HMV 531 Restoration

[PJL]

My dissipation comment is rubbish - C25 will reduce dV/dt in choke hence limit the reverse voltage which might otherwise be 1kV+....the solution still holds though...Peter

[PaulR]

Many thanks Peter. I have removed the block and will follow your suggestion. On reflection I had already decided to open it up and so retain it with new caps. I hope SWMBO is in an understanding mood!!

Paul

[PJL]

Put it in the freezer for a day. The bitumen will then chip out or if your lucky it will fall out as a single lump. No smell or mess but clean it up quickly before it thaws.

[PaulR]

Thanks Peter

That sounds a much better plan than the hot water method I used before. I waited until my wife had gone out for a few hours but the evidence remained...

Paul

[Ian B]

I second the freezer suggestion. I found that a few gentle taps around the sides followed by a whack on the bottom (ouch!) of the block usually results in the whole lot coming out in one lump. Just remember to do it outside or over a bin. or the whole thing will probably shatter into hundreds of little pieces when it hits the floor. Probably ok if you do it over the living room carpet though.........

It's a rare and up-market (in its day) gram, well worth doing a proper job to make it sound like it did when new IMHO.

Ian Blackbourn

[PaulR]

It is in the freezer as I type!

It is indeed a lovely set, particularly as it cost £10 from a local sale room. I haven't checked the tuner chassis yet, but I hope it doesn't also have a pich block.

Paul

[PaulR]

Well, I have replaced all the caps in the block and the audio coupling cap. Most of the resistors turned out to be OK. I have connected the replacement choke in the traditional place between the smooting and reservoir caps. I replaced the original U8 rectifier by a U10 with a suitable heater dropper as U8s seem to be very rare.

On applying power via a lamp limiter it started to work so I gave it full power. It worked well for a few minutes then C25 blew. It was a 450v electrolytic, but rather small in physical size and I assume that it just wasn't up to it, rather than the being a problem caused by the replacement choke.

I am having difficulty in finding a suitable, more "beefy" cap to replace C25. I cannot find anything that says what is the maximum capacitance for a U10, but I assume it is around 4uf.

Can anyone please tell me:-

Am I likely to be right in my assumption as to why C25 blew?

Will C24 be Ok - it is identical to C25?

Where can I get a suitable replacement for C25?

Thanks

Paul

[Steve_P]

C24 is 4uF.
C25 is 2uF.

If you replace the caps with ones of identical value then you should be OK.

Cheers,

Steve P.

[PaulR]

Thanks Steve

I had assumed that the value of C25 in particular would be determined by the rectifier valve and that otherwise the value wouldn't be critical.

Is it the value rather than anything else about the cap be the reason it blew?

Paul

[Patrick Dixon]

It probably blew because it was old and knacked, and hitting it with full voltage rather than gradually winding things up was the final straw.

You need to replace it with a cap of similar value (4.7uF is close enough) and at least as high a voltage rating (greater than or equal to 450V)

[PaulR]

It was actually a brand new 450v one that blew - I had replaced the whole lot. The set actually dates from 1931 and I wouldnt dare power up caps that old!

Paul

[PJL]

Hi Paul, there are two possibilities 1) Surge voltage exceeds the 450V 2) Capacitor can't cope with the ripple current.

I think it is probably both of these as this set has a field coil in the HT path and still delivers 300V anode to the PX4's.

Were they electrolytics originally? If not I'd replace them with some 630V film ones (you can get some quite cheap PCB mount 1uF types). Alternatively you will need to check the surge voltage and if it's OK make up the required ripple current requirement using parallel electrolytics.

[PaulR]

Thanks Peter

The originals were buried in the pitch block, but I don't think that they were electrolytics.

I had always assumed that electrolytics were correct to use in power supplies.

The largest value I can find in the CPC catalogue for 630v film caps is .68uf. I could use three of these for C25 as the original was only 2uf, but C24 was 4uf which is more difficult, although there is space in the large, formerly pitch filled, can. Does this cap have to be quite so robust?

There are motor run ones, but these are rated at 450v AC.

Thanks

Paul

PS The owner has found 3 and possibly 4 PX4s in a box of old valves so we have a good supply!!

[PJL]

If they were in the box they were paper. This is what I used http://uk.farnell.com/jsp/Passive+Co...sp?sku=1200810.

[PaulR]

Thanks Peter

Unfortunately the minimum order is £20. Ill have a search for an alternative supplier.

[Nickthedentist]

Originally Posted by PaulR

Unfortunately the minimum order is £20.

I may be ordering from Farnell very soon, and could forward any extra bits to you at cost.

N.

[PaulR]

Thanks Nick

Ill keep on looking and will PM you if I get anything. If you would PM me before sending in your order I would be very grateful.

Paul

[PaulR]

I have now got some caps - thanks for the offer Nick- and have fitted them. It now works but with a powerful 50 hz hum.

I have tested the h/k resistance of the MHL4 and this reads 0.2 meg. My valve tester handbook says it should be 1m for audio frequency valves so it is suspect, but before I look for a new one is there anything obvious I should look for relating to the PX4s? I haven't touched the heater wiring, and it looks original.

Thanks


Paul

[PJL]

Before you do anything check the output transformer continuity PX4 anode to anode. It's also worth checking the cathode resistors which go from the PX4 heater centre taps.

If all is OK check the current in the PX4's by measuring the heater DC volts, I guess an AVO is best as it will average out the AC, its marked 45V on the sheet I am looking at.

Does the hum change with the volume control?

[PaulR]

PS To the last posting. If i earth the transformer end of C21 the hum completely disappears.