Electronics > Repair
BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed)
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tautech:

--- Quote from: max.wwwang on December 22, 2024, 08:12:13 am ---
--- Quote from: tautech on December 22, 2024, 07:45:15 am ---Variable PSU and a current limiting resistor normally finds reverse breakdown voltage with little fuss.

--- End quote ---
Gotcha. This requires it to be taken off the board if I understand correctly.


--- Quote from: tautech on December 22, 2024, 07:45:15 am ---Connectivity issue somewhere or something intermittent with the diode or its snubber.
There's really not much to check in a standalone winding on the secondary side.

--- End quote ---
If the secondary side is all good, i.e. problem is on the primary side, is it possible to have all other voltage rails working like a dream with only one rail scrap? I just don't understand how this is possible.  |O

--- End quote ---
Can't be.

Referring to your schematic the 6V rail is a stand alone dumb rail with no feedback to the primary control side.
It is possible the secondary is open/faulty and as intermittent one might wonder.....
Dry joint would be my first suspicion.
max.wwwang:
I have not seen any different view on the location (primary vs secondary) of the problem. All I need to do is first get it back to where it was then reflow all of the joints related to the 6V rail.
max.wwwang:
Managed to trace and draw up the schematic of the daughter board, which is connected through the P190 socket (10-pin). Due to my lack of grasp of its working, the layout is certainly not the best. Insights are welcome regarding its working (or error spotted).

Clearly, this is not part of the power supply – only supplied by it.

Questions:

1. Two comparators' output pins (U2-13/14) are bound together. This doe not make sense to me. How does it work (unless their outputs are bound to sync, but in that case what's the point of having two identical outputs)? I double checked the board for multiple times and am sure they are like so on the board.

2. We know that on the main board, T125 and T126 should never be on at the same time (otherwise the rectified HVDC will be shorted). This interlock mechanism does not seem to exist on the main board. Is it on the daughter board? If so, how is this achieved?

3. A general question about comparator – I learned from Dave's video that in op-amp circuit analysis, it can assumed that the current through the input pins is zero. Does this rule apply to comparators as well?
inse:
1. comparators are usually open collector/drain output, so you can parallel them as you need
2. if you trust in your control circuit or software, you can skip a hardware interlock
3. yes, they are high impedance inputs

Reverse engineering with only partial schematics and without access to the hardware is very tiring.
If you want to gain experience, for example older HP service manuals for test gear are a valuable resource, the functional blocks are explained in detail.
Reverse engineering without any aids is then next level.
max.wwwang:

--- Quote from: inse on December 24, 2024, 09:51:06 am ---1. comparators are usually open collector/drain output, so you can parallel them as you need
2. if you trust in your control circuit or software, you can skip a hardware interlock
3. yes, they are high impedance inputs

Reverse engineering with only partial schematics and without access to the hardware is very tiring.
If you want to gain experience, for example older HP service manuals for test gear are a valuable resource, the functional blocks are explained in detail.
Reverse engineering without any aids is then next level.

--- End quote ---

Indeed it's tiring with only part of the schematics and without access to working hardware. Thanks for taking the trouble to look and answer my questions.

1. I still don't understand. IIUC, with VCC=12V (and GND), the output of a comparator is either 12V or 0V. So if one output is 12V but another gives 0V, there will be a short between these two outputs? Of course this may be avoided by software. The problem I have still is, what use can this possibly have?

2. Very true. In this case, the only pins that are logic interface with the main part of the machine is P190-1/2/3, of which 3 is output and the other two (1/2) input. It's two-way communication. Still trying to figure out how this works in principle with the rest of the machine only as a black box.

3. Thanks. This way I can figure out the voltages at the net points on a resistor ladder (lower part) +12V - 27k/27k/27k/1k//1k - GND (where '//' may be set by op-amp U1A when the motor is on, through P190-10), which I believe somehow serves as motor status indicators in various parts of the circuit.

Yes, old gear like HP and Tecktronx etc. have amazingly detailed service documentation, which is a superb source for learning and to gain experience. Will not lose the opportunity when I get my dirty hand on one, particularly for repair.

Updated the PSU schematic combined (without the daughter board).
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