1500 line tearing habit & other faults

[dazzlevision]

Hello,

Lack of width:

Is the HT correct (pin 9 of PY81 should be about 245 volts)?

The "S" correction capacitor may well be s/c - C90 (0.1uF, 400V)

The high value resistors in the width control circuit tend to go even higher with time - R130 (330k); R131 (1.8M); R129 (2.2M). The width pot is also suspect (R132, 1M). Later sets replaced R132 with a 2.2M pot and changed R130 to 680K - to reduce dissipation in the pot.

Also check R64 (39k), in the line blocking oscillator circuit - it may well have drifted very high).

I'd also replace the PL504's screen decoupler with a 0.1uF 400V. This is a mod, to replace the 1uF 350V electrololytic, which tended to "dry out" very quickly.

It is conceivable that the wrong type of EHT tripler may have been fitted. These sets had a different type between 20" (doubler - 15kV) and 24" (tripler - 20kV) sets. Although, I think there was an "interlock" between the line output transformer moulding and the tripler, to prevent the wrong type being fitted (if my memory is relaible!). Certainly, the LOPT was different between the two screen sizes, so watch out (15kV has a pink or green spot, 20kV has a white spot.

Interference lines/line pulling:

Apart from making sure the CRT's graphite coating is connected to the long spring, ensure the black wire soldered to it is plugged onto the CRT base PCB (push-on pin 22). Check the video amp and line oscillator HT decoupling capacitors by replacement - C38 & C102 - both 12uF, 350V (you can use 15uF types).

Check the coax lead contacts in the four way connector from the UHF tuner to the main PCB is making good contact. I would remove the contacts (sockets), tighten them up and re-insert them. Note there is a retaining "barb" on the contacts, which has to be flattened first, to allow removal from the plastic housing.

There should also be a brass "grounding" strip from the body of the UHF tuner to the metal reinforcing raround the edge of the main PCB.


Good luck,

Dazzlevision

[Focus Diode]

The LOPTs are very reliable in these sets thankfully, although a breakdown is not unknown to cause lack of width. It's the very last item to suspect after the width pot and resistors (dazzlevision's advice is sound and is also applicable to the 1400 chassis), PL504/PY801, scan correction capacitor, disc capacitor on LOPT and scan coils.

Should the disc capacitor need replacing (must be a disc type, not tubular), remember that heat to the LOPT tags can easilly damage the LOPT. It's best to cut the old component out without unsoldering, then lightly solder in the replacement.

Cheers,
Brian

[Hybrid tellies]

Re the suggestion from Dazzlevision concerning ragged verticals which gets worse when the brightness is increased.

Check earthing spring on the back of the crt. It should make good firm contact with the glass, but it pays to check if it is loose. Quite often all that is needed is to rapidly move the spring and contact plate up and down against the glass to re establish good contact.

Failing this (as already mentioned) suspect the doubler unit (for 17/20" 15Kv models) or tripler for the 24" or 20Kv models.

For lack of width check everything that Dazzlevision has mentioned plus C95 which is the 3rd harmonic tuning cap conected from the cathode of the PY801 to chassis. When it goes open circuit it causes lack of width and excessive EHT (which may be causing the ragged verticals problem). It is a high voltage (8kv) disc cap. On the 15Kv models it is C95 with a value of 180pf but on the 24" or 20Kv models it is C113 with a value of 220pf. When it fails it can quite often look burnt and charred.

This was a fairly common fault during my stay at Radio Rentals.