Bush DAC90A. Thank the stars I found this forum...... - UK Vintage Radio Repair and Restoration Discussion Forum

Beardyman · 28th Jan 2018, 9:49 pm

Here we are at the end of another weekend & I've finally got to the nub of the issues with my DAC90A. After the standard (or so it seems) replacement of all the caps I then measured all the resistors, they were all at the top end of their tolerances so they were replaced as well. Then on to the crumbling rubber covered wiring, truly a messy job. Then isolate pin 4 of the UL41 (just in case!) which does look rather stained but after rooting through the forum I found some very useful checks which, on the face of it, tells me it could be a good 'un, happy days. A few more checks showed the mains switch was welded, this was in keeping with the mains RF capacitor which had blown its brains out at some point! Replaced it with a NOS item from Ebay. Modified the mains input to take a fuseholder & mains lead, happy with that. Resprayed the rather dirty/manky waveband display with white radiator paint & touched up the pointer a nice deep red. Finally came the point of having to turn it on as no amount of checks would suffice! All appeared well as the variac volts came up to around the 220-230 mark then finally full power. No smoke, no bangs, very happy! Scale lamps very dim, almost non existent but I can address that later, at full volume I could barely hear it! So, after a bit more probing around & thoroughly reading the service sheet I discovered the DC resistance of the primary of the output transformer was 7-9K! Oh joy! Another atypical DAC90A fault. Never mind, RS do a very close replacement. So far I've thoroughly enjoyed myself, probably spent far too much time & money but I don't really care. Once it's up & running it'll be staying.

60 oldjohn · 28th Jan 2018, 10:52 pm

Sorry to hear the output TX is OC. When you get it working consider building the "FM to AM Converter" discussed on this forum. Get all the local radio stations broadcasting on FM on your set with no connections to the set. I use one with the older DAC90, very pleased with it just have to find a quiet spot on the dial free of computer hash, which is just about possible.

John.

Sideband · 28th Jan 2018, 11:34 pm

Quote:

Originally Posted by Beardyman

the DC resistance of the primary of the output transformer was 7-9K! Oh joy! Another atypical DAC90A fault. Never mind, RS do a very close replacement.

I've said this before....The RS component is very marginal in this set due to the anode current of the UL41 being close to the maximum rating of the RS transformer (I assume you are referring to the universal transformer). You could reduce the anode current in the UL41 by slightly increasing the cathode bias resistor (which I think is 180 ohms) so change it for a 270 ohm. It will reduce the volume slightly but will hardly be a problem on this set.

PsychMan · 29th Jan 2018, 7:52 am

I wouldn’t use NOS capacitors across the line, far better to use a new X2 part. In fact, from a regulatory perspective you absolutely have to.

Ed Dinning does great rewound output transformers for this radio

Beardyman · 29th Jan 2018, 9:18 am

My apologies if I didn't make myself clear, the RF bypass has been replaced with an X2 part, I should have made it clearer that the NOS referred to the volume control/switch! I've built a "pantry transmitter" but am interested in the FM/AM converter, I shall have a look around the forum for it. Many thanks for the tip regarding the RS TXFMR, it's the one from OEP with multiple tapping points so could be regarded as a universal type.

Once again, many thanks for all your comments and input, most welcome.

David G4EBT · 29th Jan 2018, 1:06 pm

Quote:

Originally Posted by Beardyman

I've built a "pantry transmitter" but am interested in the FM/AM converter, I shall have a look around the forum for it.

The FM/AM Converter, designed by Guy Fernando, (forum member 'i4cy') originally featured in the BVWS Bulletin last year.

The forum thread is here:

https://www.vintage-radio.net/forum/...d.php?t=137154

You can read the original BVWS Bulletin article and can still purchase the programmed PIC for £12.95 inc post at Guy Fernando's website here:

http://www.i4cy.com/m0oox/fm2am/FM%2...0Converter.pdf

Now that all BBC local radio broadcast stations have ceased transmission on M.W. the converter is even more useful.

Really, this is off topic in this thread so any further discussion on the converter needs to be in the original thread at the link above.

Hope that helps.

Station X · 29th Jan 2018, 1:16 pm

Quote:

Originally Posted by David G4EBT

Now that all BBC local radio broadcast stations have ceased transmission on M.W.....

