DAC 90A Smoothing Capacitor replacement - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

Station X · 9th Dec 2017, 12:55 pm

Jason.

I can let you have a NOS boxed Mullard UL41 for £10 including postage. I'd test it before sending it out.

PM me if you're interested.

Boater Sam · 9th Dec 2017, 1:16 pm

Good offer of a new UL41. You would be very lucky to get another even used/tested for this price.

You could just be expecting too much of this set, a slight background hum was quite normal and accepted of mains radios at this period.
Though extremely popular, the Bush DAC90A was never one of the best radios around.

JasonKillbourn · 9th Dec 2017, 1:42 pm

Oh, no worries, I wasn't going to throw the current UL41 away, but I figured a spare would be a good idea in the long run anyway, and, if the replacement cures the hum, I would box the old one, pending a future attempt at flashing it back to life, so it could serve as a viable spare.
As regards the UBC41, thank you Kalee, I'll certainly have a go at identifying pin 3 and will short it to the chassis with a crocodile clip lead to see if that works.
I've also had a good listen to YouTube vids and my DAC90A sounds quite a bit more hummy, so I'd say it does need further work.

JasonKillbourn · 9th Dec 2017, 1:48 pm

Oh dear, I had a load of phone calls that delayed me while I was writing that last reply and just saw Graham's kind offer, and I'd actually just bitten the bullet and committed to buy an NOS UL41 for quite a few quid more. That'll teach me to be impulsive.

Ed_Dinning · 9th Dec 2017, 4:42 pm

Hi Jason, as the volume appear to increase when you added the caps, can you check the voltage out of the UY41 cathode. My data says it should be about 205 VDC at this point and about 100 VDC on the smoothing cap (ie voltage readings at either end of the big 10K resistor with respect to chassis).

If the voltage is say 10 or more volts low, then this may be a sign of a tired rectifier and could possibly also increase hum slightly.

Ed

JasonKillbourn · 11th Dec 2017, 8:05 pm

Thank you for the suggestion Ed, I was away over the weekend, but have just had the chance to get the multi meter out and took a reading from either side of that 10k resistor (R15) of approx 171 VDC and 90 VDC, so the UY41 is possibly looking a bit iffy.

Ed_Dinning · 11th Dec 2017, 8:29 pm

Hi Jason, also worth doing an AC voltage check on the dropper resistors as these can get bad joints (they rarely increase in value as they are wirewound)

Ed

HamishBoxer · 11th Dec 2017, 8:37 pm

I will buy that UL41 Graham if still available as I have 3 Dac90.s that belong to me.

Station X · 12th Dec 2017, 11:15 am

OK HB.

I'll PM you once I've tested it.

JasonKillbourn · 12th Dec 2017, 10:11 pm

Just a quick update, as I had the chassis back out earlier and ran pretty exhaustive tests on all resistors and nothing startling stood out. I also remembered to try Kalee's suggestion to short pin 3 of the UBC41 to the chassis, but it had no effect on the hum.
The current state of play is: there is a question mark hanging over the UY41 following that low voltage reading the other day, so I am pondering sourcing a replacement, and I am awaiting the imminent arrival of a NOS UL41. Thank you all again for your help, and hopefully I should have more news soon.

kalee20 · 12th Dec 2017, 11:45 pm

Quote:

Just a quick update, as I had the chassis back out earlier and ran pretty exhaustive tests on all resistors and nothing startling stood out. I also remembered to try Kalee's suggestion to short pin 3 of the UBC41 to the chassis, but it had no effect on the hum.

It's an easy test to do - and as it had no effect on the hum, almost certainly the UBC41 is not the culprit. (The signal would, of course, have completely disappeared while you had the short in place).

JasonKillbourn · 13th Dec 2017, 5:17 pm

Well, the UL41 arrived today, so I am very pleased to say the hum has gone and the DAC90A sounds great

However, following that 171 and 90 VDC reading from either side of the 10k resistor, I am wondering whether I shouldn't just get hold of a replacement UY41 to be on the safe side?
Also, since I do seem to have pretty poor reception in my kitchen, where I tend to listen to the radio, does anyone ever bother adding an external aerial to these sets?

PsychMan · 13th Dec 2017, 5:25 pm

James where did you get your UL41? I got a military spec NOS one off someone on ebay but it hums like my old one despite doing all the recommended mods and replacing both smoothing caps. I've saw another thread here where someone else experienced the same.

Being a live chassis set its not a good ideal to use an external aerial. The internal loop is actually very good, reception of AM is a challenge in today's world, do you have a pantry transmitter?

