Dynatron Henley RG33W radiogram

[Michael Maurice]

I'm with Jule and Peter on this one. The transformer is expensive to rewind if it fails and you want to avoid this. I'm in Wembley, so just round the A406 from you, as I said I've got to go to Wanstead/Snaresbrook in the next week or so, let me know if you want help.

[Edward Huggins]

As this thread has now reached its 7th page, yes, do get some help as you seem to be really struggling with this. It's a lovely Gram and well worth saving. Edward

[ms660]

If the OP takes it to service engineer, then the engineer would probably do the test suggested in post# 108, it's standard procedure in cases such as this, isolate source and load then do the respective measurements.

Good luck whicheverways.

Lawrence.

[Vintage84]

Evening all.

I've now spent a few hours examining the wires and cleaning out debris in the chassis, before following instructions in post #108.

With wire link reinstated between tags 1 and 4 and with the brown wire unsoldered from tag 5, I get a reading of 7.3v between tags 1 and 5 (powered on for a few minutes with no sign of any trouble).

The resistance between the brown wire and tag 1 is around 3ohms.

[peter_sol]

If you have all the valves out including the magic eye and ECC85 then you have a short from the brown wire to chassis. you shouldn't have any reading.

[ms660]

Originally Posted by Vintage84

With wire link reinstated between tags 1 and 4 and with the brown wire unsoldered from tag 5, I get a reading of 7.3v between tags 1 and 5 (powered on for a few minutes with no sign of any trouble).

The resistance between the brown wire and tag 1 is around 3ohms.

Ok, so the transformer still survives so far.

As has already been said, there should be no resistance reading between the end of the disconnected brown wire and tags 1-4 on the transformer, you are getting a reading of 3 ohms and you stated that all the valves have been removed for that test but can you confirm that any scale bulbs have also been removed for that test? This is important.

Also I don't know if this has been covered but the ECC83 valve is a valve that can be operated with either a 6.3 volt heater supply or a 12 volt heater supply, for 6.3 volt operation (as should be the case here) pins 4 & 5 would be connected together, those two pins connected together will be connected to one side of the heater supply and pin 9 will be connected to the other side of the heater supply.

Lawrence.

[Vintage84]

I think I've finally cleared the short - no reading between brown wire and tag 1!

I've refitted some valves and they light. Do I now refit them all and scale bulbs and let it run for a while before fitting the rectifier?

Thanks for your support and patience!

[ms660]

That's good news so far, did you identify the cause of the short? If so what was it? This is important.

EDIT: I would switch the power off for now.

Lawrence.

[Nickthedentist]

Yes, looking good, but you need to ensure that whatever caused the short doesn't rear its ugly head again, or it could be curtains for the transformer.

BTW, sometimes a "beep" continuity tester and croc clips on its test leads can be really handy for tracing shorts like this: you just have to fiddle around until the beeping stops, and when it does, you know you're in the right area.

[Vintage84]

I found a couple of areas on the wire where insulation had worn through and I also had a dodgy connection on one of the scale lamps. I can't be 100% which was the issue though as I did all the repairs before testing.

[ms660]

Both those reports sound credible, so let's hope and fingers crossed, do not apply any more power yet, there are some capacitors that will need replacing on a unit of this vintage, these capacitors can go electrically leaky and cause the audio output valves to draw excessive current which can damage the transformers in the unit.

The principle ones to replace are what's known as the grid coupling capacitors to the output valves, they are connected between the control grids of the output valves and the anode circuits of the preceding stages.

So far as I understand it the output valves in this unit are ECL82's, this is two valves in one glass envelope, one is a triode audio amplifier the other a pentode audio power amplifier (the output amplifier) the audio power amplifier part of that valve is what drives the output transformer which in turn drives the loudspeaker.

So...To locate these grid coupling capacitors....quite straight forward with a schematic but if no schematic then you can refer to a valve data sheet to identify the control grid etc, this grid is referred to as g1 on the data sheet.

