Continuing Problems with 7704A Display


n4buq
 

After disassembling, cleaning (I got very tired of all the dust!), replacing the three electrolytic caps on the HV board and the six electrolytic caps on the horizontal amplifier board (mostly with slightly larger C values), and reassembling, I'm still seeing problems.

1. The striped, dashed trace (looks "snake-like"). This is affected to some degree by the graticule lamp setting (a dimmer setting seems to make it worse) and the sweep speed (again, worse at some speeds than others).

2. I can only get about -2900VDC at TP4221. R4123 is at the fully CW position. If I rotate it CCW by say, 20 degrees, the voltage drops a little (50V or 100V) but then doesn't continue to drop. I would expect to be able to get at least +/-100V (or more) from the -2960V point so I think something is probably wrong. Along with this, FOCUS is good at about the 2:00 to 3:00 o'clock position. Would this likely be the chain of R4240, R4241, R4243, and R4242 (all thick film resistors)? If I'm not mistaken, those are a custom-made, one-piece component and I'm thinking this doesn't typically go bad(?). If not those, then what else could I look for? Perhaps some of the HV caps?

3. There's a noticeable "jitter" for the readout characters. It seems a bit worse at lower brightness settings but that may only be due to brighter images tend to hide the quick fading that's present when the jittering can be seen. This may be related to #1.

4. If the vertical values for the trace cause it to swing to the top and bottom (and beyond) part of the screen, the readout characters will move vertically in sync with the + and - swings of the trace. Even stranger, if the input is a sine-wave, the individual readout characters will bounce up and down but out-of-sync with each other (e.g. the first character will appear low, the second character will display about 0.2 division higher, the third character will be at the same height as the first, etc.). This is one of the oddest things I've seen.

Hopefully there's a common thread in some/all of this. I've recapped everything I can think of that would affect this so something else is afoot. Any suggestions are welcome.

Thanks,
Barry - N4BUQ


Mark Vincent
 

Barry,

U4110 (741) could be bad. Check voltages in that circuit. Having the high voltage adjust at an extreme is not right. Ripple in the 50V supply could cause the intensity modulation you are seeing. See if you get the 73V at the collectors of Q4105 and Q4115/pin of P41I-6 of the control circuit. In mine, I changed R4108 to 1/2W. Check the string of nine 300K resistors. I changed the 22meg resistors in the dc restorer circuits to RT37 1/2W types. The thick film resistors rarely go bad.

Mark


Albert Otten
 

On Sat, Oct 9, 2021 at 05:10 PM, n4buq wrote:


4. If the vertical values for the trace cause it to swing to the top and
bottom (and beyond) part of the screen, the readout characters will move
vertically in sync with the + and - swings of the trace. Even stranger, if the
input is a sine-wave, the individual readout characters will bounce up and
down but out-of-sync with each other (e.g. the first character will appear
low, the second character will display about 0.2 division higher, the third
character will be at the same height as the first, etc.). This is one of the
oddest things I've seen.
Hi Barry,
This is "normal behavior" when the vertical amplifier LF response is not properly adjusted. The dot jump from trace to readout position has undershoot or overshoot. Similar for horizontal amplifier and position.
Albert


n4buq
 

Hi Mark,

I'm not getting 73V at the collectors of Q4105/Q4115. Here are the voltages for those two transistors:

Q4105
C: 0.5
B: 148.6
E: 149.3

Q4115
C: 0.5
B: 1.25
E: 0.48

If I remove Q4115 and leave Q4105 in place, the collector voltages stay at 151.9.
If I remove Q4105 and leave Q4115 in place, the collector voltages drop to 0.48.

