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

Mark Whittington markwhi at gmail.com
Sat Oct 3 07:19:45 EDT 2020

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

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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