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Old 30th May 2020, 7:59 pm   #1
Gabe001
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Default Strad PW461 restoration

Hello,

I wonder if I could pick your brains

I've recently acquired a lovely Strad pw461 which I'd like to restore to working condition. It didn't come with much history sadly. I have some experience fault finding and replacing the occasional component on my 2 other sets, both from the 30s, but I've never attempted something so ambitious before. I've built the lamp limiter yesterday, removed all the dirt and grime and polished the case which seems to be in good condition, and this morning took out the chassis and spent some time taking a few resistance readings which I'll post in a separate post.

The innards look relatively original but I believe (see picture) that both smoothing capacitors have been replaced in the past, probably in the 60s-70s, with a yellow Dubilier and with one I've never seen before. The valves are all present and the lineup is correct and I've got the trader service sheet from this site. I have a multimeter and a lamp limiter but no other specialist equipment like a Variac or signal generator. Having said this, I end up using these predominantly though the pu socket anyway, so if there are alignment/reception issues I won't be too disappointed.

I'm posting some pictures below. The tubular waxy capacitors will need to be replaced and I'll order those. Am I right in assuming that the low F value flat, rectangular capacitors are usually ok to keep? Anything other checks/work required before I power up (via limiter), and what are your thoughts about the replacement smoothing caps -can those stay?


Many thanks for your help,
Gabriel
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Last edited by Gabe001; 30th May 2020 at 8:11 pm.
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Old 30th May 2020, 8:09 pm   #2
Gabe001
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Default Re: Strad pw461 restoration

And here are the readings (no power applied)
L18 speaker field coil 1450 ohms
T1 primary 201
T1 secondary <1 ohm
T2 primary 40
T2 HT total 532

R1 25000
R2 95000
R3 404
R4 18000
R5 183
R6 cannot get to it
R7 33200
R8 114000
R9 1.9 million
R10 1.09 million
R11 volume control
R12 1.1 million
R13 36200
R14 47000
R15 740000
R16 222
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Old 30th May 2020, 10:21 pm   #3
cathoderay57
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Default Re: Strad pw461 restoration

Yes,the flat rectangular low value caps are usually OK. Good starting point taking the measurements, well done. The inductors (transformers etc) ) look OK. Unless you disconnected one end of the resistors before measuring them, the values measured are likely to be lower than the marked value. If they are within about 20% of the value shown in the circuit diagram they will be fine. Unless they are massively out of range the radio will probably still work. The electrolytic capacitors are a bit of a gamble unless you want to try reforming them. Don't be tempted to just try them and see because if they fail short circuit you could damage the rectifier valve or mains transformer or both. If in doubt fit replacements because they are not very expensive.
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Old 31st May 2020, 11:02 am   #4
Gabe001
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Default Re: Strad PW461 restoration

Thanks ray. I'll order them all from a single supplier and replace them, including the smoothing caps.

I note that there is a value associated with electrolytic capacitors called "lifetime" on the RS components website. This can vary between brands from 2000 to 12000 hrs. Bearing this in mind, are then any specific brands you guys like to go for, or is this a bit of a marketing gimmick?

The reason I ask is because I've had a new'-ish' capacitor fail short-circuit in one of my sets in the past, taking out the rectifier valve.

Thanks
Gabriel
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Old 31st May 2020, 2:41 pm   #5
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Default Re: Strad PW461 restoration

Hi Gabriel, the brand isn't important, it's the ripple current rating that is more so. Also you will find it easier to use more common capacitance values - while you can still get 8 + 16uF that the PW461 requires, more cost effective alternatives are to use a 10uF in lieu of 8uF and 22uF in lieu of 16uF. The high ripple cap is the one connected to the rectifier cathode, in this case 8uF. This one is quoted with a ripple current of 125mA at 120Hz which is OK https://uk.rs-online.com/web/p/alumi...itors/7567636/ . You will see others with higher ripple current ratings but the devil is in the detail as always since some of these are ripple currents quoted as high as 300mA at 100kHz. To get the actual value at (for UK mains with full wave rectification) 100Hz that figure has to be multiplied by a factor given in the datasheet of 0.3, so it's still pretty good but not as high as you might have thought! It is getting more difficult to find suitable capacitors with axial leads but I don't have a problem using the ones with radial leads. Some people like to scrape out the gunk from inside the large old electrolytic can and re-stuff with new capacitors and the radial lead types are handy for this. Personally I tend to wire them in below chassis where nobody is going to see them! Hope that helps
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Old 31st May 2020, 9:57 pm   #6
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Default Re: Strad PW461 restoration

Electrolytic lifetime is quoted at max ripple current, operating voltage and a specified ambient temperature, usually 85C or 105C.

Reducing any of the "loading factors" increases the lifetime, with temperature having the biggest effect- lifetime doubling for every 10C drop in ambient temperature.

Typical calculator:
https://www.electronicproducts.com/a...lculator.aspx#
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Old 1st Jun 2020, 6:39 am   #7
Gabe001
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Default Re: Strad PW461 restoration

Thanks ray and Chris. I've ordered a load of capacitors this morning from RS using your advice and the guide in the sticky capacitor replacement thread which is really good. Bought the radial smoothing caps in the end as there was much more choice available with better specs. Im tempted to try to re-stuff the old ones if this is an option for the waxies; I'll read up some more.

