IFR-1000s restoration, Tantalum Tantrum

 I bought a IFR-1000s "For parts not working" last year. It has been popping fuses ever time power was turned on. I first thought it was the what the IFR-1000s service manual calls the "Duty Cycle Regulator." The duty cycle regulator is a DC to DC flyback transformer. It uses a 20KHz astable multivibrator oscillator to power a transformer with 2 primaries and 3 secondary windings. It takes in around 12-15 volts, what is called "Raw DC" or the output of the 120 VAC to 15VDC transformer and bridge rectifier. The outputs of the Duty Cycle Regulator are 12V, 5V and -39 volts. The Duty cycle regulator is controlled by a comparator on the output side of the 12V. A comparator is connected to a voltage divider on the output side. When the output goes below 12 volts, it sends the 20KHz wave through the transformer to bring the voltage back up. 

I made a LT spice simulation of the duty cycle regulator to try to make sense of how it works. 

Here is a download link to the file:

https://drive.google.com/uc?export=download&id=13IjN2w7TkD26T-awUfalJqG0Q2Q-hGnD

Per John Kuivinen, WB6IQS, in his writeup "Repair and Servicing of the IFR 1000A/S Series", August 16, 2015. "The IFR 1000A/S family requires the following voltages / currents: 5 VDC regulated, 3.0 Amps. 12.6 VDC semi-regulated (max. 13.6 VDC), 3.76 Amps average, 4.2 Amps peak. -39 VDC un-regulated @ 100 mA."

You can change the output resistors in the simulation to make them provide the currents in the text above. They work with some small amount of ripple. I am not confident about the actual values of the inductance of the inductors and transformer in the simulation but they gave me the correct result so I am going to leave them alone until I can actually measure the coil inductances at some point.

However when I was first testing the Duty cycle regulator I was adjusting the 12 volt output and one of the output tantalum capacitors started smoking. Tantalum capacitors are prone to spectacular self destruction when they go overvoltage or fail due to age. They can smoke, explode, or just catch fire and cause PCB damage. They also smell quite terrible. When the capacitor in the duty cycle regulator smoked, it smelled like dead fish for days, I had to open the window and get a fan going to get the smell out. 

The dead tantalum capacitor really discouraged me from working on the IFR-1000s for a long time. I also did not have a way of producing the -39 volts. Last week though I was able to buy a couple of power supplies and keep testing. Thats when the next two tantalums blew up.

Since I believed the reason the test set was not working was because of the duty cycle regulator, I hooked up a 12v supply to the DC input of the back connector and bypassed the duty cycle regulator. Since it was a fused line and my power supply can regulate its own duty cycle I thought it would be ok.When I flipped the power switch, I quickly found the next bad tantalum capacitor. C46 on the 250KHz IF monitor Audio board caught on fire. See the image below.

It was a 47uF 35 volt Tantalum capacitor which caught on fire. I took the board out, cleaned the soot and gunk off and repaired the damage. It burned 4 resistors, the ground return path and part of the board. I bought sheets of 1oz copper foil a while ago for trace repair for just an occasion. I tinned both sides and soldered int in place. I tried just using an electrolytic for the time being but I just realized its actually in reverse polarization. I have new tantalums on order so I will just replace that when they come in.
After replacing the capacitor which I now realize is backwards I put the power supply back on and added another bench power supply to supply 5V. When I pressed the power switch, nothing blew up! So I thought everything was ok, and added the -39VDC supply. Spoiler it wasn't! Everything powered and nothing bad happened for a little bit. That is until I tried to get the oscilloscope to turn on. I was still trying to be cautious so flipped the AC inout mode switch on the scope and one more capacitor blew up. This time on the CRT power supply board.  This one make some thick purple smoke and a very loud bang noise. It did not however severely damage anything that I can tell. I too the CRT unit out of chassis to remove the capacitors and clean that board.
Pre-cleaning and cap removal above. Post clean and removal below.


I ordered some replacement capacitors. When i install them, I will try again. If you are curious about how the duty cycle regulator works try out the LTSpice file. LT spice is a free simulation tool from Linear Technologies originally before they were acquired by Analog Devices.

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