace once I understand Post #7 and reconcile that with what I've got - unless anyone has seen a 4uF/20uF dual can?
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Old 8th May 2022, 4:42 pm   #15
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Default Re: GEC BC5050 receiver.

I've never encountered an electrolytic with values 4uF 350V?

Why such a low capacitance with such a high voltage - where in a circuit were they used?
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Old 8th May 2022, 5:17 pm   #16
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Default Re: GEC BC5050 receiver.

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I've never encountered an electrolytic with values 4uF 350V?

Why such a low capacitance with such a high voltage - where in a circuit were they used?
Often doing the same function as the one in your receiver, HT de-coupling etc.

The 20uF + 4uF was used in several GEC receivers.

The 4uF needs to be rated for at least 350 Volts, especially with a directly heated rectifier as used in your receiver.

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Old 8th May 2022, 8:48 pm   #17
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Default Re: GEC BC5050 receiver.

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I've never encountered an electrolytic with values 4uF 350V?
I guess you've not seen my Murphy signal generator repair thread running in the vintage test gear section at the moment. This generator uses a couple of 4uF HT smoothing capacitors and at that small capacitance value with an electrolytic type, it doesn't take a lot of loss for them to become duff, which they are. They're not leaky or running warm, they've just dried out and are lacking capacitance.
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Old 8th May 2022, 9:08 pm   #18
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Default Re: GEC BC5050 receiver.

To be fair when repairing any AC only set I only remove the rectifier valve and plug in via the variac and just advance the output in three or four steps.
Normally if the mains transformer is duff it will have leaked wax or pitch if its seriously overheated.
It's always a good idea to leave the valves and dial bulbs in situ bar the rectifier anyway as it gives a quick indication of the health of the mains transformer.

When you say the sets been got at remember that many customers would only pay a paltry sum for repairs so bridging smoothers was common as it kept the repair costs down, often a smoother was low in value and not likely to pop its clogs so a cap would just be fitted in parallel, I've had many original smoothing caps just totally open circuit as well. If the original is swelling and leaking then it should be removed from the circuit.

Incidentally have you seen there is a Scottish Meet on the 28th of this month at my place in Kilmarnock, perhaps it would help you with repairs there are many very experienced engineers coming. See the groups and events section.
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Old 8th May 2022, 9:12 pm   #19
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Default Re: GEC BC5050 receiver.

Quote:
Originally Posted by DonaldStott View Post
I've never encountered an electrolytic with values 4uF 350V?

Why such a low capacitance with such a high voltage - where in a circuit were they used?
Don't forget that AC sets inevitably have a full wave rectifier which give a 100 hz ripple so you can use less capacitance for the reservoir capacitor, read up your theory and you'll see the reasons.
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Old 9th May 2022, 9:19 am   #20
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Default Re: GEC BC5050 receiver.

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Originally Posted by Techman View Post
I guess you've not seen my Murphy signal generator repair thread running in the vintage test gear section at the moment. This generator uses a couple of 4uF HT smoothing capacitors and at that small capacitance value with an electrolytic type, it doesn't take a lot of loss for them to become duff, which they are. They're not leaky or running warm, they've just dried out and are lacking capacitance.
Thanks Techman - I'll certainly seek out for your Murphy signal generator repair thread.

My 4uF soothing cap is in a dual can with a 20uF, the latter of which is measuring 36uF!

I struggled to understand why I couldn't get a reading for the 4uF despite trying a couple of DMMs and my ESR meter - there appears to be no capacitance at all?

It is being replaced with a 4.7uF 450V electrolytic capacitor.

Quote:
Originally Posted by murphyv310 View Post
To be fair when repairing any AC only set I only remove the rectifier valve and plug in via the variac and just advance the output in three or four steps.
Normally if the mains transformer is duff it will have leaked wax or pitch if its seriously overheated.
It's always a good idea to leave the valves and dial bulbs in situ bar the rectifier anyway as it gives a quick indication of the health of the mains transformer.
Thanks Trevor - there is certainly some wax that has escaped from the mains transformer and it has seen better days but the output voltages seem ok?

I'll refit the valves and bulbs but leave out the rectifier and see how the mains transformer performs - what should I be looking for?

Quote:
Originally Posted by murphyv310 View Post
you say the sets been got at remember that many customers would only pay a paltry sum for repairs so bridging smoothers was common as it kept the repair costs down, often a smoother was low in value and not likely to pop its clogs so a cap would just be fitted in parallel, I've had many original smoothing caps just totally open circuit as well. If the original is swelling and leaking then it should be removed from the circuit.
I don't necessarily mean 'got at' in the pejorative sense but simply that it's hard enough sometimes for me to follow a schematic and compare it with the set on the bench without having to understand any previous repair work as well.

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Originally Posted by murphyv310 View Post
Incidentally have you seen there is a Scottish Meet on the 28th of this month at my place in Kilmarnock, perhaps it would help you with repairs there are many very experienced engineers coming. See the groups and events section.
I'd have loved to attend the Scottish meet but unfortunately it coincides with the week I am away down south - maybe next time?

Quote:
Originally Posted by murphyv310 View Post
Don't forget that AC sets inevitably have a full wave rectifier which give a 100 hz ripple so you can use less capacitance for the reservoir capacitor, read up your theory and you'll see the reasons.
Thanks for that.

As they say "Every day is a School day", especially when you are an occasional hobbyist like myself - I'll check this out in my copy of Scroggie!
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