CH1 looks the same on AC as DC
CH1 and CH2 AC-GND-DC both set to GND



Both CH1 and CH2 AC-GND-DC set to DC

 

As you can observe, CH1 goes between 0V and 75mV according to your settings.

CH2 is between -30mV and +30mV. Hence it is AC coupled.
I suspect that you have lost connection in the AC-GND-DC switching somewhere.
You will have to open the scope and start hunting around while using CH1 as a working scope.
We are here to guide you.

 

As you can observe, CH1 goes between 0V and 75mV according to your settings.

CH2 is between -30mV and +30mV. Hence it is AC coupled.
I suspect that you have lost connection in the AC-GND-DC switching somewhere.
You will have to open the scope and start hunting around while using CH1 as a working scope.
We are here to guide you.

Just a reminder THERE IS HIGH VOLTAGE INSIDE THAT BOX. Don't touch anything that looks rubbery or is covered with a lot of fine, dark dust. Please be careful and stay safe.

 

First step, get a 1.5V and a 9V battery and let's check the voltage calibration on CH1.

 

With x10 probe and VOLTS/DIV set to 5mV we should be reading 50mV/div.
Hence I am seeing CH1 amplitude to be about 1.5 divisions = 75mV which is about x4 lower than 0.3V CAL OUT.

What does CH1 look like on DC vs AC? Does it look the same?

Download the SS-5702 Service Manual off the internet. We will be looking at the CH1 and CH2 input attenuation section for faults. CH2 is showing AC coupling (high-pass filter) with an RC time-constant of about 100μs, i.e. you have lost straight through DC coupling.

Set both CH1 and CH2 AC-GND-DC to GND and adjust the trace to some horizontal reference lines on the grid so that we know where is the 0V reference. Then set AC-GND-DC to DC and show us the actual DC levels with respect to GND.

View attachment 236916

Where did you find the manual? I downloaded a copy buy it was scanned to a PDF and most of the schematics are cut off on the edges.

Thanks!
Terry

 

I used my benchtop power supply set to 1 and 10 volts.
The trace was exactly on the line on the center of the screen.
The volts on the scope set to 0.5v
1 Volt


10 volts

 

Is channel 2 set to inverted ? I am asking this as channel 2 looks like a differentiated version of an inverted channel 1 waveform.
I agree with Yaakov in using channel 1 to trace the path through the channel 2 amplifier.

Les.

 

If I remember correctly channel 2 was set to inverted.

 

Is channel 2 set to inverted ? I am asking this as channel 2 looks like a differentiated version of an inverted channel 1 waveform.
I agree with Yaakov in using channel 1 to trace the path through the channel 2 amplifier.

Les.

Good eye. I did not notice that. Push CH2 POLARITY to NORM.

 

IWATSU SS-5702A Service Manual
http://www.radiopistoia.com/pdf/iwatsu.pdf

 

Take well focused photos of the PCB on the left side. We want to see the CH2 input and VOLTS/DIV switch selection.

 

Getting there.

 

There are some good closeups here. This is the kind of photo resolution that would be nice.
Notice the CH1 VOLTS/DIV rotary switch is broken.

The layout of CH1 and CH2 are identical.
CH1 components are numbered 0xx and CH2 are 1xx. For example, R064/R164, C012/C112.
Let's get some closeups of both CH1 and CH2 so that we can have different viewing perspectives.

http://www.noobowsystems.org/backtolife/ss-5702/ss-5702.html

 

Thanks everyone for your help. I will get some pictures of the circuit board in the next couple days. I have a tremendous amount on the go right now.
This morning while I was trying a few things I noticed that all of a sudden I had the same wave form
from the calibration output on both channels but the channel 2 was reading a way higher voltage.
Then all of a sudden the voltage dropped to be close to the same as CH1. If I very gently touch the AC/GND/DC switch on channel 2 the waveform goes back to the one pictured in post 21 .
I suspect there are some problems with the contacts on the switch.
Here is a picture of what I have now. Voltage is set to 5mv on both channels.
The top pair are CH1 and the bottom are CH2.
How do I go about cleaning all the contacts on the switches.

Thanks!
Terry

 

In that case, ignore what I have posted and don't bother with taking more photos.
You have identified the problem with the CH2 AC-GND-DC switch.
Open the unit and inject or spray isopropyl alcohol on to the switch contacts and work the switch while the alcohol has not yet evaporated.
You should have your problem solved.

 

In that case, ignore what I have posted and don't bother with taking more photos.
You have identified the problem with the CH2 AC-GND-DC switch.
Open the unit and inject or spray isopropyl alcohol on to the switch contacts and work the switch while the alcohol has not yet evaporated.
You should have your problem solved.

I would vote for investing some Deoxit contact cleaner (https://www.amazon.com/Hosa-D5S-6-DeoxIT-Contact-Cleaner/dp/B00006LVEU) which is very good and will help with the maintenance of good connections. Don't spray it all over the inside, just on the contacts of the wafer switches, then operate them several times. This is really good stiff in my experience. It's not just cleaner, it has a contact enhancer in it. It's like Stabilant ut much cheaper.

 

I was going to ask about Deoxit and if it would be safe to use in this application.
I will see if I can get some today. Where I used to work before retirement
used to keep it in stock.

 

I was going to ask about Deoxit and if it would be safe to use in this application.
I will see if I can get some today. Where I used to work before retirement
used to keep it in stock.

It’s safe but it purposely leaves a film behind so apply it only to the contacts and wipe it off other stuff if it gets there. The film isn’t harmful but it can attract dust and make a mess

 

I was able to get a can of Deoxit today and i took the cover off and cleaned the offending switch plus a few others while I was in there. The scope works properly now.
Thanks everyone for all the help!