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TEK465 Horizontal trace


wes Bolin
 

I wrote a few weeks ago about no Horizontal trace on my TEK465. In the
meantime I have replaced bad caps and transistors to the point where I have
a sweep signal going to one of the CRT grids. I do not have a sweep signal
on the other grid. The good signal is on the Right Deflection Plate, and
no signal on the Left Deflection Plate. Good waveforms on B and E of
Q1224, no signal on B, E, and C of Q1234. DC voltages are good on Q1234.
I am stumped. I replaced Q1234 with another transistor but no joy. Maybe
someone can give me some help?


 

Is C1220 shorted?

On 1/21/2021 1:13 PM, wes Bolin wrote:
I wrote a few weeks ago about no Horizontal trace on my TEK465. In the
meantime I have replaced bad caps and transistors to the point where I have
a sweep signal going to one of the CRT grids. I do not have a sweep signal
on the other grid. The good signal is on the Right Deflection Plate, and
no signal on the Left Deflection Plate. Good waveforms on B and E of
Q1224, no signal on B, E, and C of Q1234. DC voltages are good on Q1234.
I am stumped. I replaced Q1234 with another transistor but no joy. Maybe
someone can give me some help?



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Tom Lee
 

There shouldn't be any signal of note on the base of Q1234, so that's normal (it gets its signal from the emitter here). The dc levels being correct tells you that the transistor itself is likely fine, and that it's biased correctly.

The signal on the emitter will be very small (less than a VBE). To what resolution are you saying that there's no signal there? I'm betting that you weren't looking at it with sufficient sensitivity. In any case, whatever you see there, it should be the same as what you see on the emitter of its counterpart (Q1224), so probe the emitters of both transistors (or simply check continuity of the emitter connection).

Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 1/21/2021 10:13, wes Bolin wrote:
I wrote a few weeks ago about no Horizontal trace on my TEK465. In the
meantime I have replaced bad caps and transistors to the point where I have
a sweep signal going to one of the CRT grids. I do not have a sweep signal
on the other grid. The good signal is on the Right Deflection Plate, and
no signal on the Left Deflection Plate. Good waveforms on B and E of
Q1224, no signal on B, E, and C of Q1234. DC voltages are good on Q1234.
I am stumped. I replaced Q1234 with another transistor but no joy. Maybe
someone can give me some help?




Dave Peterson
 

Wes,

You said you checked DC values on Q1234. What about the bias circuits around Q1234? E.g.: 37.5v at C1235, -7.4v at C1220?

Also wondering if issues in or around Q1236 might be a problem? If emitter of Q1236 gets pulled down will that pull it's base down and clamp this signal at the base (waveform 80)? Check the DC values around Q1236?

Dave


wes Bolin
 

I measured C1220 as being good. I also lifted the Neg lead. Slightly more
signal to Q1224 base but still no Left trace.
Q1234 base to ground reads about 24 ohms.

Wes


Michael W. Lynch
 

Wes,

I don't suppose you have a working scope? I know that you already know this, but this is where having a scope shines, nothing beats looking for the correct wave forms in a circuit.

Are your Transistors soldered or are they in sockets? Most 465's are in sockets. I have a similar issue and was able to swap transistors from one side of this circuit and find the problem. There are several interchangeable transistors in that circuit. You can swap these components and see if the problem "jumps" from one side to the other.

I also had a 465 that would not trigger and it turned out that I had a very weak transistor (Q552 on the B Trigger Gen) in the circuit. I found this problem by swapping transistors from the working "A" Trigger Circuit.

Of course, you need to proceed with caution and make 100% certain that you are "Swapping" the right parts. Your Replaceable Parts list in the Service Manual will help you identify the parts that are interchangeable,

Good Luck.

--
Michael Lynch
Dardanelle, AR


Ozan
 

Hi Wes,
We were looking at horizontal output stages for another thread so this one is another good schematic to look at. As others also commented Q1234 and Q1224 function as single ended to differential converter and single ended signal is applied to Q1224. We expect the same signal at the emitters (they are connected after all), base of Q1224 will have signal but base of Q1234 will not have any signal (or very tiny). Your observation of no signal at B of Q1234 is expected. Your reading of 24 ohms from base to ground is also reasonable (should be lower but I don't think this is the problem).

Not seeing a signal similar (but opposite polarity) at collector of Q1234 while you are seeing a signal at collector of Q1224 is a problem. You didn't explicitly say you are seeing good signal at collector of Q1224, do you see a good signal?

The other thing I found out is the first service manual I pulled from W140 didn't have voltages. This one has:
https://w140.com/tekwiki/images/c/ca/070-1861-00_C.pdf

Could you report all the DC voltages at E,B,C of Q1234 and Q1224?

As Michael wrote I heard 465 has sockets for transistors. You may want to wiggle and re-seat the transistors (with power off) for all the transistors on page 222 and 223 of the link I gave.
Ozan


Mlynch001
 

I would also check Q1274, Q1282 and Q1288 as well as all the diodes on that left side of the circuit. One good thing is that you have one side of the circuit working. You can use the “good” side as a reference for what good parts should test like.

--
Michael Lynch
Dardanelle, AR


wes Bolin
 

Thanks for all of your help. I have spent way more time and I'm out of my
element and another O'scope for around $100 looks real attractive. At
least this one is a
parts source. I'm returning it to my friend who is still looking for a
working O'scope.
I'll keep monitoring the Group. Best Regards,
Wes
Texarkana, AR