Bush BP90 trouble - UK Vintage Radio Repair and Restoration Discussion Forum

Herald1360 · 9th Jan 2020, 11:37 pm

I've got one of these on the bench at the moment. A Repair Cafe "homer job".

Interesting output valve substitution- a 1A5GT has been fitted instead of the DL35 / 1C5GT. This is pin compatible but a lower power output and a 50mA rather than 100mA filament. Whether this was done to increase battery life or just as a get it going bodge is lost in the mists.....

Leaky caps replaced and drifted resistors too.

Nice new 84V HT battery (7*8 cell AAA holders) built and a D cell for LT.

Encouraging crackles when volume control operated, ditto wavechange switch but not a trace of signal heard, not even from Wychbold only about 30 miles away. Tried a couple of spare DK32s with no improvement. Injected 663kHz at oscillator grid- instant R4 loud and clear. Rats! The oscillator clearly isn't. Poked about V1, no HT on pin 6 oscillator anode. No continuity through oscillator coil feedback windings. Capybaras!

These windings are in series with a hidden internal connection between the two somewhere.

Last chance saloon is the BP90 I picked up at RWB in December- one of the unloved woodies that I was a bit slow to jump in for so it cost me £8

Unless anyone has a good oscillator coil going spare?

Herald1360 · 11th Jan 2020, 12:13 am

Extracted the osc. coil assembly this evening. It isn't designed to be fixed!

The feedback coils are wound in fine solid wire on a small tubular former which is then inserted inside the main former before the lot is wax impregnated. Its connections come up inside the former and are terminated to the tags round the top of the assembly. It looks like the break is near the top on one lead, but to do anything with it, the inner former is going to have to come out.

Anyone know what the melting point is for the wax? Hopefully it's somewhere in domestic oven territory!

Stylo N M · 11th Jan 2020, 1:58 am

Hi Chris,

I should imagin the wax will melt quite easily in a low oven, but it may well be a bit of a struggle to get the whole coil removed.

You could try gradually melting it with a small soldering iron tip, but then the heat is going to be pretty localised, until the whole thing slowly heats up. That's going to be a bit of a pain and the smell to.

I nearly replied to you earlier last night to say start by looking at the coils.

Paul

TrevorG3VLF · 11th Jan 2020, 1:36 pm

I would try a hot air blower. Even a hair dryer may do.

Herald1360 · 11th Jan 2020, 9:37 pm

Tried the oven at 75C- success after 20 minutes or so.

Looks like the problem wire is the bottom end of the MW feedback coil. I wonder if it will work with one turn less? Or maybe I can extend it.

Now for the fiddly bit.....

Herald1360 · 11th Jan 2020, 10:54 pm

No such luck. So far I've unwound 17 turns of the LW coupling coil and found two breaks. Looks like a turns counting job and a rewind. I'll need to get some suitable EnCu wire when I figure out its gauge. So far by eye it's not dissimilar to a strand of 0.2mm wire from some hookup cable.

Maybe 36SWG?

I'll count the rest of the turns off the LW coil and measure the winding length in the wax on the former.

Herald1360 · 12th Jan 2020, 12:50 am

Hmmm, 80 turns in 11mm, more like 40 SWG. Or 36AWG.

Ordered some- it might come in useful for something else sometime, maybe......

Herald1360 · 17th Jan 2020, 1:00 am

The wire duly arrived. The coupling coils were rewound, the two parts of the coils assembly warmed up in the oven then reassembled one inside t'other.

New coil leadouts were soldered onto the tags on the main former (isn't solder through enamel wonderful?) and the whole coil assembly reinstalled in the set.

The batteries were hooked up and the tension mounted.....

Nothing. Nada. Zilch.

Close examination of the coil assembly revealed that I'd inserted the inner former 180 deg out

. Oh well, bodge time- luckily the connecting wires to the tags in question were long enough to rewire to the opposite tags from the original layout.

The batteries were hooked up again and again the tension mounted.....

Lots of MW activity (not much in English) and several LW stations in addition to R4. Don't know if they're all "real" or some are images but I'm not too bothered!

I'll replace the frayed carrying handle cover with some heatshrink (well I don't claim to be "The Repair Shop") and box it up.

I'll be able to demonstrate it to its owner but unless he is willing to part with £25 for what it cost me to make up the batteries it will have to stay mute.

Now, I wonder if the RWB one I bought has the same fault?

Tractionist · 17th Jan 2020, 4:32 pm

Well done Chris! (I usually count the turns and also measure the whole length of the [bits!] wire too ..... this usually entails clearing the breakfast bar and quite a bit of masking tape!)

