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

[Mark Whittington]

Thanks Mike!  I don't have any power resistors in my junk bin, but a friend
of mine does and I'm going to pick them up tonight and test the power
supply under load.  I'll reply again once I have the results of load
testing.

Cheers,

Mark

On Sat, Oct 3, 2020 at 3:48 PM Sentrytv <sentrytv at yahoo.com> wrote:

> I’ve used a load anywhere from 100 to 20 ohms at least 10 watts for
> computer power supply, 100 ohms @10w should be fine per 5v and 12v rails
> each to ground.
> So for the TI the same would apply 8 V and 16 V.
> Power supply
>
> The PE-box provides unregulated power, meant to be used by voltage
> regulators intalled on each card. Typically, these would be a 78x05 (+5V) ,
> a 78x12 (+12V) or a 79x12 (-12V). According to the specs, these regulators
> need an input voltage at least 3V in excess of the required output voltage.
> Therefore, the PE-box supplies them with +16V, -16V and +8V. Note that if
> you need -5V, you could use a 79x05 and feed it with the -16V supply. This
> would just mean that you need a really big heat sink, since this regulator
> is going to dissipate a lot of power...
>
> When I  actually get to my computer I will look to see what I have for
> manuals.
>
> Sent from:
> My extremely complicated, hand held electronic device.
>
> On Oct 3, 2020, at 2:10 PM, Mark Whittington <markwhi at gmail.com> 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.
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>>
>>