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4P power supply probs - HELP!!

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Gazza

Experienced Member
Joined
Jun 27, 2013
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151
Location
Sydney, OZ
G'day all, the Astec P/S on my new to me 4P is a bit cantankerous and I'm hoping someone can point me in the right direction.
This computer looks like it has spent most of it's life in a box and has done very little work.

Problem #1
Power up from cold, 240V input OK, all outputs well below their rated voltages. 5V output is about 2 and a bit volts at best, +12 about 5V and -12V about -5V approx.
Powering off and on a few times usually kicks the supply into life or if left on for 1 or 2 minutes it will fix itself and then run perfectly all day.

Problem #2, possibly related to Problem #1
Power up from cold, the outputs will reach their correct voltages and then after about 3 seconds power down, power up, power down, power up.............................
It will do this for about a minute or 2 and then come good and run reliably for the rest of the day.
Note - this only effects +5V and +12V, -12V seems to be stable.

So far I've resoldered all suspect connections, C1, C2 & C13 replaced. Physically all electros look OK and the few that I have measured seem on the money.
Being a temperature related problem I gave it the once over with the hair dryer and concentrating the heat around the area of Q2 "seems" to fix the problem/s
Q2 is an unmarked TO3. I'm guessing a mosfet but without any markings I'm not sure - any ideas?
All help greatly appreciated.
cheers
Gazza
 
Contact TRS-Ian he might have a replacement or might be able to assist. Q2 is a PNP Part # MPS-A55 according to the service manual.
 
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G'day Frank, thanks for your reply, I had a peek at the 4P service manual but it only covers the Tandy P/S not the Astec.
Q2 on the Astec is a T0-3. I removed Q2 and checked the junctions and it seems to be a NPN, no markings anywhere.
I also checked R14 that lives next to Q2
Measures approx 2 ohms in circuit. Hair dryer on medium temp, just want to warm it not fry it. After a few seconds R14 drops to zero ohms then slowly climbs to approx 4 ohms and then varies between 0 & 4 ohms approx. Remove hair dryer and R14 will slowly return to 2 ohms.
Don't know the value of R14 as it is covered in heat shrink. When I get some spare time I'll dig a bit deeper.
Cheers
Gazza
 

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Frank,
Do you have a PDF of the Manual that contains this Power Supply (Model AZ0180) ? I can't find it described in the Manuals I have.

Thanks.

Larry
 
Thanks for that info Frank.

I have a hard copy of the 4P Service Manual purchased from Tandy way back when but it only covers the Tandy P/S and I have a PDF of the Tandy 5Meg Hard Drive Service Manual but I don't know what model P/S is covered in that manual and the circuit diag has no component values.

The power supplies on both of my 4Ps and the Hard Drive are identical, so the easy way out of this problem might be to take the P/S from the Hard Drive and mount it in the dead 4P. The Hard Drive only has a Lo-Tech IDE adapter and a DOM so any 5V wall wart should do the job.
Simple, cheap
Cheers
Gazza
 
Gazza,
From your description of the cycling, it is likely that your power supply is going into current limit (Foldback). Here is a troubleshooting
chart I got from another 65 Watt Power Supply. The components are likely incorrectly numbered, but the same methods of troubleshooting should apply.

I'd look at the Bridge Rectifier, Loads (Devices) attached to the Power Supply, shorted Output Rectifiers, and Input & Output Filter Capacitors.

65Watt.jpg

Larry
 
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Update - The problem is that the fault is intermittent. Most times it will run OK and when it does play-up it fixes itself in one or 2 minutes, doesn't give me much time to see whats going on.
So I swapped the PSUs between the 4P and the hard drive and both have been running perfectly for over a week.
I give up. Covers back on and fingers crossed.
thanks for the help
Gazza
 
Intermittent connection some place would be my guess. I hope it keeps running for you. Have fun! And best of luck I hope I helped out somewhat!
 
Get a magnifying glass and look at the solder connections to all the Pins for the Edge Connector. I think you will find small circles around some Pins. I have
one of those supply's in the Model 4 I just purchased. I re-soldered those connections and that problem is fixed. I also had a stuck RESET Button. Easy fix!

Remember, when working on Power Supplies it's always good to have an Isolation Transformer. I use a 60 Watt Light Bulb in series with the HOT Line to
the Power Supply, just in case. If the Light Bulb is Bright you have a Short.

Larry
 
G'day Larry
Resoldered TB1,TB2 and the connections on T2 - all looked a bit sus. I resoldered all of these on my first 4P over 20 years ago and they still look OK, guess the factory soldering was not up to spec.
It seems to be a temperature related problem that fixes itself after a couple of minutes and then runs perfectly for the rest of the day. Since swapping the PSUs between the 4P and Hard Drive I've had zero probs. ATM it's working perfectly and has done so for over 2 weeks. Hoping it stays that way.
Many thanks to all
Gazza
 
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