9th Jan 2020, 11:37 pm	#1
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Bush BP90 trouble I've got one of these on the bench at the moment. A Repair Cafe "homer job". Interesting output valve substitution- a 1A5GT has been fitted instead of the DL35 / 1C5GT. This is pin compatible but a lower power output and a 50mA rather than 100mA filament. Whether this was done to increase battery life or just as a get it going bodge is lost in the mists..... Leaky caps replaced and drifted resistors too. Nice new 84V HT battery (7*8 cell AAA holders) built and a D cell for LT. Encouraging crackles when volume control operated, ditto wavechange switch but not a trace of signal heard, not even from Wychbold only about 30 miles away. Tried a couple of spare DK32s with no improvement. Injected 663kHz at oscillator grid- instant R4 loud and clear. Rats! The oscillator clearly isn't. Poked about V1, no HT on pin 6 oscillator anode. No continuity through oscillator coil feedback windings. Capybaras! These windings are in series with a hidden internal connection between the two somewhere. Last chance saloon is the BP90 I picked up at RWB in December- one of the unloved woodies that I was a bit slow to jump in for so it cost me £8 Unless anyone has a good oscillator coil going spare? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

11th Jan 2020, 12:13 am	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Bush BP90 trouble Extracted the osc. coil assembly this evening. It isn't designed to be fixed! The feedback coils are wound in fine solid wire on a small tubular former which is then inserted inside the main former before the lot is wax impregnated. Its connections come up inside the former and are terminated to the tags round the top of the assembly. It looks like the break is near the top on one lead, but to do anything with it, the inner former is going to have to come out. Anyone know what the melting point is for the wax? Hopefully it's somewhere in domestic oven territory! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

11th Jan 2020, 1:58 am	#3
Stylo N M Heptode Join Date: Jul 2012 Location: Derby DE1, Derbyshire, UK. Posts: 626	Re: Bush BP90 trouble Hi Chris, I should imagin the wax will melt quite easily in a low oven, but it may well be a bit of a struggle to get the whole coil removed. You could try gradually melting it with a small soldering iron tip, but then the heat is going to be pretty localised, until the whole thing slowly heats up. That's going to be a bit of a pain and the smell to. I nearly replied to you earlier last night to say start by looking at the coils. Paul

11th Jan 2020, 1:36 pm	#4
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Bush BP90 trouble I would try a hot air blower. Even a hair dryer may do.

11th Jan 2020, 9:37 pm	#5
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Bush BP90 trouble Tried the oven at 75C- success after 20 minutes or so. Looks like the problem wire is the bottom end of the MW feedback coil. I wonder if it will work with one turn less? Or maybe I can extend it. Now for the fiddly bit..... Attached Thumbnails __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

11th Jan 2020, 10:54 pm	#6
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Bush BP90 trouble No such luck. So far I've unwound 17 turns of the LW coupling coil and found two breaks. Looks like a turns counting job and a rewind. I'll need to get some suitable EnCu wire when I figure out its gauge. So far by eye it's not dissimilar to a strand of 0.2mm wire from some hookup cable. Maybe 36SWG? I'll count the rest of the turns off the LW coil and measure the winding length in the wax on the former. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

12th Jan 2020, 12:50 am	#7
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Bush BP90 trouble Hmmm, 80 turns in 11mm, more like 40 SWG. Or 36AWG. Ordered some- it might come in useful for something else sometime, maybe...... __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

17th Jan 2020, 1:00 am	#8
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Bush BP90 trouble The wire duly arrived. The coupling coils were rewound, the two parts of the coils assembly warmed up in the oven then reassembled one inside t'other. New coil leadouts were soldered onto the tags on the main former (isn't solder through enamel wonderful?) and the whole coil assembly reinstalled in the set. The batteries were hooked up and the tension mounted..... Nothing. Nada. Zilch. Close examination of the coil assembly revealed that I'd inserted the inner former 180 deg out . Oh well, bodge time- luckily the connecting wires to the tags in question were long enough to rewire to the opposite tags from the original layout. The batteries were hooked up again and again the tension mounted..... Lots of MW activity (not much in English) and several LW stations in addition to R4. Don't know if they're all "real" or some are images but I'm not too bothered! I'll replace the frayed carrying handle cover with some heatshrink (well I don't claim to be "The Repair Shop") and box it up. I'll be able to demonstrate it to its owner but unless he is willing to part with £25 for what it cost me to make up the batteries it will have to stay mute. Now, I wonder if the RWB one I bought has the same fault? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

17th Jan 2020, 4:32 pm	#9
Tractionist Heptode Join Date: Apr 2016 Location: Cheltenham, Gloucestershire, UK. Posts: 875	Re: Bush BP90 trouble Well done Chris! (I usually count the turns and also measure the whole length of the [bits!] wire too ..... this usually entails clearing the breakfast bar and quite a bit of masking tape!) __________________ Red to red, black to black. Throw the switch and stand well back!