Here's the data sheet for the ECL82:

http://www.r-type.org/pdfs/ecl82.pdf

It shows the electrical representation of that valve and the valve pin numbers as viewed from underneath, you will see that pin 3 is g1 (the control grid) of the output pentode (pentode has 5 electrodes)

You will also see that pin 9 is the anode of the amplifier triode (triode has 3 electrodes)

Identify these pins by the tags on the valve socket (as viewed from underneath) There should be a capacitor connected between valve socket pin 3 and valve socket pin 9 for the ECL82 valve(s) It might or might not be a direct connection, if its not directly connected to tag 3 it will more than likely be connected via a resistor, that's not a problem, what's important is that the correct capacitor(s) are identified, a picture of the underneath of the ECL82 valve socket would help us to confirm.

It's important that these capacitors are replaced, even if they test as no leakage on a test meter, very important.

Before you attempt to replace them in the absence of a schematic it's important to note the capacitors value in microfarads (mfd or uf) and also the capacitors maximum working voltage, these two parameters will be marked on the capacitor somewhere, I say this because trying to remove some types of old capacitors can cause the outer casing to break up and fall apart along with the said information contained thereon.

This might sound all a bit of a drag to do but it's not really.

There will be other capacitors to check as well so if the grid coupling capacitors can be identified then leave ordering them until the other suspect capacitors of this vintage have been identified then you should be able to get all the bits in one go, they're not expensive.

Just to say again, do not apply any more power yet, important.

This post might or might not cross with others.

Lawrence.

[peter_sol]

All above advice good but the capacitors used in your set may be of a type which don't need replacing
Measure the voltage on pin 3 of the ECL82's

[Vintage84]

Hi Lawrence,
We haven't been able to find a schematic for this particular model.
The capacitor between pins 3&9 on the ECL82 is one of the red/yellow/black Plesseys that someone mentioned should be replaced earlier in the thread. Which other capacitors should be checked? And where would you recommend purchasing replacements from?
Thanks

[ms660]

You might have made a mistake, the red/yellow/black Plessey's are electrolytics, electrolytics would not be used for the control grid coupling capacitors, but they would be used to bypass the cathode resistor, the cathode resistor of the pentode output section of the ECL82 will be connected to pin 2 of that valve, that's tag 2 on the valve holder, counting clockwise from the gap between the tags (1 and 9) as viewed from underneath the chassis

A photo or two should help clarify one way or the other.

Lawrence.

[peter_sol]

If you read post 132 again my advice was to measure the voltage on pins 3 of the ECL82's could you do that and report back please.
you may not need to replace any caps.

[ms660]

There appear (to me anyways) to be some black Hunts type capacitors in this unit, if Hunts are used for the output valve grid coupling but a measurement of 0 volts is shown on the control grid I would still replace them.

Lawrence.

[Vintage84]

No voltage recorded across pin 3 of ecl82.

Pin 9 connects appears to connect to the red one in the first photo.

There are a couple of small black ones, one of which doesn't look too healthy (photo 3).

[Clydeuk]

That LEMCO capacitor doesn't look healthy at all, looks like a small electrolytic is the printed value 23uF? seems an odd value maybe I'm not reading it correctly. If it is a 22uF would be the modern equivalent.

['LIVEWIRE?']

I'd certainly replace that black Electrolytic in photo 3, and any other similar ones. A modern 22 or 33uf, 25vDC Capacitor will be suitable. Ensure you connect this, and all Electrolytic capacitors with the correct polarity In valved equipment the Negative terminal of Electrolytic caps. usually connects to chassis. Replace on sight all those Red & Yellow Plessey Capacitors, and, as others have said, all Hunts capacitors

[Clydeuk]

Just to add it's a bit of a rat's nest in there, if you're not too confident I would suggest you consider the offer of help above. It would be very easy to make a mistake amongst all that, not to mention you may be needlessly replacing some components, although obviously any capacitor that has vented as in your picture will need replacing.