Wanted: PCjr Power Transformer/AC Adapter

[jmetal88]

I figured out yesterday that my PCjr is working after rigging up a homemade power supply. The only problem is my transformer is rated at about 2/3 the current the PCjr is supposed to draw and gets pretty hot while the PCjr is operating, so I'm looking for an official power adapter.

I'm hoping someone might be willing to ship me one for under $11. This might sound like an odd amount, but I found a properly rated plug-in transformer with screw terminals on ebay that I could get shipped to me for about that amount, and it's easy enough to rip apart a molex connector to construct a plug. Since I'd prefer the official one to a home construction, I might be willing to spend, say, $15, if someone isn't willing to ship it under $11. My zip code is 74006 if that'll help determine what you can do.

Thanks for your consideration!

 

[jmetal88]

Actually, I don't know if I'm patient enough to wait on this, haha.

I just burned out the transformer in my homemade supply - I tried booting off the floppy drive and I think that's what overloaded it. I'll wait 'til Sunday evening for an offer I think, and if I don't get one I'll just order the 16.5v 45w supply I found on eBay. Yeah, it's still underrated by 3w according to the sticker on the bottom of the PCjr, but considering I had moderate success with a transformer rated at only 36w which only gave up after powering the PCjr with floppy drive for an extended amount of time, I think it'll do alright.

 

[NathanAllan]

Aww, I would be seriously into making my own overrated power supply for it, complete with fans and good regulation! If I had seen this thread of yours before you posted this afternoon I would have said it then Missed it.

 

[jmetal88]

Aww, I would be seriously into making my own overrated power supply for it, complete with fans and good regulation! If I had seen this thread of yours before you posted this afternoon I would have said it then Missed it.

Well the tricky thing is the PCjr has a split power supply. The AC step down transformer is external, but the rectifier and voltage regulators are all internal.

Although, I am kind of starting to lean, for a few dollars extra, toward a 16v 56w transformer I found that I might want to mount in a project box or something. The PCjr supposedly draws 3.2a, so that transformer should do rather well.

 

[Chuckster_in_Jax]

Well the tricky thing is the PCjr has a split power supply. The AC step down transformer is external, but the rectifier and voltage regulators are all internal.

Exactly. The IBM power adapter is only a step-down transformer with a fuse on the incoming A/C line. The plastic case is permanently glued together, so if you ever have to open it, you will destroy it.

http://www.mediafire.com/i/?regyvyra55ohs7y

 

[Chuck(G)]

That may not be a bad thing--that transformer could probably use some ventilation.

 

[jmetal88]

Exactly. The IBM power adapter is only a step-down transformer with a fuse on the incoming A/C line. The plastic case is permanently glued together, so if you ever have to open it, you will destroy it.

http://www.mediafire.com/i/?njnamyedzne

Thanks for the pics! It looks like it isn't quite wired up in the way I thought it would be. Hmm. It kind of looks like they have the outer pins connected between a terminal and the center tap, and the middle pin connected straight to the mains ground. Which suggests the transformer is actually a 33.6v transformer? Although there's something connected between the center tap and the other terminal that I can't quite identify, so I don't know. I don't know much about circuit design yet, haha.

EDIT: I believe I have discovered a problem with my internal power supply. The dual choke on the AC line and the rectifier are both getting really hot now. I'm hoping the rectifier is OK, but I think I should order a new choke, as upon inspection this one seems to have lost some of its insulation (which was the previously unidentified green goo I had cleaned off the motherboard).

 

[jmetal88]

Well, I'm a bit disappointed now - I've tested the power board outside the computer and it appears to be operating normally. For some reason the motherboard is just drawing way too much current all of a sudden, and I don't know why. The composite video signal is now unstable unless I remove the sidecar and floppy drive, and the rectifier still gets way too hot even just powering the motherboard. The weird thing is, neither of the voltage regulators is getting hot, so I can't quite figure out what's going on. The power board doesn't get hot when I operate it disconnected from the motherboard, so at this point I'm assuming there's a short on the motherboard somewhere, but I don't even know where to begin looking. Can anyone experienced with this type of thing give me an idea?

EDIT: Also, when the floppy is hooked up the video is much more unstable than when it isn't. Plus, the motor that spins the disk moves much too slowly now.

EDIT 2: I think I've found three bad capacitors on the power board, two of which are connected between the +12v line and ground.

 

[mbbrutman]

When in doubt, the best source of information is the technical reference manual.

According to the tech ref, the external transformer provides 17Vac on pins 1 and 3, and a ground on pin 2. The transformer needs to provide a maximum of 0.65 AMPs at 104 Vac (which assumes it's not getting the full 120 Vac that it is supposed to get.)

