Tek 2445 Ch1 and CH2 not working (SOLVED)

[20Ola02]

Hi,
 
Ch1 and Ch2 on my scope does not work.
I bought it used from the states, when I got it I turned it on without changing the RIFA caps and the x cap C1016 and resistor R1016 on the power supply exploded 
Fixing that I turned it on without an explosion this time

When testing different functions I noticed Ch1 and Ch2 did not work.
On the A1 board 2 transistors and a resistor was cracked, check this small post(https://www.eevblog.com/forum/repair/tek-2445-broken-transistors/).
I have since replaced the transistors, the resistor and the cap C130. Ch1 and Ch2 does still not work. I do see a signal on R141 when I push various buttons.

Does anyone have a suggestion on what to test further? or to perhaps replace?

I do fear the Ch1 and Ch2 attenuators are broken, but it seems a bit weird that both of them are broken.

Anyways, thanks in advance.

-Ola

[20Ola02]

I forgot to add that there is no relay clicking, nor in startup or when I push Ch1/Ch2 mode button.

[bdunham7]

Can you be specific about what 'not working' means?  For the problem to be attenuators, you'd have to have everything else working normally and have a straight trace on the screen, which would be adjustable via the vertical position knob.  CH3 and CH4 work?

[20Ola02]

Can you be specific about what 'not working' means?  For the problem to be attenuators, you'd have to have everything else working normally and have a straight trace on the screen, which would be adjustable via the vertical position knob.  CH3 and CH4 work?

Ch3 and Ch4 works.

By not working I mean that I cant see any change in the trace for Ch1 and Ch2 when probing on the calibrator on the scope.
The calibrator works as I have tested it with both Ch3 and Ch4.

Ch1 to Ch4 has a straight trace and can all be adjusted with the vertical knobs, same for horizontal adjustment.

[bdunham7]

OK, so you have a trace and all that.  I dragged out my 2445 from under the bench so we can compare operation.

If you turn it on and select just CH1, there should be a trace plus readout numbers at the bottom of the screen--volts/div on the left and s/div on the right.  Are they there?  Adjust the readout intensity if they aren't.

Now select 1V/div, then go down to 500mV/div and back.  The readout numbers on the screen should change and there should be a click each time. 

There are selector levers below the vertical knob for CH1 and CH2, with lights that come on for DC50R, GND, DC, GND and AC (from bottom to top).  Can you change the lights and is there a click each time?

Last, but both CH1 and CH2 input selectors all the way up to AC, then simultaneously push them both up for a few seconds.  The scope should react with self-test procedure that will last a minute or so.

[20Ola02]

I got the scope at work due to limited test equipment at home. I will follow your comparison procedure tomorrow.

[SoundTech-LG]

Tried those tests on a working 2465B, passes all, as expected.

[20Ola02]

OK, so you have a trace and all that.  I dragged out my 2445 from under the bench so we can compare operation.

If you turn it on and select just CH1, there should be a trace plus readout numbers at the bottom of the screen--volts/div on the left and s/div on the right.  Are they there?  Adjust the readout intensity if they aren't.

There is a trace plus the readout numbers: volts/div and s/div.
The trace blinks when i push in Ch1 to Ch4

Now select 1V/div, then go down to 500mV/div and back.  The readout numbers on the screen should change and there should be a click each time. 

Switching from 1V/div to 500mV/div changes the readout on the crt, there is no clicking

There are selector levers below the vertical knob for CH1 and CH2, with lights that come on for DC50R, GND, DC, GND and AC (from bottom to top).  Can you change the lights and is there a click each time?

Changing between 50R, GND, DC, GND and AC. The light changes but there is no click

Last, but both CH1 and CH2 input selectors all the way up to AC, then simultaneously push them both up for a few seconds.  The scope should react with self-test procedure that will last a minute or so.

It ran DC balance that lasted 15-20 seconds.

