Troubleshooting Dead Channel-1 on old Kikusui COS5020 Oscilloscope

Thread Starter

Armando_A

Joined Mar 27, 2016
23
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.
Ok, gave that a shot, same, zero zip, nada, no display or visuals what-so-ever (for CH-1).

-AA
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
[edited]
Set up your scope as per CH-2 photo on post #7. <- CHECK
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. <- CHECK

What is the VOLTS/DIV setting? <- 1ms

Set both channels to AC input. <- CHECK
VERT MODE set to CH 2. <-CHECK
Both probes connected to CAL (Vp-p) 2V test output. <-CHECK
Confirm height of CH-2 waveform. <- Measures 1V/div, 2 divisions
IMG_20160410_144347.jpg

Now leave CH-1 probe on CAL test output. <- CHECK
Connect CH-2 probe to CH-1 OUTPUT at rear of scope. <-CHECK *see message body*

Reset CH-2 VOLTS/DIV to 100mV. <-CHECK

Edit:

Do your probes have x1 and x10 settings? <- YES
If so, set the probes to x1 for now. <- CHECK (and already were)
Notes applied inline above.

Attached is photo of rear output signal (or lack of). I disconnected the CH-1 from the calibration point a coupe times and watched the display for CH-2; no visual change in display - flat line - while doing this.

CH-1_CAL_EXT_OUT_CH-2_IN.jpg CH-1 @ Calibration, CH-2 @ EXT OUT

-AA
 
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MrChips

Joined Oct 2, 2009
30,991
Just to be certain that we get all the information correct,
when you set up the scope and probes as per photo CH-2 on post #7
what is the setting on CH-1 and CH-2 VOLTS/DIV?

Now that I have posted your photo, I can see it is 1V.

CH-2.JPG
 

MrChips

Joined Oct 2, 2009
30,991
Next step:

Have CH-2 probe not connected to anything.
Set CH-2 VOLTS/DIV to 5V.
Set CH-2 input selection to GND.
Adjust CH-2 position so that the horizontal trace is on the center line of the screen.

Change CH-2 input selection to DC.

CH-1 probe unchanged on the CAL connection.
CH-1 VOLTS/DIV unchanged to 1V.

We will use this to trouble shoot CH-1 amplifier.
 

MrChips

Joined Oct 2, 2009
30,991
We will be examining voltages on CH-1 Input Amplifier.


COS5020 CH1 INPUT_1.jpg


COS5020 VERT AMP LAYOUT_1.jpg


COS5020 VERT AMP Photo_1.jpg




Resistors of interest, R174, R175, R150 and R151 are hidden behind these cables.

Using the CH-2 probe, measure the voltages at both legs of R150 and R151 (both 22Ω resistors). (Total of four readings.)

We want to see where on the screen the line appears.

Another way of doing this is to take DC voltage measurements with respect to GROUND using a DMM.

(btw - In future if you took the photo 180° rotated, it would match the layout diagram.)
(What camera are you using? Your photos are excellent - sharp and clear.)
 
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nsaspook

Joined Aug 27, 2009
13,446
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
It helps for the analog section. Thanks

This should be the signal at P6 or the ch1 output BNC (my scope has a mod to send the digital SAVE signal to that BNC instead)


P6

0.2V/DIV cal signal input both probes.

I have little access to the preamp board from the top.

 
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Thread Starter

Armando_A

Joined Mar 27, 2016
23
Cool, things to measure! Thank you so much for your guidance!

Here's what I gathered. I didn't bother posting the oscope values as the DMM was the same but, more accurate. Plus, the DMM has micrograbbers.

DMM: (point_a, point_b, drop as measured across resistor)
R150 a) 4.16 b) 4.19 (.03 drop)
R151 a) 6.66 b) 6.66 (0 drop)

R174 a) 4.95 b) 3.74 (1.2 drop)
R175 a) 2.71 b) 2.71 (0 drop)

DMM Ohms: (scope off & unplugged)
R150 = 22.0
R151 = 22.1
R174 = 182.2
R175 = 181.1

Resistors look to be within tolerance. So, I think that those are okay, not the problem? 0v drop; does that mean it never had voltage to drop, problem prior in circuit? I'm totally guessing here, so please take my speculations with a grain of salt...

