Jumping the the two inner wires of W1011 gives me a fully adjustable trace on Ch1. However, it is slightly out of focus.

Jumping pins 1 and 9 of U145 gives a fully adjustable, out of focus trace on Ch1. This trace changes slightly when V/Div is changed, where jumping W1011 does not.

 

Let's see if we can find out what happened to the CH1 signal.

You have a working scope, right? CH2 works.

If you have a signal generator, feed a signal into CH1.
You can also use a 555 timer circuit.
Or you can put a hook-up wire jumper between the PROBE ADJUST output and the CH1 BNC input. This gives out negative 500mV at 1kHz.

Set the CH1 and CH2 inputs to AC.
Play around with the CH1 and CH2 V/div.

The signals at U145 CA3127 16-pin DIP at pins 9 to 16 are accessible with a probe.
Look for signals at these pins. You should be able to observe the CH1 signal at these points.

 

Pin 9 - Nothing
Pin 16 - around +5VDC

Pin 1 - around +5VDC (trace will change resolution if I change Ch1 V/Div)
Pin 8 - Nothing

None of the above readings changed with the signal applied to Ch1. I used the .5V 1kHz signal applied to Ch1 via a 1X probe.

(Pin 9 and 16 were on the side closest to the attenuator board). That is the reason I checked 1 and 8, just in case I had the wrong side.

 

Just bumping this thread so that I can easily find it.

I am looking at the schematics of U145 in circuit #1 - CH1 & CH2 Attenuators.

U145 is a 16-pin DIP package labeled CA3127E mounted on a postage stamp sized ceramic board and terminating into a 10-pin SIP connection.

This is going to be a pain to remove and replace because it looks like you will have to disassemble the A12-ATTENUATOR/SWEEP board and the front panel assembly.

So we need to confirm that this is the fault before we go to all the trouble.

Here is the circuit for the U145 module.

Here is the pinout of the CA3127. Unfortunately, we don't have the layout of which transistor matches which transistor in U145. That makes it difficult to diagnose the problem. Also we only have access to one side of the DIP package. I need to think this one over more carefully.

 

Jumping the the two inner wires of W1011 gives me a fully adjustable trace on Ch1. However, it is slightly out of focus.

Jumping pins 1 and 9 of U145 gives a fully adjustable, out of focus trace on Ch1. This trace changes slightly when V/Div is changed, where jumping W1011 does not.

How were you able to jumper pins 1 and 9 of U145?
U145 itself is the 10-pin ceramic strip, not the 16-pin CA3127.

 

Yes sir, I'm tracking the difference between U145 and the CA3127E.

Was under the impression you wanted me to check those two pins on U145 from post #40

But before we do that, there is one more check. If you can get at pins 1 and 9 on the 10-pin board (see circuit #1 at L3) and jumper between pins 1 and 9 (where they show 0V) this might tell us if U145 is at fault.

From post #45:

How were you able to jumper pins 1 and 9 of U145?

Very carefully. Pin 1 of the U145, I was able to get a clip on. Pin 9, some good ole manual pressure with a pointed lead.

This was very easy in comparison to getting access to the bottom row pins of the CA3127.

If it does come down to replacing the CA3127, I ordered a couple the other day and will be here Monday. If not, no biggie. Just didn't want to have to wait on those if that's what it came to.

Nothing is as easy as it seems, but removing A12 and then gaining access to remove U145 in order to replace CA3127 doesn't seem too difficult. Only problem short of a catastrophe in removing the switch components due to age/brittleness are the board interconnect pins.

 

Wow! Sounds like you know what you are doing better than I know what I am doing.

Do you really want to go to the trouble of replacing the CA3127 just to find out it wasn't the problem?

 

No sir, not at all.

As I may have been unclear in my previous post, I purchased a couple of CA3127 just in case. If they do not need to be replaced, I have a couple in my drawer for later use, trade, burn, bragging rights....

 

I pulled A12 to be able to inspect the bottom. Nothing out of sort to my eyes. Ran an iron over some of the joints t/i U145. Same behaviour upon reassembly.

I'm not in a rush for the actual use. I had only used a single channel with my old Bell & Howell since the early '80s (before it vanished that is). Only used it to look at noise on lines. Most noise probably came from the scope itself though.

One thing that is sort of funny is that over the tek 2000 series 14 year production run, there is not many specifics across the net on many specific problems that should be arising after so many years.

 

Well if you went to all that trouble you may as well replace the CA3127 and hope for the best.

 

Pulling A12 was relatively easy, only took about 5 minutes. The CA3127s did come in on Friday instead of today, just wasn't ready to go through the pain of (needlessly?) de-soldering at the time. Was working on a couple of other things this weekend any ways.

But I did replace it just a few minutes ago.

Bottom line, same problem.....

However, if pin 4 and 6 (of the CA3127) are disconnected, a perfect flat trace for Ch1 shows up in all modes as it is supposed to. It is fully adjustable vertically in ch1 and both. It will not trigger on a signal though. When 4 and 6 are connected back, old behaviour is back.

Pin 6 is the collector of Q3 in the DIP, which goes to pin 4 of U145. Couldn't find where pin 4 on the DIP goes.

Haven't pulled 4 and 6 of the DIP separately to see if it is one or both that give that behaviour (not sure if I need to anyway).

 

Just to test the old CA3127, I configured each transistor separately in common emitter config. I put 1ma at the base and got 5ma Ic going through a 1kΩ load resistor on each one.