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Old 24th Feb 2006, 4:21 pm   #1
yestertech
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Default Bush MB60, valve holder problems.

I knew this one was trouble as soon as I got the dial off without a fight ! Perhaps anyone who has restored one of these can enlighten me ref :

1. LT supply is "high" and when reaching c. 200volts AC input, the LT is already sitting at 1.4+ volts. Transf. tapping is OK. Both the 10 ohm resistors looks to be in spec and all the valves seem to be OK.

2. Having replaced "that" cap and restored the bias volts to the DL96, there is no output when injecting a 1k sine wave to g1. Valve shows good on the TAYLOR, anode /screen volts are present and when the signal is injected from anode to earth the LS produces a tone ( thus proving OPTX and speaker etc )

Rechecked a couple of times ( senility cross-check ! ) but NO AMPLIFICATION !

Double checked by applying signal to DAF 96 - predictably same result.

Are the valveholders supsect on these sets ? ( could be the culprit for both problems I guess )
Just want to canvass opinion before I tear it apart !
ta !
ANDY

Last edited by Darren-UK; 12th Jul 2007 at 12:10 am. Reason: General tidying.
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Old 24th Feb 2006, 4:37 pm   #2
Biggles
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Default Re: MB60 doing my head in

Sounds like it could be a suspect valve holder. Try measuring the voltages around the valve base for presence of ht on the anode, and also g2. Try giving the valve a good rocking in it's base, or clean it's pins?

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Old 24th Feb 2006, 5:52 pm   #3
Mike Phelan
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Default Re: MB60 doing my head in

Hi Andy
I think you have already found the problem. Most of these B7G holders have flat pins that can be closed up using a sewing needle in a wooden handle. I use a bit of chopstick or similar.
Worth cleaning the pins with Scotchbrite or a glass fibre brush.
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Old 25th Feb 2006, 12:27 pm   #4
yestertech
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Unhappy Re: MB60 doing my head in

This radio is more trouble than it's worth.

I plied the DL96 holder with some deoxit and eventually got the OPstage working. Still no signs of life on the DAF96 No amount of "wiggling" would produce an OP.

I noticed that the seating of the valve in the holder seemed wrong and that it did not seem to go all the way in. The whole base looked , well knackered ! There was nothing for it but to replace the holder.

This proved worth it, as audio was then restored. However, no IF response ! Slightly moving the DF96 produced an OP but this vanished again almost immediately. Propping the valve at an angle, the OP was back again, but the gain seemed low, so the upper IFT slug was adjusted - Phantom twiddler in evidence as the gain shot up as I turned.

Removing the sig gen clip produced radio FINALLY - but I will have to change a second valve holder. Doh !

Last edited by Darren-UK; 12th Jul 2007 at 12:09 am. Reason: General tidying.
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Old 25th Feb 2006, 8:17 pm   #5
zak
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Default Re: MB60 doing my head in

The contacts in B7G valveholders often get corroded in battery sets,but often no need to change the holder if its a McMurdo,you can just replace the contacts.
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