Tektronix 2225 very weird vertical position fault [solved]

[Musikermomo]

Hi all,

I am trying to repair my scope Tektronix 2225 and I am lost.

The problem it shows is that the vertical position of channel one is influenced by the vertical position knob of channel 2. Also channel one trace shows the inverted signal of channel 2, even when ch2 is switched off, but only if the channel 2 vertical position knob is not turned all the way up.

Kind of hard to explain so I made two videos:
https://youtu.be/AgEqlyDWqIw
https://youtu.be/sZYH9mG9tBQ

Channel two seems to Work fine.
I checked the front panel and the attenuator board and could't find any faults so I think the problem must be somewhere on the Main Board.

Ch1 seems to work fine if i disconnect J2-1. I measured the voltages surrounding this part and further up to the delay lne driver and they are almost spot on.


Appreciate any help.

Thanks!
Moritz

[akimpowerscr]

I would try to replace U180 (CA3102E)

NB: Whenever you replace an integrated circuit, always install a good quality socket.

Service manual:
https://elektrotanya.com/tektronix_2225_oscilloscope_sm.pdf/download.html#dl

[akimpowerscr]

For a better diagnosis, a lot more information would be needed:

1) CH1 only: Is the CH1 vertical position control working normally with the same deviation up and down.
Is the CH1 signal normal? (amplitude, waveform)
Does the CH2 position control have any influence?

2) CH2 only: same

3) BOTH: describe the operation in chopp and alt .... Is it different?

[Musikermomo]

For a better diagnosis, a lot more information would be needed:

1) CH1 only: Is the CH1 vertical position control working normally with the same deviation up and down.
Is the CH1 signal normal? (amplitude, waveform)
Does the CH2 position control have any influence?

2) CH2 only: same

3) BOTH: describe the operation in chopp and alt .... Is it different?

1) CH1 only: Yes to all.
Ch2 position control has influence on Ch1 position and amplitude. Ch1 trace seems to behave normal, when Ch2 Position is set fully clockwise.

2) Ch2 only: everything seems fine.

3) chop and alt make no difference regarding this problems.

If I feed a signal into ch2 it gets doublicated (but mirrored) on ch1 regardless of switches (Ch1/Both, Add/Alt/Chop). The amplitude changes when fiddling with Vert-Pos2.

[Rollin Hand]

looks like a scratchy or open potenciometer.
check values

[Musikermomo]

It is a bit scratchy, but I checked the component values and pcb traces around the position control (Front PCB) and they are fine.

[akimpowerscr]

Interesting, I have never seen such a defect.

With a slow time base, (50ms/div for example), in BOTH, what do you see on the screen in ALT and in CHOPP?

To not waste too much time in diagnosis, I would start by replacing the two CA3102E (U180 and U130)

Check also diodes CR183, CR189, CR186 and -8,6V rail

[pbarton]

You can only get interaction between the Ch1 vertical position control and the Ch2 vertical position control if you have a fault in the alternating channel logic.
Ch1 and Ch2 should never be selected at the same time. If they are, then you have the fault that you can see.
The good news is that Ch1 and Ch2 amplifiers are both OK.
The channel selection logic is U540A, pins 8 and 9 should alternate.

[Musikermomo]

Interesting, I have never seen such a defect.

With a slow time base, (50ms/div for example), in BOTH, what do you see on the screen in ALT and in CHOPP?

To not waste too much time in diagnosis, I would start by replacing the two CA3102E (U180 and U130)

Check also diodes CR183, CR189, CR186.

Yeah, it is weird... I've been searching the Internet for 2 days and have not found any similar problem.
The Scope is partly disassembled right now, but if I remember correctly, it behaves normal in slow timebases, exept the vertical position problem and ch2's signal on ch1.

I measured the diodes in-circuit. CR183 and CR189 have threshold voltages of 0,6V and resistances of 1kohm. CR186 has a threshold voltage of 0,15V and R of 100ohm. Is CR186 dead?

