Bush VHF94 - UK Vintage Radio Repair and Restoration Discussion Forum

Pilot Mariner · 3rd Jul 2020, 8:38 pm

A Bush VHF94 I am working on intermittently reduces in volume on the AM bands only, usually when fully warmed up. By injecting a signal at various stages I have narrowed the fault down to the area around the 1st IF transformer. This is also confirmed by the magic eye indicating a reduction in signal strength (the feed to the magic eye is taken from this area). The obligatory touch of a probe or screwdriver to the IF transformer connections will sometimes restore full volume.
I have removed and checked the IF transformer, at the same time replaced C39 and C40 as a precautionary measure (Using trader sheet 1334 for the identical VHF64). Sadly the fault still exists.
Do we think I am looking in the correct area or can anyone suggest any other measurement I could take.

Thanks

Mark

cathoderay57 · 3rd Jul 2020, 9:09 pm

There's lots of switchery in the circuit. Try a good blast of switch cleaner on all the relevant contact sets. Otherwise, check for dry solder joints, especially earths to chassis. Also oxidised valve pins and sockets on the IF valve in particular. Cheers, Jerry

yestertech · 3rd Jul 2020, 10:02 pm

I would agree with Jerry, theres a lot of switch contacts going back and forth. A squirt or two of Deoxit may improve things.
As the signal to the magic eye also drops we should be able to discount the second IF stage.
The problem could be further back of course.
I found while searching a note from another forum member relating to a different issue of 'frying' which I was having with one of these. He kindly pointed out that the connections to S6a ( which selects AM or FM input to the ECH81 ) were sometimes found to be badly soldered, so this might be worth checking also.
The HT supply to the IFT is decoupled via 2k2/10nf so these might be worth checking ?
I had a similar issue on a Philips 170a many years back and this turned out to be a dry joint on one of the MW coils.
Finally a jolly picture to cheer you along - gotta love these sets....good luck !

Pilot Mariner · 3rd Jul 2020, 11:10 pm

Thanks cathoderay57 and yestertech for your suggestions, I had already thoroughly dosed the switch contacts and valve pins with switch cleaner.
I had also discounted anything prior to the IF stage by removing the ECH81 and injecting a 470khz signal at the anode b pin of the empty valve holder. The 10nF (C36) cap has been replaced and the 2k2 is measuring 2K3. I will double check again for dry joints.
Yes these are great sets, I am fixing 2 VHF94s for a friend (stevehertz) and a VHF64 (my own) all in parallel. I also have 2 other VHF64s restored a few years ago. Mark

John Caswell · 4th Jul 2020, 9:58 am

Around this time with this and a lot of other makes of radios, we had a lot of problems with the polystyrene tuning capacitors inside the IF transformers, giving exactly the symptoms you talk about, and "cured" by touching one of the IF transformer pins. The only cure was to replace them with silver mica capacitors. In a lot of cases did not even have to realign the set.

John

Pilot Mariner · 4th Jul 2020, 3:16 pm

Hi yes I have already replaced these capacitors (using the same silver micas as the originals C39 & C40). I had a similar problem on a Grundig a few years ago.

cathoderay57 · 4th Jul 2020, 9:25 pm

Maybe something awry on the AGC line, decoupling capacitors replaced and resistors checked? Diodes on EABC80 OK? Last year I had intermittent diode failure on an EABC80 that only affected FM. Cheers, Jerry

Pilot Mariner · 4th Jul 2020, 10:00 pm

Yes thanks for that suggestion, I also suspected the AGC line as I have run out of ideas around the IF transformer, but all decoupling caps have been replaced and resistors checked - some slightly high in value.
I have used these valves to track down a fault in the other VHF94 and didn't notice anything strange there, but I will try another EABC80 in this set. Unfortunately the fault hasn't occurred today.

