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

[Mark Whittington]

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.
>
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