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

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 24th Jun 2012, 1:41 am   #21
G8UWM-MildMartin
Heptode
 
Join Date: Jan 2004
Location: Stockport, Greater Manchester, UK.
Posts: 827
Default Re: NAAFI receiver problem

I'd now be tempted to reduce R17, possibly tacking 150R across the original temporarily.
Does the one fitted look original or changed to the rather unlikely value specified on the circuit at some time as part of a failed repair attempt?
G8UWM-MildMartin is offline  
Old 24th Jun 2012, 8:54 am   #22
AlanC
Hexode
 
AlanC's Avatar
 
Join Date: Jul 2006
Location: Sheffield, South Yorkshire, UK.
Posts: 312
Default Re: NAAFI receiver problem

Malcolm, did you measure the resistance from the cathode(s) to ground? It is possible the 6K8 has an interelectrode leak but the 32V you measured without it is still too high. Under typical operating conditions, the 6K7 will have 3 volts on the cathode (-3V bias) and at that, the combines amode and screen currents will be about 10mA. So a 300 ohm resistor would give the correct conditions, this is what I would expect to see.

But in this case, the 6K8 shares the resistor and so its currents are added- the datasheet shows 2.5mA for the hexode anode, 6mA for the screen. 4mA for the triode, and the bias required is -3V again. So the total current in the common resistor would be 22.5mA, requiring a resistor of 133 ohms or thereabouts.

So that's what you're looking for- in practice probably 120 or 150 ohms.
__________________
Always expect the unexpected...
AlanC is offline  
Old 24th Jun 2012, 9:39 am   #23
Malcolm G6ANZ
Octode
 
Malcolm G6ANZ's Avatar
 
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
Default Re: NAAFI receiver problem

Good morning all. Alan, I've been working from a diagram that was inside the set. It looks original I put a copy of in one of my earlier posts. The cathode resistor R3 is shown as being 15K. I did replace it with a new one as the one in circuit looked a bit past it. Thanks to ypur reasoning I have looked again at the circuit values and R3 has a small decimal point in front of it. This makes the value 0.15K not 15K. I have just put the old one back in and I now get 3v on the cathodes and a signal. Many thanks all.
No doubt I'll be back as I now try and get the best out of it.

Thanks again
Malcolm
Malcolm G6ANZ is online now  
Old 24th Jun 2012, 10:43 am   #24
AlanC
Hexode
 
AlanC's Avatar
 
Join Date: Jul 2006
Location: Sheffield, South Yorkshire, UK.
Posts: 312
Default Re: NAAFI receiver problem

Fantastic! That's what we like to hear- signals!

I shall look forward to hearing from you.

Cheers,
Alan
__________________
Always expect the unexpected...
AlanC is offline  
Old 6th Aug 2012, 8:28 pm   #25
Malcolm G6ANZ
Octode
 
Malcolm G6ANZ's Avatar
 
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
Default NAAFI again

Thanks to helpful advice it now works. This was on a Radford lab pack.
The vibrator PSU was missing so I have built a mains supply.
The HT is 320v with cold valves, which drops to 250v as they warm up. This is OK, but there is a mains hum, which is noticeable at low volumes, and I think it’s causing some distortion in the audio stages.
The ripple on the HT line is about 5V. Smoothing caps are 47uF and 22uF decoupled with 220ohm. The original design was for the valve heaters to run off of a 6v car battery and the HT from a vibrator inverter. Thus there are only three wires to the receiver chassis:
1: HT +
2: LT +
3: Common
Now I have connected the HT to the HT (probably rightJ) and the common, which is the chassis, to the 0v common from the PSU. The heater is from a 6v3 winding on the HT transformer. One end I have taken to the LT+ and the other side to common, chassis.
Could this be the cause of the hum? Running the heaters from AC not DC?

Any advice welcome. The relevent part f the circuit is in the attachment

http://i131.photobucket.com/albums/p.../psuwiring.jpg

Malcolm
Malcolm G6ANZ is online now  
Old 6th Aug 2012, 8:58 pm   #26
HamishBoxer
Dekatron
 
HamishBoxer's Avatar
 
Join Date: Aug 2007
Location: W.Butterwick, near Doncaster UK.
Posts: 8,923
Default Re: NAAFI again

This chassis is meant to run on either battery or mains,as I have both models.You have of course changed o/put grid coupling? Daft question cos almost sure you have changed all waxies.
__________________
G8JET BVWS Archivist and Member V.M.A.R.S
HamishBoxer is offline  
Old 6th Aug 2012, 9:36 pm   #27
broadgage
Nonode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
Default Re: NAAFI again

I doubt that running the heaters on AC is the cause of the hum, but to find out for certain, use a 6 volt battery for test purposes and see what happens.