JasonKillbourn · 13th Dec 2017, 6:03 pm

Yes, I did question the wisdom of an external aerial for that reason. Rather annoyingly, reception is much better in my bedroom, at the other end of the flat, but is faintly crackly in the kitchen, and unfortunately I haven't got a pantry transmitter, but would be interested in getting hold of one, so have you any recommendations for an affordable option?
Anyway, my UL41 is a military spec NOS one (CV1977) from an ebay vendor in Billingshurst called dan198745, so perhaps I got lucky?

PsychMan · 13th Dec 2017, 6:13 pm

Yeah I had the same product, different seller however. Its probably just luck of the draw

I highly recommend the SSTran AMT-3000, available in kit form from the USA. Not the cheapest to buy (£100 ish with duty on top, depending on exchange rate), and expect to wait possibly months before seeing it and having all your emails to SSTran ignored - until you mention a refund/claim

That said, it sounds the best, and documentation on build and setup are outstanding. The audio processing onboard is excellent, and when set up properly should cover your home just fine. There is something similar in the Spitfire (available locally), but its lacking the audio processing, you could do your own I suppose. Aside from that there's loads of others varying in quality and range, or even have a go making your own.

TrevorG3VLF · 13th Dec 2017, 11:58 pm

The radio sitting inside a large tuned loop or a loop driven from a long wire could give more sensitivity without connection to the live chassis.

Nickthedentist · 14th Dec 2017, 10:16 am

Quote:

Originally Posted by JasonKillbourn

Rather annoyingly, reception is much better in my bedroom, at the other end of the flat, but is faintly crackly in the kitchen...

I presume you've tried rotating it for maximum signal. This often leaves the set at a strange angle to the walls, worktop, table or what have you, though.

Leon Crampin · 15th Dec 2017, 11:56 am

Before condemning the UY41 as a cause of low HT voltages, bear in mind that Bush made some changes to the DAC90A chassis and failed to update the voltage readings.

The surge limiter in the anode circuit of the UY41 was increased in value and the pilot lamp shunt resistor was reduced. These measures were, I believe taken to minimise scale lamp failure on a hot switch-on.

I would expect to see about 95V on the set's HT line and 180V at the UY41 cathode with a reasonable set of valves. This seems to apply to all the sets I've seen (after restoration) despite the mains voltage here usually being 240 - 245V.

Leon.

9th Dec 2017, 12:55 pm	#41
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: DAC 90A Smoothing Capacitor replacement Jason. I can let you have a NOS boxed Mullard UL41 for £10 including postage. I'd test it before sending it out. PM me if you're interested. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

9th Dec 2017, 1:16 pm	#42
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: DAC 90A Smoothing Capacitor replacement Good offer of a new UL41. You would be very lucky to get another even used/tested for this price. You could just be expecting too much of this set, a slight background hum was quite normal and accepted of mains radios at this period. Though extremely popular, the Bush DAC90A was never one of the best radios around.

9th Dec 2017, 1:42 pm	#43
JasonKillbourn Retired Dormant Member Join Date: Dec 2017 Location: Potters Bar, Hertfordshire, UK. Posts: 14	Re: DAC 90A Smoothing Capacitor replacement Oh, no worries, I wasn't going to throw the current UL41 away, but I figured a spare would be a good idea in the long run anyway, and, if the replacement cures the hum, I would box the old one, pending a future attempt at flashing it back to life, so it could serve as a viable spare. As regards the UBC41, thank you Kalee, I'll certainly have a go at identifying pin 3 and will short it to the chassis with a crocodile clip lead to see if that works. I've also had a good listen to YouTube vids and my DAC90A sounds quite a bit more hummy, so I'd say it does need further work.

9th Dec 2017, 1:48 pm	#44
JasonKillbourn Retired Dormant Member Join Date: Dec 2017 Location: Potters Bar, Hertfordshire, UK. Posts: 14	Re: DAC 90A Smoothing Capacitor replacement Oh dear, I had a load of phone calls that delayed me while I was writing that last reply and just saw Graham's kind offer, and I'd actually just bitten the bullet and committed to buy an NOS UL41 for quite a few quid more. That'll teach me to be impulsive.

9th Dec 2017, 4:42 pm	#45
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: DAC 90A Smoothing Capacitor replacement Hi Jason, as the volume appear to increase when you added the caps, can you check the voltage out of the UY41 cathode. My data says it should be about 205 VDC at this point and about 100 VDC on the smoothing cap (ie voltage readings at either end of the big 10K resistor with respect to chassis). If the voltage is say 10 or more volts low, then this may be a sign of a tired rectifier and could possibly also increase hum slightly. Ed

11th Dec 2017, 8:05 pm	#46
JasonKillbourn Retired Dormant Member Join Date: Dec 2017 Location: Potters Bar, Hertfordshire, UK. Posts: 14	Re: DAC 90A Smoothing Capacitor replacement Thank you for the suggestion Ed, I was away over the weekend, but have just had the chance to get the multi meter out and took a reading from either side of that 10k resistor (R15) of approx 171 VDC and 90 VDC, so the UY41 is possibly looking a bit iffy.

