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Heathkit IP20 Power Supply

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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One lead of the thermistor is disconnected, nothing is getting hot, X4 & X5 checked the same.
Good.
If I may ask what does going across a capacitor, resistor or a transistor tell you?
Somewhere in that circuit, the voltage is dropping when you try to draw current from the output. We need to know where that voltage drop is occurring, and fix it. I will explain a bit more below.
1. Across C2: No load = 28.4V; With load = 25.3V
OK, that's good. That means that the mains transformer, the bridge rectifier (D1~4) and C2 are all OK.

That part of the unit is the unregulated supply circuitry. Voltage from the mains transformer is rectified by the bridge rectifier and smoothed by C2. This forms an unregulated supply which becomes the input for the regulator.

The voltage across C2 needs to be higher than the output voltage, to give the regulator some headroom so it can keep the output voltage steady as the load current varies.

The voltage across C2 will drop when more current is drawn, because it's unregulated. Those numbers look good.
2. Between collector and emitter of X1 (positive probe to the emitter): No load = 25 mV; With load = 0.43V
OK, that's good too. X1 is the current limit transistor. If too much current is drawn, it starts to turn OFF and reduces the voltage available to the regulator. This reduces the voltage available to the load (the thing that's connected to the power supply's output) which will generally force it to draw less current. That's how current regulation works.

So there's not much voltage being dropped across X1 even when the output voltage is very low, so the current limiting circuit is not the cause of the problem.
3. Between collector and emitter of X4 or X5 (positive probe to the emitter): No load = 10.9V; With load = 0.67V
OK. This shows the operation of the regulator. When there is no load, the regulator is able to regulate as normal. There is about 11V across the regulator transistors; this corresponds to the difference between the voltage across C2 (about 28V) and the output voltage (about 15V). Some other voltage is also being dropped somewhere else, but most of this difference is being dropped by the regulator. That's correct behaviour.

When the load is increased and the output voltage drops, the pass transistors (X4 and X5) in the regulator conduct more strongly, to try to keep the output voltage at 15V. In this case they are conducting about as hard as they can, because there is only 0.7V across them. But they still aren't able to keep the output voltage correct.

So voltage is being dropped somewhere else in the circuit.

You need to set the output to 15V, connect the load resistor, and measure the voltage at each of the highlighted points in the circuit, from left to right, to find out where the voltage is being dropped. Measure with your multimeter's black probe to the 0V rail (the negative output terminal).

You should see about +25~27V at the positive end of C2 (which I've labelled "A"), and from your description, you will see less than +3V at the positive output terminal (which I've labelled "G"). Somewhere in between those points, a lot of voltage is being dropped. We need to find out where!

epoint 272122 positive feed voltage measurement points.png
 

mrmodify

Feb 13, 2010
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Evening Kris, You sir are the man, found R18 does not exist.

A = 24.4V
B = 24.4V
C = 15.5 V
D = 15V
E = 14 V
F = 2.5 V
G = 2.5 V

I was trying to get voltage readings on E & F. I followed the schematic and I found all the connections but never could find R18. I read the manual ( for the 100th time ) and looked for the placement for R18. pg 23 and looked for control FK which is the DC regulation pot. pg 22. My unit has no resistor across that potentiometer.

I don't have a .33Ω resistor and only a couple of Schottky diodes here. Tomorrow I will pick up a couple of .33Ω 2 watt resistors for R18 and whatever diodes I need.

Your last post sir has taught me alot. I am going to go through it again tomorrow so I maybe able to maintain it in my memory.

The issue with the silicon transistors, I am planning on adding a pair of Schottky diodes in line with R23 as recommended. Is this correct?

What size should the Schottky diodes be?

:)
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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OK, cool! So you'll need to install a new R18.

BTW please don't call me "sir" :)

Those voltages also show that about 10V is being dropped between B and C. This points to a problem with the current range switch, specifically the "CR-2" wafer, shown to the left of X1 on the schematic diagram. This seems to be the rear wafer, i.e. the wafer furthest away from the front panel.

