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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 31st Dec 2019, 4:02 pm   #21
Beardyman
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Default Defiant MSH902 addendum

Having completed this restoration some time earlier this year I thought I would fire up the radio over the Christmas period just to make sure everything was ok. It was, for about 20-30 minutes, then gradually started to lose the bass notes & grow quieter. This was something I thought I had fixed previously but obviously I hadn't!?! As this set doesn't have any circuit diagrams available (apart from my own hand written ones) & dozens of photos I took prior to any work being carried out I thought I'd spend an hour or two correllating the two. It wasn't unitl I had the chassis out of the case (again) that I was idly prodding around the resistors attached to the FC4 valve that I found a dodgy reading. There was supposed to be a 500K strapped across pins 2 & 6 (g1 & k), mine was reading 50K! This was a case of mistaken identiity, entriely my fault but a quick glance & it "looks" like a 510K. The body is painted a brick red & the gold tolerance band looks yellow under anything but a bright white light. Needless to say the orange band is nearly invisible against the body. I changed it for the correct value & now the set is booming nicely & has been for well over 3 hours, looks like the FC4 survived. Tough things these valves!
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Old 31st Dec 2019, 4:33 pm   #22
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Default Re: Defiant MSH902

So what was the correct value?

Lawrence.
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Old 31st Dec 2019, 6:05 pm   #23
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Default Re: Defiant MSH902

500K Lawrence. Have a good evening & New Year.
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Old 31st Dec 2019, 6:32 pm   #24
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Default Re: Defiant MSH902

500k between osc grid and cathode seems high to me:

http://www.r-type.org/pdfs/fc4.pdf

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Old 9th Jan 2020, 9:34 pm   #25
Beardyman
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Default Re: Defiant MSH902

Yes it does but I've been going through the photos I took before any work was carried out & the original resistor was a 500K across pins 2 & 6. I've also been looking through various other schematics of radios that use the FC4 (theres a fair few!) & the highest value I've found in those was 100K in an EKCO set. Unlike the valve museum datasheet the Defiant has that 500K across 2 & 6 but also a 250R with 0.1uf in parallel to chassis from pin 6 (k). I don't have enough knowledge of valves to unpick the designers thoughts on this one but the radio now works as it should. I'm sure there are other members who may shed some light on this toothsome feature.
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Old 10th Jan 2020, 3:46 pm   #26
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Default Re: Defiant MSH902 addendum

Quote:
Originally Posted by Beardyman View Post
There was supposed to be a 500K strapped across pins 2 & 6 (g1 & k), mine was reading 50K! This was a case of mistaken identiity, entriely my fault but a quick glance & it "looks" like a 510K. The body is painted a brick red & the gold tolerance band looks yellow under anything but a bright white light. Needless to say the orange band is nearly invisible against the body. I changed it for the correct value & now the set is booming nicely & has been for well over 3 hours, looks like the FC4 survived. Tough things these valves!
You say it looks like a 510k but then you say the body is red....

A 500k (ish) resistor going down to 50k in that circuit position I find very hard to believe unless it's had the wrong colour code painted on it....

I suspect the original that was fitted was either a 50k or 56k, I would go for 50k.

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Old 15th Jan 2020, 8:14 am   #27
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Default Re: Defiant MSH902

Can you advise the contact details for the co-op archive as I am looking for a circuit for a Defiant MSH547. Thanks
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Old 15th Jan 2020, 1:48 pm   #28
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Default Re: Defiant MSH902

The Avery Man,

The email address is on the Co-Op Archive website but does take a little rooting around to find so here it is: archive@co-op.ac.uk

When I contacted them their reply was as follows:

"I am afraid we do not have service sheets for any of the radios before the 1950s. The collection of material we hold on the CWS manufacture of radios and televisions is small and includes some catalogues of products from the 1930s. There are also handwritten notebooks with information about individual models. These show that the Model MSH 902 came in two versions - AC and AC-DC and were first introduced during the 1935-36 season."

I'd have said that you may find yourself in the same position as myself, no circuits other than what you may be able to come up with, I have looked on my copy of the Vintage Radio Service Data DVD & there's nothing there either.

Best of luck sir!
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Old 18th Feb 2020, 9:40 pm   #29
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Default Re: Defiant MSH902

Final (hopefully!) addendum to this thread. This radio has always had a rustling noise in the background on MW/LW, sometimes there would be an odd phasing then back to normal or random pops/clicks. I, in my ignorance, thought initially it was simply down to the age of the valves and/or coils, after all it is nearly 90 years old having been manufactured in around 1933-34. To have some noise would be forgiveable BUT it was irritating! Then I started trawling through this forum & others for the possible causes of it. It took many hours of reading but finally a trend began appearing especially in America of all places. It would seem the mica capacitor is suffering the effects of time. I then found a paper published in 1960(!) giving a LOT of detail as to their failure modes, one of which is silver migration & inclusions in the mica itself. I'm not at all sure if the document is still within copyright so I have not attached it here. The link is: https://www.google.com/url?sa=t&rct=...10e6UqL2TWJLvf
Anyway, I was becoming more convinced this was perhaps what was still wrong with the Defiant. I remembered I had left one mica capacitor in the aerial side of things. Out came the chassis, it looked good, tested ok as well, bang on value. Grab a replacement from the stock of caps, out with the old & in with the new! Switched on, the rustling/popping/f*rting had gone! It sounds so much better.
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Old 20th Feb 2020, 8:07 pm   #30
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Default Re: Defiant MSH902

I've seen a few videos of restorers in america talking about problems with silver-mica capacitors, (eg rustling and crackling); I've never experienced it in UK sets and assume its a US problem; so well done for tracking it down!
Defiant have a special allure for me due to the well-known background behind the name and operation; their well-made, honest Plessey chassis and cabinets that have that ivorine 'CWS Cabinet Works' label and the (often) high-art-deco designs. Yours is lovely, BTW
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Old 21st Feb 2020, 11:20 pm   #31
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Default Re: Defiant MSH902

Nice! Please post more photos when you have time. It is from the same year, 1934, as the HMV-468 I'm refurbishing.
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Old 24th Feb 2020, 8:59 am   #32
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Default Re: Defiant MSH902

I'd be glad to post some pictures as & when time allows. Despite there being virtually no technical information for this set it remains one of my favourites, its quite a large set but back in the day it would have been considered as furniture as well therefore had to "look the part". I have to run this one via a pantry tx as locally we have virtually no AM stations that are clear. Even R4 on LW is swamped by SMPS/LED lighting etc. Come to think of it, some FM stations struggle too! I'd read that silver migration is an issue in the states but hadn't read of many here in the UK. I'm glad I found out about it as I've got a Philco radiogram (approx. 1933ish) under the knife & will be watching out for that. Have a good day one & all.
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Old 26th Feb 2020, 6:14 am   #33
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Default Re: Defiant MSH902

Indeed, some of these radios are now old enough that mica capacitors are failing.

Silver migration / whiskers is indeed a problem, mainly in slug tuned (adjustable inductance) IF transformers which have the capacitors built-in. Those can usually be repaired by cutting out the original capacitor(s) and installing a new one(s) under the chassis.
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