Hi all.
Recently I picked up a vintage HP 37717B communications analyzer which uses one of these MML200 Coutant power supplies. Needless to say the PSU is not working, and surprisingly there is apparently nothing available on the web regarding repairing one of these. No stories, no schematics, zilch!
There are however many companies offering repair 'services', however I don't have the luxury of sending it off, so am intending to do the repair myself.
I admit right now that I am no expert on smps repair, although I do have a grasp on the principle of the general design:- Filter, rectification, chopper, feedback etc.
The PSU was totally dead on all the outputs, so, when I first opened this PSU up I quickly located two RIFA caps and an BTA12-700B thyristor (crowbar protection?) which were bad. After replacing these there was somewhere around +6vDC on one of the outputs. I assumed that the unit was still bad (although perhaps this was perhaps a standby voltage?). The output comes from three separate modules which are labeled:
6A - +12v, 0v and +5.2v, 0v.
12A - +12v, 0v, +S and -S
25A - +5v, +S, -S
God only knows what the +S and -S are supposed to be?
Anyway I thought it would help to see if we were getting any chopper signal on the gate of the single main 10A 900v 2SK1539 N-Channel FET.
Here, I made a schoolboy error.
Although the information is in my minds data banks, for whatever reason I didn't remember that some SMPS heat sinks are LIVE (in this case +362vdc). To compound matters, I used a damaged Chinese DMM's frequency option to check any Hz on the FET gate, whilst using the LIVE heatsink as the return.
Doh!
After a clear pop, all output voltage was lost and the FET was shorted across all three pins.
The FET is now replaced with something similar but the PSU pwm controller is not switching the FET gate.
Looking at the datasheet for the UC3844D current mode PWM controller it would appear that the 3844 output should be connected directly to the FET gate. Obviously sticking a high voltage onto the gate (as I did) is also sticking it onto the pwm output signal path, which is where I am now. Checking which components are on this trace in order to find out what has been damaged.
It doesn't help that I have no means to measure frequency here, other that a scope (which I've never used for this)...
Therefore I suppose my question here would be, "has anyone here ever repaired one of these, or even how to test one of these pwm controllers with only limited equipment?"
Many thanks in advance and loads more photos if requested
The last photo has the PWM controller circuit board and pin 3 is the output
Recently I picked up a vintage HP 37717B communications analyzer which uses one of these MML200 Coutant power supplies. Needless to say the PSU is not working, and surprisingly there is apparently nothing available on the web regarding repairing one of these. No stories, no schematics, zilch!
There are however many companies offering repair 'services', however I don't have the luxury of sending it off, so am intending to do the repair myself.
I admit right now that I am no expert on smps repair, although I do have a grasp on the principle of the general design:- Filter, rectification, chopper, feedback etc.
The PSU was totally dead on all the outputs, so, when I first opened this PSU up I quickly located two RIFA caps and an BTA12-700B thyristor (crowbar protection?) which were bad. After replacing these there was somewhere around +6vDC on one of the outputs. I assumed that the unit was still bad (although perhaps this was perhaps a standby voltage?). The output comes from three separate modules which are labeled:
6A - +12v, 0v and +5.2v, 0v.
12A - +12v, 0v, +S and -S
25A - +5v, +S, -S
God only knows what the +S and -S are supposed to be?
Anyway I thought it would help to see if we were getting any chopper signal on the gate of the single main 10A 900v 2SK1539 N-Channel FET.
Here, I made a schoolboy error.
Although the information is in my minds data banks, for whatever reason I didn't remember that some SMPS heat sinks are LIVE (in this case +362vdc). To compound matters, I used a damaged Chinese DMM's frequency option to check any Hz on the FET gate, whilst using the LIVE heatsink as the return.
Doh!
After a clear pop, all output voltage was lost and the FET was shorted across all three pins.
The FET is now replaced with something similar but the PSU pwm controller is not switching the FET gate.
Looking at the datasheet for the UC3844D current mode PWM controller it would appear that the 3844 output should be connected directly to the FET gate. Obviously sticking a high voltage onto the gate (as I did) is also sticking it onto the pwm output signal path, which is where I am now. Checking which components are on this trace in order to find out what has been damaged.
It doesn't help that I have no means to measure frequency here, other that a scope (which I've never used for this)...
Therefore I suppose my question here would be, "has anyone here ever repaired one of these, or even how to test one of these pwm controllers with only limited equipment?"
Many thanks in advance and loads more photos if requested
The last photo has the PWM controller circuit board and pin 3 is the output
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