[bdunham7]

Changing between 50R, GND, DC, GND and AC. The light changes but there is no click

The two attenuators function nearly independently, but what they do share is the DATA/STROBE/CLOCK from the control board.  And that is directly related to the previous repairs and problems that you mentioned.  I doubt very much that the attenuators themselves are the issue.  Perhaps have a look at the ribbon cable for J512 that goes from the control board (bottom, towards the back IIRC) to the main board, make sure it is secure. 

Do you have another scope?  You will want to look at TP18 and then the collectors of Q130 and Q131 at the same time.  I'm not sure exactly what that is supposed to look like right now, but it might be in the manual and I'm sure we can figure it out once we see it. 

I'm assuming you have tested all of the power supply voltages per the manual?  If not, do that before you get into anything else.

[20Ola02]

Changing between 50R, GND, DC, GND and AC. The light changes but there is no click

The two attenuators function nearly independently, but what they do share is the DATA/STROBE/CLOCK from the control board.  And that is directly related to the previous repairs and problems that you mentioned.  I doubt very much that the attenuators themselves are the issue.  Perhaps have a look at the ribbon cable for J512 that goes from the control board (bottom, towards the back IIRC) to the main board, make sure it is secure. 

Do you have another scope?  You will want to look at TP18 and then the collectors of Q130 and Q131 at the same time.  I'm not sure exactly what that is supposed to look like right now, but it might be in the manual and I'm sure we can figure it out once we see it. 

I'm assuming you have tested all of the power supply voltages per the manual?  If not, do that before you get into anything else.

Tested the power supply voltages and they are within spec. I have another scope to scope with, will do that over the weekend.

[20Ola02]

Scoped at TP18(Ch1), CR130 anode(Ch2) and CR131 anode(Ch3). There is a signal but it does not look exactly as it does in the manual. Look at the pictures.
The signal is the same no matter which switch/button I press.

Also, I measured the resistance of the relay coils. I did this by measuring at the cathode of the diodes going in to the relay coil.
Cr141-Cr142 = 63ohm
Cr143-Cr144 = 72.1ohm
Cr145-Cr146 = 72.5ohm
Cr147-Cr155 = 73ohm
Cr148-Cr149 = 72.9ohm
Cr150-Cr151 = 4.31ohm
Cr152-Cr153 = 72.3ohm
Cr140-Cr154 = 72.6ohm

Most look good except Cr141-Cr142 and especially Cr150-Cr151. I don't know if this is by design or if a coil or one of the IC are shorted or burnt on the inside.

While at it I also did some diode measurements.
Checking the forward bias:
Cr140-142, Cr144-149 ,Cr154-Cr155 and Cr131 had a forward voltage of anything between 0.619V-0.638V.
Cr143, Cr150-Cr153 and Cr130 had anything between 0.457V and 0.599V.

Then checking the reverse bias:
Cr140, Cr142-Cr147, Cr149, Cr151, Cr153, Cr131 had OL displayed.
Cr141, Cr148, Cr150, Cr152, Cr154-Cr155 and Cr130 measured 1.095V to 2.576V.

According to the manual all of these diodes are the same, so it looks like some of the diodes are toast.

Anyways, Thanks with the help so far bdunham7 I really appreciate the help!

[MarkL]

...
Most look good except Cr141-Cr142 and especially Cr150-Cr151. I don't know if this is by design or if a coil or one of the IC are shorted or burnt on the inside.

I would suspect U120 before a coil being bad.

You could try scoping U120 pins 13 and 14 and compare to U110 pins 13 and 14.

