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T922 Power Supply Problem

@DC912
 

Hello.

With power on, the power lamp does not light or blink.

I measure negative 700mv at the -8 test point on the power supply board.

I measure positive 700mv at the +8 test point on the power supply board,

The secondary on the power transformer for the 8v supply tests good.

I measure 82 volts at the +100v test point on the power supply board.

With the power supply board in the scope, resistance between -8v test point and ground is 180 ohms, 200 ohms between the +8v test point and ground.

With the power supply board removed from the scope, resistance between the -8v test point and ground is 35k ohms, 20k ohms between the +8v and ground.

I also removed the vertical and horizontal boards to check the resistances on the 8v and -8v rails to ground on those boards. None of the rails on the two boards measured lower than several kohms.

Any suggestions on where to go from here would be greatly appreciated.

Thanks. Dave.

Mlynch001
 

On Sun, Jul 7, 2019 at 01:37 PM,@DC912wrote:


Hello.

With power on, the power lamp does not light or blink.

I measure negative 700mv at the -8 test point on the power supply board.

I measure positive 700mv at the +8 test point on the power supply board,

The secondary on the power transformer for the 8v supply tests good.

I measure 82 volts at the +100v test point on the power supply board.

With the power supply board in the scope, resistance between -8v test point
and ground is 180 ohms, 200 ohms between the +8v test point and ground.

With the power supply board removed from the scope, resistance between the -8v
test point and ground is 35k ohms, 20k ohms between the +8v and ground.

I also removed the vertical and horizontal boards to check the resistances on
the 8v and -8v rails to ground on those boards. None of the rails on the two
boards measured lower than several kohms.

Any suggestions on where to go from here would be greatly appreciated.

Thanks. Dave.
I would check the items listed in the service manual under Service Information. U742 is part of both +8 and -8 power supplies. Service manual also mentions several other transistors. -8V check Q772-774-776 +8V Check Q752-754-756.

--
Michael Lynch
Dardanelle, AR

Harvey White
 

One immediate suggestion would be to check the capacitors on the 8 and -8 volt bulk supplies, then check the capacitors in the supplies that the -8 and 8 feed.

That you have several K ohms on the other boards to ground suggests that they're ok.

Harvey

On 7/7/2019 2:58 PM, Mlynch001 wrote:
On Sun, Jul 7, 2019 at 01:37 PM,@DC912wrote:

Hello.

With power on, the power lamp does not light or blink.

I measure negative 700mv at the -8 test point on the power supply board.

I measure positive 700mv at the +8 test point on the power supply board,

The secondary on the power transformer for the 8v supply tests good.

I measure 82 volts at the +100v test point on the power supply board.

With the power supply board in the scope, resistance between -8v test point
and ground is 180 ohms, 200 ohms between the +8v test point and ground.

With the power supply board removed from the scope, resistance between the -8v
test point and ground is 35k ohms, 20k ohms between the +8v and ground.

I also removed the vertical and horizontal boards to check the resistances on
the 8v and -8v rails to ground on those boards. None of the rails on the two
boards measured lower than several kohms.

Any suggestions on where to go from here would be greatly appreciated.

Thanks. Dave.
I would check the items listed in the service manual under Service Information. U742 is part of both +8 and -8 power supplies. Service manual also mentions several other transistors. -8V check Q772-774-776 +8V Check Q752-754-756.

@DC912
 

Thanks Michael.

I removed and tested the transistors out of circuit. All are good.

I also removed the IC, and installed a new replacement IC in a socket. No change. Same results as above.

Should I test just the 8v circuit on the power supply board out of the scope?
Dave

@DC912
 

Thanks Harvey. I replaced all of the electrolytics on the power supply board for safe measure, although they all tested okay. I also checked all of the electrolytics on the other boards, and the one tantalum I saw. None were shorted and they all test okay for capacitance and esr on my LCR meter. I’m stumped, but would really like to get this scope back in working order. Dave

@DC912
 

Forgot to mention, I also reflowed the solder on the connectors on the power supply board and interface board.

Mlynch001
 

On Sun, Jul 7, 2019 at 07:17 PM,@DC912wrote:


Thanks Michael.

I removed and tested the transistors out of circuit. All are good.

I also removed the IC, and installed a new replacement IC in a socket. No
change. Same results as above.

