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

[Glenn Roberts]

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.

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