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Old 15th Aug 2015, 8:17 am   #1
sobell1980
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Default HMV 653 Broadcaster all wave.

Looks like another long term project is about to make its way onto the bench. I purchased this HMV 653 a couple of years ago, it has been on the "to do " pile all this time as I thought after buying it it was beyond my level of ability at that time. However after completing a few valve sets now and alignments I feel ready for it.

I have removed the chassis and faced with the insulation on the wiring crumbling and exposing the wires. I then saw the mains transformer . It looked like the insulation and it's wrapping had broken apart and blistered exposing the windings. On a further poke and prod, it looks what I thought were the windings exposed is actually where the transformers insulation wrapping is tied together?
I have included some photos of this and of the chassis. If anyone would like to advise and comment further or has any experience with this set. Many thanks.
Dave
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Old 15th Aug 2015, 8:20 am   #2
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Default Re: Hmv 653 broadcaster all wave.

Some more pictures.
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Old 15th Aug 2015, 9:05 am   #3
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Default Re: Hmv 653 broadcaster all wave.

I love these old radios, they always look a mess as usually through there life there has been quick repairs (bodge)? with component connections cut off and left with a Christmas tree of components hanging in the air, they are usually quite easy to repair as long as you do not jump from one circuit to another, if the radio is in a bad way it pays to replace all the deteriorated wiring, most times wires attached to a valve base have been wrapped as well as soldered, I use a solder sucker to remove solder then with a small pair of cutters you can snip the wire loop and remove without doing any damage. The valve top cap, you clean the wire carefully then wrap thin coil of wire around it and solder with aluminium cored solder,( if the wire is long enough not a problem),then clean up the cap and refit with araldite and solder, do not use super glue. The transformer will more than likely be OK, if you have an insulation meter (megger)
you can always check it out.
John
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Old 15th Aug 2015, 11:09 am   #4
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Default Re: Hmv 653 broadcaster all wave.

Looks like the set had some rat damage, they are quite fond of some waxy insulated parts! The valve is nicely savable as John has described above, the mains TX also should be fine You can bathe it in varnish if you are worried, a good set to listen to
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Old 16th Aug 2015, 12:10 pm   #5
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Default Re: Hmv 653 broadcaster all wave.

Well, I started off yesterday by doing just some cold checks on the mains transformer. It looked in a bad way visually as you can see from the photos. Thankfully what I thought were some windings showing through the blistered insulation / paper were in fact just string ties, tieing the two halves of the insulation paper together. A light going over with a paint brush cleared the debris and muck away , leaving in front of me what I had to test. It was thick with dirt.

Firstly I tested the primary of the mains transformer which gave a reading of 35 ohms. Great! Close to what the service data quotes.
Testing the primary to chassis ground gave a reading of open circuit.

I then went onto test the various secondarys of the mains transformer.
The secondary which supplies the HT tested at 665 ohms end to end. And, 339 ohms from one end of the winding to its centre tap and 325 ohms from the other end to centre tap which makes sense. I then tested the HT secondary to ground and had a reading of 600 ohms from each end of the winding. From its centre tap to chassis ground I got 273 ohms. Ok, so no direct shorts.

I then tested the secondary winding for valve 5 cathode for the rectifier. 0.3 ohms was my reading across the winding and 53 k ohms to chassis ground. Again no shorts to ground.

I tested the secondary winding then for the dial lamp and valve heaters. My reading was 0.3 ohms across this winding. To chassis ground I also had a very low reading of 0.3 ohms. I worried but looking at the diagram this winding is very closely connected to ground itself so this I would assume is ok?

I then noticed a large square empty space on top of the chassis where something previously occupied, there was an edge line in the dirt and dust like something had been removed. This is diagonally opposite to the mains transformer. Looking at the diagram this is where C7,C11,C25,C26 and c28 would have been housed in what I can only guess as a large cap storage box. Checking under the chassis these electrolytics have at some point been changed and tied away with string and relocated under the chassis. One of which is the size of a cigarette packet 8 uF 500v! This one looks original but the other huge tubular type look like they have at some point been changes and put underneath the chassis. The large 8UF was tied up with twine and others are disconnected and swinging in the breeze! I have included a picture of where this cap box would have been located.

Seeing now my cold checks of the mains transformer appear to be ok I will now start replacing all the wiring. All of it is falling apart and crumbling. I don't think any of it will be safe to be left. Anyway, that's my progress, slow but sure but it would be pointless diving in changing caps and wiring when something as fundemental as the mains transformer is not ok. Just going to follow the circuit and power through to start with and get all that changed. I'm sure I will find other horrors on the way.
Many thanks.
Dave.
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Old 16th Aug 2015, 1:18 pm   #6
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Default Re: Hmv 653 broadcaster all wave.

You might want to make up a capacitor box to the original pattern if you can find a picture of one.

An alternative tidy solution would be to fit them all in a small ABS box and mount that in the same place as the original was.

Good luck.

Lawrence.
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Old 16th Aug 2015, 1:48 pm   #7
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Default Re: Hmv 653 broadcaster all wave.

