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

[Mark Whittington]

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.
> >>
> >>
> >>
> >>
> >>
> >>
>
>