[Heath] The Heathkit IP-27

[Kenneth G. Gordon]

On 6 Jun 2012 at 13:11, Ken Simpson, W8EK wrote:

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

Hello, Ken. I worked at the University of Idaho as the Electronic Instrument 
Specialist, Sr. for 31 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.

OK. Thanks for that info.

> 2)  The current regulator transistor (DTG-600, if I remember right)

Yes. That's correct.

> 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.

That is a particularly "weird" transistor. I have found and examined the 
characterisitcs of it, and I see nothing particularly different about those, 
except, perhaps, the Ft is much higher than the pass transistors, and the Pd 
is 106 watts. Still, there are no real subs for it either. I don't know what to 
think about it.

> 3)  The small TO-5 transistors inside, with the heat sinks on them
> (R-???)

R-265A

> 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.

BOTH of mine are leaking, one quite badly in fact.

> 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.

I have that mod in the manual I have here, thanks. I think it could be applied 
with good effect in some other cases too.

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

It does, Ken, thank you.

I am still very undecided as to whether or not this is a useful project, but it 
WOULD be fun, if I didn't have more pressing ones to consider.

Well, I'll have to do some "heavy thinking" about it.

vy 73,

Ken W7EKB