Huge Lesson Learned!!!

[pwdrive]

Y'know, I've never been really sure about the reason for fitting that cap. across the mains input: I always thought that it was primarily there to by-pass any high-frequency 'noise' - such as fast-rising spikes, random R.F. signals and such like coming down the mains lead. As for suppressing hum - as in modulation hum - the usual trick consists of capacitor(s) from the anode(s) of the rect. valve to chassis.

Have I got all that about right?

Al.

Yes, more or less I think, it's never been my strong point either, I guess the key is in the word "modulation" I have always taken that to be hum you hear when tuned into a station but then again the mains lead is in effect an aerial so it will carry all sorts of RF frequencies, no doubt modulated by 50Hz, the receiver is fitted with valves, some none linear in operation so detection/mixing can occur of the hum modulated wave and if not stopped will rear it's ugly head...

Lawrence.

[Skywave]

VHF isn't working at all.
Rob

VHF reception, lack of. I trust that you are using something suitable for an aerial, as opposed to a piece of wire draped across the bench? That question is not intended to be a slight on your technical knowledge, Rob, but valve sets at VHF are usually a lot less sensitive that modern transistorised ones: a point often overlooked.

Al.

[Bushbaby]

Doh!
No aerial at all hence no reception!!!!

My skills are lacking

Rob

[Skywave]

Whoops! Just thought I'd mention it . . . . .
At least that is one problem that is easy to fix!

Al.

[Andrewausfa]

Well done Rob.

You're not alone with forgetting to put an aerial in, it's something I seem to do with disturbing regularity. Then comes my usual puzzlement until the penny drops.

[Bushbaby]

I have a Nikkai am/fm aerial amplifier - will that do the trick??

Thanks

Rob

[Joe]

What's wrong with its own VHF aerial?

It's something I tend to forget too, along with not putting the valves back.

- Joe

[Bushbaby]

Hi Joe
What VHF aerial? I can't assume it's the same as for MW but it shows how much I know??

Regards

Rob

[Joe]

There's a loop of wire inside the back with a plug on the end, or there should be.

If that's missing try sticking a length of wire into one of the holes of the socket.

- Joe

[Yorkie]

Hi Joe
What VHF aerial? I can't assume it's the same as for MW but it shows how much I know??

Regards

Rob

If you read page 1 of the Maker's 10-page data sheet Rob, you'll note that as with many sets, it has a rudimentary VHF 'aerial' attached to the back panel of the set.

Quote:

'A shortened dipole is fitted to the cabinet back. This is connected by means of an 80 Ohm line and two-pin plug'.

End quote.

If you are testing the set with the back panel removed, there will be no VHF aerial.

There will probably be a socket to connect a 'proper' VHF aerial, but often the one on the back of the set - just two bits of insulated flex - should bring some stations in.

Check the voltages in the VHF tuner as compared to the circuit, and if they seem reasonable, and you've cleaned the wavechange switch contacts with Servisol and checked their continuity, but the VHF band still doesn't show signs of life, it may well be due to the UCH85 valve - V1. These are quite short lived, and were often the reason that VHF sets were pensioned off. As listening to MW/LW declined, (MW only in the case of the VHF90 of course) due to more and more stations going live on VHF, if the sound faded away, it was uneconomic to take the set to a repair shop and pay a service charge and for a new valve, which in total would cost far more than buying a new transistor set by 1966 when that set was ten years old.

If you've got another VHF90 set that works on VHF, try your valve out of this one to see if it works, and if not, try the working valve in this set to see if it brings signs of life.

Hope that helps a bit.

Best of luck with it.