First off it was probably not necessary to start a new thread since members are going to be a bit confused between the two. That can be remedied.

As for the power supply, the opamp is actually being operated as an opamp rather than a comparator. The voltage you measured across C1462 was around 15 volts. The trick to the -8volt regulator stage is that Q1468 is acting to control the positive-most end of the 15V secondary transformer winding so that it is exactly 7 volts positive when measured with respect to ground. If you imagine that the load on the -8V output increases then the -8volt output would drop slightly toward 0v. As the -8volts drops the positive input to the opamp begins to go positive. This causes the output of the opamp to go positive which drives the base of transistor Q1466 which in turns drives the base of Q1468 positive. With an increase in the base drive applied to Q1468, it begins to turn on harder. This action pulls the positive end of the transformer secondary supplying the 15V DC down slightly. This results in a restoration of the -8volt output so that the droop due to the increased load is cancelled.

hgmjr

Thanks! I see you merged the threads. You are a super moderator, you can do anything.

Can you suggest a good out of circuit test for that op amp? I can post a list of what I have on hand if needed. But it would help if I could get a range of resistors (for example) and I can post just what I have in the range of those parts.

I think that I would need a virtual ground in the test?

 

Here are my voltages on U1464A


U1464A

Pin 1 = .26 (should be 2.1)

Pin 2 = 0

Pin 3 = 7.5 (should be 0)

Pin 4 = .8

Pretty much the same story for the B section

Pin 5 = 11.98
Pin 6 = 0
Pin 7 = 7 .5

 

Are you planning to remove the suspect opamp and subject it to some testing on a breadboard?

hgmjr

 

Are you planning to remove the suspect opamp and subject it to some testing on a breadboard?

hgmjr

Yes.

I have a +5 fixed supply and two variable supplies all home built. One supply can go from about 1.5 to 15VDC. The other to about 25 VDC. I have a small selection of resistors and some general purpose NPN transistors.

I also have a several trim pots.

 

I would recommend you connect up the opamp as a unity gain buffer. That is the simpliest way to connect it up. Tie the opamp's output to its negative input. Power it from 15V and ground. Then you can apply any voltage between 0V and +15V to the positive and the output will be exactly the same as the input. Be sure to add a 2K or about that on the output to ground. This will make sure that it can drive load.

hgmjr

 

Here is what I had in mind.

hgmjr

 

Here is what I had in mind.

hgmjr

Yep. Your description alone was very understandable but thanks for taking the time to look for and post the link to reinforce the idea.

I will give this at this tomorrow. Time to get some sleep now. I am going to setup the RS chip first to make sure I have everything hooked up right. I will then move on to testing the "scope" chip.

 

While yanking parts, pull or test Q1464, Q14666, or, in circuit, what are the B-E, C-E, and B-C voltages on those two transistors? Or the voltage drops on the emitter or collector resistors to get an idea on current and such.

One could have developed a short causing the strange behavior. Same with the two on the second OpAmp on that IC.


<there was a bit here about using a scope to test, but then I woke up and deleted it...>

 

It is working!!!!!!!!!!!!!!

I tested the op amp per hgmjr's instructions and I found the opamp was bad. I replaced it with the one from Radio Shack and I now have a trace!!!

Thanks hgmjr and everyone else for all of your help!!!!

 

It is working!!!!!!!!!!!!!!

I tested the op amp per hgmjr's instructions and I found the opamp was bad. I replaced it with the one from Radio Shack and I now have a trace!!!

Thanks hgmjr and everyone else for all of your help!!!!

It is great when a troubleshooting outcome is successful. Nice job.

You will need to keep an eye on things for a while. The opamp failed for a reason. Your replacement could surcome to whatever killed the original device.

hgmjr

 

It is great when a troubleshooting outcome is successful. Nice job.

You will need to keep an eye on things for a while. The opamp failed for a reason. Your replacement could surcome to whatever killed the original device.

hgmjr

Yep I already had that figured.

Bet it was the 50 being high that took out the opamp. But what caused that I have no idea. I tapped, wiggled, shook, inspected and even sprayed the area with freeze spray. No matter what I did I could not get the +50 to go back up.

 

It's always cool to see feedback/function. Lots of times a question is asked, and we never find out how it ended.

Glad you have your scope back! Are all the schematics posted in a link somewhere on this (or power supply) thread so somebody else with an odd acting scope of same model (Tek 475?) can have a crack at fixing it?

 

Why does Tektronix use un-heard-of Japanese opamps instead of popular American ones?

 

It's always cool to see feedback/function. Lots of times a question is asked, and we never find out how it ended.

Glad you have your scope back! Are all the schematics posted in a link somewhere on this (or power supply) thread so somebody else with an odd acting scope of same model (Tek 475?) can have a crack at fixing it?

The public use one can be found here. It is for the 475 but works for the 475A.

But there is a much better manual here. You will have to pay for it but the scans are extremely clear.

The folks at the tekscopes yahoo group were also very helpful in tracking down schematics and giving me help.