Oh, and pictures were all from iPhone 6 with exception of the last one, I tried the Kindle HDX for grins. I built a multi-device holder for my phone/kindle to attach to my tripod in my machining class. First time I've got to try it! Thanks for the positive feedback. I'm gonna say it works... Lol But seriously, it's been a challenge getting photos, as my itsy bitsy workspace is in the basement and makes lighting very difficult.

I will do some more examining of the schematic and see what I can glean from your steps so far, and possible places to continue looking.

Thanks again!

-AA
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
It helps for the analog section. Thanks
No sweat, the more eyes the better!

This should be the signal at P6 or the ch1 output BNC (my scope has a mod to send the digital SAVE signal to that BNC instead)

I have little access to the preamp board from the top.

Interesting, I don't have the grey shield. Does it serve another purpose?

And no, I don't get anything at the output on P6. :(

-AA
 

MrChips

Joined Oct 2, 2009
30,991
Thanks for the voltage readings.

I will outline what we're trying to accomplish.

Resistors are much more reliable than semiconductors such as diodes and transistors. At this stage, we are not looking for out-of-spec resistors.
We are looking for out-of-place voltage readings to allow us to identify which stage is at fault. After that, more sleuthing will be required in order for us to pin-point the suspect component.

There is always the danger of being too eager at trouble shooting. The more you poke around aimlessly the more chances you have of having an accident and shorting something. You don't want to make the situation worse than what you already have.

At this point, we can assume that the voltages at R150 and R151 are not correct because we would expect to see the same readings.

Off the top of my head, U102 CA3086 is highly suspect.

Don't do anything until I get a chance to study the circuit and plan the next move.

In the meantime, you may want to search for a source for CA3086 which is an obsolete component. Plan on having to purchase 2 to 5 pieces to have extras for spares.
 

MrChips

Joined Oct 2, 2009
30,991
The next set of measurements will require a very delicate touch.

Set both CH1 and CH2 settings to the same.
AC-GND-DC set to GND
VOLTS/DIV set to 5

You are going to use your DVM to measure voltages at the pins of U102 and U202.

It would be best to clip the BLACK lead the GND jack on the front panel of the scope. This way you can concentrate on the RED probe alone.
Use a RED pin probe and not the grabber probe on the +ve terminal of the DVM.

When probing the IC pins, you have to be absolutely careful that the probe does not slip and short out to an adjacent pin. One slip and the IC will likely be toast.

Measure the voltages at each one of the 14 pins on U102, CA3086 with respect to GND.

Do the same with U202.

Are you familiar with the numbering of IC pins on a 14-pin DIL (dual in-line package)?
Starting with the upper left corner, the pins are numbered 1 to 7.
Proceeding with the upper right corner, the pins are numbered 14 to 8.

(It would be a good idea to have an assistant who can write down the voltage readings while you focus on keeping the probe steady on the IC pin.)


COS5020 U102.jpg


COS5020 U202.jpg
 

nsaspook

Joined Aug 27, 2009
13,446
Interesting, I don't have the grey shield. Does it serve another purpose?

And no, I don't get anything at the output on P6. :(

-AA
It's a magnetic shield for the CRT around the deflection plates, maybe from something like Mu-metal.

Magnetic

Inside with degaussing coil, the digital board has a Sharp LH0080A Z80A CPU.
 
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Thread Starter

Armando_A

Joined Mar 27, 2016
23
Measure the voltages at each one of the 14 pins on U102, CA3086 with respect to GND.

