Bush VHF54 - Shorted Trimmer (?) - UK Vintage Radio Repair and Restoration Discussion Forum

MeanDumpsterCat · 17th May 2021, 3:09 pm

Hi everyone. i'm currently working on a Bush VHF-54. It's all working with the exception of the FM section.

Work so far: replaced capacitors on the bottom tag board (c31 through to C57). Checked resistors in the FM sub chassis. Still a few caps to go but I dont believe in blanket replacing without fault finding first. I only done the one on the tag boards because they were easy to get at and I had plenty of spares.

The FM mode does nothing but hum and some faint noise comes in at the top end of the band. I swapped out the old EF80s with some known working ones just on the off chance, still no luck.

Annoyingly, some of the wires connecting the sub chassis to the main chassis broke as I was working on it, no matter how careful I was.

When poking around, I noticed that the trimmer on the side of the sub chassis was reading as a short circuit which I thought was a bit strange. I still need to disconnect it and re test to see if it's the trimmer itself, the IF can it's connected to or one of the surrounding components. But I did hear of other cases of this going short?

Is this something common with these sets and if it is indeed short, can it be easily rectified.

Test points shown below

cathoderay57 · 17th May 2021, 3:14 pm

Usually, the trimmer is wired in parallel with a coil therefore will read S/C in- circuit. Jerry

Nuvistor · 17th May 2021, 5:10 pm

Quote:

Originally Posted by cathoderay57

Usually, the trimmer is wired in parallel with a coil therefore will read S/C in- circuit. Jerry

Indeed it is L3.
You can make a note the trimmers position and turn it very slightly. I have heard of trimmers in Bush VHF units giving problems and a slight movement curing it, I don’t know it this trimmer is the same type as those.

MeanDumpsterCat · 17th May 2021, 5:17 pm

Okay, I feel stupid. I didnt realise it was in parallel with the coil even though it's plainly obvious on the diagram. Tested it out of circuit and it's fine.

I have, however, noticed that there's no heater voltage on any of the EF80s but with 250v going into the FM section. Barking totally up the wrong tree. Not sure how I didnt notice before.

17th May 2021, 3:09 pm	#1
MeanDumpsterCat Pentode Join Date: Oct 2014 Location: Selsey, West Sussex, UK. Posts: 204	Bush VHF54 - Shorted Trimmer (?) Hi everyone. i'm currently working on a Bush VHF-54. It's all working with the exception of the FM section. Work so far: replaced capacitors on the bottom tag board (c31 through to C57). Checked resistors in the FM sub chassis. Still a few caps to go but I dont believe in blanket replacing without fault finding first. I only done the one on the tag boards because they were easy to get at and I had plenty of spares. The FM mode does nothing but hum and some faint noise comes in at the top end of the band. I swapped out the old EF80s with some known working ones just on the off chance, still no luck. Annoyingly, some of the wires connecting the sub chassis to the main chassis broke as I was working on it, no matter how careful I was. When poking around, I noticed that the trimmer on the side of the sub chassis was reading as a short circuit which I thought was a bit strange. I still need to disconnect it and re test to see if it's the trimmer itself, the IF can it's connected to or one of the surrounding components. But I did hear of other cases of this going short? Is this something common with these sets and if it is indeed short, can it be easily rectified. Test points shown below Attached Thumbnails

17th May 2021, 3:14 pm	#2
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,386	Re: Bush VHF54 - Shorted Trimmer (?) Usually, the trimmer is wired in parallel with a coil therefore will read S/C in- circuit. Jerry

17th May 2021, 5:17 pm	#4
MeanDumpsterCat Pentode Join Date: Oct 2014 Location: Selsey, West Sussex, UK. Posts: 204	Re: Bush VHF54 - Shorted Trimmer (?) Okay, I feel stupid. I didnt realise it was in parallel with the coil even though it's plainly obvious on the diagram. Tested it out of circuit and it's fine. I have, however, noticed that there's no heater voltage on any of the EF80s but with 250v going into the FM section. Barking totally up the wrong tree. Not sure how I didnt notice before.