[vcf-midatlantic] TI-99/4A Peripheral Expansion Box - Voltages
glenn.f.roberts at gmail.com
Sat Oct 3 10:08:37 EDT 2020
My experience is with Heathkit, not TI, but on an H8 this would not be unusual on an unloaded chassis. With cards installed the voltages come down.
Sent from my iPad
> On Oct 3, 2020, at 7:25 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
> 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.
>> 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
>> The schematic I've been referencing:
>> http://www.mainbyte.com/ti99/schematic/peb_power.jpg (I have the -2
>> 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.
More information about the vcf-midatlantic