Bush DAC90A poor sound - UK Vintage Radio Repair and Restoration Discussion Forum

Guineafowl · 24th Oct 2018, 11:18 am

Hi all,

Next project!

Problem:
Not much reception on LW, some on MW. All stations come through with almost an echo/wobble/underwater sound. If I switch on but don’t advance the volume, the speaker hums quite loudly.

Jobs done:
Replaced RF bypass, electrolytics and waxies. Some resistors also replaced; others fine.

Diagnostics:
All voltages much lower than they should be (e.g. 48V instead of 90V).

If I boost the voltage a bit by going up a tap on the dropper, I think the stations improve a bit. Could this all be down to a duff rectifier tube? One on order, anyway.

Also, how should the IF transformers be adjusted? There’s a small screw and locknut, but no way of turning the screw politely.

Cheers
A

Sideband · 24th Oct 2018, 12:23 pm

Well unless they have been moved, I'd leave the IF transformers alone. I've restored several of these sets and none of them have needed IF alignment. With regard to hum, most likely cause is leakage in the UL41....very common especially if the original has been overrun when caps have gone leaky. Leakage in the valve will also cause the HT to be low and, much more seriously, can damage the output transformer. UL41's are becoming very scarce and as a result, expensive. You can use a 10P14 as a replacement but you'll need to increase the cathode resistor to about 470 ohms.

kalee20 · 24th Oct 2018, 12:49 pm

Try connecting the loudspeaker to something you know is good. For some reason, DAC90a loudspeakers are prone to the suspension becoming unglued.

As the HT is low, the UY41 could be weak, or the big electrolytic can capacitor could be duff (though it's somewhat unlikely). Or the smoothing resistor could be failing high.

UL41 could be duff - measure the voltage across the 150 ohm cathode resistor (and also verify that the resistor itself measures 150 ohms).

mark2collection · 24th Oct 2018, 12:53 pm

It may be worth disconnecting the components from Pin 4 of the UL41.

I usually remove the link from the tag-board to Pin 4, then move 'now' free ends of the components to where the wire link was on the tag-board, leaving Pin 4 of the UL41, free.

A couple of weeks ago I completed the repair of a DAC90A, ended up using a CV1977 in-place of a UL41 (which was missing in this set).

The output transformer wasn't duff, it was missing too! (Along with the rectifier & a few other bits).

This set I bought a few moons ago as a scrapper, it's now a fully working set and a nice example to boot

Mark

Boater Sam · 24th Oct 2018, 12:54 pm

10P14 is the octal based output valve.
10P13 is a usable substitute for UL41, not an equivalent.

Guineafowl · 24th Oct 2018, 1:26 pm

Thanks for all that.

OK - resistance between UL41 cathode and heater is about 30 Mohm (cold), using my dc Megger.
Heater is about 9V below cathode in service.
Voltage across the 150 ohm UL41 cathode resistor (which is nearly in spec at 170 ohm) is 7V.

Using a different speaker doesn’t change the fault.

Boater Sam · 24th Oct 2018, 1:47 pm

Its not necessarily heater to cathode insulation that causes the UL41 to hum but the debris stuck to the inside of the base, you can sometimes see the black deposit.
Pin 4 should have no connection, I also make sure pin 3 has no connection too.
Sparking the valve with a piezo electric lighter sometimes cleans them up, there are several threads here about how to do this.
CV1977 is a hardened version.

Sideband · 24th Oct 2018, 1:54 pm

Don't know why you measured between cathode and heater. It CAN cause a problem but UL41's are more subtle than that......the leakage is caused by internal deposits building up between the pins sometimes looking like a silvery deposit around the base inside the glass. This causes all sorts of problems and disconnecting pin 4 as mentioned above can sometimes effect a complete cure (it did on mine). Others have had some success 'blasting' the deposit away with high voltage (a few thousand volts) between the pins (after joining the heater and cathode pins together). Anyway as said above the 10P13 is a usable substitute just with an increase of cathode resistor.