DC output for the internal power supply card is +5 at a max of 3.6 amps, +12 at a max of 1.2 amps, and -6 at 0.025 amps. (I wonder if there is a typo there.) Supposedly -12V is not available, but it is available when you use a power adapter sidecar.

 

[jmetal88]

Found the problem! The center tap on my transformer got pulled loose so I had a bad ground. When I grounded it to the mains instead I didn't have any problems.

 

[Chuck(G)]

When in doubt, the best source of information is the technical reference manual.

According to the tech ref, the external transformer provides 17Vac on pins 1 and 3, and a ground on pin 2. The transformer needs to provide a maximum of 0.65 AMPs at 104 Vac (which assumes it's not getting the full 120 Vac that it is supposed to get.)

Did the Peanut have its roots in Japan? (I'm thinking about the JX) If so, that 104vac is not unusual in Japan, where the nominal line voltage is 100VAC--I used to have a color monitor with a nameplate rating at 100V input.

DC output for the internal power supply card is +5 at a max of 3.6 amps, +12 at a max of 1.2 amps, and -6 at 0.025 amps. (I wonder if there is a typo there.) Supposedly -12V is not available, but it is available when you use a power adapter sidecar.

-6 is probably the substrate bias for the DRAM, so current needs are minimal. It's not really high enough for a reliable RS232C connection.

 

[Chuckster_in_Jax]

Exactly. The IBM power adapter is only a step-down transformer with a fuse on the incoming A/C line.

http://www.mediafire.com/i/?njnamyedzne

Need to make a correction - I updated the picture of the A/C adapter to show the proper connections.

http://www.mediafire.com/i/?regyvyra55ohs7y

The left hand side of the transformer is the 120v side. The center tap is the line and the top connection is the neutral. Notice the lug that connects the A/C Neutral from the 120v source and the ground (center pin) on the 17.7v molex plug.
The right hand side of the transformer is 17.7v. The fuse is on the bottom lug and the other wire is connected to the top. Don't know if that really matters since the 17.7v molex is not keyed.

 

[Chuck(G)]

It's probably worth mentioning that the thing embedded in the varnish on the primary side of the transformer between the bottom and center terminals is a thermal protector. It goes open if the transformer temperature gets too high. If you have what appears to be bad transformer, try connecting the center lead to the lower lug--or just temporarily bridge the two lugs.

It's not unusual for these things to fail open and the last line of defense if the transformer windings should develop a short.

 

[jmetal88]

Oh, and before I forget to post, I am still looking for an official power supply. I can wait it out a little bit longer now, since I figured out what the problem with my own transformer was.

If I don't find something in a week or so, though, I might build something more permanent than what I have now.

 

[Chuckster_in_Jax]

It's probably worth mentioning that the thing embedded in the varnish on the primary side of the transformer between the bottom and center terminals is a thermal protector. It goes open if the transformer temperature gets too high. If you have what appears to be bad transformer, try connecting the center lead to the lower lug--or just temporarily bridge the two lugs.

It's not unusual for these things to fail open and the last line of defense if the transformer windings should develop a short.

I was wondering what that device was. Never seen one before.

 

[jmetal88]

It's probably worth mentioning that the thing embedded in the varnish on the primary side of the transformer between the bottom and center terminals is a thermal protector. It goes open if the transformer temperature gets too high. If you have what appears to be bad transformer, try connecting the center lead to the lower lug--or just temporarily bridge the two lugs.

It's not unusual for these things to fail open and the last line of defense if the transformer windings should develop a short.

Ah! I see, the middle lug isn't a center tap then, it's just the protected portion of the AC output. My transformer doesn't have thermal protection.

 

[Chuck(G)]

Ah! I see, the middle lug isn't a center tap then, it's just the protected portion of the AC output. My transformer doesn't have thermal protection.

Unless I'm seeing things, it's in the AC input on the left. It's particularly important with sealed wall wart-type of power supplies (and probably required by UL) because these things don't give out smoke or stink until it's usually too late (they're sealed inside the box).

Yeah, no center taps on this baby.

 

[jmetal88]

Unless I'm seeing things, it's in the AC input on the left. It's particularly important with sealed wall wart-type of power supplies (and probably required by UL) because these things don't give out smoke or stink until it's usually too late (they're sealed inside the box).

Yeah, no center taps on this baby.

Yeah, I guess you're right - the position of the fuse is what confused me.

 

[jmetal88]

Bump - I'm still looking for one of these.

 

[Chuck(G)]

Not an IBM-brand, but I wonder if This one from Jameco would work?

Mike could probably say for certain.