Gabriel
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Old 3rd Jun 2020, 10:27 am   #8
Gabe001
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Default Re: Strad PW461 restoration

Hi, is there any technical reason the original 0.005uF fixed tone corrector cap c23 is rated at 1000v? Best match from RS is a Film Radial capacitor rated 630V 4700pF. Is this ok? I'm not sure why the voltage rating is so high.
Thanks
Gabriel
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Old 3rd Jun 2020, 10:49 am   #9
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Default Re: Strad PW461 restoration

It's so high because the primary of the output transformer can just look like a large inductance, particularly if the secondary gets disconnected from the speaker. A spike signal (crackle, perhaps) can cause very high voltages to be generated.

Even in normal operation, a maximum power signal will be ac at about HT+ VPK. This needs a capacitor with a dc rating of 1000V or more for reliability.
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Old 3rd Jun 2020, 11:46 am   #10
Gabe001
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Default Re: Strad PW461 restoration

Thanks Chris. Found them from Farnell and ordered; a bit of a pain to have to use multiple suppliers. In any case, now I have plenty of spares. I think I may be addicted now.

Gabriel
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Old 6th Jun 2020, 2:19 pm   #11
Gabe001
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Default Re: Strad PW461 restoration

Just a little update. I've started replacing the capacitors on this radio today. The 3 electrolytics have been replaced so far. I was planning to finish it but upon removing and cleaning "that capacitor" from wax and muck I realised it's one of the 1000v variety; I didn't have any so ordered some ceramic replacements from RS. In any case powered it up with the lamp limiter (bulb responded as it should) and the speaker socket disconnected and getting good DC output from the rectifier.

I have LT voltage as I can see the heater of v4 light up and there is measurable voltage across the bulb sockets, but the bulbs don't light up

Does anyone have any idea where I can buy two 6.2v 0.3a screw in bulbs?

Thanks,
Gabriel
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Old 6th Jun 2020, 2:45 pm   #12
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Default Re: Strad PW461 restoration

https://www.ebay.co.uk/itm/KL-6-5V-3...8AAOSwXrdaHU7d


is just one of many possibilities for a suitable bulb.



Don't worry about the slight voltage difference- 6.5V is normally used for a pilot light on a 6.3V heater set, it'll just be a bit dimmer still on your 4V heater supply!
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Old 6th Jun 2020, 3:01 pm   #13
Gabe001
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Default Re: Strad PW461 restoration

Thanks Chris. It just occurred to me that they may not have turned on because of the lamp limiter, buy for that price I ordered them anyway.

Gabriel
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Old 13th Jun 2020, 3:04 pm   #14
Gabe001
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Default Re: Strad PW461 restoration

Hello again,

I just thought about giving you guys an update on this project.

I've finished working on the Strad today, and it now has pride of place in the kitchen .

Chris, the lamps you recommended worked perfectly. Both original bulbs were not working so they needed replacing.

I undertook a rather ambitious (for me, as it's my first) waxy capacitor replacement and I'm posting some pictures below. C25, c7, c15, c14, c8, c20, c22, c23, c21, c24 have all been replaced. The greatest challenge was replacing axial components with radial ones; the electrolytic capacitors with 500+ volts and axial leads seem to cost silly money. I've had to extend the negative (ground) pole capacitor leads to reach the original points on chassis.There was some speaker hum initially but this resolved itself as the wax capacitors were replaced, I suppose they were leaky.

Case polished with brasso and tuning dial glass removed and cleaned. Tuning cord seemed in good condition so left in situ.

I've got decent reception and am listening to talksport as we speak (even without an aerial). Sound is good enough without distortion.

Next job is a stereo jack to pu converter cable so I can put the radio to some real use.

Thanks for all your help. Please be gentle with the feedback, but do let me know if there is something untoward on the pictures. Any further advice welcome.

Thanks,

Gabriel
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Old 13th Jun 2020, 3:58 pm   #15
unitelex
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Default Re: Strad PW461 restoration

Well done Gabriel,
What will you tackle next?

Best Regards
Chris
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Old 14th Jun 2020, 8:25 pm   #16
Gabe001
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Default Re: Strad PW461 restoration

Hi Chris, I've got a cossor 464 and a regetone a353 to collect and my first week off in 3 months. I'll probably tackle the regetone first. The case needs some work as it is badly sun bleached, which makes a bit of a change after the soldering marathon.

Gabriel
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Old 16th Jun 2020, 9:19 am   #17
Gabe001
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Default Re: Strad PW461 restoration

In case it helps anyone, the battery powered Bluetooth module in the picture works well with this radio via the PU input. I've combined L+R using 330 ohm resistors and these 3mm banana plugs from china (shown in picture) worked well as wander plug substitutes. Good volume with no amplification needed. Bluetooth module self identifies as kn321 and I've got it paired up with a Google home.

G
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