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Flyback transformer question

R

rlauzon

Experienced Member
Joined
Feb 18, 2018
Messages
239
Location
Grand Rapids, MI
So I pulled my Sperry Portable down to play with it and the screen wouldn't light up. It would boot up, but nothing would display. It did work the last time I had it on the table (about 3 weeks ago).

I verified that the power supply is feeding a good 12VDC to the video board.

A visual inspection of the video board shows no leaky/bulging caps. My guess is that the flyback transformer is bad.

The only markings that I can see on it are:
FMC-0945 GL
KFB-5008
S X H N

Looking up these in Google don't really show me much. Sadly, I have not been able to locate any deep technical information for this system.

So my question is:
How can I find out what what to order to replace this transformer?
 
Exactly what Sperry Portable computer do you have?

In my opinion/experience it would be highly unlikely to be the flyback transformer itself.

More likely to be a driver transistor etc.

Dave
 
It's a rebadged Corona PPC-400.

I don't see anything there that I can recognize as a transistor.

I have it on my list to test and replace all the caps. But that's going to take a while.

1116221134.jpg
 
I can see a couple of little black transistors in the middle of the board (with the leads marked ebc).

There is a potential TO220 transistor just below the three (3) brown ceramic disk (?) capacitors.

There is also something (TO220) bolted to the black metal heatsink.

Of course, one of the TO220 devices may be a voltage regulator.

There does appear to be a Technical Reference Manual somewhere: https://www.cs.grinnell.edu/old/drupal6/node/247.html. Send them an e-mail and see if you can obtain a copy?

Dave
 
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I see the transistors now.

And there's something that looks like a voltage regulator bolted to that housing. Very similar to the other one just below the three (3) brown ceramic disk (?) capacitors.

I'll start there. Thank you.
 
No problem.

What is the part numbers on those devices?

A positive voltage regulator will be something like 78xx where xx is the voltage that the regulator operates at.

Dave
 
The 2 small transistors were both labeled "C1959"

The "voltage regulator" (since that's what it looks like to me) is labeled "BUV46". This one has some evidence of having been replaced in the past.
I was able to find the specifications for it on line, but they don't make them anymore. DigiKey had one FJP5555TU-ND TRANS NPN 400V 5A TO220-3 that matched the same specs.
 
That's not a voltage regulator, it is a power transistor - and is most likely the thing driving (or not - if it is faulty) the line flyback transformer.

Dave
 
It is a simple VDU and it would not take long to draw out its schematic, if you cannot locate it. If you can post it here. I would start with that, then formulate the required tests to locate the fault. The IC and transistor data sheets are online and that helps.

If you are not keen on that, there are places which can be scoped and tested, for example the horizontal output transistor(HOT), and obvious places where the supply rails are distributed with the meter. The transistors can be partially checked in circuit to determine if the B-E and B-C junctions are open or shorted. But if it appears that the HOT has a shorted B-E junction, don't be fooled by that, it is normally due to the low resistance winding of the driver transformer secondary. The diodes an be checked in circuit in most cases too.

Check on the board for any fusible resistors in the supply rail distribution or small axial inductors that might have gone OC. The Electrolytic caps can be basically in circuit checked with the ESR meter. Try not to unsolder components from the pcb unless there is a very good reason to suspect they are defective.

While the horizontal output transformer might have failed, it is a lot less likely than failures in other areas in the VDU's circuitry . I'm not sure why, when a VDU fails, the first thought is that the horizontal output transformer (LOPT) has failed. We have seen this before on the forum and some people remove them, start looking for a new one, when there is nothing wrong with the LOPT. Maybe people are watching too many misguided you tube videos on VDU repair, and I have seen a few. (don't worry I will eat the humble pie if it does turn out to be the LOPT)

Generally potted transistor horizontal output transformers and their integral EHT rectifiers are pretty reliable. The are proprietary parts for each VDU, seldom generic. If there is any chance of finding one it would be from the Dalbani Corp (they are on ebay). In most cases, the only way to get an exact replacement is from a defunct VDU of the same model. Other types can be adapted but it takes a lot of experience to know how to do that.

I assume when there was nothing on the display you would have checked that the CRT's heater was running ? There should be a dull orange glow seen on dim room illumination from the gun area inside the tube near its socket.
 
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Hugo Holden said:
I assume when there was nothing on the display you would have checked that the CRT's heater was running ? There should be a dull orange glow seen on dim room illumination from the gun area inside the tube near its socket.
Click to expand...

Yes, that was one of the first things I checked. There was **no** glow on from the gun area.
 
rlauzon said:
Yes, that was one of the first things I checked. There was **no** glow on from the gun area.
Click to expand...
In most small VDU's the heater is supplied directly by the 12V DC power supply, often with a series resistor. When the heater is cold, if that resistor is not there there can be an initial turn on current surge if the 12V supply has a low internal resistance. In some VDU's the heater does run from an auxiliary winding on the LOPT, not too common in the smaller VDU's though, but possible.

So given the heater is out, initial possibilities:

1) the series resistor, if there, has cone O/C
2) the CRT heater itself has gone O/C (I sincerely hope not)
3) there is a broken connection.

You can can check the heater continuity with the meter and trace the heater wiring to see where it connects.

If you could take some more photos of the pcb top above where the LOPT is and where the wires from the CRT socket connect could help.
 
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Sadly, this repair has come to an end.

The takeaway from this is to get a lock for the Man Cave - since some guests can't keep their nose out of a room with a closed door and mess around with things they know nothing about.

Long story short: Someone knocked the display tube off the workbench onto the floor over the holiday weekend.

The good news is that with a CGA card and a CGA-to-VGA converter that I had laying around, the portable is now working as a desktop.
 
rlauzon said:
Sadly, this repair has come to an end.

The takeaway from this is to get a lock for the Man Cave - since some guests can't keep their nose out of a room with a closed door and mess around with things they know nothing about.

Long story short: Someone knocked the display tube off the workbench onto the floor over the holiday weekend.

The good news is that with a CGA card and a CGA-to-VGA converter that I had laying around, the portable is now working as a desktop.
Click to expand...

What is the type number of the broken CRT? I may know where you can get another or possibly an equivalent.
 
Also, even if we cannot find a new CRT, Don't throw away the CRT's pcb because it has hard to obtain parts on it including the H.Lin coil In the rectangular black case, width coil, the LOPT and the CRT's deflection yoke are valuable too.
 
Hugo Holden said:
Also, even if we cannot find a new CRT, Don't throw away the CRT's pcb because it has hard to obtain parts on it including the H.Lin coil In the rectangular black case, width coil, the LOPT and the CRT's deflection yoke are valuable too.
Click to expand...

I've kept all the parts
 
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