Bush BP90. IFT slug temporary repairs. - UK Vintage Radio Repair and Restoration Discussion Forum

brenda · 27th Dec 2006, 1:58 pm

I finally have my Bush BP90 making radio-like noises, after replacing every tube and capacitor, and about half the resistors. The RF/IF chain is performing admirably, but the audio section is really nasty.

Though I do get audio through the 1st. AF amp (relatively clean), and even seem to have a good signal into the output tube, the output tube seems to have no gain (I've tried two new tubes). The voltages seem pretty close (-3.5 on g1, pretty much full B+ on g2 and anode). But the plate has little signal on it, ditto the speaker output. Speaker itself seems to be fine. I'm wondering if there isn't something I'm missing, or perhaps a bad output transformer?

yestertech · 27th Dec 2006, 3:03 pm

Hi Brenda. Is the primary resistance of the OPTX as specified ( 500 ohms ? ) The DL35 ought to be pulling about 4mA on its anode, perhaps both valves are shot ?
In any case, I guess this means you wont need that DF33 now ? Andy

Ed_Dinning · 27th Dec 2006, 6:15 pm

Hi Brenda, the output transformer can be checked as follows:
Completely disconnect the transformer from the set
Feed an audio signal of 0.5 to 1 volt into the transformer speaker terminals (50Hz mains via a transformer will do).
Measure voltage out of the anode winding.
The anode winding should have a voltage about 40 times greater than the speaker winding if the transformer is OK. If you have a scope it is a good way to measure these voltages.

HTH Ed

Aerodyne · 27th Dec 2006, 8:19 pm

Presumably you have changed the 'tone corrector' capacitor across the primary of the output transformer... presumably there is evident filament glow in the output valve... just in case you find no fault with the transformer! What's left after everything else must be the culprit, i.e. possibly a faulty valveholder, far-fetched as that sounds.
-Tony

brenda · 10th Jan 2007, 1:54 pm

It's finally working, but not without a bit of sleight of hand.

The audio problem did indeed turn out to be the output transformer.

So, after replacing all four valves, recapping, restringing, replacing four resistors and banging my head on my bench...

The radio would work (sometimes) while on it's side. When I placed it flat, it would quit. Put it back on it's side, sometimes it would work, sometimes not. When it didn't I would hit the back of the chassis and it would come back with various volume levels.

After turning it off and knocking it about to try to find any wire ends that might be left in, I heard what it was....

The first IF can. The top slug (it just HAD to be the TOP one) had come apart from the adjustment screw. Being as there was no way to take the coil assembly apart without chancing destroying it, I opted for the kludge.

I poked a nice clean hole into the coil form, figured which slug had broken loose, held the coil assembly in the appropriate manner, and, while feeding a signal in with a loop of wire from my sig genny, adjusted the broken part of the assembly to peak, then stuffed wax into the hole and used my soldering iron to melt it around the broken slug to hold it in place.

Thanks for your help, everyone.

27th Dec 2006, 1:58 pm	#1
brenda Retired Dormant Member Join Date: Feb 2006 Location: Seoul, R.O.K. Posts: 26	Bush BP90. IFT slug temporary repairs. I finally have my Bush BP90 making radio-like noises, after replacing every tube and capacitor, and about half the resistors. The RF/IF chain is performing admirably, but the audio section is really nasty. Though I do get audio through the 1st. AF amp (relatively clean), and even seem to have a good signal into the output tube, the output tube seems to have no gain (I've tried two new tubes). The voltages seem pretty close (-3.5 on g1, pretty much full B+ on g2 and anode). But the plate has little signal on it, ditto the speaker output. Speaker itself seems to be fine. I'm wondering if there isn't something I'm missing, or perhaps a bad output transformer?

27th Dec 2006, 3:03 pm	#2
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,130	Re: Bush BP90 audio low and distorted Hi Brenda. Is the primary resistance of the OPTX as specified ( 500 ohms ? ) The DL35 ought to be pulling about 4mA on its anode, perhaps both valves are shot ? In any case, I guess this means you wont need that DF33 now ? Andy

27th Dec 2006, 6:15 pm	#3
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Bush BP90 audio low and distorted Hi Brenda, the output transformer can be checked as follows: Completely disconnect the transformer from the set Feed an audio signal of 0.5 to 1 volt into the transformer speaker terminals (50Hz mains via a transformer will do). Measure voltage out of the anode winding. The anode winding should have a voltage about 40 times greater than the speaker winding if the transformer is OK. If you have a scope it is a good way to measure these voltages. HTH Ed

27th Dec 2006, 8:19 pm	#4
Aerodyne Octode Join Date: Jun 2004 Location: Hampton Vale, Peterborough, UK. Posts: 1,698	Re: Bush BP90 audio low and distorted Presumably you have changed the 'tone corrector' capacitor across the primary of the output transformer... presumably there is evident filament glow in the output valve... just in case you find no fault with the transformer! What's left after everything else must be the culprit, i.e. possibly a faulty valveholder, far-fetched as that sounds. -Tony

10th Jan 2007, 1:54 pm	#5
brenda Retired Dormant Member Join Date: Feb 2006 Location: Seoul, R.O.K. Posts: 26	Re: Bush BP90 audio low and distorted It's finally working, but not without a bit of sleight of hand. The audio problem did indeed turn out to be the output transformer. So, after replacing all four valves, recapping, restringing, replacing four resistors and banging my head on my bench... The radio would work (sometimes) while on it's side. When I placed it flat, it would quit. Put it back on it's side, sometimes it would work, sometimes not. When it didn't I would hit the back of the chassis and it would come back with various volume levels. After turning it off and knocking it about to try to find any wire ends that might be left in, I heard what it was.... The first IF can. The top slug (it just HAD to be the TOP one) had come apart from the adjustment screw. Being as there was no way to take the coil assembly apart without chancing destroying it, I opted for the kludge. I poked a nice clean hole into the coil form, figured which slug had broken loose, held the coil assembly in the appropriate manner, and, while feeding a signal in with a loop of wire from my sig genny, adjusted the broken part of the assembly to peak, then stuffed wax into the hole and used my soldering iron to melt it around the broken slug to hold it in place. Thanks for your help, everyone. Last edited by Darren-UK; 8th Aug 2007 at 10:55 am. Reason: General tidying.