Need Help with Model 4

[Thomas Lake]

I have a Model 4 that will boot just fine into Model III mode (both Cassette BASIC and TRSDOS 1.3) but when I try to boot into Model 4 mode, it looks like the horizontal hold is messed up. After it's been on for a while (15 min or more) it will boot TRSDOS 6.02 OK and work perfectly as long as I don't turn it off and back on. Can someone give me pointers as to what might be wrong? Also is there a horizontal hold adjustment inside the computer? If so, which trim pot is it? If all else fails, is there someone here who can work on these old machines? I'm a software person and am lost with anything but the most basic hardware fixes.

TIA
Tom Lake

[George Phillips]

On Friday, July 18, 2014 9:00:32 AM UTC-7, Thomas Lake wrote:
> I have a Model 4 that will boot just fine into Model III mode (both Cassette BASIC and TRSDOS 1.3) but when I try to boot into Model 4 mode, it looks like the horizontal hold is messed up.

80 column mode does have a trimming capacitor to adjust the timing. Details on the problem and how to fix it can be found here:

http://www.tim-mann.org/trs80faq.html#[18]

You might also want to have the Model 4 technical reference manual. Pretty easy to Google it, but here's one link:

http://www.classiccmp.org/dunfield/trs80/m4tech.pdf

[Al Petrofsky]

> On Friday, July 18, 2014 9:00:32 AM UTC-7, Thomas Lake wrote:

> > I have a Model 4 that will boot just fine into Model III mode
> > (both Cassette BASIC and TRSDOS 1.3) but when I try to boot into
> > Model 4 mode, it looks like the horizontal hold is messed

> > up. After it's been on for a while (15 min or more) it will boot
> > TRSDOS 6.02 OK and work perfectly as long as I don't turn it off
> > and back on. Can someone give me pointers as to what might be
> > wrong? Also is there a horizontal hold adjustment inside the
> > computer?

Short answer: if you're lucky, you have a late PAL motherboard and
you just need to fiddle with the knob of C210 until things work.


Long answer:

The video signal coming out of the motherboard is supposed to be
pretty much identical in the 64-column Model III and 80-column Model 4
video modes (60 hz vertical, 15840 hz horizontal) (assuming a
U.S. configuration). Therefore, the fact that there's no problem with
64-column mode indicates that there's nothing wrong with the
horizontal hold nor with anything else on the video board.

The problem is probably in the part of the motherboard that generates
the 12.672 Mhz signal that is used only by 80-column mode.

Which type of motherboard do you have? There are three relevant
categories of motherboards:

1. 8709296 REV A and earlier (early PAL)
2. 8709296 REV B and later (late PAL)
3. 8709523, all revisions (gate-array)

George Phillips can give you a Model III BASIC program that will tell
you which board you have. You can also open the case and read the
number off the board. You can distinguish gate-array boards from the
outside of the case by (among other things) the presence of an RS-232
port pointing out the back, rather than down. Early and late PAL
boards cannot be physically distinguished from outside the case (but
they can be distinguished by software).

1. On early PAL boards, the 12 Mhz signal is generated by a dedicated
crystal (Y2) and components C9, C10, C24, C111, R5, R6, R7, R23, R53,
U17, and Q2. If this is your board, I'm guessing one of the
capacitors needs to be replaced. The specs given in the service
manual are:

C9, 47pfd 50V C. Disk NPO
C10, 100 pfd 50V C. Disk
C24, 56 pfd 50V C. Disk
C111, 10 ufd 10V Tant.

2. On late PAL boards, the 12 Mhz signal is generated from the main
crystal by a phase-locked-loop consisting of an SE 564 (U203) and a
score of supporting components. The problem is likely C210. If C210
is a variable capacitor, you may just need to turn the knob to adjust
it. If it's a simple fixed capacitor (as found in the earliest of the
late PAL boards), replace it. It's 6 to 50 pF 2% NPO (if variable) or
33 pF 2% NPO if fixed.

3. On gate-array boards, the 12 Mhz signal is generated by a special
phase-looked-loop chip, U8, which only needs three supporting
components: C3, C19, and FB1. I would try replacing the capacitors:

C3, 10uF 20V Tantalum Elec. Ax. (says 20V on parts list, but 10V on
schematic)

C19, .1uF 50V Mono

George Phillips <gp2...@gmail.com> writes:

> Details on the problem and how to fix it can be found here:
>
> http://www.tim-mann.org/trs80faq.html#[18]

That's has some helpful info (especially if you happen to have a late
PAL board with a variable C210), but be warned that the author was
unaware of there being two types of PAL boards, and he confuses
gate-array boards with late PAL boards.

-al

[Thomas Lake]

Thank you George and Al! My unit is a converted Model III and it does have the tiny, orange trim pot on the motherboard. I adjusted it (more like just touched it with the adjustment tool!) and now the computer boots up fine every time.
The only thing left is to figure out how to use double-sided disk drives in this machine. I can replace the single-sided drives with double-sided and they work fine in single-sided mode.

[Michael Black]

The software, either in ROM or on disk, I'm not sure which, doesn't know
that the floppy drives are double sided.

When I got my first floppy drive, 30 years ago, for my Color Computer, the
package was from a third party and the controller came with a third party
ROM so the double sided drive could be used as a double sided drive. They
used some odd scheme, so it's not compatible with later arrangements,
which is why I have a bunch of floppy disks I can't read without finding
that ROM and hoping it still works (it was an eprom) and putting it in the
controller.

OS-9 could be patched to deal with the other side.

Michael

[turnkit]

On Monday, July 21, 2014 10:05:39 AM UTC-5, Thomas Lake wrote:
> Thank you George and Al! My unit is a converted Model III and it does have the tiny, orange trim pot on the motherboard. I adjusted it (more like just touched it with the adjustment tool!) and now the computer boots up fine every time.
> The only thing left is to figure out how to use double-sided disk drives in this machine. I can replace the single-sided drives with double-sided and they work fine in single-sided mode.

You can also put 800K 3.5" floppy drives in and run them as quad density -- 80 track, double density double sided (I think that's QD, but maybe that's the wrong term.) At any rate if you can find a 1.44 MB drive then look for a 800K jumper. There may be instructions online for this but 3.5" media might be a little more durable or easier to find and faster possibly too.