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

[Mike Loewen]

Mark:

    I ran that PEB for an hour last week, with no problems.

Mike Loewen				mloewen at cpumagic.scol.pa.us
Old Technology				http://q7.neurotica.com/Oldtech/

On Sat, 3 Oct 2020, Mark Whittington via vcf-midatlantic wrote:

> 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.