28 Jan
2011
28 Jan
'11
12:33 p.m.
Looking at photos of my system (physical hardware is
in storage), it has 6 boards:
32K CPU/ROM/RAM,
FDC/serial,
32K RAM,
VDU #1 (keyboard, modulator, composite out, 1KB RAM),
VDU #2 (composite out, 16KB RAM, 26-way header)
Bus terminator
... plus an "RGB board" mounted on the back panel, which has RGBS BNC outputs
and hooks to the 26-way header on VDU #2. VDU #1 and VDU #2 are connected
together via 16-way DIL headers on the boards.
I *think* what I've labelled as "VDU #1" is the standard 40-column board
(1KB
The text VDU board (which I suspect is 'VDU #1') is connected directly to
the CPU board, it does not connect to the bus ribbo ncable. That's one
way to qucikly recognise it.
As an aside, IIRC, the 40 column VDU has the cassette interface
circuitry, the 80 column one deosn't (80 comlumn machines had to have
disk drives).
Aotehr aside, the 32K RAM expansion board is the same PCB as the CPU
board, with may compoennts not fitted and different address decoder PROMs.
RAM isn't enough to do 80x25 text), and "VDU
#2" is a hi-res board. From
memory, there's not a lot on the little rear-panel RGB output board; just a
few passives and transistors - I certainly don't remember it having a
modulator, so I don't think my machine does hi-res PAL output (if it does it'd
have to be via the UM1111 modulator on VDU #1)
The RGB output PCB doesn't do PAL as well. The PAL encoder connects to
the same 26 pin ehader on the hi-res board -- IIRC there's a header plug
fitted on the RGB board cable to connec the PAL encoder to.
The modulator on the text video board handels the text video _only_. It
doesn't even carry a mono version o the hi-res video. IIRC the composite
outptu socket o nthe back plane has a cable ending in a min coax plug
which is connected to the text video PCB in non-hi-res machines, but to
the hi-res board when that's fitted. IIRC that 16 wire cable between the
video boards carries the text video and sync signals so the hi-res board
output is a combination of its own image and the output of the text video
board.
Yes, mains
filters can and do blow. I've replaced a few -- well,
actually, most of the time I've replaced the capacitors only (seaprate
components on a PCB). If there's potted filter module, I guess you
replace the whole thing.
Are we talking X/Y-class filters here, or primary smoothing caps just
downstream of the bridge rectifier?
I've had many ones of the former fail, but I can't think of a single machine
I've had a few fail. But unpredicably. As I said, I was uing an HP
monitor and suddenly one of the filter capacitors (separate components on
the PCB, not a module, in this unit) exploded. It had been running for a
hour when it failed, and I'd used the machine before that with no
problems. I don;t think any test would have shown that it was likely to fail
I've worked on where I've needed to replace
the latter. I've *seen* older
I id replace the mains smoothing capacitors in the SMPSU in my HP120.
They hadn;t failed, but one had a somewhat domed top, as if there had
been pressure build-up at some point. They are located so they point
straight at the CRT neck (!) and if one had blown its top, I think I
would have been looking for a new CRT too. A pair of new capacitors
seemed sensible.
equipment - 60s and early 70s - where they've been
replaced, but it's not
something I've had to do on the age of machine I normally work on.
I've changed a few, but certainly not all in the machines I own and use.
Many of my DEC and HP machines from the 1970s still have all their
original capacitors, for example/
I seem to recall a small tantalum capacitor somewhere in the PSUs of one of
the 380Zs that I've had letting go after a short while of use after being in
storage - big bang and a lot of smoke. That's the only time I've had that
happen to me, but it was a bit unnerving at the time :-)
Yes, it happnes. And I know of now way to predict it or test for it. They
just fail.
-tony