UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 5th May 2009, 9:38 pm   #1
allanmac
Triode
 
Join Date: Apr 2009
Location: Whitstable Kent
Posts: 15
Default How much hum is normal? and valve change DAC90A

I am restoring a Bush DAC90A and after replacing a few obviously faulty parts, I have it running outside the case. The sound is very good but there is a low hum heard with the volume down. It is almost unnoticeable when it is playing a strong station.
I know I still have a few more parts to replace, mostly waxy capacitors and a couple of resistors, but I have no idea if I should be getting any hum or not when its all back together.

If I have to resort to swapping valves, is there a technique to use to get them out, they look very well held in
Allan
allanmac is offline  
Old 5th May 2009, 9:42 pm   #2
Phil G4SPZ
Dekatron
 
Phil G4SPZ's Avatar
 
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,376
Default Re: How much hum is normal? and valve change DAC90A

I'm working on a similar Bush (an AC91) and replacing the main reservoir and smoothing electrolytics has completely removed audible hum.

Small modern electrolytics are usually easy to hide beneath the chassis, and I now tend to replace the originals as a matter of course if there is any hum at all.
__________________
Phil

The place where optimism most flourishes is the lunatic asylum - Havelock Ellis
Phil G4SPZ is offline  
Old 5th May 2009, 9:53 pm   #3
Boom
Nonode
 
Boom's Avatar
 
Join Date: Jan 2007
Location: Westbury, Wiltshire, UK.
Posts: 2,451
Default Re: How much hum is normal? and valve change DAC90A

If you have sensitive hearing you might hear some hum on a DAC90A. It shouldn't be objectionable but in no way will compare to a modern hi-fi which uses totally different techniques. Your best bet is to put test caps' of about 4-8uF across the smoothing can and listen subjectively as to whether it make a difference. If so you may well find the smoothing caps' faulty.

As to removing valves be very carefull, I learnt the hard way that the spring fixing rings seized up and can take the glass seals off when handled. I now carefully remove the spring clips before attempting to remove a valve.
__________________
Dave
--------------
A lovely childhood. Just me, my mother and the voices.....
Boom is offline  
Old 5th May 2009, 9:55 pm   #4
Norman Raeburn
Heptode
 
Join Date: Feb 2008
Location: Leven, Fife, Scotland
Posts: 635
Default Re: How much hum is normal? and valve change DAC90A

I had the same problem with a DAC90A I was restoring. I snipped pin 4 from the UL41 and this made a big difference but eventually had to replace the valve as it was suffering internal leakage.
Norman Raeburn is offline  
Old 5th May 2009, 10:17 pm   #5
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,519
Default Re: How much hum is normal? and valve change DAC90A

Yes, one of the pins on the UL41 valveholder is used as a convenient mounting tag. Even worse, it's grid-related circuitry! So, any internal leakage within the UL41 has maximum effect.

If you isolate this pin, you'll be doing yourself a big favour.

As for HT smoothing, you can add as much extra capacitance as you've got room for, to the smoothing capacitor (which unusually in the DAC90a is 16F and less than the reservoir capacitor at 32F). Don't increase the reservoir or the UY41 won't thank you long term. (You could however tack on 100F 300V, and run it for a few minutes, just to prove or disprove whether it's HT ripple or not).
kalee20 is offline  
Old 5th May 2009, 10:17 pm   #6
allanmac
Triode
 
Join Date: Apr 2009
Location: Whitstable Kent
Posts: 15
Default Re: How much hum is normal? and valve change DAC90A

I`d heard about breaking the glass pimple off the valve when removing it, taking the spring off seems the most logical but I havent really looked at them yet to see if they are easy to remove. I have a spare UL41 valve, I dont want to break the old one if I need to swap it. I will also be replacing the smoothing electrolytics.
I was just a bit hesitant about the valves, I havent removed one from a set since around 1964!
Allan
allanmac is offline  
Old 6th May 2009, 2:45 pm   #7
Boom
Nonode
 
Boom's Avatar
 
Join Date: Jan 2007
Location: Westbury, Wiltshire, UK.
Posts: 2,451
Default Re: How much hum is normal? and valve change DAC90A

Very easy to remove the springs Alan. Just slip a screwdriver blade under it and ease it away from the valve. After that gently rock the valve gently to get it free from the holder.

