[Heath] More on my IP-27

[Scott Cowling]

Hi Ken,

The IP-27 is one of my favorite pieces of Heath lab equipment - I use 
mine all the time.

I had some problems with IP-27 supplies years ago, and I traced them 
mostly to the transformer.

This is all from memory, and about 35 years ago, so bear with me. :-)

The problem that I had was that the reference circuit uses a 
half-wave rectifier, Zener and a large filter capacitor.  The filter 
capacitor inrush current kept burning out the rectifier when I turned 
the supply on. This effectively connected the capacitor across the 
reference winding of the transformer, burning the winding  out.  I 
actually replaced an IP-27 transformer (bought new as a replacement 
from Heathkit); what a job!

My fix was to put a resistor in series with the rectifier to limit 
the inrush current. The Zener current is not too high, so some 
resistance here will not affect the regulation at all, but will 
protect the diode and thus the transformer. Something like 10 - 47 
ohms at a couple of watts comes to mind.

I suggest that you check the voltage on the transformer taps. This 
was what got my transformer. The reference winding was still good, 
but the heat that it generated during overload ended up shorting two 
of the other windings.

Hope this helps.

73,
Scotty WA2DFI



At 21:33 2012-06-19 +0000, Kenneth G. Gordon wrote:
>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
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