[Heath] The Heathkit IP-27

[Kenneth G. Gordon]

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