[akimpowerscr]

CR186 is in parallel with a resistance of 100R, what you are measuring is OK.
If CR186 should be defect, you should measure a dead short circuit.

Check also CR136 (same as CR186) and CR189.

It is also possible that the select channel logic U540 is defect but the set/reset seems to work as you can select CH1 or CH2.

As I already asked: "With a slow time base, (50ms.div for example), in BOTH, what do you see on the screen in ALT and in CHOPP?"

NB: I am waiting you to answer me that in CHOPP, you see both traces and in ALT only one trace each time....Is this correct ?

[Musikermomo]

It took quite a while to reassemble the thing, I'm sorry.

CR186 is in parallel with a resistance of 100R, what you are measuring is OK.
If CR186 should be defect, you should measure a dead short circuit.

Check also CR136 (same as CR186) and CR189.

Both OK.

It is also possible that the select channel logic U540 is defect but the set/reset seems to work as you can select CH1 or CH2.

As I already asked: "With a slow time base, (50ms.div for example), in BOTH, what do you see on the screen in ALT and in CHOPP?"

NB: I am waiting you to answer me that in CHOPP, you see both traces and in ALT only one trace each time....Is this correct ?

This is correct.

[akimpowerscr]

Than, it seems that the select channel circuit is working....

But replace also U540A (74LS74) to be sure.

[Musikermomo]

You can only get interaction between the Ch1 vertical position control and the Ch2 vertical position control if you have a fault in the alternating channel logic.
Ch1 and Ch2 should never be selected at the same time. If they are, then you have the fault that you can see.
The good news is that Ch1 and Ch2 amplifiers are both OK.
The channel selection logic is U540A, pins 8 and 9 should alternate.

pins 8 and 9 do alternate in ALT-mode.

[pbarton]

The oscilloscope may be displaying Ch1 correctly?
However, if there is a select channel problem that enabled both Ch1 and Ch2 together (during the Ch2 display period), then the resulting Y signal would be the ‘sum’ of the two channel amplifiers, and there would be interaction between Ch1 and Ch2 vertical settings.
The observer would still see two traces in CHOP and one trace each time in ALT (at slow timebase).
That proves that CHOP and ALT modes work, but it does not prove that Ch1 and Ch2 are alternating independently.
What you are not seeing is Ch2 (on its own) during the Ch2 display period.
Could be U180 (CA3102E) or those diodes mentioned above.

[akimpowerscr]

Replace the two CA3102E (U180 and U130)

[Musikermomo]

Thank you very much for the help so far!
I think I will swap u180 with u130 first to see if they cause the problem.
Unfortunately these ICs are quite expensive.

[akimpowerscr]

The CA3102 can be replaced with three matched pairs of transistors like 2N3904's if you are desperate, but not recomended. 

If you find CA3049, that's the same but in metal can.

[pbarton]

If you look at the CA3102E datasheet, you will see that it contains two independent differential amplifiers. I would be using a meter to check the impedance, pin to pin, to determine if I can identify a faulty reading. You have two chips to compare  (U180 and U130) before removing them from the board.
The CA3102E is expensive, so I would not buy one unless I could identify an internal fault using a meter. Tektronix Part # 156-0534-00. https://www.littlediode.com/components/home.php same seller also on eBay.

[akimpowerscr]

The funniest thing is that I sold a 2235 wreck two months ago for about the price of ONE CA3102E .....

It was irreparable, and in particular the pre-regulator inductance was missing, buttons were missing or broken and the channel 1 input switch was completely worn.

But with an external 43V, it was working ...

[Musikermomo]

If you look at the CA3102E datasheet, you will see that it contains two independent differential amplifiers. I would be using a meter to check the impedance, pin to pin, to determine if I can identify a faulty reading. You have two chips to compare  (U180 and U130) before removing them from the board.
The CA3102E is expensive, so I would not buy one unless I could identify an internal fault using a meter. Tektronix Part # 156-0534-00. https://www.littlediode.com/components/home.php same seller also on eBay.

