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 15th Sep 2019, 9:48 pm   #1
lesparapluies
Pentode
 
lesparapluies's Avatar
 
Join Date: Jan 2019
Location: Liverpool, Merseyside, UK.
Posts: 121
Default Westminster S470M power supply problem?

Hello, I've got a Westminster S470M radiogram on the bench. Firstly, does anyone know if a circuit diagram might be obtainable? This brand seems to be shrouded in mystery. Completely dead, although valve heaters light up. The mains live goes to a large wire-wound resistor which has two taps, one of which supplies the valve heaters and the pilot light bulb (which work OK and light up) , and the other which goes to the rectifier valve (UY85). This part of the resistor seems to be more or less open circuit and the voltage on the anode of the rectifier is about 20 volts. There are a couple of other high wattage resistors in this part of the power supply which seem OK but no idea what the value should be...it would be fairly easy to sketch a diagram of the power supply and the whole unit is fairly basic but I need to know the value of this large resistor... anyone any info? Many thanks (when posting the photo I spotted the value is printed on the back of the resistor!!) duh.....but any more info greatly appreciated...
Attached Thumbnails
Click image for larger version

Name:	IMG_2442.jpg
Views:	410
Size:	83.8 KB
ID:	190258  
lesparapluies is offline  
Old 15th Sep 2019, 10:59 pm   #2
Graham G3ZVT
Dekatron
 
Graham G3ZVT's Avatar
 
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,675
Default Re: Westminster S470M power supply problem?

Westminster was a Currys brand apparently
I can't find a schematic for you, but it looks like you've found the main fault.

The usual practice would be to bridge the open circuit dropper section with a 10 watt wire-wound resistor of the same ohmic value.

Find the capacitor that couples the grid of the output valve with the anode of the previous stage. Change that capacitor for a modern polyester type.
__________________
--
Graham.
G3ZVT
Graham G3ZVT is offline  
Old 16th Sep 2019, 6:31 am   #3
crackle
Rest in Peace
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
Default Re: Westminster S470M power supply problem?

The solder on the resistor tags looks as if it has all evaporated due to the excessive heat.
I would try cleaning the solder tags to bright metal and recheck the resistance value of the faulty section.
It may just need a good connection being remade. Ideally it should be re soldered with high melting point solder, but if you can clamp the connecting wire with a small nut and bolt so much the better.
Then do the other checks with the smoothing capacitors, and the audio coupling capacitor going to the grid of the Output valve from the anode of the preceding valve.
Something caused the dropper resistor to overheat.
Mike
crackle is offline  
Old 16th Sep 2019, 9:10 am   #4
lesparapluies
Pentode
 
lesparapluies's Avatar
 
Join Date: Jan 2019
Location: Liverpool, Merseyside, UK.
Posts: 121
Default Re: Westminster S470M power supply problem?

Thanks for all the info, have checked what's available online component wise and my options seem to be to use two 100 ohms in series, which is going to be very bulky, or chance a 180 ohm or a 220 ohm, any thoughts on this? Points noted re the smoothing caps, both of which are almost double their stated values so probably leaky? Also will look at the coupling caps and indeed all other components... there aren't that many...
lesparapluies is offline  
Old 16th Sep 2019, 9:15 am   #5
cathoderay57
Nonode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
Default Re: Westminster S470M power supply problem?

I agree that first you need to establish why the existing dropper has burned out. Try reforming the reservoir capacitor using a separate HT supply for starters. If it is leaking like a sieve, or if there is a low resistance elsewhere across the HT line, then that could have caused the problem. Then, maybe the best thing to do is scout around for a circuit with a similar valve line-up and a UY85 rectifier and copy the value of series resistance used in that circuit. I would not have thought the resistance is too critical, as its purpose is to limit the surge through the rectifier into the reservoir capacitor at switch-on. If you can't find a similar circuit then I would play safe and start with a resistance of 270R 10W and measure the HT voltage on load. If too low then try progressively lower value series resistors down to about 100R. Cheers, Jerry
cathoderay57 is offline  
Old 16th Sep 2019, 11:26 am   #6
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Westminster S470M power supply problem?

Is the tap in the middle of the large dropper the mains in?

Lawrence.
ms660 is offline  
Old 16th Sep 2019, 1:31 pm   #7
cathoderay57
Nonode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
Default Re: Westminster S470M power supply problem?

Hang on - the resistance values are printed on the side of the dropper!
cathoderay57 is offline  
Old 17th Sep 2019, 9:28 pm   #8
lesparapluies
Pentode
 
lesparapluies's Avatar
 
Join Date: Jan 2019
Location: Liverpool, Merseyside, UK.
Posts: 121
Default Re: Westminster S470M power supply problem?

