[Heath] More on my IP-27

[Kenneth G. Gordon]

Bringing you up to date...at least, those who may be interested. ;-)

The DTG-600 was shot, as was one of the 2N301/2N2869s. I cannot now find one of the 
2N2553s to test, but the one I did find appears to be OK.

I bought an NOS/NIB DTG-600, 2 ea 2N301s (look like they are "pulls", but test OK), and 2 
NOS/NIB 2N2553s and have installed all these new transistors.

As I may have mentioned previously, this particular IP-27 is factory-built and the 
workmanship is literally outstanding. However, it came out of an Electrical Engineering Lab at 
the University of Idaho's EE Department, and obviously suffered student abuse.

I have gone through the calibration procedure and have come up with some rather interesting 
and strange symptoms, none of which are even hinted at in the Troubleshooting Chart 
included with the original manual I have.

First of all, setting the zener current works very well, and absolutely according to the manual, 
as does setting the MAXIMUM voltage which one should see at the output in each setting of 
the VOLTAGE switch. Minimum voltage is another matter entirely.

However, and here is where it BEGINS to get weird: when I set the maximum current to the 
1.5 amp setting, and set the FINE current control to maximum, and adjust FINE voltage at 
any setting of the VOLTAGE switch to any place above the bottom end, with no load, the 
over-current relay operates.

Furthermore, the LOWER voltage of all ranges can never be reached: it is always high...but 
see below.

After this discovery, I then connected a 10 ohm 100 watt resistor to the load terminals, set the 
COARSE VOLTAGE control to the 0.5 to 5 volt setting, set the COARSE CURRENT switch 
to the 500 mA setting, and the FINE CURRENT to maximum, and ATTEMPTED to set the 
FINE VOLTAGE control to 5 volts. The maximum voltage it would reach was something like 
2.87 VDC and it wouldn't rise any further. Furthermore, it appears that the overcurrent relay 
has operated. The meter reads something like 200 mA at maximum and will not move from 
there no matter what the setting of the FINE voltage control. It will read down to zero at zero 
volts.

I have the meter on the IP-27 set to CURRENT, and an external digital voltmeter connected 
across my 10 ohm load in order to be able to monitor both values simultaneously.

HOWEVER (and this is what I consider very strange), if I then rotate the FINE CURRENT 
towards the LOW end of the range, as I approach the bottom 1/3 of its range, the voltage 
reading will "flicker" once momentarily, and by then rotating the FINE CURRENT control back 
towards the high end of the scale I can bring BOTH the current reading to 500 mA AND the 
voltage up to 5 VDC (and even more) without the overcurrent relay operating.

Furthermore, when the power supply is in the 5 VDC 500 mA condition, I then CAN reach 0.5 
VDC at the lower end.

If I adjust the voltage or current above 500 mA by either very much or for longer than a few 
seconds, the current will drop to zero and I must operate the reset-standby switch to restore 
operation.

Having reached the 5 VDC output and 500 mA readings, if I then disconnect one lead from 
the 10 ohm load, the voltage shoots up to over 8 VDC, the current reading drops to zero, but 
the overcurrent relay does not operate.

According to the Troubleshooting Chart, such "Poor or no regulation" is caused by 1) Diode 
D7 (checks as new: replaced with new: no difference), 2) Transistor Q2 (replaced with new), 
3) Transistor Q3 (replaced with new), 4) Q1, Q4 or Q5 shorted to chassis through mica 
insulator (all test completely open with a digital multimeter).

There is another symptom listed in the Chart that may be significant: it is, "Relay pulls in on 
any load at over 6 volts" 1) Transistor Q2.

So, the only thing common to these two symptoms is Q2...which is new.

Lastly, adjusting the "D.C. Regulation" control as per the manual does nothing. This control 
sets the base voltage of Q2.

This is a third symptom possibly common to Q2.

At this point, I am suspecting that diode D-8 is shorted or very leaky, which would short the 
base to the emitter of Q2.

I have not yet done voltage checks as per the schematic, but will do that asap.

However, I have visually inspected every component and the only thing that appeared 
"problematic" was C8, which had some brown "goop" on it, obviously from inside the 
capacitor. Even so, it checked perfectly good an all of three different cap checkers. However, 
I replaced it with a new one. No effect.

Well, back to the bench.
]
vy 73,

Ken W7EKB