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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?

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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

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

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

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

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:


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

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

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

Getting right to it:

Wes' friend agreed to ship the scope out to me. I've got it and another 465 belly up on my bench at the moment. There are a couple/few issues: the sweep logic chip U870 is being flaky (but works when it works), Q1098 is bad, and this Horiz Amp issue.

I think I've identified one of the key issues at hand: the bias of the input to Q1224 is too negative. The signal is between -320mv and -600mv. On my working scope the input to Q1224 is 0v with a sawtooth wave of +/- 40mv.

I've also verified with an LTspice simulation. Thank you Ozan for sharing the 2235 Horiz Amp schematic. It was the basis for a rewrite to the 465 Horiz Amp. My simulation is mimicking what I'm seeing on the bench: a zero offset 80mv input signal yields nice amplification through the first stages of the amp. A large negative offset yields the disfunction seen on the failing scope. This includes no real signal at the emitters of Q1224 and Q1234, no signal at the collector of Q1234 (a small negative DC value), and a reasonable looking signal on the collector of Q1224.

Normal Q1224/Q1234 collector signals are offset +3v with a 2v signal in both simulation and on the bench. Dysfunctional Q1224 collector is +6v offset in both simulation and bench, but the signal level is somewhat better on the bench than the simulation. Not really a factor, just an observation. Better beta in real transistor than .model?

I can't figure out where this input bias is coming from. This is how I discovered Q1098 is bad. I was trying to see if the B-sweep was contributing, but it turns out no, because the emitter base junction is shorting the B-sweep signal to ground. I've removed the transistor from the scope to eliminate it as an issue. I've verified that neither the emitter nor collector sockets are shorted to gnd.

This leaves, near as I can tell, only Q1038 or sheet 9/Horizontal Amplifier as sources for a negative bias. I'm pretty comfortable saying Q1038/A-sweep are not the source as there is no negative voltage coming from there, and the A-sweep signals inside of Q1038 are fine.

I've pulled up legs of the resistors around Q1224/Q1234 to verify their values. Every resistor has checked good. The bias voltages are according to the values shown in the schematic. One result I found interesting is I tested the input signal with R1221 and Q1224 pulled. The sweep input bias goes to +8v and there's still some signal on it. But this tells me there's nothing coming from the X axis input nor position pots to pull the input so negative. I've pulled P1200 from the position pot anyway. To no avail.

So what the heck!? Everything in the input bias looks spot on. I've even replaced Q1224/Q1234 with brand new 2N3904s. I just got a roll of 100 from Mouser.

It's a puzzle. Any ideas?

Dave

Hi Dave, if you remove Q1038 and Q1098, set POSITION pots to center position, sweep through the TIME/DIV positions, no X AXIS signal, what do you have at Q1224 base?
Measure and calculate the current through R1223, 806 ohm. Calculate the various currents going to/from Q1224 base. Does anything make sense?
G?ran

I have removed Q1226/Q1236 PNPs with no effect. They are not part of the problem. This is also part of why I built the LTspice simulation - to get insight into the current switching going on. I've also tested changing horizontal position, as well as unplugging them entirely at P1200 with no effect.

My finding is that, while yes, the Q1224/Q1234 currents drive the signals through the front end of the amp, it doesn't mean that there aren't corresponding voltage signals. The simulation confirms that an excessive negative bias on the input results in the symptoms observed on the bench. It's not a matter of unobservable currents.

I've been chasing the theory that something is pulling the base of Q1224 down, but sleeping on it allowed me to realize that perhaps the issue isn't a pull down, but a weak pull up. As the PNP Q1098 has a problem, perhaps Q1038 does as well. The node at Q1224 base is a huge common point of a lot of drivers. Q1038/Q1098 are significant contributors. I need to order up some fresh 2N3906's, but it occurred to me that the system is likely designed to have Q1038/Q1098 contributing, even if B-sweep is inactive.

I'm thinking about borrowing Q1226 and Q1236 for Q1038/Q1098. I'm hesitant to take them from my working 465 lest I damage them and kill a second scope. I need to accumulate some components to order so I can mitigate the shipping charges from Mouser. Even 100 2N3906's are less than the $8.00 shipping.