Does this look like a bad Q4115?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 11:17:59 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Barry,

U4110 (741) could be bad. Check voltages in that circuit. Having the high
voltage adjust at an extreme is not right. Ripple in the 50V supply could cause
the intensity modulation you are seeing. See if you get the 73V at the
collectors of Q4105 and Q4115/pin of P41I-6 of the control circuit. In mine, I
changed R4108 to 1/2W. Check the string of nine 300K resistors. I changed the
22meg resistors in the dc restorer circuits to RT37 1/2W types. The thick film
resistors rarely go bad.

Mark



n4buq
 

Hi Albert,

I was hoping this might just be a calibration issue. No telling who has been fiddling around with those controls before I got it. I'll have to investigate that.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 12:17:19 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
On Sat, Oct 9, 2021 at 05:10 PM, n4buq wrote:


4. If the vertical values for the trace cause it to swing to the top and
bottom (and beyond) part of the screen, the readout characters will move
vertically in sync with the + and - swings of the trace. Even stranger, if the
input is a sine-wave, the individual readout characters will bounce up and
down but out-of-sync with each other (e.g. the first character will appear
low, the second character will display about 0.2 division higher, the third
character will be at the same height as the first, etc.). This is one of the
oddest things I've seen.
Hi Barry,
This is "normal behavior" when the vertical amplifier LF response is not
properly adjusted. The dot jump from trace to readout position has undershoot
or overshoot. Similar for horizontal amplifier and position.
Albert



Albert Otten
 

I was hoping this might just be a calibration issue. No telling who has been fiddling around with those controls before I got it. I'll have to investigate that.
Barry, just do a search here for "wavy" or "wavy readout". Albert


Mark Vincent
 

Barry,

The base of the bottom transistor is too far off making the transistor in full conduction. This makes me highly suspect of the 741 IC. See if the output voltage at pin 6 is what the schematic says. If that is not right, replace the IC. A possible bad Q4105 is possible, open B to C. You can check Q4115 if you want to confirm it is good or bad.

Mark


n4buq
 

Mark,

Pin 6 of U4110 checks at 10.33V. Pin 2, though, checks at 0.87V, neither of which is correct. Is the presence of that much voltage on the + input enough to cause the overly-high voltage on Pin 6 and, possibly, that's the problem or is the IC still responsible for that as well?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 2:51:31 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Barry,

The base of the bottom transistor is too far off making the transistor in full
conduction. This makes me highly suspect of the 741 IC. See if the output
voltage at pin 6 is what the schematic says. If that is not right, replace the
IC. A possible bad Q4105 is possible, open B to C. You can check Q4115 if you
want to confirm it is good or bad.

Mark



Harvey White
 

To get an idea of what the op-amp is doing, you have to measure the voltage between the + and - inputs.  (If the + input is grounded, then that's fine, but it must be connected).  The output voltage will be close to the difference between the + and - input times the op-amp gain (not the circuit gain, the actual amplifier gain, typically 100,000 or more).   That's the rule here, the op-amp doesn't have a choice.  If the output of the op-amp is too high, then the op amp is doing the best it can, and is being limited by the circuit supply voltages.  If the voltage at the output makes sense, then the op amp is ok.  Some op amps are used as comparators, which means that the inputs are deliberately overdriven, which pushes the op-amp output as far + or - as it can go.  The 741 series can generally get to within 1.5 volts of the supply.

If you have a feedback circuit (such as a gain of 10), if the output isn't being loaded too much, then given the + and - input limit, if that's bad, then the op amp is suspect.  Op amp supply voltages are important, of course.  Can't get more out of an op amp than the supply voltage (and you'd be surprised at how many simulators allow a 100 volt output from an op amp running from + and - 15.

Generally speaking, unless the op-amp is being used as a comparator, you should measure the same voltage on the + and - inputs within a few microvolts or so.

