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

[Sentrytv]

Definitely look at the link that I sent you for the power supply diagram and of course other diagrams as well before you power the box up again so you can be 100% sure you’re Checking it correctly

Mike Rosen


Sent from:
My extremely complicated, hand held electronic device.

> On Oct 3, 2020, at 4:32 PM, Mark Whittington <markwhi at gmail.com> wrote:
> 
> 
> 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.
>>>> >> 
>>>> >> 
>>>> >> 
>>>> >> 
>>>> >> 
>>>> >> 
>>>>