Troubleshooting Dead Channel-1 on old Kikusui COS5020 Oscilloscope

ian field

Joined Oct 27, 2012
6,536
Ok, thanks for the photos.
Everything seems clear now.
It appears that the Channel 1 input amplifier is shot, i.e. the amp output is way off the screen.
Or maybe the Channel 1 vertical position control is at fault.
We will have to take a look at the schematic. I have to take a break for a number of hours (time-zone difference).
In the meantime, have at look at the Channel 1 circuit board and see if you can post some clear pictures (if you haven't already)

Edit: You did mention in your first post:
6) Traced calibration signal input to CH-1, to C106 and notice signal drops to negative DC bias, probably a clue there?

Yes, that is a big clue. Will follow up on this later.
I'm too lazy to go through the technical documentation the TS has provided - but whether its a dual beam or a dual gun scope determines where to look for the fault.

With a dual gun CRT, it could be a dud cathode or heater. A dual beam scope either chops or alternates the traces depending on timebase speed. Often chop/alternate is a switch on the front panel, some scopes tie it in with the timebase speed selector, the switching is complex and worthy of examination.
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
Remove the short to GND at U102, pin-9.
Set input to GND, VOLTS/DIV to 5V.

Measure voltages at both legs of R115, R119 and R114.

Do the same for R215, R219 and R214.
Here are the results from this test:

Code:
       LEG_1   LEG_2
R112  -11.21  -11.20
R212    0       0

R114   -2.78   -2.78
R214    0       0

R115  -10.14  -10.20
R215    0.31    0

R117  -11.23   -9.53
R217  -11.19   -7.62

R119   -9.54   10.97
R219    0.63   10.86
I threw in R112 and R117 values because they were right next to the rest of those guys. Definitely see some differences there.

As always, THANK YOU!

-AA
 

MrChips

Joined Oct 2, 2009
30,922
One more thing.

Are you able to unsolder one leg of QCR101, 2SC1907 and lift the leg away from the pad?
If you have to unsolder both legs that's ok. Just record the orientation of the transistor (diode?) before removing.
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
Thanks. I think we're getting hot.

Some more voltages please, at R113 and R116.
I am suspicious of U101, LF13741N.
Ok, here's those:
Code:
       LEG_1  LEG_2
-------------------
R113  -10.20  -9.90
R213   -8.28  -7.16

R116  -10.20 -11.23
R216   -8.28 -11.19


PIN   U101    U201
-------------------
1   -11.22  -11.17
2    -2.78    0
3   -11.19    0
4   -11.23  -11.19
5   -11.22  -11.18
6    -9.92   -7.09
7    10.97   10.85
8     0       0
I threw U101 & U201, since I was right there...

-AA
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
One more thing.

Are you able to unsolder one leg of QCR101, 2SC1907 and lift the leg away from the pad?
If you have to unsolder both legs that's ok. Just record the orientation of the transistor (diode?) before removing.
Shouldn't be too much of a problem. Will need to whip out the solder station and clear some more space but, I'll get that going.

Thanks again!

-AA
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
One more thing.

Are you able to unsolder one leg of QCR101, 2SC1907 and lift the leg away from the pad?
If you have to unsolder both legs that's ok. Just record the orientation of the transistor (diode?) before removing.
Okay, got QCR101 removed, it was easier just to remove it. (Did note orientation though) Interesting, it has one leg clipped off, guessing it's the emitter? Pulled from this datasheet. Why is that?

COS5020_QCR101_REMOVED.jpg

What's next?

-AA
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
What's happening on the screen now?
Doh, didn't even try to fire it up. Ok, so now that I have, I have a signal (flat line), woot!

So I tried to measure the C1907 with my DMM and I'm not sure what to make of it. I get readings both directions with slightly different values. If I use DMM set to ohms; Negative on middle Leg-2, positive Leg-3, I get 629 ohms. Positive on Leg-2, Neg on Leg-3, I get 635 ohms.
DMM with Diode Test same hookup method: 617 / 703

Have no idea what that means in this situation?!?

Anyhow, we have a signal now on CH-1. What's next?

Regardless, thank you thank you thank you...

-AA
 

MrChips

Joined Oct 2, 2009
30,922
Now you need to go find a replacement diode.

1N914
1N916
1N4148
1N4448

1N916 or 1N4448 would be preferred over 1N914 and 1N4148.
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
Now you need to go find a replacement diode.

1N914
1N916
1N4148
1N4448

1N916 or 1N4448 would be preferred over 1N914 and 1N4148.
Ok, I definitely have 1N4148 and 1N914's.

I dug through my unsorted stuff and also found:
BAT48
1N4005
1N4007
1N4728A
1N4742A

Since I'll have to order a proper replacement is there a particular one I should order? 1N916?

I'll probably temporarily just throw in the 1N914, unless anything else in that list is better, to see what I get.

Thanks, yet again!

-AA
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
On second thoughts none of those diodes are suitable. The leakage currents are too high.

Digikey has BAV45 in stock.
MrChips,

I wanted to pop in and say thank you so much for your help with this. I will follow up with results as soon as I can. I've had a family emergency and I had to put my repair efforts on hold, temporarily. So, I hope you don't feel unappreciated, you are. And I want to follow up with a nice big "resolved" header.

Be back soon...

-AA
 

DanC

Joined Aug 14, 2017
1
Old thread, but the Way Back Machine is a wonderful thing: I just bought a COS5020 on the eBayz ($50 + shipping) and, while the seller says it works, I'm glad to have found (& downloaded) the service manual here and hope this thread is here should I need to refer to it in the future. Many thanks!
 
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