Perq 2T1 display fault

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Fri Feb 20 17:18:17 2004

> > Just to confirm, we're talking about a PERQ 1 here -- it's confusing to
> > me wether this is a PERQ 1 (verically-mounted SA4000 hard disk at the
> > left side, monito connects on a single DC37 connector) or a PERQ 2T1
>
> It's a 2T1, vertical drive mounted at the back.

Oh, right...

Can you please re-describe the 2 faults and the machines they occur on --
I was under the impression I was considering a PERQ 1 (VMI monitor) with
a small picture all round...

OK, back to the T1. I think it's the machine I worked on some years ago
(don't worry, everything is 'stock', apart from the fact you may find the
odd IC in a socket if I replaced it). The only non-standard feature is
the monitor power cable -- I couldn't find the original lead, and the
connector at the monitor end is something odd. So I removed that
connector, fitted a length of cable with a 3 pin DIN plug (to fit the
PERQ's 55V output socket) and wired the other end inside the monitor,
using a strain-relief bush in the holw where the power connector used to
be. That shouldn't cause many problems.

If it is that machine, then it has the standard KME monitor, and that's a
very conventional design.

Start by looking at the large heatssink at the back. This contains a
voltage regulator circuit to bring the 55V down to 48V. Check that
output. If it's low, then the monitor board might be drawing too much
current. Do not tweak the preset on the heatsink's PCB, though!

The circuit, as I said, is fairly conventional. The only problem is the 4
hybrid circuits. One, on the CRT base, is the video amplifier, and was
replaced with standard components in later versions of the monitor (e.g.
the PERQ 3a, which also has a KME monitor). The 3 on the main board are
the line oscillator (I think there's a SMD chip encapsulated on this
one), the vertical osciallator components, and the vertical output stage
comopnents. The latter 2 connect to presets and a TDA1170 chip. AFAIK,
the circuitry is conventional.

Most problems centre round the line output stage. This _is_ normal.
There's a flyback transformer, a conventional driver stage, a line output
transistor, etc. Look for shorted diodes in this area. Also watch out for
the grey box on the anode cap. This contains a 150M resistor (or so)
between the EHT output ('spout' of the flyback transformer, connected to
the clip in the anode cap) and the ground wire going back to the CRT base
PCB. This is the EHT bleeder, and has been known to go short-circuit.
Thls really loads the line output stage....

Remind me as to what the fault is, I'll dig out the schematics, and
suggest tests, etc. Do you have an EHT voltmeter?

>
> (there is a PERQ 1 also with a video fault which I think I mentioned;
> that's where the confusion probably arises. That one has a totally
> mangled display, squashed horizontally and vertically, very non-linear,
> and no recognisable character output on it at all)

Do you _know_ if the PERQ is giving the right scan frequencies? (64kHz
horizontal, 60Hz vertical IIRC).


-tony
Received on Fri Feb 20 2004 - 17:18:17 GMT