[vcf-midatlantic] TI-99/4A Peripheral Expansion Box - Voltages

[Mark Whittington]

Hey Mike,

I appreciate the info.  I'm sure it'll work -- the expansion cards all have
their own voltage regulators, and they're expecting unregulated voltage
from the PEB.  The apparent problem is that the PEB's unregulated voltage
rails are way out of spec -- the 8v rail is running at 14.4v, and the +/-
16v rails are running at +/-25v.  While the voltage regulators in the
expansion cards /can/ regulate those voltages down to 5, 12 and -12, and
the cards will work, they're going to be dissipating a lot more power than
they're designed for.  It may be that there's actually no problem at all --
under load the voltages might come down to within the expected ranges.  I'm
going to test that this evening.

I didn't suspect that you'd sold me a broken box :) The console itself
works great.  I just want to make sure that the system is going to continue
to work for another 30 years before I start using it for hours at a time!

Best,

Mark

On Sat, Oct 3, 2020 at 3:41 PM Mike Loewen via vcf-midatlantic <
vcf-midatlantic at lists.vcfed.org> wrote:

>
> Mark:
>
>     I ran that PEB for an hour last week, with no problems.
>
> Mike Loewen                             mloewen at cpumagic.scol.pa.us
> Old Technology                          http://q7.neurotica.com/Oldtech/
>
> On Sat, 3 Oct 2020, Mark Whittington via vcf-midatlantic wrote:
>
> > Thanks Glenn and Mike for the responses.
> >
> > I did download the schematic, and I found and read the technical
> > information here: http://www.unige.ch/medecine/nouspikel/ti99/pebox.htm
> > before powering the unit, but I was unable to find a manual.  If you know
> > where I can find one I'd appreciate a link.
> >
> > I'll check the capacitors and put a load on the various supply rails and
> > see if that brings the voltages down.
> >
> > Thanks again.
> >
> > Mark
> >
> > On Sat, Oct 3, 2020 at 10:10 AM Sentrytv <sentrytv at yahoo.com> wrote:
> >
> >> Also I?m assuming you downloaded the manuals and the schematic for the
> >> power supply first before plugging in?
> >>
> >> Sent from:
> >> My extremely complicated, hand held electronic device.
> >>
> >>> On Oct 3, 2020, at 7:24 AM, Mark Whittington via vcf-midatlantic <
> >> vcf-midatlantic at lists.vcfed.org> wrote:
> >>>
> >>> Just realized I missed a half a sentence (in my defence it's a been a
> >> late
> >>> night).
> >>>
> >>> The corrected sentence:
> >>>
> >>> 2) Is just a case of not having a load on the outputs causing them to
> run
> >>> high?  I know that the linear regulators that are likely to be used on
> >> any
> >>> cards can technically handle these inputs, but I wouldn't want to
> assume
> >>> that they have heat sinks large enough to dissipate the extra power for
> >>> very long, so I haven't been brave enough to test the system with cards
> >> in
> >>> it to present a load.
> >>>
> >>> Thanks
> >>>
> >>>> On Sat, Oct 3, 2020 at 7:19 AM Mark Whittington <markwhi at gmail.com>
> >> wrote:
> >>>>
> >>>> Does anyone here have much experience with these?  I picked one up
> >>>> recently and I've been going over it before plugging things in.  I
> >> tested
> >>>> the unloaded voltages with the transformer set to 115VAC and got the
> >>>> following on the unregulated DC output rails:
> >>>>
> >>>> +8V rail: 14.4V
> >>>> +16V rail: 25.2V
> >>>> -16V rail: -25.3V
> >>>>
> >>>> It looks like a previous owner had it apart -- there are hand-written
> >>>> labels on the various input taps for the transformer, so I thought
> that
> >>>> perhaps it was mis-wired on reassembly.  But testing the other taps
> the
> >>>> best result I can get is with the 220VAC tap at approx. +7.5V and +/-
> >> 12V
> >>>> on the unregulated outputs.
> >>>>
> >>>> My analog circuits knowledge is pretty thin, so I'm hoping someone
> here
> >>>> might be able to help out.    My questions are:
> >>>>
> >>>> 1) I've measured the output taps from the transformer for each of the
> >>>> various input taps and they match what I'd expect to see as inputs
> given
> >>>> the various rectified outputs (RMS AC -> Peak AC, minus a diode drop)
> >> so I
> >>>> don't *think* the problem is on the power supply board.  Am I missing
> >>>> something?
> >>>>
> >>>> The schematic I've been referencing:
> >>>> http://www.mainbyte.com/ti99/schematic/peb_power.jpg (I have the -2
> >>>> version)
> >>>>
> >>>> 2) Is just a case of not having a load on the outputs causing them to
> >> run
> >>>> high?  I know that the linear regulators that are likely to be used on
> >> any
> >>>> cards can technically handle these inputs, but I wouldn't want to
> assume
> >>>> that they have heat sinks large enough to dissipate the extra power
> for
> >>>> very long.
> >>>>
> >>>> 3) Is this actually normal?
> >>>>
> >>>> Thanks in advance for any help.
>