Thanks for all info - yes I realised the value was stamped on the resistor after I'd posted the photo and I mentioned it in the posting...senior moment, sorry! Have done a little more investigating and no obvious component failures to suggest what would have taken the dropper with it. It's a very similar circuit to a Philips one I've found - UY85 and UCL82. My plan is to get the HT supply running properly and then deal first with the audio output section, from the volume control forwards, then having got some sound at least from the gram I'll investigate the tuner circuit. One possible culprit could be the output transformer; again no physical signs of overheating but you never know... any idea of what kind of readings I should be getting on a test? It's a straightforward primary / secondary with no centre tap... again any info appreciated and once again apologies for not seeing the 200Ω!!
lesparapluies is offline  
Old 17th Sep 2019, 9:34 pm   #9
lesparapluies
Pentode
 
lesparapluies's Avatar
 
Join Date: Jan 2019
Location: Liverpool, Merseyside, UK.
Posts: 121
Default Re: Westminster S470M power supply problem?

Intriguingly the smoothing caps are 50 / 50 / 16 in the same can, with the 16uF section not connected at all... must have been what they had lying around at the time
lesparapluies is offline  
Old 18th Sep 2019, 10:38 am   #10
AC/HL
Dekatron
 
AC/HL's Avatar
 
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,637
Default Re: Westminster S470M power supply problem?

Component shortages probably, it would have been more expensive, even at bulk purchase prices.
AC/HL is offline  
Old 20th Sep 2019, 4:27 pm   #11
lesparapluies
Pentode
 
lesparapluies's Avatar
 
Join Date: Jan 2019
Location: Liverpool, Merseyside, UK.
Posts: 121
Default Re: Westminster S470M power supply problem?

Going to do a bit more to it this weekend. I was a bit puzzled about how to check the output transformer for short circuit because a straightforward resistance check doesn't give the full story... came across a YouTube video in which it suggests you disconnect the transformer and then apply a small AC voltage, around 5V, to the secondary winding and check that a higher voltage is present in the primary; if it is then all would appear to be OK... is this a recognised procedure or are there any other methods of checking? Many thanks as always
lesparapluies is offline  
Old 20th Sep 2019, 10:15 pm   #12
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
Default Re: Westminster S470M power supply problem?

That's as good a method as any. It's a well-known general test procedure; apply a known input to a system, measure what comes out, and use that to determine the relationship between input and output. You already know that a transformer multiplies voltages and divides current by the same amount, so you need only take one measurement to determine the relationship (although you could certainly take several readings, plot them on a graph and find the line which is the best fit through them all; its slope will give the turns ratio. You can even get a computer to do all that for you .....). You also know that the primary has more turns than the secondary; and big numbers are usually easier to measure, so it makes sense to feed the known quantity into the secondary winding and measure the output from the primary winding.

So what sort of voltage should you feed in? Based on similar sets, going flat-out, it might deliver 2W into a 3Ω speaker. We know P = V * I and R = V / I. We can rearrange the second equation as either V = I * R or I = V / R; substituting each of these into the first equation gives I = 0.82A and V = 3.67V. Now you can get a lower bound on the turns ratio from the anode current of the valve (use the maximum value from the data sheet, if in doubt). The valve only ever passes current in one direction, so the peak value of the AC portion of the primary current cannot be any greater than the average DC anode current; this means the RMS value (which is what we are interested in here) of the primary current cannot be any more than 0.707 (= the square root of 2) times the anode current. (In reality it will be even less, because the valve stops behaving linearly when it is too far away from its intended operating point.) This then tells you the minimum possible turns ratio, and therefore the minimum voltage to expect. (Clue: you wouldn't want to be holding those probes on with your bare hands .....)

Do your own calculations, post your results here; and depending how far you trust yourself, maybe proceed to experiment!

If by way of a slight refinement you feed the supply through a resistor say 5* the DC resistance of the winding (be sure to measure the input downstream of this resistor, across the actual winding), then you should be able to check for shorted turns. If creating a deliberate shorted turn by means of poking a loop of insulated "hook-up" wire through the gap between the bobbin and the stack of laminations and briefly touching the ends together -- just long enough to get a reading -- doesn't cause much or any change in the output, that means the transformer already has shorted turns and will need rewinding.
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments.
julie_m is offline  
Old 20th Sep 2019, 10:56 pm   #13
lesparapluies
Pentode
 
lesparapluies's Avatar
 
Join Date: Jan 2019
Location: Liverpool, Merseyside, UK.
Posts: 121
Default Re: Westminster S470M power supply problem?

