Troubleshooting Dead Channel-1 on old Kikusui COS5020 Oscilloscope

Thread Starter

Armando_A

Joined Mar 27, 2016
23
Hello!

I just purchased my first oscilloscope after 30 years of waiting! It was the kind of deal that I could hardly pass up, $20. Unfortunately, site unseen. For the most part it seems to look decent and it does fire up and I get a signal on channel-2 but nothing on channel-1. Well, those were the risks at that price...

None-the-less, I would like to try an fix this old girl since it appears the CRT is good and doesn't have any major physical defects. I would like to be able to use the oscilloscope for my vintage electronic tinkering and general learning of electronics. But, it looks like I need to fix this first, if possible. I have some tools (solder station, flux, wick, etc, dmm, and misc parts and pieces), some very old electronics schooling (80's), and the eagerness to (re)learn.

This may be a lot to ask but if anyone can help shed some light on how to best trouble shoot this, I'd sure appreciate it.

Here's what I know so far:
1) Unit powers on
2) Basic power supply checks with DMM, seem to be good
3) Channel-1 no display regardless of settings
4) Channel-2 display and functions work normal (proves CRT function)
5) No output on "Channel-1 Out" (@ rear of scope, w/cal input to CH-1)
6) Traced calibration signal input to CH-1, to C106 and notice signal drops to negative DC bias, probably a clue there?

That's about exhausted my abilities with the tools and knowledge I have. So, I'm stuck at this point and have no idea how to go any further. I purchased the service manual though and will attach it along with some photo's of the unit itself.

Any advise on where to look next or area to study would be welcome. I've scoured the net and not much exists on this Kikusui; mostly unanswered questions but also no schematic, so this will help someone else help me.

Thanks for reading!

-AA

(PS: This is my first post, so I apologize in advance if I've inadvertently broken etiquette)

COS5020_BOTTOM.jpg COS5020_FRONT.JPG COS5020_PCB_A1_COMPONENT_SIDE.1.jpg COS5020_PCB_A1_COMPONENT_SIDE.2.jpg COS5020_PCB_A1_COMPONENT_SIDE.3.JPG COS5020_PCB_A1_COMPONENT_SIDE.4.jpg COS5020_PCB_A1_COMPONENT_SIDE.5.jpg COS5020_PCB_A1_SOLDER_SIDE.jpg COS5020_TOP.jpg
 

Attachments

MrChips

Joined Oct 2, 2009
19,931
@Armado_A
Excellent first time post. Clear description, photos and service manual. You must have been taking notes from other posts on the AAC forums.

As #12 says, what do you mean by

3) Channel-1 no display regardless of settings

Show us a photo of scope face with Channel 2 working.
Show us a photo of scope face with Channel 1.
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
Does that mean no horizontal line or yes, horizontal line, but no vertical deflection?
For CH-1, neither horizontal or vertical.

Reading Tektronix trouble-shooting manual shows they have a "beam finder" button that looks like it would be very helpful about now, unfortunately this unit doesn't have such a feature. :-/

I should also note that when I turn "Vertical Mode" to: CH-1 reveals no trace, CH-2 acts fully normal, DUAL shows only CH-2, ADD shows only CH-2 but dimmer.

Thanks again!

-AA
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
@Armado_A
Excellent first time post. Clear description, photos and service manual. You must have been taking notes from other posts on the AAC forums.

As #12 says, what do you mean by

3) Channel-1 no display regardless of settings

Show us a photo of scope face with Channel 2 working.
Show us a photo of scope face with Channel 1.
Lol. Yes, I made sure to try and do some investigating first so as not to be a complete idiot. (still happens though)

I have attached photos. Although I just realized I didn't get one of just CH-1, doh! But, it's the same as no display (as in off).

-AA

COS5020_TRACE_CH1+2a.jpg COS5020_TRACE_CH1+2b.JPG
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
More photos, and correction to previous statement. ADD mode also reveals no display.

Settings: 1 V/DIV, 1ms, Vertical Mode Set to:

CH-1.JPG CH-1

CH-2.JPG CH-2

DUAL.JPG DUAL

DUAL_CH-2_SRC.JPG DUAL w/CH-2 as SRC

ADD_CH-2_SRC.JPG ADD w/CH-2 as SRC

Thanks again for taking time to respond!

-AA
 

MrChips

Joined Oct 2, 2009
19,931
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.
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
Oh yes, not to worry this is not high priority (mostly anxiety of new/old toy - lol). I should have gone to bed two hrs ago myself.

Take care and have good day/night.

-AA
 

Alec_t

Joined Sep 17, 2013
10,746
As per the manual the two channels are chopped/multiplexed by the Vertical Switching circuit (Fig 5.1). It's possible the fault lies with that, so it's stuck on Ch2. Check that the clock input to pin 11 of IC U301 (Fig 5.3) is present. Your DMM set to Volts should hopefully see about half the IC supply voltage if the chop has a 50% duty cycle. BEWARE high volts in the scope!
 