**Crossed with Sam!**

crackle · 24th Oct 2018, 4:22 pm

I wouldn't change the voltage tap, you will stress the valve heaters.

Mike

Guineafowl · 24th Oct 2018, 6:42 pm

Sam/Sideband - I assumed it was a heater/cathode short you were talking about. Valves are fairly new to me - my main hobby is solid-state electronic repair.

Crackle - I only did it temporarily, as a diagnostic. Tap back in place now.

I isolated the connections to pin 4 as advised, but no joy. Not sure about the references to pin 3 - this has connections to the valve, doesn’t it?

I’ve found a CV1977, so we’ll see what that brings.

cathoderay57 · 24th Oct 2018, 9:13 pm

You measured 7v on the UL41 cathode resistor so it's passing a total current (anode + screen) of 41mA which is not unreasonable. Even though those DC conditions may be OK the valve could still be at fault re: hum. What voltage are you measuring on pin 6 (grid 1) ?

Boater Sam · 24th Oct 2018, 10:04 pm

Pin 3 is no connection, or should be but there may be some makes of UL41 that have an internal connection.

I have never tried it but what would happen if you grounded pins 3 and 4?

PJL · 24th Oct 2018, 10:35 pm

Quote:

Originally Posted by Guineafowl

All voltages much lower than they should be (e.g. 48V instead of 90V).

What is the HT voltage? If it is very low and the UL41 cathode current is about right, a new rectifier is your best bet.

The warbly sound may be intermodulation distortion caused by hum being introduced in earlier stages but I would not go chasing shadows until the rectifier had been eliminated.

John10b · 25th Oct 2018, 7:37 pm

Do you have the circuit diagram, not sure from your post, but if not it is well worth getting?
Here are some voltage reading, if you don’t already have them.
V5, UY41 Rectifier: Anode Pin 2 = 220VAC. Cathode Pin 7= 205V.
V4, UL42 Output: Anode Pin 2 = 190V @ 27mA. Screen Pin 5 98V @ 4mA.
Cathode Pins 3 & 7 = 5V @ 31mA. Control grid Pin 6 = 0V.
You say you have already changed various capacitors etc.
Hope this is of some help to you.
Cheers
John

Boater Sam · 25th Oct 2018, 7:44 pm

Think, John, you meant UL41?

John10b · 25th Oct 2018, 8:19 pm

Ops, of course, UL41, thank you boatersam.
Cheers
John

Hunts smoothing bomb · 26th Oct 2018, 9:07 am

Hi all,

So it would seem that from post #11 that a total current of 41mA is passing through the UL41
where as it has been stated that in post #14 that a total of 31 mA is more the norm (27mA for the anode and 4mA for the screen grid)

Also the fact that in post #1 the OP states that the sound from the speaker has a "wobble" (Dalik like?) to it and that the HT is also low accompanied with hum from the speaker it sounds like it is pointing to too much current drawn by the UL41. Probably stating the obvious but has the UL41's grid coupling cap been replaced? If not then this is a major culprit guilty for this behaviour.

Cheers

Guineafowl · 26th Oct 2018, 10:44 am

Quote:

Originally Posted by cathoderay57

You measured 7v on the UL41 cathode resistor so it's passing a total current (anode + screen) of 41mA which is not unreasonable. Even though those DC conditions may be OK the valve could still be at fault re: hum. What voltage are you measuring on pin 6 (grid 1) ?

Pin 6 (grid) is at 6.76V, while cathode is at 6.90V (above chassis).

Quote:

Originally Posted by Boater Sam

Pin 3 is no connection, or should be but there may be some makes of UL41 that have an internal connection.

I have never tried it but what would happen if you grounded pins 3 and 4?

Grounding pin 4 doesn’t help. Pin 3 on mine (and the Trader sheet) connects cathode and supressor to chassis via a 150 ohm resistor.

Quote:

Originally Posted by PJL

Quote:

Originally Posted by Guineafowl

All voltages much lower than they should be (e.g. 48V instead of 90V).

