Hello. I am trying to learn. Didn't take too long on this project to see I need help again. Didn't know exactly where to put this, but it has an LCD so I'll start here.
This is a PS board from a 1990's Krautkramer USD15SX A-scan unit. It is much like a digital scope with a little LCD display only instead of looking at electrical traces and signals from a probe, you are looking at reflective signals generated by an electro/mechanical transducer through some medium.
The unit is set up like a cardcage containing many boards standing vertically, side by side which plug into the main board. You have very limited access to the components on the boards.
I recently acquired 6 of these which were scrapped out in a room of junk. Since I came across Badcaps, everything I stumble upon is a candidate for learning and I decided to see what was inside. Knowing my limitations I chose the simplest one first to start on. It fires up and seems to have life, but won't turn off. Simple enough so I thought.
I ordered some connectors with the plan to make an extender card of sorts so that I can operate with the PS card components accessible. But in the meantime, I could measure voltages on the PS card edge and with unit running, the voltages were as follows:
VBAK=5V -5V=-5V +27V=+25V +12V=+12V +5V=+5V UREF = 2.5V
Then I thought I would first check the on/off switch. The front panel is a membrane overlay type keypad attached to a ribbon cable which goes to a connector on the main board. Pressing the on/off button, cycles 2 of the wires in the ribbon cable. I traced those 2 wires from the keypad, through the main board, to the 32 pin Molex connector shown in the photo on the PS board. So that tells me the switch and ribbon cable are ok. So I figured that was a clue that the problem lie on the PS board itself.
Then I went to another junk unit and found one with a working power supply and removed the one I was working on and replaced it with this one. Now the unit turns on and off via the switch. So mystery solved, the original PS is faulty. So now I'd like to fix it. Before I left to start on this, I measured the voltages again on the card edge with the unit switched off. So with unit only plugged in I get:
VBAK=5V -5V=0 +27V=+27V +12V=0 +5V=0 UREF =0
The power supply has 2 large black epoxy sealed boxes on it which I removed for the photo and ease of tracing on the board. AC comes in as shown and is converted to 27V DC in one and the other steps down 27VDC to the values shown. From a lot of reading, I found that the DC/DC converter contains a remote control pin which switches it on/off when high/low is applied. I Know that the on/off switch signals go to the pins shown on the big Molex and I know where the DC voltage comes from, but that is about the extent of my electronics knowledge. I tried to trace some circuitry to help me understand, but can't. I think this to be a very basic solution to anyone who understands stuff and will do anything you tell me to and report back, but need some guidance and explanation please. It has to be one of the small smd transistors or one of those chips, I bet, but I am not theory oriented enough yet to find out. I have attached some datasheets if they might help.
This is a PS board from a 1990's Krautkramer USD15SX A-scan unit. It is much like a digital scope with a little LCD display only instead of looking at electrical traces and signals from a probe, you are looking at reflective signals generated by an electro/mechanical transducer through some medium.
The unit is set up like a cardcage containing many boards standing vertically, side by side which plug into the main board. You have very limited access to the components on the boards.
I recently acquired 6 of these which were scrapped out in a room of junk. Since I came across Badcaps, everything I stumble upon is a candidate for learning and I decided to see what was inside. Knowing my limitations I chose the simplest one first to start on. It fires up and seems to have life, but won't turn off. Simple enough so I thought.
I ordered some connectors with the plan to make an extender card of sorts so that I can operate with the PS card components accessible. But in the meantime, I could measure voltages on the PS card edge and with unit running, the voltages were as follows:
VBAK=5V -5V=-5V +27V=+25V +12V=+12V +5V=+5V UREF = 2.5V
Then I thought I would first check the on/off switch. The front panel is a membrane overlay type keypad attached to a ribbon cable which goes to a connector on the main board. Pressing the on/off button, cycles 2 of the wires in the ribbon cable. I traced those 2 wires from the keypad, through the main board, to the 32 pin Molex connector shown in the photo on the PS board. So that tells me the switch and ribbon cable are ok. So I figured that was a clue that the problem lie on the PS board itself.
Then I went to another junk unit and found one with a working power supply and removed the one I was working on and replaced it with this one. Now the unit turns on and off via the switch. So mystery solved, the original PS is faulty. So now I'd like to fix it. Before I left to start on this, I measured the voltages again on the card edge with the unit switched off. So with unit only plugged in I get:
VBAK=5V -5V=0 +27V=+27V +12V=0 +5V=0 UREF =0
The power supply has 2 large black epoxy sealed boxes on it which I removed for the photo and ease of tracing on the board. AC comes in as shown and is converted to 27V DC in one and the other steps down 27VDC to the values shown. From a lot of reading, I found that the DC/DC converter contains a remote control pin which switches it on/off when high/low is applied. I Know that the on/off switch signals go to the pins shown on the big Molex and I know where the DC voltage comes from, but that is about the extent of my electronics knowledge. I tried to trace some circuitry to help me understand, but can't. I think this to be a very basic solution to anyone who understands stuff and will do anything you tell me to and report back, but need some guidance and explanation please. It has to be one of the small smd transistors or one of those chips, I bet, but I am not theory oriented enough yet to find out. I have attached some datasheets if they might help.
Attached Files
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