Re: CBM 3032 (PET 2001N-32), no video?

Just checked the voltage on the winding for the monitor supply, it shows
16.5 Vac, which is a little lower than the 18Vac you mentioned. Is that
within tolerance? Otherwise I’ll have to check the monitor PCB, it may
indeed be a cracked solder joint… It’s my first experience with a CBM, but
other computers of that era tend to have this kind of issues as well, I
guess it’s rather common.

Valerio Castelli


On 24 febbraio 2018 a 22:37:58, Francesco Messineo (
francesco.messineo@gmail.com) scritto:

On Sat, Feb 24, 2018 at 9:02 PM, Sherry Haibara <v.castelli92@gmail.com>
wrote:
> Hello everybody! Just acquired a beautiful CBM 3032 (PET 2001N-32) today,
and
> I'm already running into problems... Which was kind of expected, given
its
> age!
> I managed to get a first successful boot of the machine with crisp video.
> BASIC initialised correctly and I was able to execute the classic "PRINT
> 'Hello world'" command with no problem.
> I then powered off the machine as I had to go away for a while. When I
came
> back, I tried to turn it on but to no avail. I hear a slight humming
noise
> from the power supply and sometimes, but not always, there seems to be a
> sound right after flipping the power switch that reminds me of
degaussing,
> but I'm unsure if that's really what's going on.

the monochromatic CRTs don't need (and don't have) any degaussing.
What you're hearing is the power transformer building it's magnetic
field and the windings move a bit.

> I don't hear any other noise, including the usual pitched noise that
comes
> from CRT monitors when they're working. I also didn't hear any obvious
> "failing noise", like crackling or pops, and I didn't see any smoke
coming
> from the machine. The fuse is fine. I can see the electron gun of the CRT
> from the back of the case, but it's staying off - while on the first
> successful power on I could see it glowing in orange.
> There doesn't seem to be any obviously burnt component on the main board,
> and I've also already reseated the ROMs. I checked the +5V voltage on the
> user port and it's correct. I also checked the voltage on the main power
> connector (J8?) and it shows -9 V, which as far as I understood should be
> the correct value. The +12V rail also seems fine.

if the CRT heater doesn't come on (that's the orange glow on the CRT
neck), check the AC secondary
output on the transformer. There's one winding for the monitor supply,
it should be about 18Vac.
If that's ok, then it's something on the power rectifier and
regulators on the monitor PCB that are failing.
The PET monitor heater is fed by regulated DC.

> The symptoms seems a little suspicious to me, in that - if I'm not
mistaken
> - if there was a problem with the ROMs or the CPU, I should at least get
a
> garbage screen of some sort, which I'm absolutely not getting. There just
> doesn't seem to be any sign of life from the CRT.

yes, on 2001 and 2001N (30xx) variants of the PETs you get at least a
garbage screen if the CPU is not starting.
However, the lack of heater glow on the CRT's neck indicates a failure
in the monitor power supply, as I already said.

> So my question is - where should I begin troubleshooting? I don't have an
> oscilloscope, but I do have a multimeter and some basic soldering
> experience. I'm not too comfortable with going around the high voltage
CRT
> video circuitry, but I know a person that may be able to perform
> troubleshooting on that part.

18Vac on the transformer, then it goes inside the monitor to a 4 diode
bridge and a big 4700 uF/25V capacitor, you should get about 18-20 Vdc
on the capacitor when the supply is working well. Then that goes to an
LM7812 that regulates the DC voltage to 12V, it's mounted on a quite
big heatsink, you have to check that its output is indeed about 12V.
Those monitor tend to develop cracked solder joints after all these
years, so you might find that the diodes and maybe the capacitor need
their solder reflowed and renewed.

HTH
Frank IZ8DWF

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