The only difference I could find so far is between pins 9/10 and 3/10
U180:
3-10 --> 1M 
7-10 --> 0,5M
U130:
3-10 --> 700k
7-10 --> 300k

The funniest thing is that I sold a 2235 wreck two months ago for about the price of ONE CA3102E .....

It was irreparable, and in particular the pre-regulator inductance was missing, buttons were missing or broken and the channel 1 input switch was completely worn.

But with an external 43V, it was working ...

This is my first own scope. I scored it on ebay for 50€ in "fully working condition". Now the seller has dissappeared

[pbarton]

The differences you have measured are small.
If you had identified a short or an open circuit between pins, that would have been a different story.

Select Ch1 (only)  on the front panel, measure the voltage on each pin of U130.
Then select Ch2 (only) on the front panel, measure the voltage on each pin of U180.
Compare results.

akimpowerscr: I use this document as a guide for those power supplies   http://www.hakanh.com/dl/docs/troubleshooting_tips_on_2200_ps.pdf

[akimpowerscr]

This is my first own scope. I scored it on ebay for 50€ in "fully working condition". Now the seller has dissappeared

Unfortunately, there are people who say that an analog oscilloscope is only worth it if you pay it very cheap.

But in life, there are no miracles.

For 50 € you only buy wrecks .... and then some people come to complain that an analog oscilloscope is unreliable.

Do you really think a seller is going to waste their time testing a device they sell for only $ 50 ? 

These devices were very expensive (2000, 3000 euros and even more) because there were expensive parts in them, like the cathode ray tube for example.

A good analog oscilloscope costs at least 150 euros, unless it is sold broken down or for parts.

You live in Germany, why didn't you buy a HAMEG HM203? They are simple and inexpensive.

Hameg has made over 150,000 HM203s.

They have sold it everywhere, industry, education, administration, hobby,  ....

[Musikermomo]

This is my first own scope. I scored it on ebay for 50€ in "fully working condition". Now the seller has dissappeared

Unfortunately, there are people who say that an analog oscilloscope is only worth it if you pay it very cheap.

But in life, there are no miracles.

For 50 € you only buy wrecks .... and then some people come to complain that an analog oscilloscope is unreliable.

Do you really think a seller is going to waste their time testing a device they sell for only $ 50 ? 

These devices were very expensive (2000, 3000 euros and even more) because there were expensive parts in them, like the cathode ray tube for example.

A good analog oscilloscope costs at least 150 euros, unless it is sold broken down or for parts.

You live in Germany, why didn't you buy a HAMEG HM203? They are simple and inexpensive.

Hameg has made over 150,000 HM203s.

They have sold it everywhere, industry, education, administration, hobby,  ....

I wanted an analog scope. I use scopes mainly for ampstuff, so no need for modern fancy-pancy scopes.
My father bought a very nice scope some years ago for 50€ from ebay, it was a Hameg, that was in working condition, as advertized.
But call me naive, I didn't know that lying is ok if you sell a tek scope...
2225ves go for just about the same prices as hm203s...

[Musikermomo]

Select Ch1 (only)  on the front panel, measure the voltage on each pin of U130.
Then select Ch2 (only) on the front panel, measure the voltage on each pin of U180.
Compare results.

I measured each pin and these are the only major differences. (All other pins are just a few mV off.)

Ch1(only):
U130: Pin4/8 --> 0V
U180: Pin4/8 --> -550mV

Ch2(only):
U130: Pin4/8 --> -10mV
U180: Pin4/8 --> -80mV

BOTH:
U130: Pin4/8 --> -5mV
U180: Pin4/8 --> -320mV

[pbarton]

Can't see anything wrong there. The manual contains lots of sample waveforms to verify correct operation.
Do you have access to another scope?