That's news to me. I'm listening to BBC Radio Norfolk on 855kHz right now. I'm sure there are other BBC local radio stations on MW too.

David G4EBT · 29th Jan 2018, 3:19 pm

My mistake Graham - enjoy it while you can - I thought they'd all gone.

They've closed down in this region (Humberside, Lincolnshire, Nottinghamshire), but not all BBC AM broadcast have gone yet.

Seven BBC local radio stations terminated their AM transmissions on January 15th 2018, with three more stations losing at least one service.

Here's the current list:

BBC Surrey, Sussex, Humberside, Kent, Lincolnshire, Nottingham and Wiltshire turned off their medium wave transmitters altogether on 15 Jan, whilst partial closures will affect BBC Radio Devon, Essex and Humberside.

BBC Surrey is closing 1368 kHz AM – but will remain on 104 and 104.6 MHz FM
BBC Sussex is closing 1161 and 1485 kHz AM – but will remain on 104.8, 104.5, 95.3, 95.1 and 95 MHz FM
BBC Radio Humberside is closing 1485 kHz AM – but will remain on 95.9 MHz FM
BBC Radio Kent is closing 1602 and 774 kHz AM – but will remain on 104.2, 97.6 and 96.7 MHz FM
BBC Radio Lincolnshire is closing 1368 kHz AM – but will remain on 94.9 and 104.7 MHz FM
BBC Radio Nottingham is closing 1584 kHz AM – but will remain on 95.1, 95.5 and 103.8 MHz FM
BBC Radio Wiltshire is closing 1332 and 1368 kHz AM – but will remain on 103.3, 103.5, 103.6 and 104.3 MHz FM
BBC Radio Devon is closing 1458 kHz AM – but will remain on 801 and 990 kHz AM along with 104.3, 94.8, 103.4, 96.0, 95.7 and 98.5 MHz FM
BBC Essex is closing 1530 kHz AM – but will remain on 765 and 729 kHz AM along with 95.3 and 103.5 MHz FM
BBC Radio Lancashire is closing 1557 kHz AM – but will remain on 855 kHz AM along with 95.5, 103.9 and 104.5 FM.

https://radiotoday.co.uk/2018/01/bbc...wave-closures/

Radio Merseyside went off air more than five years ago, back in 2012:

https://www.theguardian.com/media/20...ium-wave-radio

If the powers that be had their way, FM would be going too any time soon, were it not for the fact that most radio listening is done in cars these days, and the take-up of DAB in cars is still quite a low proportion of total cars on the road, but best not re-run that topic again, which generates more heat than light.

kalee20 · 29th Jan 2018, 3:33 pm

I listen to Radio Devon with my DAC90a on 801kHz, in Lynton FM reception is non-existent (have to go 5 miles or so towards Barnstaple), but AM on 801kHz can be read.

Tough luck on the O/C output transformer. Good news is that you can tack in almost any single-ended valve output transformer, and although it may not be the optimum match, it will work well enough to assess things.

Station X · 29th Jan 2018, 3:33 pm

Please let's not discuss the FM switch off, how crap DAB is etc. etc.

Let's get back to discussing the repair of the OP's radio.

Beardyman · 29th Jan 2018, 10:59 pm

Evening all,

Well, after an hour or two in the workshop (spare room or "Cave" as my better half refers to it!) I discovered a schoolboy error I had made. R14 (150R) which I had replaced was wrong......very very wrong! I had fitted a 150K! No wonder it was quiet! Here's the odd thing, the output transformer is still reading 7-9K but ample volume!?! There's the odd click & pop but the giveaway is the scale lamps, at first switch on from cold they are fairly bright, then they dim down as the heaters come up but just before audio comes through they jump up to failry bright again & remain there. This I am attributing to the HT temporarily arcing across in the output transformer & remaking the connection therein. Once tuned in to a local station (SMOOTH radio I think?) the tone is very pleasing. Would anyone have any objections if, at a later date, I put together a success story for this delightful little radio?

AC/HL · 29th Jan 2018, 11:59 pm

None whatsoever!

Boater Sam · 30th Jan 2018, 2:05 am

If you had arcing in the output transformer you would definitely hear it in the speaker! And it would not do it for long.
Is the transformer the original?
7K to 9K ohms is illogical.
The lamps behaviour is about normal, the cold filaments initially draw a large current, decreasing as they heat, till the rectifier passes the HT when they brighten a bit.

