Need to get MG Electronics Power Supply repaired...or NOT!?

7812 voltage regulator test results:

w/ Power off and writing facing me

Black lead on the center pin, read lead on the left pin = 1.2mV and it trickled down to 0

Black lead on the center pin, read lead on the right pin = .5mV and it trickled down to a negative number

w/ Power On and writing facing me

Black lead on the center pin, read lead on the left pin = 30V

Black lead on the center pin, read lead on the right pin = 12.04v

I re-tested the D313 for continuity (after the unit has been turned on and off)

There is NO continuity between the base and the emitter or collector

There IS continuity between the emitter and the collector

I retested the corroded diode and it is ok.  The corrosion was giving me a bad reading previously but I scraped it off and the diode shows a voltage reading of .519v in one direction and nothing in the other.

All sounds good and normal except the D313. We need to test that one out of circuit and verify if it is good or bad.

At some point I mistyped Read when I meant to type Red. I mention it so that we don't start a new convention for what is the correct nomenclature.  Sorry about that. I always try to proofread but at 1:30 AM I may miss things.

John

Yes.

Here are more photos of the potentiometer, which is the most complex one I have ever seen, wire with two resistors.  Last picture shows the capacitor.

Diode Tests look normal.

John

The Ceramic disk capacitors all look normal.

John

The component in the picture is the range switch which chooses 0 to 7 Volts - 7 to 12 volts and 12 to 20 volts. The potentiometer will be the control to the right as you look at the front panel. It should turn smoothly without clicking and it has a black ramp painted on the front bezel to denote a change in the voltage as it is turned.

John

Here is one oddity I noticed.  It looks like the yellow lead to the center terminal of the pot was in contact with the terminal to its right as pictured.  So much so that I thought it was welded to both.  But I pushed it with some tweezer and it separated as pictured in the last two photographs

It's difficult to test because I don't have three hands, but it seems like the R value between the two outside terminals on the pot is 1.6 kOhms.  The value between the red outside lead and the center yellow lead ranges from .9Ohms to 1.6k Ohms.

Hi Matt,

The readings on the potentiometer appear to be normal and acceptable. I would not have been surprised if the yellow was connected to both terminals. This potentiometer is being used as a resistive trimmer in either configuration. I am waiting to see what voltages you measure on the leads of the two LM 741 ICs.

John

Hi Matt,

Here is another experiment I would like you to try. There are two large capacitors on the board with the diodes. I would like you to set your digital meter to DC volts. Turn on the power supply and measure the voltage across the capacitor leads in each of the settings of the range control on the front panel. We may get different voltages and we may get only one voltage. This may tell us something about the actual circuit design. Do this test for each of the two filter capacitors. Note the lead that is closest to the line on the side of the capacitor is the negative. Let me know what you find out.

John

The diode test on the power transistors yields .010 volts between the base and the emitter in both polarities while the transistor is in circuit.  All four power transistor yield the same results

The diode test on the power transistors yields .010 volts between the base and the emitter in both polarities while the transistor is in circuit.  All four power transistor yield the same results

The diode test isn't going to tell us much on the power transistors. You could try to test them with the analog meter. They could be hooked in parallel and if one was bad but not shorted you would not see it.

John

John,

As you well know this isn't my area of expertise.  Too bad there is no schematic.  Some thoughts on things I didn't see but may have missed:

• have all of the electrolytic caps been checked
• can the transformer be isolated and checked
• if the caps, diodes, transistors, assorted ICs, and transformer are good then is it worthwhile to go through the circuit with an iron and touch up all the joints to make sure all the connections are good

Frank

Thanks for the input Frank, Shabaz, and Gene. I will keep your suggestions in mind.

John