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-   -   Bush FM motorboating. (https://www.vintage-radio.net/forum/showthread.php?t=7270)

kevinb 30th Jan 2006 1:48 pm

Bush FM motorboating.
 
My Bush VHF54 works ok on AM but on FM it motorboats if you turn volume up. I have tried all valves and previous suggestions, any help would be appreciated.

Hunts smoothing bomb 30th Jan 2006 1:52 pm

Re: Bush VHF54 FM Problems
 
Hi,

I had a set that did this and I changed the caps in the FM section around the UCC85/ECC85, also the small value electrolytic in the FM discriminator circuit and the set plays very well to this day.8-)

Regards
Lee

twynham 12th Feb 2006 11:37 pm

Re: Bush VHF54 FM Problems
 
I've just fixed my one tonight... :D Wasn't getting anything on FM either, other than some rude noises and the TI blowing its top from the 25v+ being delivered by the EABC80.

Had already replaced the FM Electrolytic. Then bit the bullet and completely rebuilt the FM front end last night to no particular avail. >:(

Tonight, replaced decoupling caps C39, C40, and C41 around the IF amp (V4). Absolute sods to get to. Complete change of personality. Radio 4 now coming in *loud and clear* with 2ft piece of wire stuck in the RF input.

I guess the IF was unstable at the higher frequency.

Twynham 3, Hunts 0.

Good luck!!

TimS 20th Feb 2006 10:29 pm

Re: Bush VHF54 FM Problems
 
Is "motorboating" a "put.....put.....put...." sound?

I'm getting something like that on my VHF61 (FM only, and it doesn't start until a few moments after VHF is selected).

Is the VHF54 similar to the VHF61?

-- Tim.

Jim - G4MEZ 26th Feb 2006 3:05 pm

Re: Bush VHF54 FM Problems
 
I have the same problem on my VHF64. +27V across the discriminator reservoir etc. I found that if the VHF and MW (or LW but not SW) keys are held down simultaneously it bursts into life on VHF. As far as I can see, doing this removes the 27V of VHF AGC from the second IF amp and tames it down a bit, perhaps stopping it from taking off.

Not sure if you can play the same trick on other Bush VHF series as an aid to fault-finding but loading down the anodes of the IF Amps with a highish value resistor (or even an AVO) will damp it down.

I'm off to change all the decouplers around the IF stages and see if it cures it.

Jim

PaulR 6th Mar 2006 7:44 pm

Re: Bush VHF54 FM Problems
 
Get rid of all those Hunts caps - you know it makes sense!! My VHF 61 works a treat now they have all gone!

Paul

Jim - G4MEZ 6th Mar 2006 7:50 pm

Re: Bush VHF54 FM Problems
 
Quote:

Originally Posted by PaulR
Get rid of all those Hunts caps - you know it makes sense!! My VHF 61 works a treat now they have all gone!

Paul

Seconded. In my VHF64 I found all the anode and screen decouplers on the Mixer/1st FM IF and the two IF stages either O/C or a few pF.

It may be my imagination but the brown Hunts seem worse than the black ones. It seems too that a Hunts that has an easy life (e.g. decoupling the heater chain or AGC line) seem to live forever. Those running close to their rated voltage seem to die quickest. Again, it may be my imagination..

Jim

Paul Stenning 6th Mar 2006 10:45 pm

Re: Bush VHF54 FM Problems
 
Thirded, from dealing with several Bush VHFxx sets (in particular the VHF61 and VHF80).

jay_oldstuff 7th Mar 2006 5:05 pm

Re: Bush VHF54 FM Problems
 
It's all my VHF54 needed to bring FM crashing back in be warned though some are a swine to get at:wall:

These sets do sound good on FM when working so its worth persevering with.

Jay


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