Can a bad monitor damage a PSU?

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Can a bad monitor damage a PSU?

Sorry for the long story, but... I picked up a IIe from a thrift store a couple of weeks ago. Looks like it's from 1985. I got it home, hooked it up to a small portable B&W TV that had a composite input, and it seemed to work fine. I got the "beep", ran/passed the diagnostic test, switched to ROM BASIC and ran a few short programs. No disks to test the drives yet.

Later I bought a Sanyo DM 4512 security monitor off of eBay. It's also from around 1985. I eventually tracked down the adapter so I could plug an RCA plug into it. (All these old Sanyos seem to have RF jacks in the back, but it's really composite input, isn't it? I hope???) I powered it on not hooked up to anything and it didn't seem to show any immediate problems.

Of course, I knew I had to replace the Rifa cap in the PSU (AA11040-B), so I ordered a variety pack of X2 capacitors off Amazon and swapped in a new 0.1u capacitor, which is the correct capacitance for C1 from what I gathered. I first double-checked the replacement's capacitance with my meter and it was right on the money. (Interestingly, I tested the original Rifa after I got it out, and it clocked in at 0.13u. Not sure if that means anything.)

The computer powered on with my little TV as a monitor and the new PSU cap. LED light on motherboard came on. Got the beep, etc. So far so good...

Then I powered down and hooked up the Sanyo to the Apple for the first time. I think I powered on the Sanyo, then the Apple, but it might have been the other way around.

Anyway, I got nothing. Not a pop. No smoke. Just a dead computer. No LED, no beep. I disconnected the monitor and tried again a few days later with no montor attached. Still nothing. 

Checked the power rails via one of the expansion slots and also a couple of rails at some of the chips (where is the best place to check power on the motherboard anyway?) Nothing. Then checked voltages at the end of the PSU-to-Motherboard plug (so, not under load), and got nothing. Confirmed that the power cable from mains to the back of the PSU was carrying power.

Opened the PSU and checked the fuse. It's not blown! The cap looks fine, but my next step is to pull the PSU board out and poke around a bit.

So... did I just get a bum replacement capacitor that could only withstand one power-on before failing? So the Sanyo monitor is just a coincidence? (But that capacitor failing wouldn't take down the whole PSU, would it?)

Or did the monitor do something terrible (I don't know, pull a ton of current??? Send voltage out the video input???) that hosed the PSU in general--or worse, the motherboard--and the new capacitor is just a coincidence?

 

 

 

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Start troubleshooting

Start troubleshooting logically from where the power comes in. Disconnect the psu from the board and power on the psu. Check the voltages at the plug. 

 

 

If everything is good, you can check for shorts on the power rails on the board. I believe on the IIe there are 4 axial capacitors near the plug. Each one filters a different voltage rail. Check across both leads of each cap to make sure there is no shorts.

 

 

If nothing is shorted, plug the psu back in and power on and check the voltages at those caps. Look at the schematic to see which cap should be which voltage. 

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The chance of a monitor

The chance of a monitor killing a poer supply are pretty remote.

 

Disconnect your power supply from the computer and check for the voltages on its pins.  You're looking for +5V, -5V, +12V, and -12V.

 

If you get nothing then chances are (if you have an Astec-branded power supply) that C7 (a small 220 uF capacitor) on the power supply (it's a smaller one that sits all by itself about midway down the PS circuit board) is blown.  

C7 is the only capacitor (other than the RIFA filter cap) that ever fails on Astec supplies.

 Without it, the power supply can't start up, so you'll get no voltages.

 

This does not apply to DynaComp branded power supplies.  Their failure modes are different.

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nick3092, thanks! As I

nick3092, thanks! As I mentioned in my (admittedly long/dense) post, I  did check power at the power cable (good) and at the PSU (bad). So the PSU is really most sincerely dead.

 

But good tip on those four capacitors! Luckily they're not shorted, so I'm feeling better about my odds that the motherboard isn't the problem (and the cause of frying the PSU). And good to know there's a convenient place to check all four voltages once I get the PSU back up.

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It's an Astec

baldrick, yup, it's one of the later Astecs (AA11040-B). I'll give the C7 a look, thanks! Those other large caps on the AC side are holding charge pretty well, even after 48 hours. Ask me how I found out... :-)

In the meantime I wound up ordering one of those new PSU kits from ReactiveMicro, where you keep the old case and just swap out the guts. Arrives next week. I suppose if I get the old one working, I could return the kit.

