Restoring my first vintage television

[BassoonBloke]

Just realised youv'e already passed this hurdle (DOH!!),

Alan.

[Radio_Dave]

Originally Posted by Steve_P

Were the valves and CRT heaters lit?

As far as I could tell, yes, apart from the U25 . The neck of the CRT is covered in a fabric tape and I can't see anything, but there was a voltage there.

Originally Posted by Steve_P

Was this whistle from the Power Supply or the Line Output Area?

They're both in the same place so I can't tell.

Originally Posted by Steve_P

Measure the HT on the caps in the PSU.
Do you get any noise from the speaker when the channel changer is operated?

I've ran out of time to play for today, I'll see if I get time tomorrow to check these

Originally Posted by BassoonBloke

Looking at your original picture of the chassis, i notice that the mains dropper resister has been repaired/bypassed in the past

I've fixed the mains dropper

Regards
David

[Radio_Dave]

Originally Posted by ppppenguin

Beware of the EHT terminal. The capacitor formed by the inner and outer graphite coatings can recharge itself even when power is not applied. It's not harmful unless it makes you drop the tube.

I want to un-solder, remove and check the EHT rectifier valve (U25) because I can't see its heaters glowing. Do I need to discharge anything?

Thanks
David

[ppppenguin]

Before you try to remove the U25 have you checked for EHT at each end of it? With a well insulated screwdriver you should be able to draw an arc, maybe up to 0.5" long, from the anode. The arc should be blue-ish and vigorous. Don't earth the screwdriver! Your body capacitance is more than enough. The cathode should give a thin yellow arc, you may have to earth the screwdriver for this.

Before you touch the U25 discharge the CRT by earthing the cathode of the U25. You could remove the EHT lead fromt he CRT but remember that the CRT might recharge itself. Just discharge it again with an earthed lead before touching.

I've probably got a spare NOS U25 if you need it. PM me if necessary.

[Radio_Dave]

I can't get a spark from the cathode or the anode of the U25 and the heater is definately not lit. I guess the LOPTx is OC?

Regards
David

[tvden]

Originally Posted by Radio_Dave

I can't get a spark from the cathode or the anode of the U25 and the heater is definately not lit. I guess the LOPTx is OC?

Regards
David

this does not always mean that the lopt is oc have you first tried checking the continuity of the U25 heater, if the heater is supplied from the lopt check this winding

[ppppenguin]

Den, with respect, that's misleading. Even if the heater is o/c or the U25 completely removed you should get a spark from the anode terminal. The problem may well be elsewhere in the timebase though it could be the LOPT. Is anything overheating? If so, it could be a leaky U25 heater winding, faulty LOPT or one of several other faults. If no overheating, can you draw a spark from the LOP anode? If not then the LOP screen resistor may be high or LOP valve faulty.

[Steve_P]

Wait a minute! Don't do that yet.

The EHT won't be present unless the line O/P valve is working and has drive.

The first thing to check is the Line Oscillator. V11 is the Line Output Valve, 30P4. Check that there is a negative voltage on Pin 5. If there isn't, head back to the line oscillator stage, and check here. This is one of the functions of V7 - 30FL1.

If there is, go to pin 4 of V11. Should be 100v or thereabouts.

Rock the line hold control about, listen for any change in the whistle you hear. Set it about midway.

Try replacing V11 and the efficiency diode V13.

Check the stage carefully before you go any further.

Cheers,

Steve P

[Radio_Dave]

OK, here's the results

V11 (30P4) pin 5 = -16V and pin 4 = 140V (too high?)

Twiddling the line hold control alters the pitch of the whistle dramatically, is this good or bad?

Thanks
David

[Steve_P]

All good! Now, check the EY51 heater winding. If this is bad, change it. A little coax inner in the sleeving should do.

If this is OK, can you draw a spark from the Anode of the EY51. If yes, change the valve.

If no, well inspect the LOPT for bad joints.

Cheers,

Steve P.

[Radio_Dave]

Checked the heater on the EHT rectifier valve and the continuity of the heater windings on LOPTx and they're both OK.

I powered up the telly and gave it a longer run, but I was only rewarded with smoke. The mains dropper is so close to the LOPTx there's a chance the smoke came from there, but I don't think I'm that lucky

Regards
David

[Steve_P]

Hmm - this sounds LOPTish to me. Do you still get the line whistle?

Have you used powerful enough dropper resistors on the dropper? If not, one might have given way a bit.

Replace R71 and see if that 140v is dropped back down. If not, then I think you need a LOPT rewind. Happens! If so, remove the LOPT, make good diagrams and if you've got a camera, pictures. There are two people who do rewinds:

Ed Dinning and Murphymad (Mike Barker).

Cheers,

Steve P.

[Radio_Dave]

Yes, still got the whistle but it sounded lower and a bit shaky. This has started since I removed and replaced the U25, so I can't help thinking that I've messed something up!

R71 measures spot on and looks healthy so I think it's ok.

It bothers me that I can't see the heater glow in the U25, or would I see anything (it's only a 2V heater)?

One more question (sorry), how does the EHT lead connect to the tube and how do I remove it? It seems to disappear into a hole in the crt, but it's very loose and sloppy.

Thanks
David

[Steve_P]

Is there a metal cover over the tube?

The connection you refer to goes into a socket on the tube face. There should be a rubber cover over the plug and the wire to the LOPT.

Get an insulated screwdriver, push it under until it makes contact with the EHT contact. Push and lift the connection off. Takes practice!

Then get the transformer out and get it seen to!

Cheers,

Steve P.