Bush FM motorboating. - UK Vintage Radio Repair and Restoration Discussion Forum

kevinb · 30th Jan 2006, 1:48 pm

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

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.

Regards
Lee

twynham · 12th Feb 2006, 11:37 pm

I've just fixed my one tonight...

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

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

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

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

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

Thirded, from dealing with several Bush VHFxx sets (in particular the VHF61 and VHF80).

jay_oldstuff · 7th Mar 2006, 5:05 pm

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

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

Jay

30th Jan 2006, 1:48 pm	#1
kevinb Pentode Join Date: Dec 2005 Location: Leicestershire, UK. Posts: 211	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. Last edited by Darren-UK; 12th Jul 2007 at 12:53 am. Reason: General tidying.

30th Jan 2006, 1:52 pm	#2
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,369	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. Regards Lee __________________ Lee

12th Feb 2006, 11:37 pm	#3
twynham Diode Join Date: Sep 2005 Location: Cheshire Posts: 5	Re: Bush VHF54 FM Problems I've just fixed my one tonight... 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!! __________________ -- Stewart Twynham

20th Feb 2006, 10:29 pm	#4
TimS Pentode Join Date: Jun 2005 Location: Edinbane, Isle of Skye, UK. Posts: 180	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.

26th Feb 2006, 3:05 pm	#5
Jim - G4MEZ Tetrode Join Date: Feb 2006 Location: Newport Pagnell, Buckinghamshire, UK. Posts: 85	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

6th Mar 2006, 7:44 pm	#6
PaulR Nonode Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 2,030	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

6th Mar 2006, 10:45 pm	#8
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 7,348	Re: Bush VHF54 FM Problems Thirded, from dealing with several Bush VHFxx sets (in particular the VHF61 and VHF80). __________________ Paul Stenning Forum Admin/Owner and BVWS Committee Member

7th Mar 2006, 5:05 pm	#9
jay_oldstuff Octode Join Date: Jan 2005 Location: Hyde, Greater Manchester, UK. Posts: 1,074	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 These sets do sound good on FM when working so its worth persevering with. Jay Last edited by Darren-UK; 12th Jul 2007 at 12:51 am. Reason: General tidying.