Then again, a repaired 37-year-old PSU is still a 37-year-old PSU. So there's that. Hmmm...

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bzzt

An F connector (the kind with threads around the sleeve) is never used for composite video. To be used with computers, that type of TV/monitor requires an RF modulator. I hope the "adapter" you used was a modulator?

Sets from that era can be "live chassis" units where nothing is grounded (such units cannot be converted to have AV inputs without creating electric shock hazards). It is possible that line voltage came through the RF jack and popped the video output transistors Q1 or Q2 (or even other components on +5 and -5 volt rails). Isolating the computer equipment from possible live-chassis TV sets is one of the necessary functions of a modulator.

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Hmmm...

Well that's what I was afraid of. It was sold to me as a "composite" monitor, and I just assumed those security-type monitors used weird jacks. Plus I've seen tons of pics of Apples hooked up to those various Sanyos, so figured it would work. Shows what I know.

I'll plug the sucker back into the wall and test the video in jack for any voltages coming out. Fingers crossed...

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robespierre wrote:An F
robespierre wrote:

An F connector (the kind with threads around the sleeve) is never used for composite video. To be used with computers, that type of TV/monitor requires an RF modulator. I hope the "adapter" you used was a modulator?

Sets from that era can be "live chassis" units where nothing is grounded (such units cannot be converted to have AV inputs without creating electric shock hazards). It is poss ible live-chassis TV sets is one of the necessary functions of a modulator.

While you may be correct with your comments about certain un-grounded vintage televisions (and the particular case you're describing about line voltage leaking into the 75 ohm RF connector is almost as rare as hens' teeth), the Sanyo DM4512 (actally I think it is a VM4512) security monitor does not have RF inputs.

Typically that monitor (of which there were dozens of variations on the same chassis) had BNC video inputs (they're actually twist-lock baynoet type connectors) as would be typical in many closed-circuit TV camera systems.  I have a variation of that monitor on an identical chassis (with a different brand name) and it has BNC inputs as well as a single RCA composite input.  Many versions of that monitor had only BNC inputs.

While those BNC inputs are different form factors to the RCA inputs on a domestic composite monitor, they would be compatible.  And there are lots of BNC-toRCA adapters available, which is likely what robespierre bought.  Stray voltage on that input is highly unlikely.

 

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Yeah, I took a closer look at

Yeah, I took a closer look at the pictures from that listing. It's incorrectly listed as a "DM" 4512. The correct model is VM 4512 and there should be a service manual out there somewhere (maybe it hasn't been scanned in yet). It has two coaxial inputs labelled "VIDEO", so definitely baseband and not RF, plus two switches. The connectors look to be SO-239 ("UHF") style, not BNCs and not F connectors either.

So without a manual, I couldn't say what the functions of the switches are, or why there are two input connectors when there doesn't seem to be an easily accessible input switch. It may be a feed-thru arrangement where they are for video in and out, with 75Ω termination switchable by one of those unlabelled switches. The other switch may be for disabling AGC or some such.

I wouldn't rule out (mains) electrical leakage current as the cause of the problem. If nobody has been inside this unit in 40 years and no electrical safety tests have been performed, there are all kinds of things that may have happened to break the isolation.

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More on the monitor

Well, just to be clear, there might not be any problem at all with the monitor. I did check for stray voltages yesterday and didn't find anything. For all I know my PSU just decided to coincidentally fail at the same time I tried out the monitor, and the two things may have nothing to do with each other.

The monitor is indeed a VM4512. The video jacks are threaded, like the sort of regular coax connections I'm used to seeing on cable boxes, etc., but they're about 15mm in diameter, or 50% wider.

Here's a (poor) pic of the back.

  • I've got the RCA adapter on the video input.
  • The other jack is an output.
  • The impedence switch is labeled 75 ohms and "high" (300 ohms, I assume?)
  • The switch on the far left is "DC Restoration" off/on. Anyone know that that does?

 

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Your monitor is the same as

Your monitor is the same as mine, only different connectors.  Looks like RG59 coaxial conectors.

It has an impdance switch that switches between 75 ohms and something higher - probably 150 ohms, depending on the video source.

Come to think of it I have seen (and we have at our curling club) closed circuit composite cameras with RF barrel connectors pushing composite (not RF) video. 

CCTV is a wierd and wonderful world.  But I can pretty much guarantee that your monitor did not damage your power supply.