What is the HT voltage? If it is very low and the UL41 cathode current is about right, a new rectifier is your best bet.

The warbly sound may be intermodulation distortion caused by hum being introduced in earlier stages but I would not go chasing shadows until the rectifier had been eliminated.

Voltage on the first electrolytic is 148V, and on the second, after the 10K smoothing resistor, is 48V. Lots of voltage drop there.

Quote:

Originally Posted by John10b

Do you have the circuit diagram, not sure from your post, but if not it is well worth getting?
Here are some voltage reading, if you don’t already have them.
V5, UY41 Rectifier: Anode Pin 2 = 220VAC. Cathode Pin 7= 205V.
V4, UL42 Output: Anode Pin 2 = 190V @ 27mA. Screen Pin 5 98V @ 4mA.
Cathode Pins 3 & 7 = 5V @ 31mA. Control grid Pin 6 = 0V.
You say you have already changed various capacitors etc.
Hope this is of some help to you.
Cheers
John

Yes, I have the Trader sheet 1161.

Voltages (my values in brackets):
V1 mixer anode: 98 (48)
V1 osc anode: 47 (27)
V1 screen: 47 (27)
V1 cathode: 0.8 (0.45)

V2 anode: 98 (48)
V2 screen: 47 (27)
V2 cathode: 1.0 (0.4)

V3 anode: 74 (38)
V3 cathode: 1.0 (0.4)

V4 anode: 190 (115)
V4 screen: 98 (48)
V4 cathode: 5.0 (6.78)

V5 anode: 222ac (180ac)
V5 cathode: 205 (150)

Quote:

Originally Posted by Hunts smoothing bomb

Hi all,

So it would seem that from post #11 that a total current of 41mA is passing through the UL41
where as it has been stated that in post #14 that a total of 31 mA is more the norm (27mA for the anode and 4mA for the screen grid)

Also the fact that in post #1 the OP states that the sound from the speaker has a "wobble" (Dalik like?) to it and that the HT is also low accompanied with hum from the speaker it sounds like it is pointing to too much current drawn by the UL41. Probably stating the obvious but has the UL41's grid coupling cap been replaced? If not then this is a major culprit guilty for this behaviour.

Cheers

V4 grid coupler was the first one I replaced.

So V4’s grid should be at 0V, and the cathode 5V above that. My V4’s grid is only 0.14V below the cathode.

Assuming the resistors and caps around the grid are OK (which I think they are), then we seem to be overdriving V4, or V4 is overdriving itself due to an internal short (Grid to cathode?).

Guineafowl · 26th Oct 2018, 10:53 am

Oddly enough, pin 4 of V4, which has no connection in the valve and is now isolated on my set, is at 6.3 V. Is this a stray voltage from internal leakage?

cathoderay57 · 26th Oct 2018, 11:48 am

Pins 3 and 4 of UL41 are shown as internal connection, (not no connection), on most valve datasheets. In others (such as Trader Sheet 1161) Pin 3 is shown as cathode/Grid3 as you say. Pin 7 is also cathode. It is best not to connect anything to Pin 4 in my view. It is not unusual to find a positive voltage on Grid 1 of UL41, as you have found, even with a good decoupling capacitor from the previous valve anode. As you also have distortion one suspects it's a dodgy UL41. See what happens when you plug in your CV1977. If the voltages return to normal, and the distortion disappears, then happy days. I can probably find you a decent UY41 if needed. Cheers, Jerry

24th Oct 2018, 12:23 pm	#2
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,572	Re: Bush DAC90A poor sound Well unless they have been moved, I'd leave the IF transformers alone. I've restored several of these sets and none of them have needed IF alignment. With regard to hum, most likely cause is leakage in the UL41....very common especially if the original has been overrun when caps have gone leaky. Leakage in the valve will also cause the HT to be low and, much more seriously, can damage the output transformer. UL41's are becoming very scarce and as a result, expensive. You can use a 10P14 as a replacement but you'll need to increase the cathode resistor to about 470 ohms. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

24th Oct 2018, 12:49 pm	#3
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Bush DAC90A poor sound Try connecting the loudspeaker to something you know is good. For some reason, DAC90a loudspeakers are prone to the suspension becoming unglued. As the HT is low, the UY41 could be weak, or the big electrolytic can capacitor could be duff (though it's somewhat unlikely). Or the smoothing resistor could be failing high. UL41 could be duff - measure the voltage across the 150 ohm cathode resistor (and also verify that the resistor itself measures 150 ohms).

