Bush A774

[EnyaFox]

Hello everyone!

I am having trouble with restoring this A774 Chassis. I had first powered it up but got no EHT and sound. I had done some research and looked in a servicing book. I had found out that sound will not work as the transistor stages are powered off a 20v line from line output transformer.

The EHT rectifier has been replaced with a rod. The capacitor and the resistor ontop of the transformer seem out of place. I have had a look at a few photos and have struggled with figuring out where they go and I have attached an image.
The PL504 seems to also be red plating but only when it's top cap is on.

The whole TV was recapped before I had obtained it and they seem to be the right caps.

[Welsh Anorak]

Did it work before you got it? I'm always supicious of 'recapping', especially as on this chassis there are only a couple I'd change.

Check the boost capacitor (0.1 uF, 1000V) for a short, then if OK disconnect the EHT stick and see if the valve cools down.

Now check for a healthy negative voltage at the control grid of the PL504. You can disconnect the screen grid feed resistor (2k2,5W) to stop the 504 getting angry while you're testing. Then, I'm afraid, it's starting to look like everyone's favourite A774 component - the transformer.

[EnyaFox]

Also to note, there is some sort of popping noise in the speaker when the PY88 cathode is connected. It does seem to also come from something else.

[EnyaFox]

Originally Posted by Welsh Anorak

Did it work before you got it? I'm always supicious of 'recapping', especially as on this chassis there are only a couple I'd change.

Check the boost capacitor (0.1 uF, 1000V) for a short, then if OK disconnect the EHT stick and see if the valve cools down.

Now check for a healthy negative voltage at the control grid of the PL504. You can disconnect the screen grid feed resistor (2k2,5W) to stop the 504 getting angry while you're testing. Then, I'm afraid, it's starting to look like everyone's favourite A774 component - the transformer.

The TV wasn't working when I got my hands on it. I have checked the boost capacitor, that's not shorted. Disconnecting the EHT stick doesn't cool the valve down. The resistor did desolder when testing without the EHT stick

[gallowfields]

Is that the correct LOPTX? All the A774's we had were fitted with a DY87 and the problems we had were with the heater winding to it. Used to replace it in the customer's house. It looks like it's out of a later version which could explain your thoughts about the resistor and capacitor mounting.

[FERNSEH]

The Bush A774 chassis employs a sine wave line oscillator transformer, this component has been known to fail. A high-slope RF pentode type EF184 used as the oscillator valve.
The line output transformer in your set is a later service component. Even later around 1980, replacement line output transformers employed an EHT tripler.

DFWB.

[EnyaFox]

Originally Posted by Welsh Anorak

Did it work before you got it? I'm always supicious of 'recapping', especially as on this chassis there are only a couple I'd change.

Check the boost capacitor (0.1 uF, 1000V) for a short, then if OK disconnect the EHT stick and see if the valve cools down.

Now check for a healthy negative voltage at the control grid of the PL504. You can disconnect the screen grid feed resistor (2k2,5W) to stop the 504 getting angry while you're testing. Then, I'm afraid, it's starting to look like everyone's favourite A774 component - the transformer.

The voltage on the control grid is around -70V DC
Would you know how that would be fixed?

[EnyaFox]

Nope, now back down to -30v

[its ur aerial]

-30v drive, from memory sounds about right , and if the Boost cap's OK, I would then suggest that, Glyn in correct...LOPT, they were a very common failure, back in the day, sometimes it was just the over wind to the DY86/7 that cooked up, but obviously not applicable in this example using a Stick Rec.

Ken G6HZG.

[Hybrid tellies]

If you are getting -30v line drive and the boost cap and stick EHT rectifier are not causing the overload then it’s looking like it’s the LOPT. But there is a few things you should check before condemning the LOPT.
With the TV switched off check the resistance from the top cap of the PL504/PY88 to chassis. If you are seeing a short circuit or low ohms reading the high voltage tuning capacitor could be short or leaky. It’s connected between the top cap of the PY88 and chassis. It will probably be mounted on the LOPT itself. This capacitor only used with line output stages that use EHT sticks, doublers or triplers.
Another thing we would check is the S correction capacitor connected between the LOPT and scanning coils. If it goes short circuit it produces a low resistance to chassis via the scanning coils.