Harvey

On 10/9/2021 6:04 PM, n4buq wrote:
Mark,

Pin 6 of U4110 checks at 10.33V. Pin 2, though, checks at 0.87V, neither of which is correct. Is the presence of that much voltage on the + input enough to cause the overly-high voltage on Pin 6 and, possibly, that's the problem or is the IC still responsible for that as well?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 2:51:31 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Barry,

The base of the bottom transistor is too far off making the transistor in full
conduction. This makes me highly suspect of the 741 IC. See if the output
voltage at pin 6 is what the schematic says. If that is not right, replace the
IC. A possible bad Q4105 is possible, open B to C. You can check Q4115 if you
want to confirm it is good or bad.

Mark





Mark Vincent
 

Barry,

The 741 is bad. Replace it. Mr. Harvey White is right about the proper operation of an op amp.

Mark


n4buq
 

Mark,

You mention 22M resistors in the DC restorer circuits. I see R4239 which shows as a 22M on the schematic and 22M in the parts list. The similar resistor appears to be R4216 which shows as 44M on the schematic in the hard copy manual I have but shows as a 22M in the parts list. A PDF of the manual I have shows R4216 as 22M on the schematic.

Those are the two resistors to which you referred, correct?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 11:17:59 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Barry,

U4110 (741) could be bad. Check voltages in that circuit. Having the high
voltage adjust at an extreme is not right. Ripple in the 50V supply could cause
the intensity modulation you are seeing. See if you get the 73V at the
collectors of Q4105 and Q4115/pin of P41I-6 of the control circuit. In mine, I
changed R4108 to 1/2W. Check the string of nine 300K resistors. I changed the
22meg resistors in the dc restorer circuits to RT37 1/2W types. The thick film
resistors rarely go bad.

Mark



Mark Vincent
 

Barry,

The 22meg resistors are R4216 and R4239. The 44meg in the hard copy is a misprint. If you want/need the d-c restorer diodes, use 1N4937. These diodes can be used elsewhere as well.

Mark


n4buq
 

Mark,

I thought the 44M was a typo. Both resistors in my HV supply are 22M. I'll get that 741 ordered/replaced and go from there.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 10:49:58 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Barry,

The 22meg resistors are R4216 and R4239. The 44meg in the hard copy is a
misprint. If you want/need the d-c restorer diodes, use 1N4937. These diodes
can be used elsewhere as well.

Mark



n4buq
 

Hi Harvey,

Thanks for that explanation. I have a very basic understanding of op-amps but it helps to have clarifications like this. If I understand correctly, without a feedback circuit (resistor), then the op-amp operates in a "wide-open" mode and amplifies as much as possible. Is that correct? This circuit has the - (inverting) input is grounded through a 470k resistor (bypassed with a 0.001 cap). The + (non-inverting) input is tied to the HV adjusting resistor and the following chain of high-resistance resistors. That input is labeled as having a typical voltage of -0.01V and the output of the op-amp is shown with a typical output voltage of 0.86V. Both of those voltages are significantly incorrect. Hopefully replacing the IC will resolve those voltages.

Thanks agin,
Barry - N4BUQ

----- Original Message -----
From: "Harvey White" <madyn@dragonworks.info>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 6:09:04 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
To get an idea of what the op-amp is doing, you have to measure the
voltage between the + and - inputs.  (If the + input is grounded, then
that's fine, but it must be connected).  The output voltage will be
close to the difference between the + and - input times the op-amp gain
(not the circuit gain, the actual amplifier gain, typically 100,000 or
more).   That's the rule here, the op-amp doesn't have a choice.  If the
output of the op-amp is too high, then the op amp is doing the best it
can, and is being limited by the circuit supply voltages.  If the
voltage at the output makes sense, then the op amp is ok.  Some op amps
are used as comparators, which means that the inputs are deliberately
overdriven, which pushes the op-amp output as far + or - as it can go.
The 741 series can generally get to within 1.5 volts of the supply.

If you have a feedback circuit (such as a gain of 10), if the output
isn't being loaded too much, then given the + and - input limit, if
that's bad, then the op amp is suspect.  Op amp supply voltages are
important, of course.  Can't get more out of an op amp than the supply
voltage (and you'd be surprised at how many simulators allow a 100 volt
output from an op amp running from + and - 15.

