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Old 1st Dec 2021, 2:21 pm   #21
Gabe001
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Default Re: Bush DAC90A hum - AGAIN!

If you disconnected the lead from IFT to VC and the hum persists, it's not that lead.

It's probably something in the output stage (from VC onwards) too close to an AC supply. This could be from mains, heater, or scale lamp circuits. You may need to do some careful prodding with a chopstick -move things a bit and see what happens to the hum. A quick test to see if your vc is the problem is to bypass it.
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Old 1st Dec 2021, 2:44 pm   #22
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Default Re: Bush DAC90A hum - AGAIN!

If the set's been fitted with a non-original volume control/on-off switch, then the first thing I'd do is be looking in this area and particularly completely disconnecting the mains from the switch tags and bypassing the switch altogether and connecting direct to the set, well away from the pot and noting the result.
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Old 1st Dec 2021, 5:25 pm   #23
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Default Re: Bush DAC90A hum - AGAIN!

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 1st Dec 2021, 5:34 pm   #24
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Default Re: Bush DAC90A hum - AGAIN!

In that case do a continuity test from the potentiometer metal casing to ground and also screen to ground. I think the problem is likely to be in that area - I can't remember if you said the hum was there from the start or if it was ok at first and then recently started. It'll turn out to be something really simple, just a case of diagnosing the cause.

Last edited by Techman; 1st Dec 2021 at 6:00 pm. Reason: a slight re-write.
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Old 1st Dec 2021, 5:59 pm   #25
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Default Re: Bush DAC90A hum - AGAIN!

Disconnect the end of R17 the heater dropper, the connection that goes to the rectifier heater, the first one in the chain, whilst the set is switched on. If the hum goes, then it is a heater chain problem. If the hum dies as the cathodes go cold then the hum is coming from elsewhere. Don't try this if you are not used to working on live sets.
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Old 3rd Dec 2021, 11:09 am   #26
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Default Re: Bush DAC90A hum - AGAIN!

Systematically working through all suggestions as to the possible source of the hum, next in line was to check that there wasn't any pick-up from within the volume control potentiometer having previously by-passed the mains connections to the on/off switch.

Volume pot duly disassembled:-

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The interior was first cleaned with IPA and this removed any crud on the carbon track and contacts. The carbon track was then cleaned with a light application of DeoxIT Fader F5 NOT D5! The inner metal ring was cleaned up with a fibre glass pencil as were all contacts. An overall clean up of the external tabs and everything reassembled and back in the set.

And the result - no change to the hum, which was disappointing!

A few more things to try, especially around the heater chain, but we are running out of options ...

I fully appreciate that tracking down the source of hum in a vintage valve radio is a bit like chasing unicorns but as always this has proved to be an educational, albeit frustrating, journey.
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Old 3rd Dec 2021, 11:28 am   #27
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Default Re: Bush DAC90A hum - AGAIN!

I feel disappointed for you Donald, l just hope you didn't buy the fader lube just for this one problem as I've just been looking for it and it's £21! Once I've got over the shock l think I'll ask Santa to put some in my stocking as l do tend to blast all my pots with Servisol D5, which I'm thinking might be a bit unkind.
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Old 3rd Dec 2021, 1:09 pm   #28
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Default Re: Bush DAC90A hum - AGAIN!

Quote:
Originally Posted by theredhouseinn View Post
Disconnect the end of R17 the heater dropper, the connection that goes to the rectifier heater, the first one in the chain, whilst the set is switched on. If the hum goes, then it is a heater chain problem. If the hum dies as the cathodes go cold then the hum is coming from elsewhere. Don't try this if you are not used to working on live sets.
John.
Just a quick update to mention that I tried this suggestion from John (theredhouseinn) whilst taking suitable precautions as I was working on a live set!

The hum very gradually faded once the first link in the heater chain was disconnected which corresponds with the cathodes going cold - so it's not the heater chain either?

Quote:
Originally Posted by Kentode View Post
I feel disappointed for you Donald, l just hope you didn't buy the fader lube just for this one problem as I've just been looking for it and it's £21! Once I've got over the shock l think I'll ask Santa to put some in my stocking as l do tend to blast all my pots with Servisol D5, which I'm thinking might be a bit unkind.
Don't worry Ken (Kentode) - I already had DeoxIT Fader F5 as I'd had some bad experiences previously when spraying pots with D5 which is fine for switches.

