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Old 5th Dec 2021, 12:14 pm   #41
Ambientnoise
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Default Re: Bush DAC90A hum - AGAIN!

Try these pages

https://www.vintage-radio.net/forum/...d.php?t=111365

https://www.vintage-radio.net/forum/...ad.php?t=42302

https://www.vintage-radio.net/forum/...d.php?t=154697
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Old 5th Dec 2021, 3:07 pm   #42
DonaldStott
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Default Re: Bush DAC90A hum - AGAIN!

Quote:
Originally Posted by snowman_al View Post
Alistair,
If you have not already, swop the CV1977/UL41.
https://www.vintage-radio.net/forum/...d.php?t=124729
Thanks snowman_al - already tried that with a UL41 from another DAC90A but no change.

Measuring across the entire heater chain from Pin 1 of V5 to Pin 1 of V3 I'm getting 141.0V AC so slightly low as the Bush Service Instructions say 116.6V AC.

Can someone remind me why I cant get continuity across the heaters pins (1 and 8) of a UL41? Pins 3-7 have continuity because of the internal valve connection.
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Old 5th Dec 2021, 3:31 pm   #43
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Default Re: Bush DAC90A hum - AGAIN!

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Originally Posted by DonaldStott View Post
Measuring across the entire heater chain from Pin 1 of V5 to Pin 1 of V3 I'm getting 141.0V AC so slightly low as the Bush Service Instructions say 116.6V AC.

Can someone remind me why I cant get continuity across the heaters pins (1 and 8) of a UL41? Pins 3-7 have continuity because of the internal valve connection.
1) Looks considerably high to me....

2):

https://www.vintage-radio.net/forum/...d.php?t=170189

Lawrence.
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Old 5th Dec 2021, 3:38 pm   #44
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Default Re: Bush DAC90A hum - AGAIN!

Thanks for those links which Ive encountered before - the thought of all that high-voltage zapping fills me with dread and I'd be concerned about making things worse, not better!

One of the links did, however. allude to the fact that the UL41 lacks a cathode bypass capacitor and has significant impedance cathode-chassis? Has anyone tried adding such a bypass cap? I note that the DAC90A mods by kalee20 includes the following changes associated with the UL41: -

Click image for larger version

Name:	UL41 Mods.JPG
Views:	43
Size:	26.1 KB
ID:	247330

The relevant Thread is here and has the following explanation (my emboldening): -

'The solution I adopted, as there is plenty of reserve gain, was to leave the current feedback in place (the cathode resistor unbypassed), add a bit of voltage feedback (a gain reduction of a couple of db), and inject into the cathode a fraction of HT ripple voltage. In theory this can neutralise the hum completely. I found empirically that 7k from the HT rail (via a DC blocking capacitor) gave a hum null, and therefore used 7.5k as the nearest standard value.'
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Old 5th Dec 2021, 3:39 pm   #45
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Default Re: Bush DAC90A hum - AGAIN!

Quote:
Originally Posted by ms660 View Post
1) Looks considerably high to me....

2):

https://www.vintage-radio.net/forum/...d.php?t=170189

Lawrence.
Apologies Lawrence, just a typo - it should read 114.0V AC

And thanks for the link to one of my own Threads - I knew I'd asked about this before!
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Old 6th Dec 2021, 6:42 am   #46
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Default Re: Bush DAC90A hum - AGAIN!

Applying negative feedback to mask the hum isn't solving the problem. If the hum gets worse you'll run out of negative feedback to apply.

Try scoping the signal path (or use the Velleman signal tracer) and see when the hum appears.
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Old 6th Dec 2021, 9:12 am   #47
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Default Re: Bush DAC90A hum - AGAIN!

Quote:
Originally Posted by DonaldStott View Post
Thanks for those links which Ive encountered before - the thought of all that high-voltage zapping fills me with dread and I'd be concerned about making things worse, not better!