PsychMan · 30th Jan 2018, 11:18 am

The primary of my DAC90A output transformer also read high dc resistance, along with the odd pop and crackle from the speaker. That did prove to be a breakdown in the transformer primary, I got a rewound one from Ed and it’s worked perfectly ever since. Before then I had assumed if the TX was at fault it would go open circuit quite quickly

Boater Sam · 30th Jan 2018, 11:37 am

I too would have thought it would die very quickly, ya larn summat every day.
Worth remembering too.
Seems Beardyman that you need a new or a rewind transformer, Ed's doing a good business with these!

Beardyman · 30th Jan 2018, 10:42 pm

Once again, many thanks for all your input.
I agree it is most illogical! I decided a "kill or cure" approach tonight.
It's been on all evening, a total of 5 hours so far & its working brilliantly.
I, at the moment, simply cannot explain why the transformer primary is reading so high, both the service & trader sheets state 500R or thereabouts.
The DMM I used was a Metrix MTX3281, I do have an AVO8 somewhere so might just do the measurement again with that. I can't imagine it will read any different. You never know!

PsychMan · 30th Jan 2018, 11:16 pm

Are you disconnecting one side of it to take the reading?

7k sounds odd for a healthy output transformer. I checked what I wrote at the time and mine was reading 1.2k and definitely produced audible crackling - almost a raspy noise that was fairly regular

Boater Sam · 31st Jan 2018, 12:51 am

I can understand 1.2k, I've seen them around that when the green spot has got into them, but they fail very quickly when you wind up the watts, UL41 will churn a good bit out, 4W, even though Bush upped the cathode resistor slightly.
But 7k is crazy, if you reckon only 40mA anode current that's over 11 watts being dissipated in the primary, it won't stand that for too long. And max is 53mA!

Herald1360 · 31st Jan 2018, 1:29 am

Could just be DMM not liking highly reactive load on its resistance range. Try measuring the dc drop across the primary when the set's running at zero volume. If it's around 12-14V you're in luck. If it's much higher, tough!

(Anode current for UL41 in Bush datasheet is around 27mA)

Boater Sam · 31st Jan 2018, 1:36 am

Wow, Bush ran the UL41 at half maximum power? I never knew that, I try not to fix DAC90As, detestable common sets.
And they still cook out the output valve and transformers.
I knew the transformers were on the limit, they must be wound with candy floss.

28th Jan 2018, 9:49 pm	#1
Beardyman Hexode Join Date: Jan 2018 Location: Portsmouth, Hampshire, UK. Posts: 430	Bush DAC90A. Thank the stars I found this forum...... Here we are at the end of another weekend & I've finally got to the nub of the issues with my DAC90A. After the standard (or so it seems) replacement of all the caps I then measured all the resistors, they were all at the top end of their tolerances so they were replaced as well. Then on to the crumbling rubber covered wiring, truly a messy job. Then isolate pin 4 of the UL41 (just in case!) which does look rather stained but after rooting through the forum I found some very useful checks which, on the face of it, tells me it could be a good 'un, happy days. A few more checks showed the mains switch was welded, this was in keeping with the mains RF capacitor which had blown its brains out at some point! Replaced it with a NOS item from Ebay. Modified the mains input to take a fuseholder & mains lead, happy with that. Resprayed the rather dirty/manky waveband display with white radiator paint & touched up the pointer a nice deep red. Finally came the point of having to turn it on as no amount of checks would suffice! All appeared well as the variac volts came up to around the 220-230 mark then finally full power. No smoke, no bangs, very happy! Scale lamps very dim, almost non existent but I can address that later, at full volume I could barely hear it! So, after a bit more probing around & thoroughly reading the service sheet I discovered the DC resistance of the primary of the output transformer was 7-9K! Oh joy! Another atypical DAC90A fault. Never mind, RS do a very close replacement. So far I've thoroughly enjoyed myself, probably spent far too much time & money but I don't really care. Once it's up & running it'll be staying.

28th Jan 2018, 10:52 pm	#2
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 3,988	Re: Bush DAC90A. Thank the stars I found this forum...... Sorry to hear the output TX is OC. When you get it working consider building the "FM to AM Converter" discussed on this forum. Get all the local radio stations broadcasting on FM on your set with no connections to the set. I use one with the older DAC90, very pleased with it just have to find a quiet spot on the dial free of computer hash, which is just about possible. John.