Thanks Lawrence. Yes when I get that far I will consider the options you kindly mentioned . For now it's cleaning and rewiring.

Is everyone concurring with my main transformer measurements?
Many thanks
Dave.
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Old 16th Aug 2015, 2:00 pm   #8
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Default Re: Hmv 653 broadcaster all wave.

Your transformer resistance measurements seem ok to me, hopefully the transformer will be ok under normal voltage and loading conditions..So far so good.

Lawrence.
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Old 16th Aug 2015, 11:44 pm   #9
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Default Re: HMV 653 Broadcaster all wave.

Hi Dave
The capacitor box contains C10/11/13/25/26, modern equivalents would be
3 x 4.7uf, 1 x 8uf, 1 x 16uf, these could all be mounted on a tag strip and placed in a painted Colmans mustard tin to make it look authentic, (just an idea)?
John
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Old 17th Aug 2015, 9:40 am   #10
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Default Re: HMV 653 Broadcaster all wave.

Very nice John.

Good luck with that set, Dave. I would remove the valves, check for obvious shorts on the secondary and than apply mains (via a modern 3-core flex and 1A fuse and RCD) to the transformer to see if it overheats or trips the RCD before spending ages on the rest of the set.

Nick.
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Old 17th Aug 2015, 6:51 pm   #11
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Default Re: HMV 653 Broadcaster all wave.

Hi Nick,

Thanks for the interest in my new project. I have checked the secondaries in depth as much as possible if you want to take a look at my findings in post #5. Only one of the secondaries for the dial lamps and valve heaters concerned me with a reading of 0.3 ohms from the secondary to ground. But, looking at the diagram one on the ends of the secondary is very close in regards to chassis ground so I thought this to be ok.
Regarding your suggestion of powering up the mains transformer, yes I have already soldered in a new 3 core flex with a 1 amp fuse. However, there probably is not one piece of wire on the set that the insulation hasn't crumbled ex posing the wire to others or short to ground. It really is that bad. Are you suggesting that I disconnect all of the secondaries from the set so in effect the mains transformer is running out of circuit? ?? This would kind of test the transformer in a stand alone situation? I would then be able to test the output voltages from the TX gor the three separate secondaries . There is no way I could safely test the transformer in circuit at the moment even with the valves removed. I could however disconnect all the secondaries and power it up that way. Many thanks Nick. Thanks also to all so far who have contributed to the repair with some very worthwhile ideas.
Regards,
Dave.
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Old 17th Aug 2015, 7:25 pm   #12
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Default Re: HMV 653 Broadcaster all wave.

Hi Dave, Nick is exactly right when he says remove all the valves and then basically give it the mains. You’ve done as many DC tests as you can and by their very nature they will never really give conclusive results, just giving you only a very rough idea of the state of the windings. If you can confirm that your radio has a VALVE rectifier, then it will be quite safe to remove it for the transformer test, as this will isolate the whole of the HT system in the receiver. A radio which you wouldn’t do this type of test with would be one with a ‘solid state’ type rectifier like for example those big Grundig radios from the 1950s. With these radios you would have to do some un-soldering of connections from the rectifier.

It’s a great shame you’ve cut through that original lacing and insulating paper on that transformer. What you should have done, after confirming with these tests that you’re about to do that the transformer is electrically sound, would be to blow and carefully brush all the muck off it and then if you wanted to tidy it up then treat it with some insulating varnish as was discussed recently on another thread.

So, as said, remove ALL the valves – you could leave the dial lights as they’ll act as an indication when you apply power. If you have a Variac, you could use this to wind the power up slowly with an old AVO on a current range in series to monitor anything funny going on with the current. You can only use an analogue type meter for this type of test as you can’t follow digital numbers properly and fast enough to be able to spot a problem quick enough. As nick says about using a 1 amp fuse, yes, if you have one, but a 3 amp will blow quick enough if there’s a short.
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Old 17th Aug 2015, 7:40 pm   #13
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Default Re: HMV 653 Broadcaster all wave.

As an additional note on the above (I thought I’d put this in a separate post). Don’t put an AVO or any other ammeter in series with the supply if you’re just going to apply straight mains. Only monitor current when winding up the voltage slowly from zero with a Variac. Obviously, you’ll need the meter set on an AC current range. Start with it on the 1 amp AC range and if all seems well, you can then reduce the range. With all the valves out and a good transformer the current will be VERY low. A burnt out transformer will very quickly cause the reading to rise to several hundred milliamps as the Variac is increased. If you were to put a meter in series with the supply when connecting straight to 240 volts mains, you would run the risk of blowing up the meter long before a fuse would blow if there happened to be a fault.
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Old 18th Aug 2015, 12:19 am   #14
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Default Re: HMV 653 Broadcaster all wave.

Sounds like a useful exercise for a lamp limiter to me........
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Old 18th Aug 2015, 10:41 pm   #15
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Default Re: HMV 653 Broadcaster all wave.

To minimise the amount of copper, mains transformers are designed with a primary inductance that will draw current even when there is no load.

