A2E Astec AA11040B or C power supply - need help to repair it

[TRS-Ian]

Hi everyone, greetings from Tandyland

I'm trying to fix a AA11040B PSU for an A2E. I've checked the fuse, electrolytics, and the globar resistor. I've replaced the Rifa, but the PSU still shows no signs of life. Does anyone have links to a step-by-step way to check and fix the PSU?

I am testing it plugged into an A2E motherboard, it's my understanding that like most Astec PSUs of the era it needs to be under load to function.

Thanks in advance,

Ian.

 

[ldkraemer]

Ian,
Here is a bit of information on the A2E Power Supply.
https://ia801002.us.archive.org/1/items/Astec_Power_Supplies_Aug82/Astec_Power_Supplies_Aug82.pdf

https://www.applefritter.com/conten...chirping-and-gives-fluctuating-voltage-output


I also have saved this from the Model 3 Manual for the Astec Power Supply that is in the TRS-80 Model 3.

ASTEC TRS-80 Model 3 Power Supply

NOTE: Use an isolation transformer when servicing the power supply.
Disconnect Connectors SK2, SK3, and SK4 to
avoid possible damage to the Computer from high voltages
that may be produced while servicing the power supply.


Power Supply Load Values:
.................+5.....................+12................-12
min.............45A..................3A..................0A
Safe............11.11 OHM........40 OHM.........0
PWR...........5 Watts.............8 Watts..........0
MAX............2.5A.................2.02A............1A
PWR............2 OHMS..........24.24 OHM......120 OHM
Safe PWR....25 Watts..........50 Watts.........2 Watts

Use a 60 Watt Incandescent Light Bulb in Series with the Line (HOT)
to protect from a possible shorted Supply. If there is a defect
the Light Bulb will "Shine Bright", or if there is a problem it may
be half bright, depending on the malfunction.


REF: Model 3 Service Manual
Check Fuse Resistor R25. If R25 is open check for shorts at
the Bridge Rectifier (DB1). Check for a possible shorted
Driver Transistor (Q2). If R25 is good apply 120V AC power
and check for 120V AC across capacitor C2. If 120 AC is
missing check the Line Choke (L1), Power Switch (SW1) and
check connector SK1 for good connections & Safe LOADS. If
120V AC is present at Capacitor C2, check for 163V at cathode
of Diode D2 (use emitter of Oscillator Transistor Q1 as
reference). If 163V is missing, check Bridge Rectifier
(DB1). If 163V is present, check the waveforms at the base
of oscillator Transistor (Q1) and Driver Transistor (Q2) to
determine if the power supply is oscillating. If the power
supply is not oscillating check the voltages and com-
ponents associated with Transistors Q1 and Q2, check
Diodes D1, D2, and D3 and check for possible open windings
on Transformers T1 and T2. If the power supply is
oscillating, check the source voltages at connector SK2
(-12.8V at Pin P1, 12.0V at Pin P2 and 5.0V at pin P4). If the
voltages are not regulated, check the voltages and comp-
onents associated with the Regulator IC (IC1) and
Regulator Transistor (Q3). Check Transformer T1 for poss-
ible open windings. If the 12V source is missing, check
Diode D4, Coil L4, Electrolytics C11 and C15 and check the
windings from Pin 7 to Pin 9 of Transformer T2. If the -12.8V
source is missing, check Diode D7, Coil L6, Electrolytics
C14 and C16, and check the windings from pin 6 to 8 of Trans-
former T2.



Larry

 

[David_M]

The most common cause of the symptom you describe is an open winding on the feedback transformer. That winding with the 68 Ohm resistor across it is prone to failure. C7 (220uf) will sometimes cause the problem I generally recap before powering them on so it isnt usually an issue. A few times I've found OC resistors in the primary side and once it was a transistor Q1. Failure of the main switching transistor or the bridge is quite rare nowadays but it was fairly common back in the 80's.

As I said that transformer is most likely to be the problem. You can't buy new ones but you can repair them, get yourself some enameled copper wire wind about 35 turns over the old windings and terminate your windings on the posts the bad winding terminates on. I always do my windings in the same direction as the old winding, I don't know if it makes a difference but I do it anyway.


It's not pretty but it works.

 

[David_M]

In case anyone is curious, Ian and I did a part swap and part of the deal was a changeover of the faulty power supply with a refurbished one.

It turned out to be a quick recapping exercise. There was some minor leakage of a few filter capacitors but the fault was caused by C7 (220uf), it was totally shot.