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Type: Posts; User: Hugo Holden

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  1. I cannot think of any checks on the IC that would...

    I cannot think of any checks on the IC that would be useful out of circuit, though if it was out of circuit it could be a useful time to check the passive components, capacitors and resistors & diode...
  2. IC301 a TDA1170, maybe the TCA1170 is a typo. The...

    IC301 a TDA1170, maybe the TCA1170 is a typo. The feedback around it is only for linearity control, not to sustain oscillation. The oscillator section of it is independent of the feedback. It would...
  3. The vertical oscillator has stopped and/or the...

    The vertical oscillator has stopped and/or the vertical output stage is not working or driving any current via the yoke. Time to check that IC & its voltages. For the oscillator, the IC pin provides...
  4. ...everything you have seen so far though is...

    ...everything you have seen so far though is still consistent with a fracture to the connections of the V hold potentiometer on the solder side of the PCB, have you been able to inspect these...
  5. Well, I guess so far we know that the vertical...

    Well, I guess so far we know that the vertical timebase (which is part of the IC) was running at half the normal frequency and now it appears to have stopped altogether, and the V hold pot and the...
  6. No, an electrolytic is not suitable for a timing...

    No, an electrolytic is not suitable for a timing application like this. Generally they use a polystyrene type capacitor, but any non electrolyic polyester/mylar cap will be good enough. The schematic...
  7. This is consistent with C302 not charging at all....

    This is consistent with C302 not charging at all. The V hold pot, or the pcb connections to it are likely open circuit. You also saw two modes of operation manipulating the pot. This sort of thing...
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    The two pieces you refer to are stuck (presumably...

    The two pieces you refer to are stuck (presumably spot welded ) together on my SOL.
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    When resistance develops between the dissimilar...

    When resistance develops between the dissimilar metals of the IC pin and the socket pin, it causes everything from a near open circuit, through to non linear effects (like a rectifier junction)...
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    It's always possible bleeder resistors can go...

    It's always possible bleeder resistors can go open. They have a penchant for it as all resistors which operate at high applied voltages seem to. Fortunately electrolytic caps are intrinsically leaky...
  11. Yes I think it unlikely C302 would have gone...

    Yes I think it unlikely C302 would have gone leaky too, and it's very difficult for a capacitor to double its value, it generally would never happen with a Mylar or poly type. If it is not charging...
  12. Well the pot probably is ok, but worth testing on...

    Well the pot probably is ok, but worth testing on the meter.

    So the capacitor C302 is slow to charge up, In theory at least if you put a scope on it it should be charging up to about the same...
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    Normally the high voltage caps on the primary...

    Normally the high voltage caps on the primary side have bleeder resistors, just leave it off for 10 min before touching it.

    The charge in a CRT bulb is only a worry if you have to remove the EHT...
  14. Will rotation of the pot throw the image out of...

    Will rotation of the pot throw the image out of vertical lock and cause a rolling image, or have any subtle effect on the vertical position at all ?

    The reason is that to get the effect there...
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    .....normally for turning the GCA R, G and B...

    .....normally for turning the GCA R, G and B signal into a composite level at the resistor matrix in the base circuit of the video output transistor, IBM at least mixed the green to be about 55% the...
  16. The schematic mostly shows the V hold pot as a...

    The schematic mostly shows the V hold pot as a variable resistor,likely though it is wired in circuit with the wiper connected to one adjacent leg. This means of course if it turned out that when the...
  17. Yes, the V hold pot wiper could simply have gone...

    Yes, the V hold pot wiper could simply have gone O/C try to exercise the pot & check its connections.
  18. This is a fun fault. All that has happened is...

    This is a fun fault. All that has happened is that the vertical scan rate has dropped in half (and it's still managed to stay in V lock on alternate V sync pulses and keep the correct height) so you...
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    That is very good news in that the circuit can be...

    That is very good news in that the circuit can be drawn out and the resistors measured and on top of that the transistors could, if required, be replaced with modern surface mount types. So it's...
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    I don't know , but it sounds similar to the way...

    I don't know , but it sounds similar to the way IBM combined the RGBI signals with a resistor mixer at the base of a video output transistor. The transistor acts in a emitter follower mode. More than...
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    I guess the thing to remember is that adding the...

    I guess the thing to remember is that adding the isolation transformer doesn't in fact add any safety, what it does do is that it allows parts of the primary circuitry, on the line power side, to be...
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    Thanks Bob (Saver of lost Sols, I like that)...

    Thanks Bob (Saver of lost Sols, I like that)
    Hugo.
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    That is really great, Thanks!

    That is really great, Thanks!
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    This is a challenging one. If I were repairing it...

    This is a challenging one. If I were repairing it I would copy out the schematic completely and for any device, like those IBM devices just draw them as squares with input-output connections etc. I...
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    I still think it is an uphill battle without a...

    I still think it is an uphill battle without a decent scope trying to help repair a computer by remote control. My advice would be to down tools until the OP acquires a decent analog scope with at...
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