bush vhf41

[tiv48]

mistake in above post the cap that has been changed is the grid coupling cap for the el41
(should of gone to specsavers)

[tiv48]

have now fitted a new 3 core mains lead .i also found some of the rubber covered wire to the output tx needed to be replaced.
on re aplying mains the lamp limiter li up very brightly and there was a smell of hot wax.
this turned out to be the ez 80 going short.
after this diversion i have the audio stage working but no sign of any signals on AM or VHF. there is a click when changeing the wave change switch.with the signal injector i had a loud out put at the volume control ,and a low output at t13 then my injector died .
time to try something else ,i hooked the scope up to C31 (the ocs coupling cap) and found the ocsolator worked bu oml on long wave.
i pluged a wire into the aerial socket and was rewarded with a low leavel radio 4 and a french station .
so it looks like at least 3 faults to cure
1.why osc only works on LW
2. why the low gain.
3why no vhf?
rob t

[Steve_P]

The IF on these for VHF is not 10.7 Mhz on these. It is in fact 19.5 Mhz. Maybe the IF has been tweaked - it does happen!

This was the first VHF radio and rather then the dubiously reliable ECC85, there are 2 of EF80's and they need to be top notch - well the first one in particular. Swap them over (does work sometimes) and then try a big antenna on it.

As with all such jobs, clean the wavechange switch as they are the root of all evil.

For the AM, I would look round the oscillator stage. What are the volltages on LW and MW? Try a spare ECH81 and check the MW components after checking the IF - 470 kHz.

Cheers,

Steve P.

[tiv48]

thanks for that steve i must admit the switching looks quite complex. as the osc works ok on LW i think i must look closer at S7 and S10
i have cleaned the wave switch with servisol .
i have tried a new ech81 no change.
the voltage an the triode anode (ech81) is a bit low at 75 volt (avo 8) R11/C19 maybe .
i think it may be better to do a mas change of the remaining hunts caps most have cracked cases but read ok.
also i yet to change c35 its wax but again reads ok.
rob t

[Steve_P]

It's not the value of the capacitance that matters. It's the ESR - Effective Series Resistance. If there is not any leakage of course.

Any that are cracked - change them. Don't bother metering them.

Something else - servisol is not always enough. Check the wafers for conductivity. And clean them again.

Do you have the Bush VHF41 service data?

Cheers,

Steve P.

[tiv48]

the data i have is from pauls cd i agree that the swiches need checking with a meter
i do have a peak ers meter but only tend to use it on high value psu caps.
the bigest problem is access and no so good eye sight.
it may be misalighned as i was told some one had tried to repair it before
rob t

[Joe]

At the risk of teaching my grand-mother to suck eggs I'd change the lot as that will rule out a lot of problems and save wild goose chases. After that I'd be thinking alignment if I knew somebody had had a pop at it before. I've never had to re-align an un-messed with Bush though.

- Joe

[tiv48]

thanks joe i would not look forward to realignment and some of those caps look a swine to get at .
my next job will be to get the osc working on both bands .imay be doing this wrong ? maybe i should work on the low gain?
rob t

[Radio Fixer]

Given up on Servisol switch cleaner and now use De Oxit (no connections other than being a happy user). Its expensive and took a lot of ooing and ahhing before I spent out for it. But it really is the best and lasts a long time as only a little is needed.

Servisol seemed to work initially but in 6 months switches needed squirting again which I dont find with De Oxit.

There is a school of thought that any cleaner should only be minimally applied to Pax wafers, particularly if they have high voltages on them. I read that it can get into the material and cause breakdown. I dont use excess and have never had a problem. Perhaps wafers that breakdown after using it were already carbonised from previous arcing and were a hazard anyway.

Gary

[tiv48]

i agree with you about deoxit but the price!
if i can i use one of those fibreglass pens to remove any oxidation then a quick sqirt of servisol.
i try and keep switch cleaner away from ht contacts.
.
rob t