The DC restoration affects video output, I'm not sure exactly what it does, but the output is notably lower when that switch is flipped.  There was a video about a similar monitor on the Youtube channel for Adrian's Digital Basement:  https://www.youtube.com/watch?v=KrX3s_Dp7kc

My money is still on your power supply's C7.

 

 

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Ha!

I watched that video a couple of months ago when it came out, and apparently filed away exactly none of the infomation at the time. Thanks for the reminder. If only mine had turned out to secretly be amber too. Oh well.

About to dig through my parts bin to see if I've got a 220uf cap in there somewhere...

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No joy

Swapped out C7 and tested at the PSU output plug (no load). Bupkis. I couldn't test the capacitance in the old C7, but I did check that it wasn't shorted.

I guess I could keep re-capping. But probably going to throw in the towel and just wait for the replacement PSU to come in.  ¯\_(ツ)_/¯

In the meantime, I might dig around in the attic for something "disposable" that puts out a composite signal, to test the monitor more.

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75 Ω vs high impedance

75 Ω vs high impedance controls whether the video signal is being internally terminated. When switched to "high impedance", the only thing connected to the video input-output is the amplifier circuit, which has a high input impedance, meaning it has negligible effect on the signal. You could connect several of these monitors in series to the same video signal, and since their amplifiers have a high impedance, the quality of the picture wouldn't change much whether there were 1 or 10 displays. When the switch is in the 75 Ω position, a resistor of around 75 Ω is shunted between the input and ground. This is intended to be used only on the end of the signal chain, i.e. the last monitor hooked up in series to the same video source. It prevents the signal from reflecting back along the cable which would cause ghosting. The 75 Ω value is used because the type of coax for video signals has a 75 Ω characteristic impedance, so a resistor with that value will absorb the energy moving down the cable. If a more lossy type of cable is used it isn't required to have termination because reflections would be damped by the cable loss.

The DC restorer circuit is needed because many video sources are AC-coupled: effectively there is a high-pass filter that separates the source and the display. One benefit of this is reduced hum, which appears as ugly "jail bars" if there is line-frequency interference coupled into the signal. But as a consequence of having no connection at DC, the relative level of the signal needs to be corrected by the display so that black level stays consistent. If you have a DC-coupled video source you can turn the DC restorer off.

You should only change the setting of those switches when the display is off or disconnected from any input signal. Because of the inductance of the coax cable, flipping the terminator on and off might cause electrical overstress to the amplifier.

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Good info

Thanks, Robes.

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robespierre wrote:75 Ω vs
robespierre wrote:

75 Ω vs high impedance controls whether the video signal is being internally terminated...

 

Robespierre is spot on with his explanantion. Except perhaps with the last statement. Nothing needs to be disconnected or turned off when changing the switches. That would be like saying don't connect or disconnect a video cable while power is on. Trust me, that is not an issue.

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PSU Swap Time

Well the replacement PSU guts arrived today from ReActiveMicro. Swapped 'em out, checked the voltages, and it seemed to working like a champ.

Hooked it up to the motherboard, turned it on, and I got the happy beep. Power rails look okay.

Haven't hooked up a monitor yet, but my hopes are high. I still have some work on the keyboard before I put it all together.

I tell ya, this new PSU is something else. We've come a long way in the past 30-40 years...

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It's a really nicely designed

It's a really nicely designed, well thought out piece.

It's been 100% reliable in my own computers, including one that runs a BBS 24-7-365.

 

 

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The Conclusion to the Story

Just to wrap everything up:

  • Looks like the original power supply died because its time was simply up. A couple of basic recaps couldn't save it.
  • The replacement PSU "guts" from ReActiveMicro were fairly easy to swap in, and they run great. I love that I still have the vintage look of the old case, but with modern, reliable components tucked inside.
  • The Sanyo monitor was not the problem, as it turned out. The eBay Gods were on my side this time, because it actually works/looks fantastic (see pics). The fact that the old PSU gave up the ghost when I first tried the monitor out was apparently just a coincidence.
  • The weird input on the back of the Sanyo is not a regular coax (N or BNC) connector but rather an "UHF" or PL-259 jack that is about 50% bigger than those. Nonetheless, it takes a good old-fashioned composite signal, requiring only an RCA-to-PL259 adapter. I picked up a couple on Amazon.
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Thanks for the followup.

Thanks for the followup.  Many times advice is given and the original poster disappears and we're left not knowing the outcome.

 

And those old security monitors have pretty fantastic resolution, too.

 

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