Bush SRP31D Rectifier Modification Help Please

[_Cosmic_]

I have a, now familiar and well loved by those on the forum, Bush SRP31D record player.
I have repaired a broken tweeter which seems to work nicely and repaired quite a lot of other items on it.
I noticed that the bass control wasn't really functioning so I read up on the threads in the forums that suggest that the selenium rectifier was probably in need of an upgrade.
I measured the voltage on the electrolytic can red tag that read about 260V on switch on that reduced steadily down to 112V over about 30 seconds and seemed to stabilise there.
To carry out the suggested modification, I removed the grey wire from the RHS tag on the rectifier. I removed the other end of that wire that was attached to the main transformer. I attached the 1N4007 diode with the line end of it closest to the red tag onto the electrolytic capacitor can as suggested on one of the threads.
I soldered a 220k resistor to the end of the diode and the other onto the transformer where the grey lead to the rectifier previously was attached
I left the red wire attached to the selenium rectifier LHS tag in place as the red wire goes down through the board and onto the transformer attached to the Celestian speaker.
After checking everything several times, with trepidation I switched on the power and noticed that the voltage initially read about 260V and reduced down to 220 after about 15 seconds and seemed to stabilise so all seemed to be well.

When I put it all back in the case and switched on again. After about 10 seconds i heard some crackling from the speaker then the red light on the front went off and now everything seems dead.
I investigated today to find the main fuse ok, the internal fuse bulb is ok, everything seems normal there is no burning or anything obvious wrong.

I'm not an electronics engineer although I am pretty good at fixing things have O level electronics and am reading Morgan Jones book on valve amplifiers for fun, so know a little bit. I'm not that confident with mains electricity and am very cautious. I have a decent multimeter and soldering iron but not a esr meter or oscilloscope..
Here's a photo of the temporary modification, I know there's no insulation on the wires yet.
I have a feeling the fault has something to do with the remaining wire on the rectifier. I can't find anywhere in the forum that advises what to do with it. And I can't find any clear photos of the modification.
I'm a little reluctant to turn it on again until I know if I have the wiring right.

I wonder, could a member be kind enough to advise.
many thanks
(Cosmic) Chris

[Station X]

Did you mean 220k? 220R sounds more likely. That's what the colour code appears to indicate. What wattage is it?

[_Cosmic_]

Yes of course, quite right, it's 220R.
one watt.

[Station X]

Circuit in this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=86032

The marked end of your diode should have continuity to C16, R23, R21 and the output transformer centre tap.

The pilot light is fed from the LT tap on the mains transformer secondary and is in parallel with the heater.

[Techman]

It sounds to me that you didn't read up on the power rating for the resistor. You'll need at least a 7 watt (10 watts even better) type, as any less will get very hot and either blow or catch fire. Make sure the resistor isn't close to any wiring or other components, as even the correct type can run quite hot.

[_Cosmic_]

There's continuity from the transformer to C16(large can) red tag (is that the anode) all way through along the red wire going down through the chassis to the centre tag on the output transformer.

I just noticed that the newly installed diode has perhaps got damaged as it responds both ways when applying the multimeter continuity leads. The remaining new ones I have left don't.

The pilot light is showing closed circuit so that hasn't blown.

[_Cosmic_]

Julie m on 9th Apr 2011, 5:07 pm says to use 1 watt resistors so that's what I did.

[_Cosmic_]

Thanks for that Graham. I have a circuit diagram, its a photo clearly showing the modification to follow I can't seem to see anywhere. If that makes sense.

[Techman]

Originally Posted by _Cosmic_

Julie m on 9th Apr 2011, 5:07 pm says to use 1 watt resistors so that's what I did.

That's completely wrong!

Can you post a link to that thread for verification, please?

[_Cosmic_]

There you go
About 2/3rd way down.
https://www.vintage-radio.net/forum/...ad.php?t=67864

[Techman]

Originally Posted by _Cosmic_

There you go
About 2/3rd way down.
https://www.vintage-radio.net/forum/...ad.php?t=67864

Thanks for that.

You've been very unlucky picking that particular thread with incorrect information in it, as there must be dozens more with the right information in them.