You only make the expensive mistake of breaking the pips off once.
__________________
Dave
--------------
A lovely childhood. Just me, my mother and the voices.....
Boom is offline  
Old 6th May 2009, 4:06 pm   #8
allanmac
Triode
 
Join Date: Apr 2009
Location: Whitstable Kent
Posts: 15
Default Re: How much hum is normal? and valve change DAC90A

Quote:
Originally Posted by David Tilley View Post
Very easy to remove the springs Alan. Just slip a screwdriver blade under it and ease it away from the valve. After that gently rock the valve gently to get it free from the holder.

You only make the expensive mistake of breaking the pips off once.
Thanks, sounds easy enough.
Allan
allanmac is offline  
Old 6th May 2009, 6:20 pm   #9
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 5,879
Default Re: How much hum is normal? and valve change DAC90A

I think there is a thread on this somewhere. Some DAC90's don't have the springs on the valveholders. If yours does and you want to retain them, it's best to clean and grease them before refitting. Most people just remove them. It's OK if the set is not moved about or bumped too much and as most are in static collections, it will be fine.



Rich.
__________________
On Spike Milligan's headstone......I told you I was ill...
Sideband is online now  
Old 6th May 2009, 9:34 pm   #10
Nickthedentist
Dekatron
 
Nickthedentist's Avatar
 
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 15,141
Default Re: How much hum is normal? and valve change DAC90A

Quote:
Originally Posted by Richard_Newman View Post
Some DAC90's don't have the springs on the valveholders. If yours does and you want to retain them, it's best to clean and grease them before refitting.
...or just temporarily remove them whenever you want to remove/replace a valve, then pop them back on afterwards

My DAC90A barely hums at all.

Good luck.
Nickthedentist is offline  
Old 7th May 2009, 4:37 am   #11
dominicbeesley
Octode
 
dominicbeesley's Avatar
 
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,848
Default Re: How much hum is normal? and valve change DAC90A

I'd also be interested to hear from anyone who has the back off their set in the future as to an actual measurement of hum from their set, i.e. taken in volts RMS (or peak to peak if easier) across the field coil of the set or anode of the output valve. When I get back from my hols I intend to have a quick service of my set.

My set also hums a little (messing with filtering components exhausted but not tried the UL41 tag dodge). The trouble is I've no idea what is a 'little hum' having only heard any one elses DAC90A playing in a crowded auction hall where the hum is completely masked by background noises.

Cheers

Dom

PS: As to the expensive valve pip breaking excercise - some idiots take more than one go to work out why all their valves are going gassy - three ECH42s in a Pye P76F before I clocked!
dominicbeesley is offline  
Old 7th May 2009, 3:37 pm   #12
johnlees
Hexode
 
johnlees's Avatar
 
Join Date: Oct 2004
Location: Cheltenham, Gloucestershire, UK.
Posts: 283
Smile Re: How much hum is normal? and valve change DAC90A

There is softly audible hum on my DAC 90A when it's off-station. It does not vary with the volume control and is almost a comforting reassurance that the set is working.

When tuned to any station it is completely inaudible, however, so I'll leave all that's well alone.
__________________
Now that's the kind of Long Wave I like ! (Severn Bore near Gloucester)
johnlees is offline  
Old 7th May 2009, 4:55 pm   #13
Boom
Nonode
 
Boom's Avatar
 
Join Date: Jan 2007
Location: Westbury, Wiltshire, UK.
Posts: 2,451
Default Re: How much hum is normal? and valve change DAC90A

It would be an extremely rare ac mains set that had no hum at all. I have never come across one. All 11 of my DAC90A's have a slight background hum as do my DAC90's. The TV22 from the same stable also has a slight audio hum which I asked someone for a second opinion on and they said it was perfectly normal.