3rd Jul 2020, 8:38 pm	#1
Pilot Mariner Hexode Join Date: Jun 2003 Location: Burntwood, Staffordshire, UK or Kabaty in Warsaw Poland. Posts: 438	Bush VHF94 A Bush VHF94 I am working on intermittently reduces in volume on the AM bands only, usually when fully warmed up. By injecting a signal at various stages I have narrowed the fault down to the area around the 1st IF transformer. This is also confirmed by the magic eye indicating a reduction in signal strength (the feed to the magic eye is taken from this area). The obligatory touch of a probe or screwdriver to the IF transformer connections will sometimes restore full volume. I have removed and checked the IF transformer, at the same time replaced C39 and C40 as a precautionary measure (Using trader sheet 1334 for the identical VHF64). Sadly the fault still exists. Do we think I am looking in the correct area or can anyone suggest any other measurement I could take. Thanks Mark Attached Thumbnails

3rd Jul 2020, 9:09 pm	#2
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,382	Re: Bush VHF94 There's lots of switchery in the circuit. Try a good blast of switch cleaner on all the relevant contact sets. Otherwise, check for dry solder joints, especially earths to chassis. Also oxidised valve pins and sockets on the IF valve in particular. Cheers, Jerry Last edited by cathoderay57; 3rd Jul 2020 at 9:14 pm.

3rd Jul 2020, 10:02 pm	#3
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,130	Re: Bush VHF94 I would agree with Jerry, theres a lot of switch contacts going back and forth. A squirt or two of Deoxit may improve things. As the signal to the magic eye also drops we should be able to discount the second IF stage. The problem could be further back of course. I found while searching a note from another forum member relating to a different issue of 'frying' which I was having with one of these. He kindly pointed out that the connections to S6a ( which selects AM or FM input to the ECH81 ) were sometimes found to be badly soldered, so this might be worth checking also. The HT supply to the IFT is decoupled via 2k2/10nf so these might be worth checking ? I had a similar issue on a Philips 170a many years back and this turned out to be a dry joint on one of the MW coils. Finally a jolly picture to cheer you along - gotta love these sets....good luck ! Attached Thumbnails

3rd Jul 2020, 11:10 pm	#4
Pilot Mariner Hexode Join Date: Jun 2003 Location: Burntwood, Staffordshire, UK or Kabaty in Warsaw Poland. Posts: 438	Re: Bush VHF94 Thanks cathoderay57 and yestertech for your suggestions, I had already thoroughly dosed the switch contacts and valve pins with switch cleaner. I had also discounted anything prior to the IF stage by removing the ECH81 and injecting a 470khz signal at the anode b pin of the empty valve holder. The 10nF (C36) cap has been replaced and the 2k2 is measuring 2K3. I will double check again for dry joints. Yes these are great sets, I am fixing 2 VHF94s for a friend (stevehertz) and a VHF64 (my own) all in parallel. I also have 2 other VHF64s restored a few years ago. Mark

4th Jul 2020, 9:58 am	#5
John Caswell Hexode Join Date: Jul 2009 Location: Wokingham, Berkshire, UK. Posts: 437	Re: Bush VHF94 Around this time with this and a lot of other makes of radios, we had a lot of problems with the polystyrene tuning capacitors inside the IF transformers, giving exactly the symptoms you talk about, and "cured" by touching one of the IF transformer pins. The only cure was to replace them with silver mica capacitors. In a lot of cases did not even have to realign the set. John

4th Jul 2020, 3:16 pm	#6
Pilot Mariner Hexode Join Date: Jun 2003 Location: Burntwood, Staffordshire, UK or Kabaty in Warsaw Poland. Posts: 438	Re: Bush VHF94 Hi yes I have already replaced these capacitors (using the same silver micas as the originals C39 & C40). I had a similar problem on a Grundig a few years ago.

4th Jul 2020, 9:25 pm	#7
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,382	Re: Bush VHF94 Maybe something awry on the AGC line, decoupling capacitors replaced and resistors checked? Diodes on EABC80 OK? Last year I had intermittent diode failure on an EABC80 that only affected FM. Cheers, Jerry

4th Jul 2020, 10:00 pm	#8
Pilot Mariner Hexode Join Date: Jun 2003 Location: Burntwood, Staffordshire, UK or Kabaty in Warsaw Poland. Posts: 438	Re: Bush VHF94 Yes thanks for that suggestion, I also suspected the AGC line as I have run out of ideas around the IF transformer, but all decoupling caps have been replaced and resistors checked - some slightly high in value. I have used these valves to track down a fault in the other VHF94 and didn't notice anything strange there, but I will try another EABC80 in this set. Unfortunately the fault hasn't occurred today.