MrChips

Joined Oct 2, 2009
19,931
We are going to use your working CHAN 2 to trouble shoot CHAN 1.
Before we begin, some words of CAUTION.

You are going to test the circuit with the POWER ON and this requires special CAUTION.

There are lethal voltages in an oscilloscope.

1. Remove the grounding clips on both probes. You don't want a dangling ground clip to accidentally short out.
2. Remove the cap on the probe connected to CHAN 2. You will use only the pin tip of the probe for probing.
3. No personal jewelry allowed. Remove all finger rings, wristwatch, bangles, wristbands,etc.
4. Get into the habit of putting your non-dominant hand in your pocket. That is, if you are right-handed, put your left-hand in your left pant pocket when probing the circuit. This will reduce the potential of sending any lethal voltages and currents across your chest.
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
Thanks for the safety tips. All things I "should" know but I've re-read because I know how this goes... I'm in school (again) learning to be a machinist, so safety is extremely important me!!! I'd like to keep all my limbs and parts and pieces while I'm alive.
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
As per the manual the two channels are chopped/multiplexed by the Vertical Switching circuit (Fig 5.1). It's possible the fault lies with that, so it's stuck on Ch2. Check that the clock input to pin 11 of IC U301 (Fig 5.3) is present. Your DMM set to Volts should hopefully see about half the IC supply voltage if the chop has a 50% duty cycle. BEWARE high volts in the scope!
Measuring .39 volts on DMM at pin 11. And +5 for Vcc (pin 14).

I suspect it's before this because I don't get output on my EXT-OUT for Channel-1. But, that purely speculation from my understanding of the block diagram.

-AA
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
We are going to use your working CHAN 2 to trouble shoot CHAN 1.
You are going to test the circuit with the POWER ON and this requires special CAUTION.

There are lethal voltages in an oscilloscope.

1. Remove the grounding clips on both probes. You don't want a dangling ground clip to accidentally short out.
2. Remove the cap on the probe connected to CHAN 2. You will use only the pin tip of the probe for probing.
3. No personal jewelry allowed. Remove all finger rings, wristwatch, bangles, wristbands,etc.
4. Get into the habit of putting your non-dominant hand in your pocket. That is, if you are right-handed, put your left-hand in your left pant pocket when probing the circuit. This will reduce the potential of sending any lethal voltages and currents across your chest.
1. Done
2. Done
3. Done (this was easy, have none)
4. Yes (need to repeat outloud)

-AA
 

nsaspook

Joined Aug 27, 2009
6,729
I have a working DSS5020 (digital storage version) that's very similar to the COS5020. Not sure what I can do to help but thanks for the service manual Armando as the front-end preamp section looks to be nearly identical.

 

SLK001

Joined Nov 29, 2011
1,543
First, turn screen illumination as far down as it will go (you want it "OFF").
Second, turn the focus to the worst it can get.

With channel 1 set to sweep, look for an electron "glow" at the top and bottom of the screen. If you can see a "glow" then Mr. Chips is probably correct in that the vertical amp is shot (this is the most suspect part anyway). Also check the channel 1 vertical position pot, as someone suggested. A bad pot could also manifest these symptoms.
 

MrChips

Joined Oct 2, 2009
19,931
We'll begin by looking for a signal on CH1 OUTPUT at the rear panel BNC connector.

Set up your scope as per CH-2 photo on post #7.
Set both CH-1 and CH-2 VOLTS/DIV to the same setting, turn the RED knobs fully clockwise until it clicks, RED knobs pushed in.

What is the VOLTS/DIV setting?

Set both channels to AC input.
VERT MODE set to CH 2.
Both probes connected to CAL (Vp-p) 2V test output.
Confirm height of CH-2 waveform.

Now leave CH-1 probe on CAL test output.
Connect CH-2 probe to CH-1 OUTPUT at rear of scope.

Reset CH-2 VOLTS/DIV to 100mV.

Edit:

Do your probes have x1 and x10 settings?
If so, set the probes to x1 for now.
 
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Thread Starter

Armando_A

Joined Mar 27, 2016
23
I have a working DSS5020 (digital storage version) that's very similar to the COS5020. Not sure what I can do to help but thanks for the service manual Armando as the front-end preamp section looks to be nearly identical.
Nice! When I was trying to locate the schematics for this Kikusui I tried to look for clones as well but I wasn't completely sure about the other models mentioned. Information seemed pretty sparse regardless. So, I bit the bullet and purchased the official service manual. Hope it helps! I have a slight better version, as I had to optimize that PDF in order to get it under the size limitations for attachments here. PM me if you want a theoretically better one. (However, I can't see a huge difference. 13MB to <10MB)

BTW: The service manual has a few glaring typos, so double check it and yourself when something doesn't add up. i.e. two channel-1's in the diagram. I saw others too, so watch out.

-AA
 
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