There may be a problem with the contacts on that wafer, or perhaps R8 (0.82Ω 2W) is damaged or missing. If R8 is OK, see if you can upload some close-ups of the rotating contact and the contacts that touch it. You should be able to clean it if you're very careful not to bend those springy contacts but please let us have a look first.

You can buy a couple of 1N5817, 1N5818 or 1N5819 Schottky diodes (all are equally suitable for this application) if you want, but I think we should wait until the rest of the unit is working properly before dealing with that part of the circuit.
 

mrmodify

Feb 13, 2010
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Evening Kris,

R8 does not exist either, I will pick up some tomorrow. I can see where it was removed from the switch.

Sorry about the sir.

I did clean the wafer switch with some contact cleaner when I first openned the unit. It probably did not do a good job of cleaning.

I have taken picture but they did not come out clear and you really can't see much I am going to try to get better pics tomorrow.
 

mrmodify

Feb 13, 2010
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Evening,
I had to order the R8 resistor, should be in in a couple of days.

I have attached the pictures to this post of the CR2 wafer switch.
 

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mrmodify

Feb 13, 2010
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Evening,
I have installed R18, R8 and have reconnected R23 and placed R23 back into the heatsink.

I installed a 10Ω 25 watt resistor and put the unit on my dim bulb tester with a 150 watt bulb. Power up unit all was fine. Then to a 60 watt bulb, then to the mains.

Unit will put out 15.4 volts @ 1.5 amps with the 10Ω load with a 10 amp meter in series with the load.

R16 will change the voltage some as it is turned while doing the fine adjustment precedure.

R2 the current limit switch will not raise or lower the amperage. I am assuming that R2 should control the amount of output amps.

Outputs never got hot at any point while I had unit powered up.
 

mrmodify

Feb 13, 2010
157
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Evening all,

D6 was missing and I have finaly recieved D6 from the parts house. It took long enough.

I have installed D6 and did the final adjustment procedure.

Things are better but not fixed.

When loaded with a 10Ω 25 watt resistor the voltage will fall from 15 volts to 11 volts and the amperage will only go up to a little over 1 amp.

X1 (OB2) is getting warm which I think would be normal, All other parts are ambient temperture.

The D.C. regulation 50Ω rheostat does not change anything. I have cleaned it and have checked the operation with a ohm meter and it checks good.

Does anyone have any suggestions on what to check next?

I don't remember if I checked D7 & C4 before. I will also check the voltages that KrisBlueNZ had me check earier to see what I have.

Thanks John
 

mrmodify

Feb 13, 2010
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Starting to work on my power supply again. Can i substitute a 500 ohm resistor in the place of the thermistor just for testing purposes? The thermistor got fried by accident. I know the resistance of the thermistor is 500 ohms. I still have a issue somewheres and I would like to keep on workin on it till the new thermistors come in.
 

73's de Edd

Aug 21, 2015
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.


Sir modifier . . . . . . . .


Why 'soitanly

But keep a finger moving to the x4 transistor case, for an ongoing temperature check, in case you end up pulling some serious wattage thru the supply and get the X4-5 transistor pair warming up a bit.
'ole Germanium transistors gain parameters shift quite a bit at different temperature points.
Sooooooo . . .usual design practice is to use a temperature sampling diode / transistor junction or thermistor.
Your situation being the final one.

Ayyy mate . . . . .your units design also seems to be Ausss-strayyyyyyy-lee-yun in its respect of having the regulation portion
being in the negative power buss . . . . ."upside down" to speak.

All of the switch associated circuitry tends to fool one into thinking the unit is more complex that it actually is.


73's de Edd



.
 
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mrmodify

Feb 13, 2010
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Good Evening,
I have found that X4 is open and so is R22. R22 is a 500 ohm thermistor with no valid P/N. My question is do I need a PTC or a NTC thermistor to control the bias of X4 & X5? View attachment 24686
 

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mrmodify

Feb 13, 2010
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Power Supply is done, Fixed !!!, 15volt 1.5 amp test has passed. :D:D:):)
Thanks very much to all that have helped giving input to this project!!
In a few weeks will go and repair my other power supply which should be a breeze now.
And a very special thanks to the late KrisBlueNZ!!:(
 
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