[bdunham7]

I wouldn't jump to the conclusion that the diodes are bad and I agree with MarkL that U120 is worth a look.  The schematic is incomplete in that it doesn't show the ground and power pins.  These Darlington arrays don't use the power directly, but when used to drive relays they typically connect the power pin so that the freewheeling diodes can pass any spikes back to the supply.  The arrays are unlikely, AFAIK. to short the outputs to one another, but any particular output could be shorted to ground or to power.  Shorted to power would be easy to spot--the outputs of U120 should never be continuously high, but for testing shorted to ground, you would have to compare the input and output--any input that is low should have an open output.  A low-impedance probe (0.5-1k)--even homemade--is handy for testing this type of output.  Don't overlook U130.

I'm not sure why you don't get any relays clicking--I see the outputs from Q130/131 seem to sag, but not always.  One working theory would be that if U120 pin 13 and 14 were both shorted low, that would soak up all the power from Q130/131 since the strobe/relay combo only works if one of the pair is low.  Perhaps I haven't thought it through enough.  I haven't needed to work on this section of these scopes before so it is new to me too.

Here's an internal diagram of the MC1413 (U110/120/130).  The one in the scope is an MC1413PDS which I couldn't easily find a datasheet, but this should give you an idea what it does.

https://www.mouser.com/datasheet/2/308/1/MC1413_D-2315660.pdf

[20Ola02]

Scoped pin 13 and 14 on both U120 and U110 using normal oscilloscope probes. There are two minor differences: U110 is peaking more than U120 and has a higher output voltage then U120.
U110, U120 and U130 has +15V on pin 9.

[bdunham7]

At this point I'd recommend breaking out the soldering equipment.  Remove or lift one leg of CR150 and CR151 and then recheck the resistance measured across that coil wherever is convenient, probably pins 13 and 14 of U120.  Then try to operate the scope again and see if you get any of the attenuator relays clicking.

Where are you grounding your probes?

[20Ola02]

I'm grounding the probes at the nearest point which is the metal tabs on the shield on the Ch2 attenuator.

Lifting the anode of both CR150 and CR151 and then measuring on the cathodes I got 72Ohms between them.
No clicking of the relays.

Edit:
The diodes are now measuring 0.636 and 0.628V on forward bias and OL on reverse bias

[bdunham7]

Edit:
The diodes are now measuring 0.636 and 0.628V on forward bias and OL on reverse bias

That's what I was going to ask you to check next.  Hmmmm.   I'm going to have to think about this and I'm still on my first cup of coffee.  I don't see how you could have had that low reading before with the diodes and U120 being good--but they don't appear to be bad now.  Can you check at pins 13 and 14 (or wherever you are measuring the coil resistance) for continuity to ground? 

Edit: And always remember to check continuity both ways (reverse the polarity of the test leads) when testing devices like this.

[MarkL]

On those captures, pins 13 and 14 (and probably the others) should have a high voltage of around 13.3V.  It's odd that they're all over the place, like they're floating.

Could you verify you have +15V on both sides of R144 when idle?

And on the emitter side of R144 you should see about a 1V drop as various relays are switched in and out.  Screen shot attached.

EDIT: Fixed collector --> emitter.

[20Ola02]

Can you check at pins 13 and 14 (or wherever you are measuring the coil resistance) for continuity to ground? 

Edit: And always remember to check continuity both ways (reverse the polarity of the test leads) when testing devices like this.

Measured the resistance between the pins of U110-U130 and ground on the Ch2 attenuator shield.


com is the common on the multimeter(a fluke 83)
U110 with com to ic pins:
Pin 10-12 = 31k
Pin 13,14 = 15k
Pin 15,16 = 210k

U120 with com to ic pins:
Pin 10 =120k
Pin 11,12 = 297k
Pin 13 in circuit = 66ohm , Out of circuit = 66ohm
Pin 14 in circuit = 138ohm, out of circuit = 70ohm
Pin 15 =64ohm
Pin 16= 134ohm

U130 with com to ic pins:
Pin 15 = 120k
Pin 16 = 31k

U110 with com to ground:
Pin 10-12 = 43k
Pin 13,14 = 39k
Pin 15,16 = 550k

U120 with com to ground:
Pin 10 = 120k
Pin 11,12 = 297k
Pin 13 in = 66ohm and out of circuit = 67ohm
Pin 14 in = 70ohm and out of circuit = 71ohm
Pin 15 = 64ohm
Pin 16 = 138ohm

U130 with com to ground:
Pin 15 = 122k
Pin 16 = 43k

Could you verify you have +15V on both sides of R144 when idle?

