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VT320 Modern Replacement Power Supply

Alegend

Experienced Member
Joined
Jun 13, 2017
Messages
56
Location
Israel
After failing to fully cure the wobbly display on one of my VT320s (Thread) I have decided enough time was wasted fiddling with the old, overcooked PCB.
I have tried replacing the main DC/DC covnerter IC, as well as several capacitors, but the display still had a small, annoying, noticeable horizontal wobble.

So I have designed and built a replacement power supply board for the VT320.

All the production files have been placed on github.

This board is a drop-in replacement for the DEC 50-17278-01 board that supplies +5V, +12V and +18.5V to the main VT320 board.
It is based on off-the-shelf parts (mainly Astrodyne TDI board-mount power modules) that total under $60 in small quantities.
My replacement board supplies +18V instead of the original +18.5V, which seems to work just fine.
Note that, at least according to available schematics, there were two versions (pinouts) of the DC power connector. I have designed the board according to the terminal that I needed to fix. But a different-pinout VT320 can easily be changed to connect to my new board by replacing the order of pins on the female connector. All is needed is a small flat head screwdriver.
Some photos below.
Board Connected, Mounted and Powered.jpgBoard Rev 1_0 Mounted.jpgAssembled Board.jpg
-Alon.
 

Alegend

Experienced Member
Joined
Jun 13, 2017
Messages
56
Location
Israel
Wow -- Great job!! Are the power supplies in the VT320's that unreliable?
Thanks! don't know much about this board's failure rate in the general wild population of VT320s, one bad one was enough for me...
-Alon.
 

Roland Huisman

Veteran Member
Joined
Mar 24, 2011
Messages
1,394
Location
The Netherlands
Wow -- Great job!! Are the power supplies in the VT320's that unreliable?
Recently fixed one for a friend... This one had a quite common chain reaction but I got it back online. Capacitors were in very bad shape and that leaded into a disaster blowing up the switching fet, regulator IC, diode, power rectifier and mains fuse.


1.jpg

Here I've drawn the schematics and error current flowing trough the power supply. I've done that to explain these typical switching power problems to that friend. The capacitor on the regulator side C513 was especially bad. This caused the uc3842 regulator IC to have a very unstable power supply and has insufficient power to drive the power mosfet. The whole regulating gets instable, the mosfet starts to heat up because it will not fully conduct any more because of the lack of driving power.. The FET gets shorted, a high voltage appears on the gate because of the internals being fried. That current flows into the UC3842 trough the 1N4148 diode. The shorted mosfet now shorts the full 320VDC power supply trough the transformer. The transformer will survive this, but the bridge rectifier and mains fuse will be blown too. So you have to check/replace the full circuit before turning it on with new components.

VT320 voeding.jpg

But nice job to create a new power pcb. You never know when this might help others...

Regards, Roland
 

Alegend

Experienced Member
Joined
Jun 13, 2017
Messages
56
Location
Israel
Recently fixed one for a friend... This one had a quite common chain reaction but I got it back online. Capacitors were in very bad shape and that leaded into a disaster blowing up the switching fet, regulator IC, diode, power rectifier and mains fuse.


View attachment 1242398

Here I've drawn the schematics and error current flowing trough the power supply. I've done that to explain these typical switching power problems to that friend. The capacitor on the regulator side C513 was especially bad. This caused the uc3842 regulator IC to have a very unstable power supply and has insufficient power to drive the power mosfet. The whole regulating gets instable, the mosfet starts to heat up because it will not fully conduct any more because of the lack of driving power.. The FET gets shorted, a high voltage appears on the gate because of the internals being fried. That current flows into the UC3842 trough the 1N4148 diode. The shorted mosfet now shorts the full 320VDC power supply trough the transformer. The transformer will survive this, but the bridge rectifier and mains fuse will be blown too. So you have to check/replace the full circuit before turning it on with new components.

View attachment 1242399

But nice job to create a new power pcb. You never know when this might help others...

Regards, Roland
You clearly have better repair skills and probably more patience as well... I have replaced five capacitors including the two high voltage ones, as well as three in the low voltage and filter section and the IC. I got suspicious of the mosfet or maybe the 18.5V filtering coil. But buying more parts and spending many more hours of work, no success guaranteed, made less sense to me than to design and build a new power supply.
 

Roland Huisman

Veteran Member
Joined
Mar 24, 2011
Messages
1,394
Location
The Netherlands
Well I have a lot of fun repairing these things. So I thought lets explain what can go wrong in a worse case... But I'm sure your new design can be a great help for others as well. So well done! These things do not have an endless life. So a brand new power supply will live quite long I think :)
 
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