My trans sister is smoking hot.

[socketa]

Hi guys,
Tektronix 465M
Looks like 95V is supposed to be derived from the 32V Unreg
But only getting 11V
Is the transformer shorted?
And what could be causing Q522 to overheat and smoke? (fried 2 of them so far)

[R_J]

Re: My trans sister is smoking hot.

The +95 volts comes from the transformer pin2, check cr582 and cr584, also check whats supplied by the +95 volts, having a short, or heavy load on the +95 would put quite a load on the Q552 transistor
check zener diodes VR552 & VR553 and diode CR552
I see you measure +52 up near the fuse, that could be correct. on pin 4 of t550 in the circle does that say +51.6?

Maybe post the manual

[ReeceyBurger123]

Re: My trans sister is smoking hot.

lol

[CapLeaker]

Re: My trans sister is smoking hot.

lol

[ReeceyBurger123]

Re: My trans sister is smoking hot.

I was about to come here and make some harsh comment to OP but then realized what he did had a good laugh

[socketa]

Re: My trans sister is smoking hot.

The link for the manual is:

PDF Page 166 (schematic 8)
The 95V rail also feeds VR524 at the top of that schematic.
Why is there 0v on pin 2 of the transformer?

Those 5 diodes, that you mentioned, seem to test OK with a multimeter on diode test mode, and none of them are shorted

According to page 4-25 (PDF page 53) of the manual, VR552 and VR553 provide overvoltage protection, and when they conduct, they cause Q552 to turn on and blow the fuse F558 (interestingly, when Q552 is smoking, the 0.25A fast fuse doesn't blow, but it could just be because it's a cheap fuse)

Note: this was not working when i got it, but have since managed to get the low voltage supply (+5V, -5V, +32V Unreg) working properly after replacing a couple of components.

[R_J]

Re: My trans sister is smoking hot.

When you said Q522 did you mean Q552, what is the number on the transistor, the manual only lists a tek pt#

You would measure 0 volts dc on pin 2 because its a/c (15 to 20 khz) then it gets rectified by CR582 to become 95 volts dc
You could have a bad high voltage transformer or a bad high voltage module
You could check the voltage at Q548, it should be low, around .5 volts
Did you check Q556 to make sure its not shorted or open?
Is there any voltage on TP -2kv? should be negative 2000 volts.

[eccerr0r]

Re: My trans sister is smoking hot.

I highly doubt there will be -2KV at the TP if +95 is dead. I'd start disconnecting stuff from the secondary first to see if there's an overload condition, at least to help locate the problem. Do you have another oscilloscope - anything showing up at TP552 (waveform 57)?

Hope that it doesn't turn out to be the same problem like my scope (Tek 2465 that I had to debug HV as well.)

How is Q556?

Also you may need to do dim bulb to help debug this, might save a few transistors or fuses. I'm surprised the fuse is ineffective, my scope kept blowing fuses.

[CapLeaker]

Re: My trans sister is smoking hot.

I was about to come here and make some harsh comment to OP but then realized what he did had a good laugh

I had to laugh hard as soon as I saw that title.

[R_J]

Re: My trans sister is smoking hot.

I would'nt expect there to be full -2kv but if there is 0 volts I would suspect shorted caps in hv module but if there is lets say -100 volts at least that line is not shorted
You could disconnect 3,5 & 11 of the hv module and see if the +95 comes up

[eccerr0r]

Re: My trans sister is smoking hot.

I saw the title "My trans sister is smoking..." and didn't really think much of it, I actually thought transistor as expected...

but once I clicked I saw the full title "My trans sister is smoking hot." ... then that was funny.

[socketa]

Re: My trans sister is smoking hot.

Glad to provide a bit of a laugh

"When you said Q522 did you mean Q552" - Yes
And the fuse did blow - i didn't look carefully enough, and have since replaced it.
i found out Q522 is a D44H11 transistor (the original one was not present when i got it)
"Did you check Q556 to make sure its not shorted or open?" - it's not shorted (reluctant to remove it since two of the PCB copper inserts came out with with other transistor)
TP552 - there is the same voltage there, that is at the 32V Unreg fuse.
i don't have a high voltage probe to check the 2kV

[budm]

Re: My trans sister is smoking hot.

The E-C Voltage of Q548 is 0V (3,2V at Emitter, 3.2V at the Collector), are you sure it is not shorted out, Q548 provides the DC bias for the Q552.
Also check Q554.

https://www.badcaps.net/forum/attach...1&d=1488337902

[socketa]

Re: My trans sister is smoking hot.

The E-C Voltage of Q548 is 0V (3,2V at Emitter, 3.2V at the Collector), are you sure it is not shorted out, Q548 provides the DC bias for the Q552.
Also check Q554.

Yes, am very sure.
In diode test mode, for each transistor, i get 0.66v for testing B-E and then B-C, and no reading when i reverse the leads.

and voltages are:

Q544
B = 2.99v
C = 2.44v
E = 2.35v

Q548
B = 2.43v
C = 3.18v
E = 3.22v

[eccerr0r]

Re: My trans sister is smoking hot.

The transistor sounds like it's still good then, if it's showing a ~0.6V from B-E and B-C but no drop in reverse.

Right now it still smells like an overload/short somewhere outside the transistors which are showing the signs of it. Every situation is different, I'd start disconnecting things to see if anything would stop the smoking/overheating. My similar Tektronix issue I was able to start to zero in on the real problem when I disconnected the CRT from the HVPSU (which stopped the burning of fuses), but this is only a suggestion and may not give any useful results if the problem didn't lie in the CRT.

[petehall347]

Re: My trans sister is smoking hot.

you should always check c to e on transistors . open both ways is good .
low voltage low current testing isnt the best way to test them ..sometimes pass all tests and still bad under load .

[socketa]

Re: My trans sister is smoking hot.

"but this is only a suggestion and may not give any useful results if the problem didn't lie in the CRT."

the CRT was disconnected and the vertical board was removed when the transistor got fried

Couldn't the problem lie in the lack of a 95V?, because if there is no 95v supply, then the output of Q544 and Q548 would be mucked up , which would incorrectly bias Q552.
Looks like the voltage at the collector of Q548 is supposed to be -0.8V, but it is actually 3.18V.
Could this cause Q548 to be too strongly forward biased?

And it sure seems like there is no voltage coming from pin 2 of the transformer, because when i remove the leg of the diode that's connected to it, i still only get 11V on the non-transformer side of the diode; so this 11v must be coming from the other other direction, and therefore there is no voltage being generated on pin 2 of the transformer.

[R_J]

Re: My trans sister is smoking hot.

The reason for the 11 volts comes from the +32 and a small amount of current will flow through R572 (200k pot) and r542 (100k) resistor
You either have a bad high voltage transformer or a bad u550 high voltage module
Can you disconect pin 10 pin 9 and pin 8 of the transformer from the high voltage module and see if the +95 returns, If it does then the transformer is ok and the module is bad.
How are you trying to measure the voltage on pin 2 of the transformer? Most meters won't read high frequency a/c (you need a scope) catch22

[socketa]

Re: My trans sister is smoking hot.

Measured pin 2 of the transformer with another scope - flat line
(but even without the scope, shouldn't it have showed up on the other side of the diode CR582?)

Disconnected pins 8, 9 and 10 of the transformer and still only get that same 11v on the 95V rail.

[socketa]

Re: My trans sister is smoking hot.

Does this mean that the HV module is probably good, and that the transformer is probably bad?