Generally speaking, unless the op-amp is being used as a comparator, you
should measure the same voltage on the + and - inputs within a few
microvolts or so.

Harvey


On 10/9/2021 6:04 PM, n4buq wrote:
Mark,

Pin 6 of U4110 checks at 10.33V. Pin 2, though, checks at 0.87V, neither of
which is correct. Is the presence of that much voltage on the + input enough
to cause the overly-high voltage on Pin 6 and, possibly, that's the problem or
is the IC still responsible for that as well?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 2:51:31 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Barry,

The base of the bottom transistor is too far off making the transistor in full
conduction. This makes me highly suspect of the 741 IC. See if the output
voltage at pin 6 is what the schematic says. If that is not right, replace the
IC. A possible bad Q4105 is possible, open B to C. You can check Q4115 if you
want to confirm it is good or bad.

Mark







Harvey White
 

A slight correction.  The op amp *always* amplifies as much as it can.  The feedback circuit forces the resultant input signal  to be amplified only so much.  So the circuit is the important part for controlling how things work, and understanding what the op amp tries to do is critical to what the circuit actually does.

You're right, if there is no feedback from the output to the - input, then the op-amp runs wide open, more like a comparator (and can be used like that).

As you describe the circuit, it would be running wide open.  I suspect your explanation flips the + and - inputs.  The + input grounded through a resistor and bypassed by a capacitor is there to correct an offset caused by the input current to the transistors (in theory, zero or close to it.)  In older amps, it's larger and needs to be compensated for.

What I suspect the circuit is:  an inverting amplifier with a gain of -86.  (- input of 0.01, output of 0.86).  The gain in this case is -Rf/Ri where the Ri is the input resistor from a voltage source to the - input, and Rf is the effective resistance between the output and the - input signal.  Check the basic theory on inverting op-amps, I won't go into it on the list.

Pin 3 is the + input, pin 2 is the - input, pin 6 is the output if I remember the 741 style connections properly.

Harvey

On 10/10/2021 9:27 AM, n4buq wrote:
Hi Harvey,

Thanks for that explanation. I have a very basic understanding of op-amps but it helps to have clarifications like this. If I understand correctly, without a feedback circuit (resistor), then the op-amp operates in a "wide-open" mode and amplifies as much as possible. Is that correct? This circuit has the - (inverting) input is grounded through a 470k resistor (bypassed with a 0.001 cap). The + (non-inverting) input is tied to the HV adjusting resistor and the following chain of high-resistance resistors. That input is labeled as having a typical voltage of -0.01V and the output of the op-amp is shown with a typical output voltage of 0.86V. Both of those voltages are significantly incorrect. Hopefully replacing the IC will resolve those voltages.

Thanks agin,
Barry - N4BUQ

----- Original Message -----
From: "Harvey White" <madyn@dragonworks.info>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 6:09:04 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
To get an idea of what the op-amp is doing, you have to measure the
voltage between the + and - inputs.  (If the + input is grounded, then
that's fine, but it must be connected).  The output voltage will be
close to the difference between the + and - input times the op-amp gain
(not the circuit gain, the actual amplifier gain, typically 100,000 or
more).   That's the rule here, the op-amp doesn't have a choice.  If the
output of the op-amp is too high, then the op amp is doing the best it
can, and is being limited by the circuit supply voltages.  If the
voltage at the output makes sense, then the op amp is ok.  Some op amps
are used as comparators, which means that the inputs are deliberately
overdriven, which pushes the op-amp output as far + or - as it can go.
The 741 series can generally get to within 1.5 volts of the supply.

If you have a feedback circuit (such as a gain of 10), if the output
isn't being loaded too much, then given the + and - input limit, if
that's bad, then the op amp is suspect.  Op amp supply voltages are
important, of course.  Can't get more out of an op amp than the supply
voltage (and you'd be surprised at how many simulators allow a 100 volt
output from an op amp running from + and - 15.