[EnyaFox]

Originally Posted by Hybrid tellies

If you are getting -30v line drive and the boost cap and stick EHT rectifier are not causing the overload then it’s looking like it’s the LOPT. But there is a few things you should check before condemning the LOPT.
With the TV switched off check the resistance from the top cap of the PL504/PY88 to chassis. If you are seeing a short circuit or low ohms reading the high voltage tuning capacitor could be short or leaky. It’s connected between the top cap of the PY88 and chassis. It will probably be mounted on the LOPT itself. This capacitor only used with line output stages that use EHT sticks, doublers or triplers.
Another thing we would check is the S correction capacitor connected between the LOPT and scanning coils. If it goes short circuit it produces a low resistance to chassis via the scanning coils.

Everything seems to check out unfortunately...
I guess the LOPT has gone bad as I did hear some sort of arcing for the first time today.

Is there an easy replacement for it?

[dazzlevision]

The only easy replacement for the A774 chassis LOPT is the correct part for this particular chassis.

Here is a list of the versions that I have come across:

A774 chassis (DY802), A775 (made by RBM) - as originally fitted

A774 chassis (DY802), BEAB approved Z734/Z735 (made by RBM) - modified type, to meet British Electrotechnical Approvals Board (BEAB) safety standards

A774 chassis, with ITT type 11TEZ solid state EHT doubler (might have been produced by RBM)

A774 chassis (with EHT doubler), "LOPT 4", (made by Ampmace, Coventry)

A774, Willow Vale Electronics stock number 26-021A (a pattern part), with a solid state EHT "stick" rectifier, instead of a DY802 valve.

[EnyaFox]

As I probably wouldn't be able to find a replacement, what would be the best way to continue? I don't know if I should try and rebuild/rewire it or not.

This is the first valve/hybrid TV I have worked on. I do know the dangers and high voltages associated with these TV's. I know how to work safely with them.

[Welsh Anorak]

I think rewinding it would be very difficult because of the construction.

I know some forum members have had success using a Thorn jellypot from a 1500 chassis to replace faulty LOPTs. As this is also a single standard TV it should be possible, but you'll need to do some studying of the relevant circuit diagrams to get the puls feeds and third harmonic tuning correct. Also the LT supply would be best sourced from a separate supply.

[dazzlevision]

Originally Posted by EnyaFox

As I probably wouldn't be able to find a replacement, what would be the best way to continue? I don't know if I should try and rebuild/rewire it or not.

Try a request for an A774 LOPT in this Forum's "wanted" section.

[Freya]

Mark Hindes is listing a NOS LOPT for the 774, http://markhindes.easywebstore.co.uk/A774_A3Z4E.aspx

[EnyaFox]

So, I have obtained another A774 TV, I hope the LOPT isn't too much of a hassle on this one. It will either need a full restoration or use both TV's to make a working set out of the two.

[slidertogrid]

These Transformers were truly dreadful even when the sets were still in service. The Heater winding Loop would arc to the frame, the overwinds failed like many other RBM LOPTs, But on the A774 even the primary would go shorted turns occasionally. I used to turn the boost/width pot down as low as possible and use the raster correction magnets to get the picture to fill the screen even if this was at the expense of straight verticals at the sides of the screen. The owner usually didn't notice that but they did notice if the LOPT failed and the set went off! Great sets to have in a collection now though!

[EnyaFox]

So I powered the new A774 using a lamp limiter. Lamps slightly lit but I get this. I know the frame timebase isn't running at it's correct speed but I've got sound and a picture.

[Welsh Anorak]

That's great - and surprising - news! Which do you think is the better set, cabinet and tube wise?
Oh - I'd snip that mains filter capacitor out - big bangs aren't good for the nerves...