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New Member Dead 7704


volvoboy242@...
 

Yes I did that. I found nothing alarming until Q480 and Q482.
I'll check again tomorrow evening and post my findings.
The voltages on the schematic, I noticed after I had made my measurements and installed the covers back on the scope.
All voltages seemed reasonable, after the fact and from memory, until I saw the 25.3V value.
Checking the stages will be much more meaningfull now that I know what I'm supposed to be seeing.


volvoboy242@...
 

With all due respect gentlemen, you have me confused with someone else.
By my third post down this thread, I had already documented the CRT lit bright and sharp.
Raymond, I'll have to digest what you just told me regarding the amp tomorrow. It's long past my bed time.
I will take another stab at it following the suggestions you and Dennis just made.
PROMISE!!


volvoboy242@...
 

I went through the schematic and penciled a list of the transistor refdes in each stage. I measured at the emitter (or as close as I could get) of each one and charted my readings next to the numbers.
Most of my measurements were fairly close to spec.
I did find a discrepancy of almost 2V at Q468 which is in the second push pull amplifier.
Also Q436 wouldn't give a constant reading at which point, I noticed that the transistors and a cap or two in the second stage were getting hot. There is a heat sink clamped around Q466 and Q468 together.


I have to stop for today but I think I need to measure the voltages at all three legs of each transistor...without burning myself.


I'll elaborate later.


John Griessen
 

On 01/12/2017 04:29 PM, volvoboy242@... [TekScopes] wrote:
Most of my measurements were fairly close to spec.
I did find a discrepancy of almost 2V at Q468 which is in the second push pull amplifier.
Also Q436 wouldn't give a constant reading at which point, I noticed that the transistors and a cap or two in the second stage
were getting hot.

Now this is close to the kind of observation that some of us looking at the same schematic
can help you with. If you report this kind of info and also try to give enough to "mentally reproduce" your
measurement you may get some good suggestions and get closer to a fix.

When you say "a 2V discrepancy", it would be better to use shorter words and say, "The base of Q468 was 2V higher than
a schematic note on page xx of manual version xx dated x/x/1968 etc...

With the words "discrepancy of almost 2V at Q468" we're still in the dark as to what to test next.

You've started to wear out the patience of some on the list, but there are still some that will help.
You'll get more response and more progress not making posts that have "not enough volts observation info" and
"too much information" of side interest in one post.

When you say, " I think I need to measure the voltages at all three legs of each transistor" it makes me think
of accidentally shorting them together while touching a probe tip. Instead, you can use the
schematic to find a conductor that touches a transistor
emitter base or collector that are easy to probe compared to touching the
transistor legs as they are plugged into sockets of a Tek scope.

John


volvoboy242@...
 

"You've started to wear out the patience of some on the list, but there are still some that will help."


I don't know what you are all reading because I just don't see where I've been dismissive, reckless, obtuse or anything but grateful and attentive.


I re read the whole thread and I can see how some words might have been misinterpreted like reading "CRT" when I wrote "display" (I should have written "readout").

Or when some replies crossed over others in a short time.
There was one reply where I was asked to post a video and someone else replied that they had already removed the control board or something.


I haven't gone off on wild goose chases.
I absolutely haven't ignored any advice at all. I wouldn't be here if I didn't need it.


I asked at the very beginning if anyone would be willing to give me guidance while I did whatever checks they needed.
I'm not asking for step by step, sure fire scope fixing procedure that will magically work for any scenario possible.


All I wanted was for someone to work with me, one step at a time.


Tell me to check voltage at the emitter of Q480.
I'll check it.
I'll post back with my reading.


The first advice I got was to read the manual.
I did that.
Step one; check power supplies.
Did that....Took a couple days because I was being very careful not to disturb anything.
Second, third, eighth and seventyth piece of advice; Check for loose connections.
I'm doing that carefully as I go.
Someone wanted me to check the plastic clips on the plugin slots;
Did that....took pictures.
Someone wanted me to clean the plugin contacts with contact cleaner;
Did that...reported back.


I had at least two guys telling me to stop messing with the CRT.
I have no idea why they thought I was doing anything but making sure it was plugged in.


I'll post back later with actual numbers.
Bear with me


volvoboy242@...
 

Here are the measurements I took today.
I removed the lexan cover from the vertical amp to access the board.
Emitter from Q468 measured 8.52V. It's spec was 6.3V.
Q434 and Q436 gave me unstable readings. Also the spec voltage is illegible on my copy of the manual.
I'll look into it more tomorrow. That could have been operator error. I was getting tired and stupid by then and my kids came flying through the door.
I was forgetting numbers that I had looked at 10 seconds prior and had to keep looking back at the manual.
I figured I had reached the point of diminishing returns and quit.
Also the heat that I was feeling from the second stage was worrying me.
I'm now afraid to run the scope for more then ~10 minutes in it's current state.


I don't think my measurements of just emitters will give a complete picture of what's happening.
I'd like to pull three measurements off all 4 transistors in all three stages.
Also I forgot to take a macro (that's a close up picture) of a disk cap that was adjacent to where I noticed things getting hot.
I believe it was C471 but I'll have to double check that tomorrow.
It looks slightly heat damaged to me but I'd like someone here to give me an opinion, plus it may be a clue.


I WON'T TRY TO UNSOLDER IT! i JUST WANT SOMEONE TO LOOK AT A PICTURE OF IT!


These are my notes.
https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1304650996/lightbox/663731500?orderBy=ordinal&sortOrder=asc&photoFilter=ALL#zax/663731500 https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1304650996/lightbox/663731500?orderBy=ordinal&sortOrder=asc&photoFilter=ALL#zax/663731500
I'm not sure how critical the specified voltages are.
I did try turning the vertical position pot as I took some of the measurements.
Nowhere in the manual does it specify where the vertical position knob should be set for these values.
I assumed the pot should be centered so that's where I left it.


Night night all.