7B50 - No SweepCR262


Mark Vincent
 

Barry,

I apologize for the mistake I made in the diagnostics. I went by the initial statement and previous problems I had. Albert is is right about asking about TP725. Thank you Albert about inquiring about this test point. Knowing that first, I would have given a different answer.That having a small signal will eliminate CR262 as a problem. The other two transistors being on will make it appear the second CR to be bad. That may not be so. See if the switch is dirty or something else is making these turn on. The second CR may still be good. I have NO intentions of intentionally misleading anyone on this group.

Let us know if the new 741 in your 7704A makes a difference in the high voltage once you get it installed. I have prayed you will get your 7704A and plugins working to your satisfaction. I know you will be so happy having a working scope and plugins. Please do not think I will ever knowingly mislead you or anyone else on this group.

Mark


n4buq
 

Mark,

No need to apologize! I have benefitted from your advice here many times and would not think you're trying to mislead anyone. I'm sure I could have given more information that would have helped you analyze the problems.

Since LOCKOUT may also be an issue, I started looking at TP436, TP434, TP432, and TP416. None of those have any activity on them. They're either stuck HI or LO but not other signal is present on those. I presume that's why LOCKOUT is holding off the sweep but I'm not sure yet.

Regarding the 741, unfortunately that hasn't made any difference. I'm seeing about -2915V as a maximum negative voltage. Something else must be going wrong but haven't done much other analysis on that yet. BTW, my scope is an earlier serial number and I think would have had the 670-1855-00 HV board; however, mine has a 670-1855-01 board in it. I presume that shouldn't matter and may even be a good thing but thought I'd mention it.

Thanks again for all your help.
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, October 14, 2021 7:58:56 PM
Subject: Re: [TekScopes] 7B50 - No SweepCR262
Barry,

I apologize for the mistake I made in the diagnostics. I went by the initial
statement and previous problems I had. Albert is is right about asking about
TP725. Thank you Albert about inquiring about this test point. Knowing that
first, I would have given a different answer.That having a small signal will
eliminate CR262 as a problem. The other two transistors being on will make it
appear the second CR to be bad. That may not be so. See if the switch is dirty
or something else is making these turn on. The second CR may still be good. I
have NO intentions of intentionally misleading anyone on this group.

Let us know if the new 741 in your 7704A makes a difference in the high voltage
once you get it installed. I have prayed you will get your 7704A and plugins
working to your satisfaction. I know you will be so happy having a working
scope and plugins. Please do not think I will ever knowingly mislead you or
anyone else on this group.

Mark



Mark Vincent
 

Barry,

Nothing stands out yet what is the problem. Start checking d-c voltages. Something will be off. This is what I would do. I see R313 is to a switched voltage. See if you get the -15V on one end of that resistor. This is a point to check. Sometimes writing down voltages to compare has to be done when enough numbers are needed. There may be someone else in this group that may have had the same problem and can tell you what that problem was.

The 741 in the scope would have been the first thing I would have put in to see if that was the problem. I have had this number give problems in the past. The string(s) of resistor(s) can be checked to see if they are too far out of tolerance. I know it is unlikely, check the 470,000 ohm resistor on the input with the IC out. The inverter control and B+ adjust pots affect each other. You may have said what the voltages at the + and - inputs are. They should be almost zero. That IC gives the appearance of a transconductance circuit by the two resistors from the output to ground. I not stating this is the type of circuit, only looking similar. I have no other ideas of the two problems you have. Further information from you will help.

Mark