DER 5649

The picture you have shows the classic signs of a dead cathode decoupler in the frame output stage...

I know, but there isn't one. The cathode goes straight to ground on the frame output

Huh?! Oh, hang on, I remember this one.

IIRC the idea is that the heaters are fed via a diode acting as a wattless dropper, and by deriving the bias from the heater chain it would show up as a massive hum bar if the diode went short, thus alerting the user to a fault condition.

This arrangement was also seen in the 1400 chassis, a good idea in principal but if the heater rectifier went s/c most people would ignore the rolling mess on the screen and continue to listen to the sound resulting in severely overrun heaters. The later 1500 went one better where the LT supply for the tuner and IF strip was obtained from the bottom end of the heater chain. If the rectifier went s/c you lost the LT supply which removed both picture and sound leaving a blank raster.

John.

Excellent results, well worth saving this set
Don't come across many DER sets
Well done !

Chris

Excellent results, well worth saving this set
Don't come across many DER sets

Chris

Furthermore, I can't imagine there were many 405 line only sets still being produced by 1969.....this must surely be one of the last?

Great result! I'm sure I was told that these were one of the last 405 only sets. They are really good picture wise, and stay stable for hours on end, in fact I had one running all day at a vintage show in York a few years back, it worked faultlessly all day!

Regards,
Lloyd.

There is an account of the one I restored here.

Much the same sort of problems. Mine is still working well but I think I may need a new boost diode.....

Thanks Sideband, an interesting post and yes, very similar faults.

I also had soft focus (hard to see in the previous photo's) and it turned out to be a 1meg (R99) supplying boost HT to the focus taps that was open circuit.
I also found the width kept on increasing as the set warmed up and this turned out to be R106 and R107 that had gone quite high in value, a couple of 2m2 from the wiper of the width control.
Its now stable after replacing them.

The picture is more or less sorted now, but I have an unusual problem regarding the sound.

The primary winding of the sound output transformer is intermittently open circuit.
Placing a few volts DC across the primary, with the set switched off of course, there is crackling from the speaker.

The primary is meant to be 600 ohms, but a cold resistance check reads open circuit.
Bother !!

Check the leads from the tags on the transformer incase they're dry jointed. I had similar problems with a frame output transformer once, finding it cured an intermittent frame collapse problem.

Excellent result, well done.
Brian

Thanks Brian,

I did take the speaker out with the transformer, but it is certainly a dud primary
(You can see it in situ in my third and fourth pics at the beginning of the post).

Will have a rummage round and see if I can find something to replace it.

Unfortunately, I don't have anything suitable, so rather stuck at the mo.

I have a scrap radio chassis that uses a UL84 output valve, I don't know the sound output valve in the TV but if the TX I have is a possibility I can check the windings to see if they are intact. If its good and any use I can post it.
Frank
Edit, if you are going to run the set any length of time with the o/c TX I would remove voltage from the output valves screen grid or you will need a new valve as well.

These transformers seldom if ever fail but there is a first time, especially through age! If the problem is intermittent it could be close to the tags. It would do no harm in going over them with a hot iron. As I mentioned earlier a stubborn case of intermittent frame collapse I experienced was cured on doing this to the frame tx.

I have a BRC 1400 sound output transformer I could send you if you need it.

Cheers
Brian

Do you have a close photo of the transformer?
I'll have a look to see if I have something that will do the job

Chris

Thanks chaps....

I will post a photo of it later when at home this evening

Transformer failure as far as o/c primary is unlucky and not that common, as suggested no harm in checking the lead-out wires from the bobbin to the tags.
However I have found quite a few speaker transformers faulty in radio and record player amplifiers. Not o/c but causing low level rustling noises when the amp is running at low volume. I have had this at least three times over the years, once in a DAC90a which was very intermittent.
I would say any transformer that uses the same output valve would do providing you can mount it on the speaker.
Rich.

Thanks for all the kind offers everyone.
Focus 2 (Brian) is kindly sending me a tx from a 1400 chassis, so i'm hoping it will fit
The hole to hole gap is approx. 5.5cm
Here is a picture of the duff tx....

I would say any transformer that uses the same output valve would do providing you can mount it on the speaker.
Rich.

That could be rather tricky as the sound output valve is a PCF80 believe it or not.
Why on earth they didn't use say a PCL83, I cannot imagine.

I would say any transformer that uses the same output valve would do providing you can mount it on the speaker.
Rich.

That could be rather tricky as the sound output valve is a PCF80 believe it or not.
Why on earth they didn't use say a PCL83, I cannot imagine.

If you think that weird, my HMV 1824 (TV set) uses an EF80 as its audio amp/output valve! So that's an RF/IF amplifier valve doing the job of both, AF amplifier, and AF output in one....

At least a PCF80 looks a bit like an audio amplifier valve!

I guess you can get away with it for low power audio use.
In a small set, the need for a massive volume and floor shaking bass is simply not there lol.

Here's the sound output transformer from the 1400. Hopefully it'll be ok. The speaker connection tags are at the far right and left tags.
I'll get it in the post tomorrow.

Cheers
Brian

Thanks Brian

Today, I removed the tube and cleaned all cosmetic parts, re-fitted and it looks rather smart in the dark green rexine.
Got Mrs Mills to tinkle the ivories for me
Shame I can't hear her lol.

(By the way, I have put a temporary resistor across the primary to save the pcf80 from damage )