Generally speaking, unless the op-amp is being used as a comparator, you
should measure the same voltage on the + and - inputs within a few
microvolts or so.

Harvey


On 10/9/2021 6:04 PM, n4buq wrote:
Mark,

Pin 6 of U4110 checks at 10.33V. Pin 2, though, checks at 0.87V, neither of
which is correct. Is the presence of that much voltage on the + input enough
to cause the overly-high voltage on Pin 6 and, possibly, that's the problem or
is the IC still responsible for that as well?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 2:51:31 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Barry,

The base of the bottom transistor is too far off making the transistor in full
conduction. This makes me highly suspect of the 741 IC. See if the output
voltage at pin 6 is what the schematic says. If that is not right, replace the
IC. A possible bad Q4105 is possible, open B to C. You can check Q4115 if you
want to confirm it is good or bad.

Mark







Göran Krusell
 

Hi, I would like to comment on this, the U4110 gain is infinite (or 100 000) since there is no feedback resistor from pin 6 to pin 2. The value -0.01V is not significant. At pin 2 you should have close to 0V unless the amplifier is badly over-driven. I suggest that you exchange the opamp U4110.

I think I had a similar problem long time ago but don’t remember the details. However I moved the diodes CR4121 and CR4122 to ground instead of +/-15 V for better protection.

Carry on
Göran


Ulf Kylenfall
 

Following this thread with interest since I have also been restoring an old 7704A mainframe.

I thaught that the CRT in my mainframe had reached EOL but after replacing almost all
capacitors the CRT proved to be in good shape. I do however see some jitter in the
readout. Unless I find any 741's in my junk box, any suggestion for a better
replacement for the U4110?

Cheers

Ulf Kylenfall
SM6GXV


SCMenasian
 

Almost any more modern general purpose opamp which will take sufficiently high power supply voltages and have the same pinout. TL061, TL071 and TL081 come to mind, Also, the even more modern tle2072, etc. Also LF356 and similar. CA3140 also comes to mind. The most important thing to watch out for is bandwidth - in case the circuit is prone to oscillation. This list is not complete - it just reflects my current stock and data sheets. I find no reason to use a 741 now; OTOH, I am not compulsive about restoring ancient instruments to original - often, improved performance (better then original) can be had using modern components.

Stephen Menasian


Albert Otten
 

On Sat, Oct 9, 2021 at 08:51 PM, n4buq wrote:

If I remove Q4115 and leave Q4105 in place, the collector voltages stay at 151.9.
If I remove Q4105 and leave Q4115 in place, the collector voltages drop to 0.48.
Hi Barry,

Here you see the extremes of the voltage at which a fixed (negative) negative voltage is added to get the CRT cathode voltage. You might measure the cathode HV itself for these two conditions to see the possible HV adjustment range. If -2960 V is not within that range then don't blame U4110.
Note that a deviation in the +54V produces a roughly 60 times large deviation in HV voltage.

Albert


Tom Lee
 

There are certainly far better op-amps than the 741. That said, there are always fewer questions to consider if one replaces like with like. Given that the 741 is still in production and widely available at low cost, I'd advise the OP to stick with the original. Often "better" is the enemy of "best".

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/10/2021 14:01, SCMenasian wrote:
Almost any more modern general purpose opamp which will take sufficiently high power supply voltages and have the same pinout. TL061, TL071 and TL081 come to mind, Also, the even more modern tle2072, etc. Also LF356 and similar. CA3140 also comes to mind. The most important thing to watch out for is bandwidth - in case the circuit is prone to oscillation. This list is not complete - it just reflects my current stock and data sheets. I find no reason to use a 741 now; OTOH, I am not compulsive about restoring ancient instruments to original - often, improved performance (better then original) can be had using modern components.

Stephen Menasian