Are the power supply capacitors big enough ? If similar in capacity to the originals, then they may not be.
Remembering that the original vibrator power supply probably ran at more than 50 cycles and therefore needed smaller capacitors.
broadgage is offline  
Old 7th Aug 2012, 9:01 pm   #28
Malcolm G6ANZ
Octode
 
Malcolm G6ANZ's Avatar
 
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
Default Re: NAAFI receiver problem

Thanks for the suggestions.
Hamish, your'e right, I have changed all the wax caps.
Broadgage, the caps are 47u and 22u @400v in the new PSU and there are the 8u and 4u @ 400v in the rx chassis. These two I have rebuilt with new parts in the old can.
I'll take another look at the ripple and try to find out where the hum is getting in.

malcolm
Malcolm G6ANZ is online now  
Old 7th Aug 2012, 10:34 pm   #29
Sean Williams
Dekatron
 
Sean Williams's Avatar
 
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
Posts: 3,517
Default Re: NAAFI receiver problem

Malcolm,

220r as a smoothing resistor? sounds a bit low to me - try a bigger resistor, say 1k (will need to be rated at a few watts)

This will reduce the ripple.

Or seeing as the set is HUGE, use a choke in the PSU instead...
__________________
Engineers make things work and have spare bits when finished
Sean Williams is offline  
Old 9th Aug 2012, 9:27 pm   #30
Malcolm G6ANZ
Octode
 
Malcolm G6ANZ's Avatar
 
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
Default Re: NAAFI receiver problem

I've been looking at this set again. Sean I put in a 1K smoothing resistor---no change.
Thinking there might be a heater cathode leak I swopped out each valve in turn ;again no change. All the origonal valves are now back in. Thinking further, may be I'm being too picky inthe amount of hum there is. With the volume at minium there is a small background hum, just as I would expect. Then as the volume is increased without a station being tuned in, the hum increases but dosn't sound like mains, its more like hum and noise from the gain stages at high gain. When a station is tuned in it sounds ok and there is no background hum unless the volume is very low when the power supply hum is evident
Am I over concerned or should I start looking at the AGC area?? The radio is quite sensitive with a long wire aerial on it.

Malcolm
Malcolm G6ANZ is online now  
Old 10th Aug 2012, 7:51 am   #31
ajs37
Heptode
 
ajs37's Avatar
 
Join Date: Jun 2003
Location: Bristol, UK.
Posts: 900
Default Re: NAAFI receiver problem

Hi Malcolm - just a thought from our brief chat last night (thanks for the BBC Master - it does seem to work!) - have you tested the valves in a valve tester? I have been caught out a couple of times by trying to substitute what I thought was a 'good' valve for a 'bad' one - only to discover that both were 'bad'.
Cheers
Andy
ajs37 is offline  
Old 10th Aug 2012, 8:42 am   #32
Malcolm G6ANZ
Octode
 
Malcolm G6ANZ's Avatar
 
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
Default Re: NAAFI receiver problem

Hi Andy, Glad the master works. The one piece of test kit I dont have is a valve tester! All I was looking for was h/k leakage, and as all the replacement valves performed in exactly the same way as those in the set I think the fault,if there really is one, is not a valve.
Malcolm
Malcolm G6ANZ is online now  
Old 10th Aug 2012, 9:22 am   #33
ajs37
Heptode
 
ajs37's Avatar
 
Join Date: Jun 2003
Location: Bristol, UK.
Posts: 900
Default Re: NAAFI receiver problem

well if you need to use a valve tester, I have recently restored my Mullard HSVT - any time you are in the area!
Cheers
Andy
ajs37 is offline  
Old 11th Aug 2012, 7:02 pm   #34
Malcolm G6ANZ
Octode
 
Malcolm G6ANZ's Avatar
 
Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
Default Re: NAAFI receiver problem

Thanks for that offer Andy.
I think I've been too critical over the amount of hum on this set. Listening to it warm up there is first a quiet mains hum,which does not change with volume level, then as it comes to life there is more 'hum and noise. this is probably the receiver running without a signal and increasing all the gains thus giving a noise and 'hum' output. I'm putting 'hum' in inverted commas as it's not pure mains but a mix. Looking at the AGC line this moves correctly as stations are tuned in and out, the stronger the station the more negative the AGC line.
So I'll call it working, pack it back into its large case and return it to its owner.
Many thanks to all who have helped me on this.

Malcolm
Malcolm G6ANZ is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 10:09 am.


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 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.