Do the same with U202.
Ok, so I carefully measured U102 and U202. Below is a chart of what they read (DIFF being U102 relative to U202):

Code:
    PIN #    U102    U202    DIFF
    1        4.82    5.16    0.34
    2        3.60    3.50   -0.10
    3        2.83    2.75   -0.08
    4        3.58    3.50   -0.08
    5        5.62    5.20   -0.42
    6        0.00    0.00    0.00
    7       -0.75   -0.73    0.02
    8        3.33    3.31   -0.02
    9       -0.28    0.00    0.28
    10      -1.00   -0.73    0.27
    11       3.46    3.31   -0.15
    12      -5.91   -5.88    0.03
    13      -6.67   -6.64    0.03
    14      -1.29   -1.15    0.14
I see some very interesting patterns in the DIFFerences. I feel some progress here but I don't know what this tells me yet.

Thoughts?

As always, thank you!

-AA
 

MrChips

Joined Oct 2, 2009
30,991
No, we're not really looking at differences between U102 and U202. We are looking at U202 as a reference to see what would be normal.

More importantly, U102 and U202 are used as a differential amplifier. There are certain pins that are expected to have the same voltage.

Namely, pins 2 and 4 are the inputs of the amplifier. They should have same voltage, such as 3.50V on U202.

Pins 1 and 5 are the output pins of the amplifier. They too should have the same voltage.

Keep the CH 1 and CH 2 settings and probes as last instructed.

Set the CH 1 position to the 12 o'clock position.
Set VERT MODE to CH 1 or DUAL (try both positions and see the effect while you do the adjustment below).

Make a note of the current position of RV105.
With a small flat screw driver, adjust RV105 and see if the trace appears on the screen.
If the trace appears then mission accomplished.

Set RV105 to bring trace in middle of screen,


COS5020 RV105.jpg
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
...pins 2 and 4 are the inputs of the amplifier. They should have same voltage, such as 3.50V on U202.

Pins 1 and 5 are the output pins of the amplifier. They too should have the same voltage.

Keep the CH 1 and CH 2 settings and probes as last instructed.

Set the CH 1 position to the 12 o'clock position.
Set VERT MODE to CH 1 or DUAL (try both positions and see the effect while you do the adjustment below).
Ok, so I tried that but it didn't make any difference. Screen showed nothing for CH-1, and 5VDC for CH-2 (in dual mode). I think the problem lies elsewhere because I can't adjust RV105 enough to match U102 to U202 on either pins 2 or 4, and when I come close it upsets the balance of pin 4 when measuring pin 2, and visa versa. So I'm gathering that RV105 is working correctly and should be set to make pins 2 & 4 equal (they are inversely proportionate). Did I explain that right?

I put RV105 back into balance (~3.59/3.60) which brings me back to my earlier observation of having a negative DC offset of something like -.28VDC when I traced the signal up to C106 (see post #1 bullet #6). Is this clue of use here?

Thanks again for your time!

-AA
 

MrChips

Joined Oct 2, 2009
30,991
With the POWER OFF and power cord unplugged, get a short piece of wire-wrap wire and jumper U102 pin-9 to GND.

AC cord plugged in and power on.
See if CH1 trace appears on the screen. You may have to adjust CH1 POSITION.

Measure the voltages on U102.


COS5020_PCB_A1_SOLDER_SIDE_U102_pin-9.jpg
 

Thread Starter

Armando_A

Joined Mar 27, 2016
23
That is what we are doing now. Pin-9 on U102 should be at 0V when the input and trace is balanced.
Ok, so I jumpered U102 pin #9 to GND and I have a signal on my CRT!!!

Hmm, I wonder if this could be modified to make a "beam finder" on this scope, thinking that's what the Tektronix's one's must do (I'm assuming anyhow, haven't seen one in 20+ years).

COS5020_TRACE_CH1_PIN9_GND.jpg

I haven't taken all measurements on U102 yet. I'll get to that tomorrow. But I did measure the two points circled in red; which were zero (0).

Off to get some much needed sleep...

This IS progress! Thank you!

-AA
 

MrChips

Joined Oct 2, 2009
30,991
This narrows down the fault to the CH1 input preamp.

Forget about installing a "BEAM FINDER". Setting the input switch to GND would have the same effect if everything was working correctly.

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.
 
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