CR 1020 Power supply gremlin

[Nuklhed]

Hello team, trying to restore a basket case 1020 using jayfree boards. Can’t get out of protection. The following readings for your consideration:

+NB at J1 41.4vdc
-NB at J1 - 42.8vdc
+25 is 26.6 vdc
-25 is - 18.2 vdc
-13 is - 11.1 vdc
TB is 12.9 vdc (schematic 12.7)
MB is 13.0 vdc (schematic 12.8)
PL is 1.9 vdc (schematic 20)
Protection relay coil 0.45 vdc

Readings taken to chassis ground, volume zero, with load resistors.

Any help is greatly appreciated!

 

[avionic]

+25 is 26.6 vdc
-25 is - 18.2 vdc

There is one issue.. About 8 volts of DC offset..

 

[Nuklhed]

Can you point me to the secret pot where I can just adjust this away!
Hello again my friend…..
Huddling here in my South Carolina lab waiting for the hurricane. Figured I’d pull the 1020 off the shelf and have another go.

 

[arhargi]

Whenever I have Protection Mode issues on my CR-2020 (which is virtually the same design as the CR-1020), the first place I go to look is the main capacitor board. I check for lifted/broken traces, disconnected components, all of the solder joints, and the soldered connections that plug into the power supply pcb.

 

[Nuklhed]

Thanks arhargi! You and I are on the same page. I just finished swapping the ecap board with a known good one. No changes in the readings. I’m thinking it’s gotta be on the relay board somewhere.

 

[avionic]

Can you point me to the secret pot where I can just adjust this away!

This has no " adjustment ". Likely a failed component in the +/- 25v regulated power supply. On the relay/power supply board. Your +/- NB looks OK
So the issue is in this circuit.

 

[avionic]

+NB at J1 41.4vdc
-NB at J1 - 42.8vdc

These originate on the E-cap board . This tells me that the problem never was the E-cap board. These two voltages are inputs to the +/- 25 volt power supply. +/- 25 volt is the outputted regulated voltage.

 

[avionic]

Find R725 and R726. Measure the voltage on both ends of each one of these resistors.

 

[Nuklhed]

R725:
TR 712 side: 28.25 vdc
TR 707 side: 26.64 vdc

R726:
TR 715 side: - 19.58 vdc
TR 708 side: - 18.23 vdc

 

[avionic]

R726:
TR 715 side: - 19.58 vdc
TR 708 side: - 18.23 vdc

ECB voltages of TR714

 

[Nuklhed]

TR714
E -19.54 vdc
C - 42.10 vdc
B - 20.10 vdc

 

[avionic]

Pull and check R720, 716 and 718..

 

[Nuklhed]

R716 100
R718 10k
R720 18.2k

 

[avionic]

R716 100
R718 10k
R720 18.2k

These are fine.

What is the emitter voltage of TR703 ?
And ECB of TR704

 

[avionic]

Have you replaced zener diode D715 ?

just in case .. Depends on voltage at TR703 emitter. If it is around 7 volts the zener is OK.

https://www.mouser.com/ProductDetail/Taiwan-Semiconductor/MTZJ7V5SC-R0G?qs=Mv7BduZupUhhyP8sxGvuCg%3D%3D

 

[avionic]

I'm thinking TR704 is dumping 10vdc to ground. I'd replace it.

 

[Nuklhed]

TR703 E 7.09 vdc
TR704 B -0.63 vdc
C -20 vdc
E 0 vdc

 

[avionic]

TR704

B -0.63 vdc
C -20 vdc
E 0 vdc

Replace it..
2SA844 -- KSA1015Y

https://www.mouser.com/ProductDetail/onsemi-Fairchild/KSA1015YTA?qs=ljbEvF4DwOP4wK7PS53%252BtA%3D%3D

 

[Nuklhed]

Replaced TR704 with a KSA1015Y….
Still only - 18 vdc at the - 25 vdc terminal.

 

[avionic]

Only thing left is capacitor C716 .01µf probably a ceramic disk..