I agree with johnlees. I find it quite pleasing.
__________________
Dave
--------------
A lovely childhood. Just me, my mother and the voices.....
Boom is offline  
Old 9th May 2009, 7:30 am   #14
allanmac
Triode
 
Join Date: Apr 2009
Location: Whitstable Kent
Posts: 15
Default Re: How much hum is normal? and valve change DAC90A

Quote:
Originally Posted by kalee20 View Post
As for HT smoothing, you can add as much extra capacitance as you've got room for, to the smoothing capacitor (which unusually in the DAC90a is 16F and less than the reservoir capacitor at 32F). Don't increase the reservoir or the UY41 won't thank you long term. (You could however tack on 100F 300V, and run it for a few minutes, just to prove or disprove whether it's HT ripple or not).
I assume then that a 32F + 32F See here would be a good replacement for the original?
Allan
allanmac is offline  
Old 9th May 2009, 9:52 am   #15
plaka78
Pentode
 
plaka78's Avatar
 
Join Date: Jul 2007
Location: Salisbury, Wiltshire, UK.
Posts: 159
Default Re: How much hum is normal? and valve change DAC90A

I had a look at a DAC 90a that I picked up recently, Changed grid Capacitor etc. The Hum was very bad and I had a reading of 7 volts on the grid of the UL41 (Valve faulty) Snipped off pin 4 the sound was hum free and the grid voltage went down to 5 mV. It the first time I had ever tried this mod and was amazed at the results.
plaka78 is offline  
Old 9th May 2009, 5:21 pm   #16
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,519
Default Re: How much hum is normal? and valve change DAC90A

Quote:
Originally Posted by allanmac View Post
I assume then that a 32F + 32F See here would be a good replacement for the original?
Allan
Yes, a very good replacement. Hum will be lower, at no detriment to the rectifier valve. It'll even fill the chassis hole nicely!
kalee20 is offline  
Old 14th May 2009, 4:50 pm   #17
allanmac
Triode
 
Join Date: Apr 2009
Location: Whitstable Kent
Posts: 15
Default Re: How much hum is normal? and valve change DAC90A

After replacing all the wax capacitors and testing the resisters, which, spookily, were almost all within tolerance, I was left with the humming, which got a little louder when the chassis was back in the cabinet. I ordered a new dual section electrolytic cap and while waiting for it to arrive I fitted a new UL41 and the hum has gone, well, if you put your ear on the case you can just hear a very faint background hum, which I assume is normal.
It now has a beautiful rich clear sound that puts many modern sets to shame.

Thanks for everybodies input

Now for a beer

Allan
allanmac is offline  
Old 14th May 2009, 5:03 pm   #18
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,519
Default Re: How much hum is normal? and valve change DAC90A

Definitely leakage within the UL41 then. But, do the pin 4 mod and prolong the useful life of your new UL41.

Is there any chance of having the old UL41, if you've got no further use for it? I'll pay p & p of course!
kalee20 is offline  
Old 14th May 2009, 5:36 pm   #19
allanmac
Triode
 
Join Date: Apr 2009
Location: Whitstable Kent
Posts: 15
Default Re: How much hum is normal? and valve change DAC90A

Your very welcome to the old one. PM me your address and I'll send it to you.
I'll treat you to the P+P.
Allan
allanmac is offline  
Old 16th May 2009, 11:38 am   #20
allanmac
Triode
 
Join Date: Apr 2009
Location: Whitstable Kent
Posts: 15
Default Re: How much hum is normal? and valve change DAC90A

Posted Friday, should be with you early next week.
Allan
allanmac is offline  
Closed Thread

Thread Tools



All times are GMT. The time now is 8:05 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.