Repairing 5151 monitor

[rvdbijl]

Hi All,

I've started working on an old 5151 monochrome monitor that worked for a little while once I got it. I left it on for 30 mins, walked away and came back to a nice burnt electronics smell...

I started diagnosing and replacing components. First thing that was broken was the main 22V transformer. I replaced that and the monitor still would not fire up. Measuring voltages when AC power was present, but no video signal looked good. When applying a video signal, the secondary VAC from the transformer dropped to 17V (from 21V) and something started smelling .. warm.
I found what was getting warm, and it was component D505 (diode, RGP15J or NTE552 based on the replacement component type list). Looks like this is a standard 600V 1A diode. Could I replace it with a 1N4005 (also specified at 600V, 1A)? Or does it need to be something specific?

Also - I have some trouble imagining that this was just a bad diode.. So if this guy is getting awful hot - what else could be the cause? Anyone have any ideas?

 

[Hugo Holden]

I think in the 5151, I would have to look at the circuit again, it does not have an independent Horizontal oscillator. Its H scan drive, to the H output transistor, is derived from the input connector from the computer's video card along with the vertical signal and the video. This was to some extent a potentially tricky situation because if there is anything wrong with the H sync being fed to the monitor , for example, the wrong timing (frequency) or pulse width, then the horizontal output transistor can be destroyed or the horizontal output stage (transistor and H output transformer) can draw very high currents from the power supply, loading it down.

When you say "no video signal" presumably that means no video signal and no V & H sync signals as well, or do you mean that you just disconnected the video signal link leaving the syncs alone ?

I would check very carefully the output waveforms from the video adapter card, with the scope, first to see if they are in spec, especially the H sync.

Many other types of monitors have their own independent H scan oscillator, it simply sync locks to the incoming (or separated) H sync and if the H sync is corrupt in some way, it cannot harm the monitor.(this is the way its done in TV's too). But unfortunately it is a different story with some early computer video monitors like the 5151. Though they did attempt to improve the situation in the 5151 by transformer coupling to the output transistor, but this does not solve the fact that the peak current in the H output transformer's primary is determined by how long in each cycle that the output transistor is switched on for, so if the transistor is turned on too long per cycle, the currents and the EHT also increase. So its a riskier proposition to let the H sync pulse be in control of that and it probably only saved a few $ worth of parts doing it that way.

So check out the H sync first before moving on to any remaining faults would be my advice.

 

[rvdbijl]

Hugo,

Thanks for your response!

I know the video card is OK since I have another 5151 that works fine. This is my 2nd 5151 and I'd like to fix it because, unlike my first one, this one didn't have any burned in patterns.

So I'm pretty sure the HV sync is fine, as is the video signal. When the diode (D505) was burning up, it seemed to overheat TR24 (transistor) as well, but not as dramatically. It appears that D505 is connected into one of the pins of the HV transformer.

I did test quickly with the (half burned) diode removed, and all the voltages going to the HV transformer looked good (except the pin that this diode was connected to).

 

[Hugo Holden]

Ok, If D505 is cooking up, most likely it is C509 (0.022uF 630V cap) shorted out, its less likely the high value resistors also it feeds have gone very low. Also check D505 itself, that it only conducts in one direction on the meter.

 

[rvdbijl]

Ok, If D505 is cooking up, most likely it is C509 (0.022uF 630V cap) shorted out, its less likely the high value resistors also it feeds have gone very low. Also check D505 itself, that it only conducts in one direction on the meter.

Tested C509 after desoldering it. In spec at 21nF on the meter. Definitely not shorted out. Tested a few of the resistors in the area (10MOhm & 100kOhm) and they look good too. D505 is total toast as it literally swelled up and turned completely black.

I bought 10 replacements for that one and will try replacing it (measuring it now, it conducts in both directions). If it smokes again, at least I'll be able to do that 10 times. They're supposed to be in next week.

If you have any other suggestions on what to test, let me know!

Thanks!

 

[Hugo Holden]

A good diode to replace that one with would be a UF4007, it never hurts to go up a little on the diode's rated voltage, and in this application a fast diode is better than a standard one. Most likely the original one had its peak reverse voltage exceeded by the H output transistor having an incorrect drive signal, probably only briefly, at some point.

 

[rvdbijl]

Hugo,

That did the trick. Monitor is back up and running. Damage was: Bad transformer, bad diode and a bad electrolytic cap.

Thanks for your help!