Should I test just the 8v circuit on the power supply board out of the scope?
Dave





Dave,

I am certainly not an expert, but if it is possible to operate the LV supply without a load, it would help you to determine if the problem lies with the supply or one of the other boards in that scope. I have restored a couple of T922/935 scopes to operation, but never had any issues with the LV power supply. Have you checked the voltages at the various components in the supply against the schematic? How about VR746? This is a 9.1 Zener that tends to fail quite often. Any of those many diodes failing could cause problems. I have seen several ZENER and other diode failures in older equipment that I have repaired, most recently in my Type 576 Curve tracer. You replaced C738,758,778 along with C722, 742 and 743 ?

--
Michael Lynch
Dardanelle, AR

Harvey White
 

OK, then there's several things to think of.

1) with no power on the other boards, and a decent (several K resistance) to ground on the power leads on the boards themselves, I'm not sure what's holding down the power supply lines on the boards.

2) I assume that without any boards plugged in, the main voltages are down.

with the main voltages down, and having replaced *every* capacitor (and <I'll never tell> all the capacitors in the right way), I'll assume that all the resistance readings are decent.  Then the question is:  *what* if shorted, would pull the voltages down.  You can tell if the voltages are down by looking at the current sense resistors (if any), measuring the voltage across them, and then seeing  what the current is from that.  If the current sense resistor is connected between the base and emitter of a transistor, any reading over about .6 to .7 volts indicates that the supply is trying to current limit.

That indicates that something is pulling too much current.  If the supply feeds another one, check that other supply too.

Harvey

On 7/7/2019 8:37 PM, @DC912 wrote:
Forgot to mention, I also reflowed the solder on the connectors on the power supply board and interface board.



Mlynch001
 

On Sun, Jul 7, 2019 at 07:57 PM, Mlynch001 wrote:

Dave,

I am certainly not an expert, but if it is possible to operate the LV supply
without a load, it would help you to determine if the problem lies with the
supply or one of the other boards in that scope. I have restored a couple of
T922/935 scopes to operation, but never had any issues with the LV power
supply. Have you checked the voltages at the various components in the supply
against the schematic? How about VR746? This is a 9.1 Zener that tends to
fail quite often. Any of those many diodes failing could cause problems. I
have seen several ZENER and other diode failures in older equipment that I
have repaired, most recently in my Type 576 Curve tracer. You replaced
C738,758,778 along with C722, 742 and 743 ?

--
Michael Lynch
Dardanelle, AR
Also check VR762, which is the 5V reference for U742 and the 8V supply. Also, check to see if someone has added a 10uF 20V cap between the junction of VR762 and R762, (this "field modification" is penciled in on one of my schematics) could this cap be present and shorted? If you can test the voltages at the various points in the circuit and compare them to the schematic, it would be helpful. At this point, without some numbers, this becomes pure conjecture. Harvey is correct, either something is loading the supply or fooling the circuit into thinking that it is loaded.
--
Michael Lynch
Dardanelle, AR

@DC912
 

Thank you both gentlemen. There are two small 125v 1.5A pc board mounted fuses on the power supply board that do not appear on the circuit diagram in the copy of the service manual that I have. I had initially dismissed them, wrongfully presuming they were in the 100v volt supply. After studying the board, I realized they were in the secondary of the 8v supply, and that they were open. I tacked in new fuses with an ammeter connected in order to carry out a test, and everything is now working (replacing the electrolytics must have corrected the cause of the failure of the fuses). Thanks for all of your help. Dave.

Mlynch001
 

On Mon, Jul 15, 2019 at 10:04 AM,@DC912wrote:


Thank you both gentlemen. There are two small 125v 1.5A pc board mounted fuses
on the power supply board that do not appear on the circuit diagram in the
copy of the service manual that I have. I had initially dismissed them,
wrongfully presuming they were in the 100v volt supply. After studying the
board, I realized they were in the secondary of the 8v supply, and that they
were open. I tacked in new fuses with an ammeter connected in order to carry
out a test, and everything is now working (replacing the electrolytics must
have corrected the cause of the failure of the fuses). Thanks for all of your
help. Dave.
Dave,

Don't you just love it when you find a problem and fix it! Glad you were successful. PS, if you need any parts for the 922 or similar scopes, I have a working 922 parts scope which I might be able to pull parts from.

--
Michael Lynch
Dardanelle, AR