See here:-

https://www.youtube.com/watch?v=0ADYRx2jQSE
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Last edited by DonaldStott; 3rd Dec 2021 at 1:19 pm.
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Old 3rd Dec 2021, 6:38 pm   #29
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Default Re: Bush DAC90A hum - AGAIN!

Hi.
Where has the radio been used? One fairly common issue is when it's used in a damp or steamy environment. This can effect valve bases and paxolin boards, it's a somewhat gradual issue and can take time to be noticed. The boards absorb moisture and become very slightly conductive as can valve bases, even the plastic variety believe it or not, had this issue at various times over the last 52+ years.
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Old 3rd Dec 2021, 7:38 pm   #30
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Default Re: Bush DAC90A hum - AGAIN!

Thanks Trevor - Iíve no information on the setís provenance before it came into my possession about 18 months ago? Since then itís not been anywhere that is damp or steamy.

I note what you say about the valve bases and paxolin boards becoming slightly conductive - any way to test for that?
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Old 3rd Dec 2021, 9:21 pm   #31
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Default Re: Bush DAC90A hum - AGAIN!

Sometimes you can.
Use a high impedance meter, preferably a VVM, one lead on the Chassis and poke around the board its self not the contacts, any voltages picked up will indicate a leak. Also worth looking at this on a scope, it might surprise you.
Ken on the forum sent me a Pye radio last year with an obscure issue, the problem was the valve base on the FC, it was leaking from the heater connections, a new base effected a cure.
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Old 3rd Dec 2021, 10:04 pm   #32
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Default Re: Bush DAC90A hum - AGAIN!

I had a conductive stretch of paxolin on a kbmr10 a few months ago. It was the waveband switch and was measurable with a multimeter. I think the resistance, which should have been infinite, measured about 300k.
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Old 4th Dec 2021, 12:30 pm   #33
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Default Re: Bush DAC90A hum - AGAIN!

Although I don't have a VMM (?) I spent the morning prodding around the main tag board, valve bases etc but could find no evidence of voltage leaks or resistance readings - my DMM goes up to 200M.

It would be good to at least find the source of the hum! By disconnecting the lead from the IFT to the volume control we have hopefully demonstrated that the problem lies in the output stages forward of the volume control? I also by-passed the switch and refurbished the volume control pot to rule those out as well.

When I turn the set on but leave the volume control at the minimum setting we get the usual low level audio breakthrough but the hum takes several seconds after that to build up. This seems to be indicative (to me) of a valve(s) heating up? It would be good to isolate V5 (UY41), V4 (UL41) and V3 (UBC41) in turn although these have all been replaced? Advice and guidance on how to do this would be appreciated - thanks.

I'm still hopeful that Techman is correct "It'll turn out to be something really simple, just a case of diagnosing the cause" which is why I'm careful to follow a systematic approach and not just randomly trying possible fixes.

Supplementary Question - while I appreciate that the valve heaters are connected in series I'm often confused by the seemingly random connections between the heater pins on valves and associated chassis links. Does anyone have a heater chain wiring diagram for the Bush DAC90A?
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Old 4th Dec 2021, 12:54 pm   #34
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Default Re: Bush DAC90A hum - AGAIN!

Do you mean the point to point?

Or just the circuit?

They are in that order to keep the valve where hum would be most of a problem (V3) closest to chassis and the valves with the best Heater Cathode insulation at the "hot" end.

Cheers

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Old 4th Dec 2021, 1:02 pm   #35
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Default Re: Bush DAC90A hum - AGAIN!

Quote:
Originally Posted by DonaldStott View Post
Supplementary Question - while I appreciate that the valve heaters are connected in series I'm often confused by the seemingly random connections between the heater pins on valves and associated chassis links. Does anyone have a heater chain wiring diagram for the Bush DAC90A?
I don't have a wiring diagram but there's lot's of under chassis photo's on here and other places, looking at one in particular here's what I can make out starting from the dropper output to the heater chain:

From dropper output to Pin 1 of V5, Pin 8 of V5 to Pin 1 of V4, Pin 8 of V4 to Pin 1 of V2, Pin 8 of V2 to Pin 1 of V1, Pin 8 of V1 to Pin 8 of V3, Pin 1 of V3 to chassis.

