[Heath] The Heathkit IP-27

[Ken Simpson, W8EK]

The college where I taught (before I retired) used IP-27 power 
supplies for at least 20 years.

Several things we found (that I remember):

1)  The pass transistors on the back were the most common failure. 
They could be replaced with generic NTE or ECG equivalents, and would 
work OK, but 2N301s were better if you could get them.

2)  The current regulator transistor (DTG-600, if I remember right) 
was very critical, and a substitute would not work.  A substitute 
might let the power supply look like it was working, but it would not 
current limit properly or reliably.

3)  The small TO-5 transistors inside, with the heat sinks on them 
(R-???) were critical for leakage.  Since they were PNP Germanium, 
they often leaked.  Most replacements would not work properly, 
although I think we finally found one that would "sort of" work, but I 
don't remember its number.

4)  An important mod was recommended by Heath.  It basically put a 56 
volt zener, and a 0.01 ceramic cap, across the C E junction of the 
pass transistors.  This solved about 95% of the failures we had.  What 
happened is that if the student turned the supply off too fast, the 
inductive kick from the power transformer would wipe out the pass 
transistors (see #1 above).  The zener took care of this transient.

This may not answer your question exactly, but should give you some 
useful information.

73,

Ken, W8EK



Kenneth G. Gordon wrote, On 6/5/2012 7:04 PM:
> 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