Dave

I read somewhere that you get free shipping from DigiKey if you prepay with a check.
Bob

Or Amazon Prime! LOL.

I guess I have a bias against Amazon for such things. Kind of like Radio Shack. But why? I guess as long as I shop carefully and understand I might not be getting the best quality product. Why not?

I see packages of assorted types for cheap. Might be worth having a variety. Thanks for kicking me in a different direction Bob!

Dave

I quickly scanned your post, I didn't see anything about Q1218 so if you tried already you can ignore the following comment: The voltages you measured say pretty much all the available current is pulled out of the base node of Q1224. I looked at the timebase circuit briefly, the worst that can happen looks like a short to ground (not pulling below). That leaves Q1218 and the circuit around Q1218 as the suspect. It could be as simple as a dirty switch (S1150 going to the diodes).

What voltage do you measure at the emitter of Q1218?

Ozan

Yeah, I didn't get into Q1218. It did have me running in circles a bit too because of its -8v at the collector.

I did a couple of things: removing it and trying running the scope without it. No effect.

I also measured voltages while switching S1150/Time/Div (to X-Y is cam switch 14). Mostly "correct" and following the engagement/disengagement of S1150/14 (X-Y mode). I say "correct" because the Q1224 base node is connected too, and it throws off the voltages around R1202, R1219, Q1218e.

What I thought was very telling was lifting R1221 resulted in Q1224 base going all the way up to +8v. No hope of the circuit working, but told me there was nothing else pulling that node down. Not Q1214/Q1218 nor the horizontal position pots.

That's when I ran out of ideas and hollered for help.

I've ordered a box of 250 assorted BJTs from Amazon with free delivery for tomorrow. So I'll have some fresh 2N2906s to try. I'll probably (impatiently) swap Q1226/Q1236 into Q1038/Q1098 this afternoon to see if that brings Q1224base back up. Or releases the magic smoke.

Dave

Looks like I found the problem!

I'll have to add Q1038 and Q1098 to my horizontal amp simulation.

Clearly both of these transistors need to be in place and working for the correct DC level on Q1224base.

I swapped Q1226/Q1236 (2N3906s) into Q1038/Q1098 and the sweep sawtooth into Q1224 is now correct and their collectors are now correct too. Of course there's no second stage loading the first, and who knows what else is going on with this scope. I'm supposed to get my collection of various transistors from Amazon tomorrow and I'll find out if I can get a display back on this old bird.

I suspect that it's only Q1098 that's bad, and the lack of it's pull up contribution to the Q1224base node is what's causing the negative bias. A simulation will help clarify that. I need a proper transistor tester. A DMM shows it good. Clearly it's not.

So that's been satisfying. Just learned more about the sweep circuits than I ever would have just staring at schematics.

Dave

Wes,

If you're watching this group and thread still, I found another disfunction that was confounding the trace:

The vertical amp input, Q4402, was dead, and caused the vertical plates to be stuck high. So the trace would only show up if BEAM FIND was half pressed.

After pulling the transistor out I found that the passivation in the can was only half filled. So it was just a bad part that must have failed at some point not being sealed.

It also appears at least one tunnel diode is dead. I used some 2N3904s in place of Q4402/Q4412 for testing, and I'm able to get a waveform, but only with the trigger holdoff tuned just right.

I have parts in hand and on the way. I'm going to have fun fixing all the issues this scope has, but it's pretty funny that there's just about nothing on this scope that hasn't suffered a failure in some way. It'll be fun to rehab it to full working condition again. It's going to take a while.

Dave

On Thu, Mar 25, 2021 at 06:55 AM, Dave Peterson wrote:

After pulling the transistor out I found that the passivation in the can was
only half filled.

What does that mean? I know glass passivation but "filled"?

Raymond

I am as intrigued by Dave's comment as you are, Raymond. I have never seen a production JFET that wasn't built in a planar process. They existed, but by the time the 465 was in design, non-planar JFETs had long since died out. The passivating goop that Dave is referring to was common in the alloy-junction and mesa days (and it didn't really work well), but one of the main selling points of the planar process was its oxide passivation -- no need for the goop. There should be no goop-filled parts in a 465.

So I'm very curious!

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070