EDIT: Post crossed.

Lawrence.

Last edited by ms660; 4th Dec 2021 at 1:11 pm. Reason: one to 1
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Old 4th Dec 2021, 1:16 pm   #36
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Default Re: Bush DAC90A hum - AGAIN!

Quote:
Originally Posted by DonaldStott View Post
Quote:
Originally Posted by theredhouseinn View Post
Disconnect the end of R17 the heater dropper, the connection that goes to the rectifier heater, the first one in the chain, whilst the set is switched on. If the hum goes, then it is a heater chain problem. If the hum dies as the cathodes go cold then the hum is coming from elsewhere. Don't try this if you are not used to working on live sets.
John.
Just a quick update to mention that I tried this suggestion from John (theredhouseinn) whilst taking suitable precautions as I was working on a live set!

The hum very gradually faded once the first link in the heater chain was disconnected which corresponds with the cathodes going cold - so it's not the heater chain either?

Quote:
Originally Posted by Kentode View Post
I feel disappointed for you Donald, l just hope you didn't buy the fader lube just for this one problem as I've just been looking for it and it's £21! Once I've got over the shock l think I'll ask Santa to put some in my stocking as l do tend to blast all my pots with Servisol D5, which I'm thinking might be a bit unkind.
Don't worry Ken (Kentode) - I already had DeoxIT Fader F5 as I'd had some bad experiences previously when spraying pots with D5 which is fine for switches.

See here:-

https://www.youtube.com/watch?v=0ADYRx2jQSE
Cheers Donald, that's a brilliant link.
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Old 4th Dec 2021, 8:08 pm   #37
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Default Re: Bush DAC90A hum - AGAIN!

It doesn't look like it's the heater chain then. Maybe you could try the velleman signal tracer next? Start at the volume control and work your way forward along the signal path.

G
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Old 4th Dec 2021, 8:21 pm   #38
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Default Re: Bush DAC90A hum - AGAIN!

I think this has been mentioned but the UL41 is def prone to hum due to internal leakage. I see you have changed it but is the replacement new ? They can be zapped to fix this, do a search for “zapping a ul41” or similar, in the forum. A DAC90A of mine was bad until I replaced the UL41 with a new one. I guess the valve base could give the same problem so if the valve is def OK, a new base could be the next move.
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Old 5th Dec 2021, 10:37 am   #39
DonaldStott
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Default Re: Bush DAC90A hum - AGAIN!

Thanks to Mike T (Cobaltblue) and Lawrence (ms660) for the heater chain information.

Quote:
Originally Posted by Ambientnoise View Post
I think this has been mentioned but the UL41 is def prone to hum due to internal leakage. I see you have changed it but is the replacement new ? They can be zapped to fix this, do a search for “zapping a ul41” or similar, in the forum. A DAC90A of mine was bad until I replaced the UL41 with a new one. I guess the valve base could give the same problem so if the valve is def OK, a new base could be the next move.
I've gone back and re-read everything I could find on UL41 internal leakage but can't find anything definitive on how to test for that? My idea (!) was to somehow test each valve out of circuit in turn, but in the absence of a valve tester?

I mentioned earlier that V4 had been replaced with a CV1977 and that I had rechecked all my valve voltages, including heaters, and there were no surprises.

Thanks to Tony (boxdoctor) I understand that the '...colour of the getter is often a good pointer to the state of the valve, and kind of life it has had. Usually valves with a good silver getter are more likely to be OK, assuming no O/C heaters or other mechanical faults.' My CV1977 has a nice shiny getter flash on the inside of the valve envelope.
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Last edited by DonaldStott; 5th Dec 2021 at 10:42 am.
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Old 5th Dec 2021, 11:57 am   #40
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Default Re: Bush DAC90A hum - AGAIN!

Alistair,
If you have not already, swop the CV1977/UL41.
https://www.vintage-radio.net/forum/...d.php?t=124729
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