Re: CBM 710: Replacing Power Supply - Help Needed

Den Fri, 13 Apr 2018 19:04:24 +0000 skrev Micah Bly
<micahbly@gmail.com>:
> Hi All,
> 
> I am trying to get a CBM 710 into working shape. I recently acquired
> this from a collector, and my understanding is that is has never been
> up and running, it was incomplete from the start. There were a few
> missing chips and ROMs, but I think I'm basically down to the last
> part: new PSU.
> 
> I am the US, the machine is from Germany. 240V (or maybe it's 220V)
> 50Hz vs 120V and 60Hz.
>
> I have enlisted the help of a long-time Commodore technician (I don't
> mean he worked for Commodore, I mean he has been repairing them for
> 20+ years and has his own shop still). I am entirely useless when it
> comes to anything more than the most basic circuit.
> 
> I have been reading through a few relevant historical posts on CBM
> hackers, and looking through other stuff that Google reveals. So far,
> we have assembled the following:
> - ELM440 frequency generator.
> https://www.elmelectronics.com/ic/elm440/
> - NTSC color burst crystal
> - Mean Well RT-125B power supply.
> http://www.meanwell.com/productPdf.aspx?i=491
> 
> The PSU seems like it should be fine for the power side of thing, but
> could someone help us with designing a circuit for the timing? I
> showed this diagram to the gentleman helping me, and he is scratching
> his head pretty furiously about it.
> http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/b/cbm700-ceag-PSU.jpg

I would modify the existing PSU instead of replacing it.

My understanding of north American electics is that you usually have
two "hot legs" feeding a residential panel, and some stuff like a
cooktop, an owen, a washing machine, a dryer, an AC and so on, is fed
by 240V AC. If you have any way of temprorary gaining access to 240V
AC, you could test the existing PSU that way. Make sure you have an
adequate fuse or circuit breaker. The buildt in fuse in the 710 machine
only protects one of the two incoming wires while the 2.2µF "Y2" noise
suppresion capacitor is connected directly from the other incoming wire
to ground.

Judging by the schematics, it would be rather simple to convert it to
110-120V AC operation.

If the PSU works fine with 240V, I'd start with replacing all the
electrolytic capacitors except the 100µF 385V one which should be
replaced with two with another value. Test the PSU again after
recapping every other capacitor. This way you can be more sure that
nothing else went bad just by coincidence while you work on the PSU.

replace the 100µF 325V capacitor with two 200µF 200V capacistors in
series.

Replace the 360k 1W resistor with two 180k 1/2W resistors, connected
over each of the two new 200µF capacitors.

Desolder one AC leg of the rectifier (or cut a trace on the PCB). Hook
up that desoldered leg of the rectifier to the other AC leg of the
rectifier.

Solder a wire onto the PCB where you just removed one leg of
the rectifier, and solder the other end of that wire to the point where
the two new capacitors meet.

Now we are mostly done. The only things left to do is to modify the
startup and frequency generation circuit, and fix the fan. The best and
most simple way to fix the first part is to get a suitable transformer
120V - 11.3V good for at least 2VA, and replace the existing
240V/11.3V 2VA transformer. It's probably best to just replace the
fan/fans with 120V 60Hz ones.

----------------

If you anyway want to use the new PSU you have aquired, you could copy
the circuit for the 50/60Hz signal generation from the schematics of
the original PSU. It's just a small transformer, one diode, three
resistors, one transistor and one small capacitor.
 
> My goal here, aside from getting the CBM II back up and running, is to
> document it in case somebody else wants to do something similar in the
> future. I don't have a web site, so I am using a blog, here:
> http://www.vintageisthenewold.com/not-your-daddys-128-retrochallenge-2018-04-update-1/

I like your ambition!
Btw it's me who already have made some comments on your blog posts :) 


-- 
(\_/) Copy the bunny to your mails to help
(O.o) him achieve world domination.
(> <) Come join the dark side.
/_|_\ We have cookies.