3 What are these diodes?
I am repairing a Pioneer PDP-50MXE1 with a failed power supply. I've identified the area of failure (blown IC MIP3E3 - DIP-7 MOSFET type IC for a Switching Power Supply Control, with DRAIN Voltage 700V and Control Voltage 8V ) and have sourced a replacement for that, but am having trouble identifying some diodes that have been damaged by the burnout.
Does anyone know where I can find a schematic of the PSU, or can anyone help me with identifying replacements for the blown or suspect diodes? The board is marked "AXY803" and according to the PDP-50MXE1 service manual, is replaced as a whole rather than serviced - therefore no schematic for it. Here's a photo of the board, board markings and blown area.
And here's a rough(and incomplete) schematic I've drawn of the blown area by following traces - some of the suspect diodes look to me like back emf / surge / transient suppression across transformer windings. The diodes in question are circled on the schematic, with mock-ups of their body markings.
Here is a photo of the diodes in question, showing (left to right) D139 marked "Z100 4E" (100V zener?), D149 marked "Z150 4.D" (150V zener?), D133 marked "74 90", D141 marked " B3710 45" (200V zener? maybe 1n3710B?) and D205 marked "24B2" (24V zener? maybe NTE5031E?). I don't know what the two cathode bands on D141 mean either.
D133 has unusual markings and I can't make head nor tail of them. reading one weay up, it has numbers 7 then 4 then at right-angles "90". Reading the other way up, it has numbers "06" then "4" and "7" at right angles. here are some photos that might help identify it.
Cathode at the bottom, rotating the body left-to-right:
Cathode at the top, rotating the body left-to-right:
Any help or insights greatly appreciated.
Ist dies eine gute Frage?
5 Kommentare
A photo would be good. Incidentally, I've never seen a Zener spec'd at above 20V or so - I suspect D139 and D149 are something else.
von Philip Le Riche
@serafis post some good images of your board with your question. Make sure that the board marking show up anywhere. The power supplies on most service manuals are not included since they are manufactured by somebody else. So we need to see those board numbers to possibly find a schematic. use this guide for that Bilder zu einer vorhandenen Frage hinzufügen
von oldturkey03
It looks like some, at least, of those diodes are avalanche TVS diodes - see https://www.allaboutcircuits.com/technic... - but hard to tell what their rating should be without knowing the expected voltages in the circuit. Certainly if one of them went short circuit it'd take out the others in series with it.
von Philip Le Riche
Thanks Philip, from what I've read a TVS diode is similar to two back-to-back zeners, and two zeners in series act like a single zener of the sum of the two voltages. If my understanding of this is correct (and Google is correctly identifying Z100 and Z150 as zeners), then the diodes across the transformer windings would be equivalent to a 250V zener back-to-back with another high voltage diode (B3710), which would have a similar effect to a TVS.
But you may well be right. Perhaps the double-cathode-band diode (B3710) is a TVS rather than a zener. Hard to know. In any case I may just try a couple of high voltage zeners and/or TVS, and see what happens - it can't be much more broken than it is now!
The one I'm really struggling with is D133 (74 - 90 or whatever the markings mean). There are more of these on the board elsewhere, near 200V and 280V electrolytics, so I think it's high voltage but beyond that, no idea.
Anyway, thanks for your insights and any further help greatly appreciated.
Cheers, Jon.
von Jon Frere
The other two diodes with the strange "74-90" markings are in the rectification area for the VSUS voltage, which I think from research is typically around 200V or greater.
Each of these "74-90" diodes sits in parallel with a 10uF 250V electrolytic, after the main VSUS rectification and before 3x parallel 800uF 280Vsmoothing caps.
From their small physical size and where they're located (across capacitors), I'm thinking they are more likely to be just high voltage ultra-fast recovery diodes, something like SF28 (600V 2A ultrafast) or MUR460 (600V 4A Ultrafast), rather than zeners, TVS or Schottky.
Any thoughts?
Thanks.
von Jon Frere