One of the links did, however. allude to the fact that the UL41 lacks a cathode bypass capacitor and has significant impedance cathode-chassis? Has anyone tried adding such a bypass cap? I note that the DAC90A mods by kalee20 includes the following changes associated with the UL41: -

Attachment 247330

The relevant Thread is here and has the following explanation (my emboldening): -

'The solution I adopted, as there is plenty of reserve gain, was to leave the current feedback in place (the cathode resistor unbypassed), add a bit of voltage feedback (a gain reduction of a couple of db), and inject into the cathode a fraction of HT ripple voltage. In theory this can neutralise the hum completely. I found empirically that 7k from the HT rail (via a DC blocking capacitor) gave a hum null, and therefore used 7.5k as the nearest standard value.'
Adding NFB in my opinion is a bodge. It didn't hum initially or when new so it's got a fault. Find the fault, repair it and Bob's your uncle.
A systematic approach will find the issue. Being honest this kind of fault can lead you on a merry dance and get you nowhere.
Personally if I get a set with a stinker of an issue I leave it and go back a day later, this clears the thought process and you start again. Surprisingly you often find the issue in a very short time.
The other thing is often another pair of eyes can help.
I'm always willing to guide someone in the best way to fault find, I've trained apprentices in the past and others so if you get really bogged down I'm open to help but after Christmas.
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Old 6th Dec 2021, 9:40 am   #48
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Default Re: Bush DAC90A hum - AGAIN!

R8 and C15? Jerry
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Old 6th Dec 2021, 10:37 am   #49
Colin Boggis
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Default Re: Bush DAC90A hum - AGAIN!

Co-ax should be fine in place of single screened cable. The suggestion of pickup in or near the volume control is a very good point. I've had that myself and a new control cured it.
You could perhaps disconnect the mains wiring from the switch and connect it remotely? With care of course. If this solves it then it confirms leakage within the pot or just very poor screening between the track & switch.
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Old 6th Dec 2021, 10:48 am   #50
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Default Re: Bush DAC90A hum - AGAIN!

Quote:
Originally Posted by Colin Boggis View Post
You could perhaps disconnect the mains wiring from the switch and connect it remotely? With care of course. If this solves it then it confirms leakage within the pot or just very poor screening between the track & switch.
Post#23.

Quote:
Originally Posted by DonaldStott View Post
I bypassed the on/off switch completely and wired the mains directly to the set - no change whatsoever to the hum level?

Tomorrow I'll disassemble the volume pot and closely examine the tracks, contacts etc. - no harm in cleaning everything when I'm in there!
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Old 6th Dec 2021, 12:03 pm   #51
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Default Re: Bush DAC90A hum - AGAIN!

I believe he did the "disconnect mains wiring from volume pot switch" test already and it made no difference
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Old 6th Dec 2021, 12:22 pm   #52
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Default Re: Bush DAC90A hum - AGAIN!

Is it possible the hum is being induced on the mains supply?
Have you tried the radio on another socket in a different location?

Regards,
Symon
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Old 7th Dec 2021, 3:26 pm   #53
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Default Re: Bush DAC90A hum - AGAIN!

Having rewired, replaced or refurbished almost every component under the chassis I think it's time to press PAUSE on this Project and put the DAC90A back on the shelf until the New Year!

Everything on the upper side of the chassis had similar work done earlier on in this restoration but come 2022 I will go back and check everything again - just in case.

Thanks to everyone for their helpful suggestions, advice and help but we still have an annoying hum, the source of which still eludes me?
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Old 7th Dec 2021, 4:20 pm   #54
cathoderay57
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Default Re: Bush DAC90A hum - AGAIN!

Hi Donald, it sounds symptomatic to me of a bad ground circuit for either the volume control or the grid of V3 the UBC41. As suggested in Post #48 check R8 and C15; also the 2.2M R6 which might have drifted very high. Component numbers refer to the Bush Service Sheet. Jerry
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Old 7th Dec 2021, 8:26 pm   #55
DonaldStott
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Default Re: Bush DAC90A hum - AGAIN!

Quote:
Originally Posted by cathoderay57 View Post
Hi Donald, it sounds symptomatic to me of a bad ground circuit for either the volume control or the grid of V3 the UBC41. As suggested in Post #48 check R8 and C15; also the 2.2M R6 which might have drifted very high. Component numbers refer to the Bush Service Sheet. Jerry
Thanks Jerry - R6, R8 and C15 have all been replaced and tested.

I agree that the problem is symptomatic of a bad ground connection. The signal path from the grid (Pin 3) of the UBC41 (V3) through R7 and C14 to the volume control seems ok?

It's back on the shelf now until the New Year!
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