Many thanks to julie_m for that last posting, very informative. I knew all the theory but that was so well explained in practical terms so very grateful. Now just one more thing - the capacitors (see photo) - are the red ones tubular ceramics? - they all seem to be close to their marked values And the green, marked "Erie" - I thought at first glance they were electrolytics but the value (0.047uF) and the lack of any polarity markings suggest otherwise...(I'm kind of coming back to this game after a lifetime spent in a different direction, built a few valve amps in the late 60s / early 70s so it's re-learning old stuff and the advice is very welcome)
Attached Thumbnails
Click image for larger version

Name:	IMG_2446.jpg
Views:	155
Size:	125.9 KB
ID:	190560  
lesparapluies is offline  
Old 21st Sep 2019, 10:16 am   #14
'LIVEWIRE?'
Rest in Peace
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
Default Re: Westminster S470M power supply problem?

Those red capacitors are (tubular)ceramic types. AFAIK they are NPO (Negative temperature coefficient)types. The Green ERIE cap. is certainly not electrolytic.
'LIVEWIRE?' is offline  
Old 5th Oct 2019, 4:50 pm   #15
lesparapluies
Pentode
 
lesparapluies's Avatar
 
Join Date: Jan 2019
Location: Liverpool, Merseyside, UK.
Posts: 121
Default Re: Westminster S470M power supply problem?

Update - have spent a few hours on the amplifier section (disconnected the radio section altogether for the moment), checked output transformer, which seems OK, replaced the open circuit mains dropper resistor and the power supply caps, and have tested the amp - progress made, there is sound, ... however there is still a problem... the HT voltages show a good 300v half a minute or so after switch-on, but these voltages gradually drop over the first few minutes to less than 150V and the performance deteriorates accordingly. I have no means of checking the valves, other than a basic multimeter test to check for shorted pins; am I right to assume a faulty rectifier valve, or is there some other classic symptom that I'm missing? I've sketched a diagram of the section I'm working on and as usual, any thoughts appreciated, thanks, MD
Attached Thumbnails
Click image for larger version

Name:	S470M002.jpg
Views:	129
Size:	62.9 KB
ID:	191342  
lesparapluies is offline  
Old 5th Oct 2019, 5:37 pm   #16
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default Re: Westminster S470M power supply problem?

There should be a cathode resistor on the UCL82 pentode section or some other means of biasing! And a resistor from the grid of its triode section to ground. The NFB arrangement is unusual but looks like it should work OK.

Have you checked for any positive voltage on the UCL82 pentode control grid? Replaced its grid coupling capacitor?

Try monitoring the voltage across the output transformer primary- if the O/P valve is running away this will increase as the HT falls away.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 5th Oct 2019, 7:04 pm   #17
lesparapluies
Pentode
 
lesparapluies's Avatar
 
Join Date: Jan 2019
Location: Liverpool, Merseyside, UK.
Posts: 121
Default Re: Westminster S470M power supply problem?

Thanks for the quick reply. Just checked and yes I've overlooked a 10M resistor between pin 1 and the chassis; pin 8 is connected straight to the chassis, will double check the rest of my diagram to make sure I haven't missed anything else.
lesparapluies is offline  
Old 5th Oct 2019, 7:55 pm   #18
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default Re: Westminster S470M power supply problem?

Pin 8 (kt) can be direct to chassis- the triode is "contact potential" biassed. The pentode cathode on pin 2 needs a resistor to provide 12-16 volts of bias (bypassed by an electrolytic around 25uF or so).
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 5th Oct 2019, 8:38 pm   #19
lesparapluies
Pentode
 
lesparapluies's Avatar
 
Join Date: Jan 2019
Location: Liverpool, Merseyside, UK.
Posts: 121
Default Re: Westminster S470M power supply problem?

Just done a revised diagram; I think the first one was a bit confusing with the 2 cathodes back to front... hopefully this one will be a bit clearer...I'm taking all this on board but I've just checked the unit and definitely the only connection to pin 2 is to the resistor and capacitor which go to the transformer secondary...
Attached Thumbnails
Click image for larger version

Name:	S470M004.jpg
Views:	155
Size:	56.4 KB
ID:	191362  
lesparapluies is offline  
Old 5th Oct 2019, 8:42 pm   #20
lesparapluies
Pentode
 
lesparapluies's Avatar
 
Join Date: Jan 2019
Location: Liverpool, Merseyside, UK.
Posts: 121
Default Re: Westminster S470M power supply problem?

Had a quick look at some other designs which use the same valve combination and yes they all have the resistor / capacitor to chassis from pin 2 - do we assume that in this one the connection to ground is made via the speaker and the NFB line?
lesparapluies is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 4:18 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 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.