I would go with Herald1360 suggestion and use a lamp limiter rather than a Variac and meter. To confirm there is no major earth leakage I would also suggest you connect the chassis to mains earth and use an RCD protected mains socket for your test.
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Old 19th Aug 2015, 9:22 pm   #16
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Default Re: HMV 653 Broadcaster all wave.

Yes, of course Dave could use a series lamp to limit the current. I don’t tend to use one as I’ve always had Variacs to hand and with a meter in line I can see exactly what’s going on.

It’s not quite true that I never use a lamp limiter. I don’t have a purpose built one made up, but sometimes crock clip a bulb in series with the supply if I need one. An ‘off topic’ example of this was when the 3 speed switch control unit for the central heating pump totally burnt out and all I was left with was a set of blackened wires emerging from the body of the pump. I needed to ‘hard wire’ it to work on a fixed speed to get it up and running again and making a wiring mistake and burning out the pump windings was NOT an option. I seem to remember that something like a 100 watt bulb worked very well as a limiter as I confirmed the correct connections.

I can’t remember whether Dave has a Variac or not, so using a ‘lamp limiter’ would be as good as anything to prove the serviceability of his transformer and I should have mentioned it in the above post, but forgot.

I would advise testing the transformer in position and not go disconnecting the wires as these may be a bit ‘delicate’ where they enter the transformer and the poor old thing has already had enough stick in that respect. I think there’s a very good chance that the transformer is perfectly ok. As you get more experienced with these things and have seen a lot more old radios, you’ll realise that most old transformers in old radios tend to look like the one you have there, so you can’t always go on looks, although you can sometimes go on smell.

I’ve got a burnt out transformer kicking around under the bench that’s come out of someone’s radiogram and I should have chucked it ages ago. If you really want typical ammeter readings from a burnt transformer, then I can always connect it up with the Variac and Avo and report the readings – I seem to remember that it was around half an amp at full mains – ‘fizzling’ and getting very hot, with a little smoke, I think! I could then do the same with a good transformer off load and report the reaings – as said, it’ll be something like just a few milliamps.
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Old 23rd Aug 2015, 6:54 pm   #17
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Default Re: HMV 653 Broadcaster all wave.

Good evening all.
Thank you all so much for your replies to this thread. Sorry for not replying sooner, I have been visiting my friend in hospital. Your interest in the project is greatly appreciated.
Well today I rigged up a very crude lamp limiter on some crock clips, valves removed and power applied. No fuses blown or nasty smells or overheating of the mains transformer. I then applied full mains, again with valves removed. I could hear a slight rapid clicking from the mains transformer as if I could hear the 50 Hz, a bit like what you hear standing under a mains pylon! This disappeared after a short time to silence. With my meter in circuit I measured the current draw of the mains transformer over a period of 5 mins. It was a steady 94 milli amps with full mains. This was measured in series with the live lead to the mains transformer with my meter obviously set to measure ac not dc. I'm happy with this reading over 5 mins or so. The transformer did not get hot or smell and the mains plug fitted with a 1 amp fuse.
I m happy to continue with the rewire if we all concur with my findings?

Techman, I appreciate your valued guidance with the set and your advice with the mains transformer. I would like to say I have not cut the paper insulation, this is where the two halves of paper insulation were braided together. This is going to carefully be braided back together now I have seen the error of my ways!!! Plz forgive a beginner. This is the first time I had experienced a mains transformer in this state. As you can see from the photos, the two halves of insulaton still have their holes to be braided together and have not been cut. I have taken note and heed of your advice for future reference.

Many thanks as always.
Dave.
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Old 23rd Aug 2015, 7:24 pm   #18
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Default Re: HMV 653 Broadcaster all wave.

Hi Dave,

I think I was looking at the paper below the laced card, but I’m sure you’ll be able to make it all good again.

I’ve got to be honest and say that 94 milliamps seems a bit high. When I read your post I just grabbed a couple of random transformers from under the bench and got readings of around 25 milliamps with a dodgy old Avo 7 that happened to be handy. I then tried that known burnt out transformer and got around 400 milliamps. It was a very quick check and I’ll go back in a bit and verify the accuracy of the AC current ranges on that meter. I wouldn’t worry too much as the transformers I tested were a fairly modern (1970s) Douglas and a fairly large ex-military high quality impregnated type, whereas yours is very old and likely to have a lot more losses than these.

I think you’ll be alright, but see what others think.
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Old 23rd Aug 2015, 7:36 pm   #19
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Default Re: HMV 653 Broadcaster all wave.

This reading was taken with my expensive automotive snap on calibrated DMM, if that makes any difference to the readings. Thanks Techman for your reply, let's hope for the best.
Dave.
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Old 23rd Aug 2015, 7:40 pm   #20
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Default Re: HMV 653 Broadcaster all wave.

Just as an addition to the above, when there’s shorted turns, the reading generally won’t stay steady, but will vary as things heat up, so a steady reading over five minutes is good.

The Avo is not far out and just reading a few milliamps low.
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