What's even more shocking is that no one seemed to pick up on the fact that the power rating was wrong. Someone suggested 2 watts, but even at that I wouldn't expect the resistor to last very long!

You might be alright with 5 watts, but the higher the better for reliability and cool running - it needs to be a ceramic wire wound type.

[_Cosmic_]

I've read that thread over and over again. I really believed it to be true as it's been on so long and must have 'helped' hundreds of people..can't it be removed or something..I absolutely trust this forum. I have been reading up on things for weeks before I eventually posted.
Anyway do you think that's why the diode has been damaged?
Does my wiring look correct apart from the resistors? They don't look burnt or damaged.
I posted an image of my modification right at the start that looks pretty clear to me.
I will get some more ordered and try again..Its not that you can walk into maplins now, everything has to be ordered and every little thing costs postage on top doesnt it....a newcomer like me hasn't got a shed full of spare components..

[Techman]

Originally Posted by _Cosmic_

..can't it be removed or something..

I agree, I think it certainly needs editing by a moderator.

I don't actually ever remember seeing that particular thread before. However, the poster that made that statement is a very knowledgeable person and wouldn't normally make a technical mistake such as that. There's an ohms law calculation that can be made taking into account the current passed by the four sections of the two valves that can be used to work out the theoretical power rating of that resistor, but even that would work out at far more than 1 watt. This resistor in this particular position is one of those cases where, for reasons I'm not going to go into, is where you basically throw ohms law out of the window, or work out a number and then treble it.

As for the diode - I would have thought that it should be fine...anyway, these usually come in packs of 10, so you should have another 9 spare!

[Half a Mullard]

Cricklewood Electronics are good for parts and are reasonably priced. As suggested a 10 watt resistor should do the trick, like this one

https://www.cricklewoodelectronics.com/10W220R.html

[Techman]

I agree, Cricklewood are very good - I've used them for components in the past.

I also agree regarding a 10 watt part!

As an observation - the diode looks like it's a bit close to that green wire wound resistor which could run hot and damage it - keep the diode well away from hot components!

[Michael Maurice]

Originally Posted by Techman

I agree, I think it certainly needs editing by a moderator.

I've edited the original thread.

[Herald1360]

Originally Posted by Techman

There's an ohms law calculation that can be made taking into account the current passed by the four sections of the two valves that can be used to work out the theoretical power rating of that resistor, but even that would work out at far more than 1 watt. This resistor in this particular position is one of those cases where, for reasons I'm not going to go into, is where you basically throw ohms law out of the window, or work out a number and then treble it.

As for the diode - I would have thought that it should be fine...anyway, these usually come in packs of 10, so you should have another 9 spare!

The reason Ohm's law doesn't appear to work here is that unless you can calculate the true rms value for the 'orrible peaky pulsating dc from the rectifier into the reservoir capacitor any power calculation is nonsense. The current through the resistor isn't just the sum of all the dc loads, it also contains the reservoir capacitor high peak charging pulses.

Reading the bits about low resistances, sounds very much like a dead (short circuit) reservoir capacitor or a short somewhere further down the HT line. That would eat the resistor and diode in double quick time!

[_Cosmic_]

Thanks to everyone for all your contributions and advice. I ordered 10w resistors from switch electronics before I saw your contributions. I will read everything through again in the morning.
Thanks guys.
I managed to order a couple of MES 6V 0.5v fuse bulbs on an auction site..One for this project and one for a spare chassis I have....just in case..quite hard to find...

[Richardgr]

Out of interest, are you able to put your hands on an old style 60W bulb, and make a dim bulb tester. I always start everything that way, and it sounds like you might need to be prepared for other issues in the power supply.

I was amazed to be able to walk into my local light shop and still be able to buy one over the counter. Hopefully still obtainable in the UK.

[ms660]

Rectifier surge limiter power dissipation calculation (approximate)

Figure out the DC load current then double that value then use that value for I in the Isquared*R power calculation formula.

Then double the result and use that as a guide for the minimum power rating of the surge limiter resistor, that way the resistor will have a happy life as will any components or other bits that are close to it.

Lawrence.