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Astec AC8151 PSU giving low output (Osborne, TRS80 etc)

Witchy

Experienced Member
Joined
Oct 11, 2015
Messages
361
Location
Flatlands, UK
Hi folks,

These PSUs seemed to be a default go-to for anyone back in the 80s who needed a +5/+12/-12 ~2A supply for anything from a computer (Osborne), disk drive (TRS80, Memotech FDX) or video terminal (Falco), and there's a good schematic and parts list available, even a servicing guide. But! Nothing I've found says what the main chopper transistor is other than 'Q2' - 'Power transistor'.

I have one here out of an STC Executel 3910 'executive telephone' that's producing little to no output even with a load attached and I've run out of things to check so I'm looking for ideas. Some caps on it had burst when I first uncovered it, as had its RIFA .1uF mains filtering cap so I replaced them all with modern and reasonably similar sized equivalents. Ohmmeter across the outputs with it disconnected doesn't show any shorts and because I can I've removed all the major components to test them, they all look OK. The only things I've not looked at fully yet are the small resistors, major transformer and coils.

Anyone? I know the Executel itself powers up since I've got another 8151 PSU that works but I don't want to start pulling that to bits. Yet :)
 

ldkraemer

Veteran Member
Joined
Mar 14, 2013
Messages
2,256
Location
Chaffee, MO
I checked the TRS-80 Model 3 & Model 4 documents and their ASTEC AA11320 & ASTEC AA12090 is similar in design. I found this troubleshoooting
information that might be of help. (The components might be labeled a bit different.)

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.

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. 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.

Perhaps you can determine that the Q2 Transistor is fully functional
with the above information.


PM me with an email address and I'll send you some schematics and
documentation on the above supply's.


Larry
 
Last edited:

Witchy

Experienced Member
Joined
Oct 11, 2015
Messages
361
Location
Flatlands, UK
Hi Larry,

Thanks for that, it's very similar to the 8151 service manual. I'm trying to find a 100x probe for my scope since I need that for testing the waveforms, though I took Q2 out of circuit and it tests OK as a transistor. I also checked DB1 which looked OK. In the Apple][ world they use a very similar PSU layout (also Astec) and the low voltage output is down to a 330uF feedback capacitor going out of spec but I've already changed these ones. Unless I've managed to get a brand new failure of course :)

Cheers
 

Witchy

Experienced Member
Joined
Oct 11, 2015
Messages
361
Location
Flatlands, UK
Going to bump this since I now have a 100x probe. The Service Guide says I need an isolating transformer (500VA) and optionally a Variac. I have a Variac and a step-down transformer for US-based stuff which is a 450VA unit - I can reconfigure the PSU to be 120V. The Service Guide says 500VA is necessary 'to keep the AC waveform from being clipped off at the peaks' so am I wasting my time with the step-down?

I'm also wondering if I should've recapped using high-grade Panasonics and not just matching ratings....

Cheers!
 
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