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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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5th Dec 2021, 1:14 pm | #41 |
Heptode
Join Date: Feb 2006
Location: Marlborough, Wiltshire, UK.
Posts: 915
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Re: Bush DAC90A hum - AGAIN!
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5th Dec 2021, 4:07 pm | #42 | |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
Posts: 1,840
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Re: Bush DAC90A hum - AGAIN!
Quote:
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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5th Dec 2021, 4:31 pm | #43 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: Bush DAC90A hum - AGAIN!
Quote:
2): https://www.vintage-radio.net/forum/...d.php?t=170189 Lawrence. |
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5th Dec 2021, 4:38 pm | #44 | |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
Posts: 1,840
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Re: Bush DAC90A hum - AGAIN!
Quote:
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: - 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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5th Dec 2021, 4:39 pm | #45 | |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
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Re: Bush DAC90A hum - AGAIN!
Quote:
And thanks for the link to one of my own Threads - I knew I'd asked about this before!
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6th Dec 2021, 7:42 am | #46 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
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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. |
6th Dec 2021, 10:12 am | #47 | ||
Dekatron
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,420
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Re: Bush DAC90A hum - AGAIN!
Quote:
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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Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member |
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6th Dec 2021, 10:40 am | #48 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
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Re: Bush DAC90A hum - AGAIN!
R8 and C15? Jerry
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6th Dec 2021, 11:37 am | #49 |
Pentode
Join Date: Jun 2008
Location: Christchurch, Dorset, UK.
Posts: 237
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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. |
6th Dec 2021, 11:48 am | #50 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Bush DAC90A hum - AGAIN!
Quote:
Lawrence. |
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6th Dec 2021, 1:03 pm | #51 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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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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6th Dec 2021, 1:22 pm | #52 |
Nonode
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,562
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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 |
7th Dec 2021, 4:26 pm | #53 |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
Posts: 1,840
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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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7th Dec 2021, 5:20 pm | #54 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
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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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7th Dec 2021, 9:26 pm | #55 | |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
Posts: 1,840
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Re: Bush DAC90A hum - AGAIN!
Quote:
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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