[Heath] The Heathkit IP-27

[Ron]

I have an IP-20 (which is similar), with bad Ge output and driver 
transistors.
Replacing the transistors turned out to be more expensive than the 
supply
was Worth. I tried replacing with Si PNP transistors, but these 
failed almost
immediately. I should analyzed the design to see what mods were 
necessary
to support Si transistors, but never had the time.

I suspect that restoring these supplies is not worth the expense 
and/or the
trouble. If you discover otherwise, I'll be grateful for your comments.

-- Ron




On 6/5/2012 7:04 PM, Kenneth G. Gordon wrote:
> I have one of the subject power supplies here which was given to me. It was obviously
> factory-built and is immaculate inside, although the outer case is very dirty.
>
> The trouble is, it appears that more than one of the original Germanium PNP transistors in it
> have either deteriorated, or are bad, as it doesn't work properly, especially at voltages above
> 25 volts where the "Fine" voltage adjust control has no effect on output voltage, although it
> DOES vary the output voltage over a narrow range at lower voltages output.
>
> Furthermore, the over-current relay keeps activating, even with no load.
>
> According to the "Troubleshooting Guide" in the manual, both of these indicate problems with
> the transistors.
>
> I have researched the original transistors, both as to specifications, and as to availability and
> cost.
>
> Cost is simply astronomical, WHEN they can even be found!
>
> Although there are NTE cross-referenced subs for most of these, I have always been a bit
> disappointed in NTE subs as they don't always exhibit the same characteristics as the
> originals. Besides, those aren't any less expensive than the originals.
>
> I am now toying with the idea of completely reversing the polarity of every polarity-sensitive
> device in this power supply, and substituting Silicon NPN transistors with almost identical
> specifications for ALL the PNP Ge devices.
>
> The power supply has very good characteristics, and would be very useful in my work here.
>
> Furthermore, the circuit is quite simple, and doesn't have any trick circuits included.
>
> What do you folks think of this idea? Am I considering making a silk-purse out of a sow's
> ear? ;-)
>
> I have yet to find a decent schematic of a more modern design for a switchable, variable-
> voltage variable-current supply with current-limiting, or I would simply use the main power
> supply as the source and replace everything else with a more up-to-date circuit.
>
> Output voltages from this thing are 0.5 VDC to 50 VDC in 5 volt increments, and current-
> limits are able to be set to from zero to 50 mA, 150 mA, 500 mA,and 1.5 A. Ripple is
> miniscule and voltage variation, no-load to full-load, is +/- 15 mV over the entire range, or can
> be set to zero (!) for any single voltage over the entire current range up to 1.5 A.
>
> Comments?
>
> Ken W7EKB
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