help with Tektronix 2213 power supply

[soundaudiorepair]

Hi all,

I'm trying to fix my Tektronix 2213, I'm pretty sure I've isolated the problem to the preregulator circuit. Schematic is attached, as is the theory of operation

The output of the triac should be around 45-50vAC, and it is 12v.

I've replaced every cap, diode, and transistor in the part of the circuit that generates the control pulse for the triac:
CR903-906, CR917
VR913, VR914, CR915
C912, C915, C917
Q921, Q918

Which leaves the triac itself (Q925), the pulse transformer (T925), and the opto coupler (U931), or I suppose a resistor. What can I do to isolate the problem? I don't have an 2nd oscilloscope.

[oldway]

Did you checked the fuse F937 0.5A on A19 current limit board ?

[soundaudiorepair]

Did you checked the fuse F937 0.5A on A19 current limit board ?

Yes

[tppc]

Hi soundaudiorepair,
back in 2008 I repaired a unit which had similar problem with the power supply.

As far as I remember, in my case, C957 was found guilty   .
Try replacing C956 too.

Well this worked for me, not sure it will solve your problem. Anyway, give it a try...

Regards.

[lordvader88]

A triac in a scope PSU !!, something else to watch out for in the future.

[soundaudiorepair]

Hi soundaudiorepair,
back in 2008 I repaired a unit which had similar problem with the power supply.

As far as I remember, in my case, C957 was found guilty   .
Try replacing C956 too.

Well this worked for me, not sure it will solve your problem. Anyway, give it a try...

Regards.

I think the problem is in the preregulator and not the inverter but it never hurts to replace electrolytics I suppose.

Sam

[soundaudiorepair]

A triac in a scope PSU !!, something else to watch out for in the future.

The PS is nuts! It goes from AC to DC, back to AC, then back to DC. I honestly don't really understand why.

Sam

[floobydust]

Tek had some unusual PSU designs for low EMI. They are not simple to troubleshoot and take patience.

You've replaced almost everything, it must be frustrating.

I would confirm the regulation loop with opto U931 is not commanding the low 12V output, i.e. if C951 is leaky the opto would be on at low voltages. You can measure the opto's LED voltage. Or connect a variable bench PSU to the +42.5V rail and see when the opto is turning on (2213 is disconnected from mains). If it's low, like 12V then the problem is not on the triac-side.

[LateLesley]

A triac in a scope PSU !!, something else to watch out for in the future.

The PS is nuts! It goes from AC to DC, back to AC, then back to DC. I honestly don't really understand why.

Sam

If you think about it, nearly all Switched mode PSUs follow that chain - AC to DC, to high frequency AC, then back to DC again. They do it for efficiency, and to be able to make transformers smaller. It seems they can make them smaller using the higher frequencies. So I suspect that is the reason for it, even if it seems counter-intuitive.

https://electronics.stackexchange.com/questions/144329/transformer-size-vs-frequency

[tppc]

Hi soundaudiorepair,
back in 2008 I repaired a unit which had similar problem with the power supply.

As far as I remember, in my case, C957 was found guilty   .
Try replacing C956 too.

Well this worked for me, not sure it will solve your problem. Anyway, give it a try...

Regards.

I think the problem is in the preregulator and not the inverter but it never hurts to replace electrolytics I suppose.

Sam

If I were you, I would try replacing C957 anyway...

Regards.

[soundaudiorepair]

Tek had some unusual PSU designs for low EMI. They are not simple to troubleshoot and take patience.

You've replaced almost everything, it must be frustrating.

I would confirm the regulation loop with opto U931 is not commanding the low 12V output, i.e. if C951 is leaky the opto would be on at low voltages. You can measure the opto's LED voltage. Or connect a variable bench PSU to the +42.5V rail and see when the opto is turning on (2213 is disconnected from mains). If it's low, like 12V then the problem is not on the triac-side.

Hey thanks for the ideas. I measured voltage from pin 1 of U931 to ground. 0v. My bench supply only goes up to 20v, so I don't know if hooking it up would do any good for testing.

I replaced or tested with a ESL meter C945, C947, C957, and C951. No change.

[soundaudiorepair]

Hi soundaudiorepair,
back in 2008 I repaired a unit which had similar problem with the power supply.

As far as I remember, in my case, C957 was found guilty   .
Try replacing C956 too.

Well this worked for me, not sure it will solve your problem. Anyway, give it a try...

Regards.

I think the problem is in the preregulator and not the inverter but it never hurts to replace electrolytics I suppose.

Sam

If I were you, I would try replacing C957 anyway...

Regards.

I replaced or tested with a ESL meter C945, C947, C957, and C951. No change.

[Ray Gianelli]

Testing with an ESR meter won't tell you the whole story.  They won't catch leakage, for instance.

If you haven't done so, replace C945, C947, C957, and C951.

[soundaudiorepair]

Testing with an ESR meter won't tell you the whole story.  They won't catch leakage, for instance.

If you haven't done so, replace C945, C947, C957, and C951.

Ok. The only one I didn't have something on hand for was C945. Will order that along with all the Zeners.

Sam