29th Jan 2018, 7:52 am	#4
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,765	Re: Bush DAC90A. Thank the stars I found this forum...... I wouldn’t use NOS capacitors across the line, far better to use a new X2 part. In fact, from a regulatory perspective you absolutely have to. Ed Dinning does great rewound output transformers for this radio

29th Jan 2018, 9:18 am	#5
Beardyman Hexode Join Date: Jan 2018 Location: Portsmouth, Hampshire, UK. Posts: 430	Re: Bush DAC90A. Thank the stars I found this forum...... My apologies if I didn't make myself clear, the RF bypass has been replaced with an X2 part, I should have made it clearer that the NOS referred to the volume control/switch! I've built a "pantry transmitter" but am interested in the FM/AM converter, I shall have a look around the forum for it. Many thanks for the tip regarding the RS TXFMR, it's the one from OEP with multiple tapping points so could be regarded as a universal type. Once again, many thanks for all your comments and input, most welcome.

29th Jan 2018, 3:19 pm	#8
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,763	Re: Bush DAC90A. Thank the stars I found this forum...... My mistake Graham - enjoy it while you can - I thought they'd all gone. They've closed down in this region (Humberside, Lincolnshire, Nottinghamshire), but not all BBC AM broadcast have gone yet. Seven BBC local radio stations terminated their AM transmissions on January 15th 2018, with three more stations losing at least one service. Here's the current list: BBC Surrey, Sussex, Humberside, Kent, Lincolnshire, Nottingham and Wiltshire turned off their medium wave transmitters altogether on 15 Jan, whilst partial closures will affect BBC Radio Devon, Essex and Humberside. BBC Surrey is closing 1368 kHz AM – but will remain on 104 and 104.6 MHz FM BBC Sussex is closing 1161 and 1485 kHz AM – but will remain on 104.8, 104.5, 95.3, 95.1 and 95 MHz FM BBC Radio Humberside is closing 1485 kHz AM – but will remain on 95.9 MHz FM BBC Radio Kent is closing 1602 and 774 kHz AM – but will remain on 104.2, 97.6 and 96.7 MHz FM BBC Radio Lincolnshire is closing 1368 kHz AM – but will remain on 94.9 and 104.7 MHz FM BBC Radio Nottingham is closing 1584 kHz AM – but will remain on 95.1, 95.5 and 103.8 MHz FM BBC Radio Wiltshire is closing 1332 and 1368 kHz AM – but will remain on 103.3, 103.5, 103.6 and 104.3 MHz FM BBC Radio Devon is closing 1458 kHz AM – but will remain on 801 and 990 kHz AM along with 104.3, 94.8, 103.4, 96.0, 95.7 and 98.5 MHz FM BBC Essex is closing 1530 kHz AM – but will remain on 765 and 729 kHz AM along with 95.3 and 103.5 MHz FM BBC Radio Lancashire is closing 1557 kHz AM – but will remain on 855 kHz AM along with 95.5, 103.9 and 104.5 FM. https://radiotoday.co.uk/2018/01/bbc...wave-closures/ Radio Merseyside went off air more than five years ago, back in 2012: https://www.theguardian.com/media/20...ium-wave-radio If the powers that be had their way, FM would be going too any time soon, were it not for the fact that most radio listening is done in cars these days, and the take-up of DAB in cars is still quite a low proportion of total cars on the road, but best not re-run that topic again, which generates more heat than light. __________________ David. BVWS Member. G-QRP Club member 1339.

29th Jan 2018, 3:33 pm	#9
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: Bush DAC90A. Thank the stars I found this forum...... I listen to Radio Devon with my DAC90a on 801kHz, in Lynton FM reception is non-existent (have to go 5 miles or so towards Barnstaple), but AM on 801kHz can be read. Tough luck on the O/C output transformer. Good news is that you can tack in almost any single-ended valve output transformer, and although it may not be the optimum match, it will work well enough to assess things.

