Dead IBM 5151 MDA Monitor

[3lectr1c]

No logic probe either I'm afraid. Never had to troubleshoot a CRT before.
I am young enough to hear the normal high-pitched CRT noise, is that what you're referring to here? I hear nothing from the 5151, no crackling, nothing.
I'll try to get it open soon.

 

[Hugo Holden]

No logic probe either I'm afraid. Never had to troubleshoot a CRT before.
I am young enough to hear the normal high-pitched CRT noise, is that what you're referring to here? I hear nothing from the 5151, no crackling, nothing.
I'll try to get it open soon.

Either there is no H drive pulse, or the H driver & H output stage in the VDU has died, or the VDU's PSU is out.

 

[3lectr1c]

Someone’s been in here…

That someone also took out the covered screws…

 

[3lectr1c]

It would appear that this fuse hidden in here is blown. It’s a bit tricky to get my probes in there, but I think it’s got no continuity. This is right near where the mains wire goes, I suspect it goes through this fuse.
Heater does not glow.

Question then: how would I go about finding a replacement, and also, what caused it to blow? If it’s something within the monitor, I’d assume that may need addressing. Everything else inside looks pretty normal though. Dusty, cobweb-y, but normal.

 

[modem7]

It would appear that this fuse hidden in here is blown. It’s a bit tricky to get my probes in there, but I think it’s got no continuity. This is right near where the mains wire goes, I suspect it goes through this fuse.

That fuse must be the one designated 'F1', shown at the far left of the circuit diagram at [here].

Heater does not glow.

Powered from the +15V out of the power supply - see [here].

Question then: how would I go about finding a replacement,

A quick online search showed that fuses with wire are still being sold. Example at [here]. Obviously, you will be verifying that the fuse is blown before acquisition.

... and also, what caused it to blow? If it’s something within the monitor, I’d assume that may need addressing.

Assuming a blown F1:
- Maybe it blew from old age, and/or some transient surge.
- Maybe a bridge rectifier diode went short-circuit.

 

[Hugo Holden]

In the case of the 5151, the leading cause of the fuse blowing (apart from the usual suspects) is an incorrect horizontal drive waveform. This can massively increase the current drawn by the H output stage. It typically destroys the H output transistor (HOT), though its possible the fuse could save it. You will know when you replace the fuse, if it blows immediately with no H drive signal sent to the VDU, probably, the HOT is shorted out. You could confirm that by disconnecting the HOT's collector terminal and trying another fuse. But of course, as mentioned by Modem7, there are many other causes for fuse blows.

 

[3lectr1c]

Thanks for the great info. So - is the fuse linked by modem7 above is the proper replacement, not just an example of that style?

 

[modem7]

So - is the fuse linked by modem7 above is the proper replacement, not just an example of that style?

An example. The SAMS Computerfacts document lists the 5151's fuse as 750 mA / 250V / fast acting.

It would appear that this fuse hidden in here is blown. It’s a bit tricky to get my probes in there, but I think it’s got no continuity.

Is the fuse blown or not ?

 

[3lectr1c]

I won't be able to tell for sure until I get the board it's on out, but I'm about 95% sure I had my probe properly touching the hard to reach side with it in there, and I got no continuity.

 

[modem7]

Regarding a possible replacement fuse: If you have decent soldering skills, an option is what is pictured at [here]; a 'standard' 3AG sized fuse that has solid wire soldered to the caps. The caps are lightly filed beforehand to facilitate the take of solder.

 

[3lectr1c]

Does this one that states "Board Mount" mean that it comes with the wires?

https://www.mouser.com/ProductDetail/Bussmann-Eaton/BK-GMA-V-750MA?qs=sGAEpiMZZMsIz3CjQ1xegWrlMY7vL59hMQx1FaPQCkY%3D

Others say "holder/clip", which I'd assume to be the type without wire?\

Your suggestion could definitely work too if I can't find a wired fuse.

 

[modem7]

Does this one that states "Board Mount" mean that it comes with the wires?
https://www.mouser.com/ProductDetai...=sGAEpiMZZMsIz3CjQ1xegWrlMY7vL59hMQx1FaPQCkY=

It certainly sounds like it.
The datasheet that the web page points to includes, "Optional axial leads available."
Maybe send a query to the supplier (to be sure).

Others say "holder/clip", which I'd assume to be the type without wire?

I assume the same.

 

[Hugo Holden]

I detest those soldered in fuses, it is a lazy cheap way to add a fuse and its a real pest if it needs replacing. The better fuses are either fitted inside a panel mount line IEC connector, or a panel mount fuse holder, or at least a chassis mount fuse holder.

One quick way to solve the dilemma in this case is to just fit a high voltage rated line fuse holder across the two terminals by soldering its wires there on the tag strip. So if your fuse blows you can just replace it asap with a standard fuse, without getting out the soldering iron:

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