24th Oct 2018, 12:53 pm	#4
mark2collection Hexode Join Date: Jul 2010 Location: Royal Berkshire, UK. Posts: 471	Re: Bush DAC90A poor sound It may be worth disconnecting the components from Pin 4 of the UL41. I usually remove the link from the tag-board to Pin 4, then move 'now' free ends of the components to where the wire link was on the tag-board, leaving Pin 4 of the UL41, free. A couple of weeks ago I completed the repair of a DAC90A, ended up using a CV1977 in-place of a UL41 (which was missing in this set). The output transformer wasn't duff, it was missing too! (Along with the rectifier & a few other bits). This set I bought a few moons ago as a scrapper, it's now a fully working set and a nice example to boot Mark

24th Oct 2018, 12:54 pm	#5
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Bush DAC90A poor sound 10P14 is the octal based output valve. 10P13 is a usable substitute for UL41, not an equivalent.

24th Oct 2018, 1:26 pm	#6
Guineafowl Tetrode Join Date: Sep 2018 Location: Inverness, Scottish Highlands, UK. Posts: 74	Re: Bush DAC90A poor sound Thanks for all that. OK - resistance between UL41 cathode and heater is about 30 Mohm (cold), using my dc Megger. Heater is about 9V below cathode in service. Voltage across the 150 ohm UL41 cathode resistor (which is nearly in spec at 170 ohm) is 7V. Using a different speaker doesn’t change the fault.

24th Oct 2018, 11:18 am	#1
Guineafowl Tetrode Join Date: Sep 2018 Location: Inverness, Scottish Highlands, UK. Posts: 74	Bush DAC90A poor sound Hi all, Next project! Problem: Not much reception on LW, some on MW. All stations come through with almost an echo/wobble/underwater sound. If I switch on but don’t advance the volume, the speaker hums quite loudly. Jobs done: Replaced RF bypass, electrolytics and waxies. Some resistors also replaced; others fine. Diagnostics: All voltages much lower than they should be (e.g. 48V instead of 90V). If I boost the voltage a bit by going up a tap on the dropper, I think the stations improve a bit. Could this all be down to a duff rectifier tube? One on order, anyway. Also, how should the IF transformers be adjusted? There’s a small screw and locknut, but no way of turning the screw politely. Cheers A

24th Oct 2018, 1:47 pm	#7
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Bush DAC90A poor sound Its not necessarily heater to cathode insulation that causes the UL41 to hum but the debris stuck to the inside of the base, you can sometimes see the black deposit. Pin 4 should have no connection, I also make sure pin 3 has no connection too. Sparking the valve with a piezo electric lighter sometimes cleans them up, there are several threads here about how to do this. CV1977 is a hardened version.

24th Oct 2018, 1:54 pm	#8
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,572	Re: Bush DAC90A poor sound Don't know why you measured between cathode and heater. It CAN cause a problem but UL41's are more subtle than that......the leakage is caused by internal deposits building up between the pins sometimes looking like a silvery deposit around the base inside the glass. This causes all sorts of problems and disconnecting pin 4 as mentioned above can sometimes effect a complete cure (it did on mine). Others have had some success 'blasting' the deposit away with high voltage (a few thousand volts) between the pins (after joining the heater and cathode pins together). Anyway as said above the 10P13 is a usable substitute just with an increase of cathode resistor. Crossed with Sam! __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

24th Oct 2018, 4:22 pm	#9
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Bush DAC90A poor sound I wouldn't change the voltage tap, you will stress the valve heaters. Mike

24th Oct 2018, 6:42 pm	#10
Guineafowl Tetrode Join Date: Sep 2018 Location: Inverness, Scottish Highlands, UK. Posts: 74	Re: Bush DAC90A poor sound Sam/Sideband - I assumed it was a heater/cathode short you were talking about. Valves are fairly new to me - my main hobby is solid-state electronic repair. Crackle - I only did it temporarily, as a diagnostic. Tap back in place now. I isolated the connections to pin 4 as advised, but no joy. Not sure about the references to pin 3 - this has connections to the valve, doesn’t it? I’ve found a CV1977, so we’ll see what that brings.

