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Radio B7G Valve Bases

 
AmRadioFan
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This in my first post so please forgive me if it is in the wrong place or covers an issue that has already been dealt with.

I have just started work on an Ever Ready Sky Casket that is in stunning cosmetic condition but less impressive electronically.

After changing the usual suspect capacitors I powered it up to total silence. 

I checked the LT current and instead of the expected 125ma I got just 50ma. Removing the DL96 and DAF96 made no difference to the current drawn. The 50ma was what I would expect the DK96 and DF96 to take. Having established that the filaments of the DL96 and DAF96 are intact, I checked the LT voltages appearing at the two valve bases and they were as expected.

I can only conclude that for some reason the valve pins are not making proper contact with the valve base contacts even though contact can be made with a piece of thin wire.

I would really appreciate any suggestions on how to fix this issue, ideally without changing the valve bases.

Many thanks in advance

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Topic starter Posted : 17/08/2021 6:48 pm
Nuvistor
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@amradiofan 

Are the valve bases “McMurdo” these are known to give problems, the connections are a forked prong and the prongs break off?

Other types may well just need cleaning and tighten up.

Anyway welcome to the forum.

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Posted : 17/08/2021 7:27 pm
Lloyd
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Welcome!

Just had a similar problem with an Ultra Coronation Twin, dodgy connections in the valve bases, but annoyingly when I tried to tweak the valve holder's fork, one of it's prongs snapped off! Fixed it in the end by pulling an unused fork from another socket in the set and replacing it.

Regards,

Lloyd

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Posted : 17/08/2021 8:53 pm
Boater Sam
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The fork type connectors are known to be poor once strained. Also the fumes off dead batteries causes corrosion on most metals.

Are the valve pins clean?

It may be necessary to replace the valve bases.

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Posted : 18/08/2021 1:17 pm
AmRadioFan
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Thank you all for your suggestions - much appreciated.

Yes they are McMurdo bases and on closer examination some of the connectors are broken. 

I'll try Lloyd's idea of changing the folks first but failing that It will be a fun job to change two valve bases at the end of the gardening season.

Peter

 

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Topic starter Posted : 18/08/2021 3:37 pm
Nuvistor
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@amradiofan 

Well worth replacing individual forks if you can, save a lot of work.

Your set is around 60 years old, these valve bases were giving the same problem in various products in the 1960’s and 70’s, not surprising they are giving problems after so many years.

 

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Posted : 18/08/2021 7:18 pm
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PYE625
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It can often occur in B8A and B9A sockets of the same make. They all use the same type of forked pins and can be replaced on an individual basis. Simply desolder the pin, straighten the lug and push up through the base. In the past, I have needed to replace a few like this. A scrap chassis containing similar such valve holders is worth it's weight in gold.

Here is an example photo of a B8A base that has the same style of pin I believe...

s l1600
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Posted : 18/08/2021 7:59 pm
AmRadioFan
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My Sky Casket again ......

By cannibalising a new B7G base I've managed to replace three broken pins in the DL96 base. To complete the job I need to do another in the DAF96 holder but that is impossible to get at without removing the front panel. 

Removing the front panel requires the volume control and wave change knobs to be removed. As no grub screws are visible I assume they are both of the push on / impossible to get off variety.

I would really appreciate any hints on how to get this sort of knob off without actually breaking them.

Many thanks in advance

Peter

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Topic starter Posted : 30/08/2021 2:48 pm
PYE625
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Posted by: @amradiofan

I would really appreciate any hints on how to get this sort of knob off without actually breaking them.

With difficulty sometimes, but I find an old cloth or a piece of string placed under the knob around most of it's circumference and simply pulled upwards will do the trick. Sometimes, in stubborn cases, a tiny drop of WD40 applied where the shaft enters the knob may help. Don't spray the stuff or it will get everywhere and you don't want that, believe me. Don't be tempted to use a metal implement, it will very likely chip or break the knob.

 

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Posted : 03/09/2021 9:25 pm
AmRadioFan
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@pye625 

Thank you for that suggestion, I've tried it and it works, the knobs are off.

I have now managed to replace the broken part of the DAF96 valve holder but the set still does not work fully. Putting my finger on the DAF91 grid produces a loud hum so at least the two stages are working. This radio is still a 'work in progress' job.

Once again thanks for the advice

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Topic starter Posted : 05/09/2021 6:19 pm
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sideband
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You'll probably find that the other valveholders have suffered the same fate. You need to check each fork with a pin or something similar. You should soon find any that are weak or broken. At least  you have made progress and the audio stages are working.

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Posted : 06/09/2021 10:25 am
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AmRadioFan
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Six new capacitors, one resistor and six valve holder pin sockets replaced.

Now I have a working radio - Thanks for the advice on how to fix it 👍 

Repair2
SkyCasket
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Topic starter Posted : 22/01/2022 8:22 pm
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Nuvistor
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@amradiofan 

Excellent, the radio does look nice.

 

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Posted : 22/01/2022 11:05 pm
sideband
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Well done! Some radios are like that....! Broken valve holder pins are not just restricted to battery sets either although they do seem to be the worst offenders. You've made a good job of changing those capacitors.

You might want to check that Plessey electrolytic....the one on the left in bright yellow with the red and black end caps. They had a poor reputation in the trade even back in the 60's. If it's OK, fine but they can be troublesome.

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Posted : 23/01/2022 5:08 pm
AmRadioFan
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@sideband

I did wonder about the electrolytic so I switched the radio on with no LT supply and 92 volts HT and monitored the leakage current ....... At the end of one hour it had reduced to 11.0 micro amps which equates to a resistance of about 8.4 Meg ohms which seems reasonable. Having said all that I'll still probably replace it next time as a precaution. 

Thanks to you all for your kind comments 

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Topic starter Posted : 01/02/2022 7:37 pm
Katie Bush
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I'd also be looking at ousting that 'Orrible 'Unts, top edge, just under the tuning gang, they were the worst of all caps. - "Mouldseal" I think they were called.

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Posted : 02/02/2022 5:13 pm
Cathovisor
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I'm looking at that 10MΩ resistor and wondering "how close to 10MΩ is it?"

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Posted : 02/02/2022 9:37 pm