Ok folks, this may be of interest to any owner of a 004A/005A and potentially a 006A/007A as well
I got Kris's A11 unit here to our shop yesterday.
The first initial test / trouble shooting were done this morning at 09:00 Hours sharp.
The results are as follows:
1. Kris wiring of the new replacement / substitute T2 with inline 2AG Fuse is good ( He did a good job soldering the burnt 18ga Airframe Wires to Fork Terminals )
2. The 6 Diodes mounted on a Heat sink on the A11 Chasis checked out good.
3. The 4 Filter Capacitors on the Filter Unit E2 are suspicious ( 0.01uF Capacistance checks OK, but NO ESR)
4. C2, the large Capacitor mounted to the A11 Chasis is toast
5. The initial Check of the original VR Board checked out OK, but needs further testing after C2 has been replaced
The root cause of T2 burning up:
Transformer usually never burn up unless their internal wire lacquer coating breaks down from time / heat / humidity.
In this case here specific to A11 it is C2 which dries out over time. C2 has a 20uF / 400V rating and when tested with a Capacitance / ESR tester the value starts out at a very low capacitance and then increases rapidly way above 40uF as the test frequency applied increases (should stay the same)
As Higher the Capacitance with frequency gets, as more the Capacitor becomes a short - This is when an AC Signal is applied - it checks out OK with DC (no leaking, no short)
However, C2 is connected to X1 and X3 on T2 and can get up to 124VAC when T2 has 208VAC applied on H1 and H2
This High AC Voltage is then applied to the two SCR's (with Heat Sink) on the VR Board - they supply the VR Board with operating power and start switching this entire circuit
When C2 starts to fail, this short is seen by the secondary side of T2 X1 and X3 and because T2 has no fuse on the primary side in the original design, starts to overheat T2, eventually roasting T2 to well done.
This is a gradual failure and can first be noticed by increased Output Voltage Fluctuation (Your Voltmeter's Needle starts to bounce more and more on your Front panel).
So, rather then suspecting a bad Voltage Regulator Board when this starts, replacing C2 first will, to 98%, fix the problem and prevent overheating of T2
The problem here is the aging of the components looking at the time when these units where originally built.
C2 is actually so bad, that it winded up overloading my ESR Tester as the frequency applied during testing increased - my ESR tester is now toast too, just like T2
But Kris had offered me a BIG Steak Dinner when I come to Huston before it blew anyway! So, I will not dwell on it.
