Grundig 2012 intermittent volume.

[pilot mariner]

A Grundig 2012 I am working on intermittently changes volume. This only happens on VHF, with M L and SW unaffected, and the volume changes are always to the same level. The slightest touch with a probe or screwdriver virtually anywhere in the circuit restores normal volume which may last for a few minutes before failing again. I have carried out all the usual checks ie switches, dry joints etc. The only suspect left is the EC92 in the VHF box. As I don’t have a spare to hand, are there any further tests I can make to confirm this. I can post a circuit if required.
Thanks
Mark

[Eddystonepete]

Hi Mark, does this Grundig have a a magic eye? If so does the display change under fault condition? A test of the EC92 could be, disconnect the aerial and connect a few feet of wire direct to the next stage (mixer grid) to see if the fault recurs. You could also unplug the EC92 during this test but a quick search for a Grundig 2012 shows a radio with a UC92, series heater.

Peter

[pilot mariner]

Hi Peter

Yes it does have a magic eye, and you're right it doesnt move under fault conditions. At the moment I am struggling a bit because the circuit I have found on the net is different to the set I have here. It does show an EC92 though but my set has an additional EAF42 in the IF stages. I will try your suggestion and report back. Just wondering if the fault could be in the ratio detector circuit?

Mark

[AlanBeckett]

So is yours a 2012 - E Valves - or a 2012GW - U Valves?
They're both in the Radio Museum. There's also a Table Gram Version - http://www.radiomuseum.org/act_main.cfm
Neither has an EF42.

Alan

[pilot mariner]

Hi Alan

Thats very strange then its definately a 2012 with E valves, EC92, ECH81, EF41, EAF42, EABC80, EL41, and EM34. The radiomuseum states 2 EL41s so maybe someone has changed one for a EAF42? But the circuit I have only shows 1 EL41 so Im confused.
Mark

[pilot mariner]

Further to the fault, I tried Peters suggestion but it was inclonclusive with no sound at all. However, I have tuned into the vhf box on a scanner at 10.7mhz and got a good signal with no fault and a very reduced signal during fault condition. This confirms my initial suspitions that the fault is with the vhf tuner, possibly the valve.

Mark

[Eddystonepete]

Hi Mark, good lateral thinking with the scanner !.
Peter

[pilot mariner]

Thanks Peter.
One of my above posts should read EF41s not EL41s
looks like I need an EC92 then for the next step.
Also is an EAF42 a substitute for an EF41.?

Cheers

[AlanBeckett]

It's got a Diode in it. Best check the Pinouts.

Alan

[AlanBeckett]

Not unless it's been rewired.
http://www.r-type.org/exhib/aad0127.htm
http://www.r-type.org/exhib/aai0082.htm

Alan