11th Dec 2017, 8:29 pm	#47
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: DAC 90A Smoothing Capacitor replacement Hi Jason, also worth doing an AC voltage check on the dropper resistors as these can get bad joints (they rarely increase in value as they are wirewound) Ed

11th Dec 2017, 8:37 pm	#48
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,932	Re: DAC 90A Smoothing Capacitor replacement I will buy that UL41 Graham if still available as I have 3 Dac90.s that belong to me. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

12th Dec 2017, 11:15 am	#49
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: DAC 90A Smoothing Capacitor replacement OK HB. I'll PM you once I've tested it. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

12th Dec 2017, 10:11 pm	#50
JasonKillbourn Retired Dormant Member Join Date: Dec 2017 Location: Potters Bar, Hertfordshire, UK. Posts: 14	Re: DAC 90A Smoothing Capacitor replacement Just a quick update, as I had the chassis back out earlier and ran pretty exhaustive tests on all resistors and nothing startling stood out. I also remembered to try Kalee's suggestion to short pin 3 of the UBC41 to the chassis, but it had no effect on the hum. The current state of play is: there is a question mark hanging over the UY41 following that low voltage reading the other day, so I am pondering sourcing a replacement, and I am awaiting the imminent arrival of a NOS UL41. Thank you all again for your help, and hopefully I should have more news soon.

13th Dec 2017, 5:17 pm	#52
JasonKillbourn Retired Dormant Member Join Date: Dec 2017 Location: Potters Bar, Hertfordshire, UK. Posts: 14	Re: DAC 90A Smoothing Capacitor replacement Well, the UL41 arrived today, so I am very pleased to say the hum has gone and the DAC90A sounds great However, following that 171 and 90 VDC reading from either side of the 10k resistor, I am wondering whether I shouldn't just get hold of a replacement UY41 to be on the safe side? Also, since I do seem to have pretty poor reception in my kitchen, where I tend to listen to the radio, does anyone ever bother adding an external aerial to these sets?

13th Dec 2017, 5:25 pm	#53
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: DAC 90A Smoothing Capacitor replacement James where did you get your UL41? I got a military spec NOS one off someone on ebay but it hums like my old one despite doing all the recommended mods and replacing both smoothing caps. I've saw another thread here where someone else experienced the same. Being a live chassis set its not a good ideal to use an external aerial. The internal loop is actually very good, reception of AM is a challenge in today's world, do you have a pantry transmitter?

13th Dec 2017, 6:03 pm	#54
JasonKillbourn Retired Dormant Member Join Date: Dec 2017 Location: Potters Bar, Hertfordshire, UK. Posts: 14	Re: DAC 90A Smoothing Capacitor replacement Yes, I did question the wisdom of an external aerial for that reason. Rather annoyingly, reception is much better in my bedroom, at the other end of the flat, but is faintly crackly in the kitchen, and unfortunately I haven't got a pantry transmitter, but would be interested in getting hold of one, so have you any recommendations for an affordable option? Anyway, my UL41 is a military spec NOS one (CV1977) from an ebay vendor in Billingshurst called dan198745, so perhaps I got lucky? Attached Thumbnails

13th Dec 2017, 6:13 pm	#55
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: DAC 90A Smoothing Capacitor replacement Yeah I had the same product, different seller however. Its probably just luck of the draw I highly recommend the SSTran AMT-3000, available in kit form from the USA. Not the cheapest to buy (£100 ish with duty on top, depending on exchange rate), and expect to wait possibly months before seeing it and having all your emails to SSTran ignored - until you mention a refund/claim That said, it sounds the best, and documentation on build and setup are outstanding. The audio processing onboard is excellent, and when set up properly should cover your home just fine. There is something similar in the Spitfire (available locally), but its lacking the audio processing, you could do your own I suppose. Aside from that there's loads of others varying in quality and range, or even have a go making your own.

13th Dec 2017, 11:58 pm	#56
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: DAC 90A Smoothing Capacitor replacement The radio sitting inside a large tuned loop or a loop driven from a long wire could give more sensitivity without connection to the live chassis.

15th Dec 2017, 11:56 am	#58
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: DAC 90A Smoothing Capacitor replacement Before condemning the UY41 as a cause of low HT voltages, bear in mind that Bush made some changes to the DAC90A chassis and failed to update the voltage readings. The surge limiter in the anode circuit of the UY41 was increased in value and the pilot lamp shunt resistor was reduced. These measures were, I believe taken to minimise scale lamp failure on a hot switch-on. I would expect to see about 95V on the set's HT line and 180V at the UY41 cathode with a reasonable set of valves. This seems to apply to all the sets I've seen (after restoration) despite the mains voltage here usually being 240 - 245V. Leon.