And on the emitter side of R144 you should see about a 1V drop as various relays are switched in and out.  Screen shot attached.

With a multimeter:
R144 both side 14.99V when idle.
When pressing a button the 15V side goes down to 14.93V and the side that goes to the emitters goes down to 12.18V

With a oscilloscope:
R144 both side is 15V when idle
When pressing a button the 15V side goes down and the side that goes to the emitters goes down to 12.12V

Sidenote:
Using a scope I only see a pulse up to 4.3ish volts on the pins of U120 and 5ish volts on U110.
Is this to low?

Edit: minor corrections

[bdunham7]

U120 with com to ic pins:
Pin 10 =120k
Pin 11,12 = 297k
Pin 13 in circuit = 66ohm , Out of circuit = 66ohm
Pin 14 in circuit = 138ohm, out of circuit = 70ohm
Pin 15 =64ohm
Pin 16= 134ohm

Well then!

Does 'out of circuit' just mean you lifted a leg of the diodes for that pin?  The next thing to do is to lift a leg for CR150/151/152/153 to isolate the shorted sections of U120 and see if it comes back to life.  If not, lift CR147/148/149 as well.

[20Ola02]

Well then!

Does 'out of circuit' just mean you lifted a leg of the diodes for that pin?  The next thing to do is to lift a leg for CR150/151/152/153 to isolate the shorted sections of U120 and see if it comes back to life.  If not, lift CR147/148/149 as well.

Out of circuit means I lifted a leg of a diode for that pin. I should've made that clear in the reply.
I have ordered three UN2003A to replace the MC1413 if necessary.

[MarkL]

...
Sidenote:
Using a scope I only see a pulse up to 4.3ish volts on the pins of U120 and 5ish volts on U110.
Is this to low?

Yes, it should be much higher at around 13.3V.  On both.

Given those resistance readings on U120, it's also not a surprise that U120 has a slightly lower peak (like one diode drop).  Hopefully you've found the culprit with U120.

[20Ola02]

Should I socket the ic and change the diodes?

[MarkL]

Should I socket the ic and change the diodes?

It wouldn't be a terrible idea to socket the IC since it's not completely clear why Q131 and Q130 blew up in the first place.  I'm thinking: If it was just because of a shorted U120, why aren't your replacements blowing up now?  Without knowing the root cause of the failure, the new U120 could be at risk.

I think you already showed the diodes were ok.  As long as they have reasonable forward voltages around 0.6V (and there will be minor variations), I would not bother to do a wholesale replacement.

EDIT: I think bdunham7's suggestion to isolate the dead sections of U120 is a good idea, or remove U120 completely.  It will show if the rest of the attenuator control is still working.

[20Ola02]

Well I messed up

Taking the board as instructed in the manual I managed to snag W916 on the shield of the CRT braking it in half and unwinding the coil

Cant find any information about that part in the manual, but using different calculators online I found it to probably have an inductance of 39-43nH.
To be sure I used a LCR meter(Agilent U1731C). After nulling it out and then connecting the other coil(W917) I measured 0.01-0.02uH.

Replacing those I figured I could use these: https://no.mouser.com/ProductDetail/API-Delevan/1026R-06K?qs=sGAEpiMZZMv126LJFLh8y0dLG8tusDUNPnCZuVkxmtA%3D
Or these: https://no.mouser.com/ProductDetail/API-Delevan/1026R-00M?qs=xY1IUdBv1vR4CNNxLAcGzA%3D%3D

But I don't know