24th Oct 2018, 9:13 pm	#11
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Bush DAC90A poor sound You measured 7v on the UL41 cathode resistor so it's passing a total current (anode + screen) of 41mA which is not unreasonable. Even though those DC conditions may be OK the valve could still be at fault re: hum. What voltage are you measuring on pin 6 (grid 1) ?

24th Oct 2018, 10:04 pm	#12
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Bush DAC90A poor sound Pin 3 is no connection, or should be but there may be some makes of UL41 that have an internal connection. I have never tried it but what would happen if you grounded pins 3 and 4?

25th Oct 2018, 7:37 pm	#14
John10b Nonode Join Date: Nov 2016 Location: Aberaeron, Ceredigion, Wales, UK. Posts: 2,884	Re: Bush DAC90A poor sound Do you have the circuit diagram, not sure from your post, but if not it is well worth getting? Here are some voltage reading, if you don’t already have them. V5, UY41 Rectifier: Anode Pin 2 = 220VAC. Cathode Pin 7= 205V. V4, UL42 Output: Anode Pin 2 = 190V @ 27mA. Screen Pin 5 98V @ 4mA. Cathode Pins 3 & 7 = 5V @ 31mA. Control grid Pin 6 = 0V. You say you have already changed various capacitors etc. Hope this is of some help to you. Cheers John

25th Oct 2018, 7:44 pm	#15
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Bush DAC90A poor sound Think, John, you meant UL41?

25th Oct 2018, 8:19 pm	#16
John10b Nonode Join Date: Nov 2016 Location: Aberaeron, Ceredigion, Wales, UK. Posts: 2,884	Re: Bush DAC90A poor sound Ops, of course, UL41, thank you boatersam. Cheers John

26th Oct 2018, 9:07 am	#17
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Bush DAC90A poor sound Hi all, So it would seem that from post #11 that a total current of 41mA is passing through the UL41 where as it has been stated that in post #14 that a total of 31 mA is more the norm (27mA for the anode and 4mA for the screen grid) Also the fact that in post #1 the OP states that the sound from the speaker has a "wobble" (Dalik like?) to it and that the HT is also low accompanied with hum from the speaker it sounds like it is pointing to too much current drawn by the UL41. Probably stating the obvious but has the UL41's grid coupling cap been replaced? If not then this is a major culprit guilty for this behaviour. Cheers __________________ Lee

26th Oct 2018, 10:53 am	#19
Guineafowl Tetrode Join Date: Sep 2018 Location: Inverness, Scottish Highlands, UK. Posts: 74	Re: Bush DAC90A poor sound Oddly enough, pin 4 of V4, which has no connection in the valve and is now isolated on my set, is at 6.3 V. Is this a stray voltage from internal leakage?

26th Oct 2018, 11:48 am	#20
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Bush DAC90A poor sound Pins 3 and 4 of UL41 are shown as internal connection, (not no connection), on most valve datasheets. In others (such as Trader Sheet 1161) Pin 3 is shown as cathode/Grid3 as you say. Pin 7 is also cathode. It is best not to connect anything to Pin 4 in my view. It is not unusual to find a positive voltage on Grid 1 of UL41, as you have found, even with a good decoupling capacitor from the previous valve anode. As you also have distortion one suspects it's a dodgy UL41. See what happens when you plug in your CV1977. If the voltages return to normal, and the distortion disappears, then happy days. I can probably find you a decent UY41 if needed. Cheers, Jerry