29th Jan 2018, 3:33 pm	#10
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Bush DAC90A. Thank the stars I found this forum...... Please let's not discuss the FM switch off, how crap DAB is etc. etc. Let's get back to discussing the repair of the OP's radio. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

29th Jan 2018, 10:59 pm	#11
Beardyman Hexode Join Date: Jan 2018 Location: Portsmouth, Hampshire, UK. Posts: 430	Re: Bush DAC90A. Thank the stars I found this forum...... Evening all, Well, after an hour or two in the workshop (spare room or "Cave" as my better half refers to it!) I discovered a schoolboy error I had made. R14 (150R) which I had replaced was wrong......very very wrong! I had fitted a 150K! No wonder it was quiet! Here's the odd thing, the output transformer is still reading 7-9K but ample volume!?! There's the odd click & pop but the giveaway is the scale lamps, at first switch on from cold they are fairly bright, then they dim down as the heaters come up but just before audio comes through they jump up to failry bright again & remain there. This I am attributing to the HT temporarily arcing across in the output transformer & remaking the connection therein. Once tuned in to a local station (SMOOTH radio I think?) the tone is very pleasing. Would anyone have any objections if, at a later date, I put together a success story for this delightful little radio?

29th Jan 2018, 11:59 pm	#12
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Bush DAC90A. Thank the stars I found this forum...... None whatsoever!

30th Jan 2018, 2:05 am	#13
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Bush DAC90A. Thank the stars I found this forum...... If you had arcing in the output transformer you would definitely hear it in the speaker! And it would not do it for long. Is the transformer the original? 7K to 9K ohms is illogical. The lamps behaviour is about normal, the cold filaments initially draw a large current, decreasing as they heat, till the rectifier passes the HT when they brighten a bit.

30th Jan 2018, 11:18 am	#14
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,765	Re: Bush DAC90A. Thank the stars I found this forum...... The primary of my DAC90A output transformer also read high dc resistance, along with the odd pop and crackle from the speaker. That did prove to be a breakdown in the transformer primary, I got a rewound one from Ed and it’s worked perfectly ever since. Before then I had assumed if the TX was at fault it would go open circuit quite quickly

30th Jan 2018, 11:37 am	#15
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Bush DAC90A. Thank the stars I found this forum...... I too would have thought it would die very quickly, ya larn summat every day. Worth remembering too. Seems Beardyman that you need a new or a rewind transformer, Ed's doing a good business with these!

30th Jan 2018, 10:42 pm	#16
Beardyman Hexode Join Date: Jan 2018 Location: Portsmouth, Hampshire, UK. Posts: 430	Re: Bush DAC90A. Thank the stars I found this forum...... Once again, many thanks for all your input. I agree it is most illogical! I decided a "kill or cure" approach tonight. It's been on all evening, a total of 5 hours so far & its working brilliantly. I, at the moment, simply cannot explain why the transformer primary is reading so high, both the service & trader sheets state 500R or thereabouts. The DMM I used was a Metrix MTX3281, I do have an AVO8 somewhere so might just do the measurement again with that. I can't imagine it will read any different. You never know!

30th Jan 2018, 11:16 pm	#17
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,765	Re: Bush DAC90A. Thank the stars I found this forum...... Are you disconnecting one side of it to take the reading? 7k sounds odd for a healthy output transformer. I checked what I wrote at the time and mine was reading 1.2k and definitely produced audible crackling - almost a raspy noise that was fairly regular

31st Jan 2018, 12:51 am	#18
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Bush DAC90A. Thank the stars I found this forum...... I can understand 1.2k, I've seen them around that when the green spot has got into them, but they fail very quickly when you wind up the watts, UL41 will churn a good bit out, 4W, even though Bush upped the cathode resistor slightly. But 7k is crazy, if you reckon only 40mA anode current that's over 11 watts being dissipated in the primary, it won't stand that for too long. And max is 53mA!

31st Jan 2018, 1:29 am	#19
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Bush DAC90A. Thank the stars I found this forum...... Could just be DMM not liking highly reactive load on its resistance range. Try measuring the dc drop across the primary when the set's running at zero volume. If it's around 12-14V you're in luck. If it's much higher, tough! (Anode current for UL41 in Bush datasheet is around 27mA) __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

31st Jan 2018, 1:36 am	#20
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Bush DAC90A. Thank the stars I found this forum...... Wow, Bush ran the UL41 at half maximum power? I never knew that, I try not to fix DAC90As, detestable common sets. And they still cook out the output valve and transformers. I knew the transformers were on the limit, they must be wound with candy floss.