Dear All,
I am struggling with this unit for some time.
The waveform at TP305 is correct, according to the manual.
However I can not obtain more than 5 steps at TP-306.
I have checked the voltage at U305B-3 which should "define" the maximum number of steps and it varies from 0.05 to 4.75V , as expected.
Just as a desperate measure I have alredy replaced U305, U220, U350, Q306 and Q324 without success.
I have created a photo album with the waveforms at TP-305, TP-306 , tthe 577 display as well as the pertinent schematics sheets.
See /g/TekScopes2/album?id=279289.
Any help will be welcome.
Thanks,
Roger

The step generator looks OK, the step amplifier has a problem. Take some more measurements at U350 pins 1 and 7. Are there more steps present there?

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I agree checking pin 7 of U350 is a good idea. That said pin 1 of U350A is the output of a unity gain amplifier summing in a DC voltage ?and should not have any steps. It should reflect the same DC voltage as the wiper of the offset multiplier R350. ? ?On the step generator schematic check the emitter of Q310 to see if you have all ?10 steps there.?

Thank you for the replies.
I am away from the lab until Monday afternoon.
But I am almost sure that the signal at the emitter of Q310 is correct.
Roger

Dear all,
I came back and checked the signal as suggested.
The signal at the Emitter of Q310 is correct.
However I get the same "plateu" of the staircase signal at pin 7 of U350B.
I have added a photo of this signal in the photo album?/g/TekScopes2/album?id=279289.
Any other suggestion?

On Tue, Oct 18, 2022 at 02:10 PM, Rogerio O wrote:

Dear all,
I came back and checked the signal as suggested.
The signal at the Emitter of Q310 is correct.
However I get the same "plateu" of the staircase signal at pin 7 of U350B.
I have added a photo of this signal in the photo album?/g/TekScopes2/album?id=279289.
Any other suggestion?

---
Hi Roger,
There are only few components around U350B so this is a real puzzle.?

I suggest we keep the scope at 1V/div, DC-coupled, and set trace on X axis when input is grounded. This way we can check if anything is wrong in DC sense. As far as I can tell the scope shots so far has AC coupling selected.

Then in two channel mode:
1) Look at pins 7 and 6 of U350B. We expect no signal at pin 6, and approximately 0V DC.
2) Look at pins 7 and 5 of U350B. We expect no signal at pin 5, and approximately 0V DC.
3)?Look at pin 7 of U350B and left side of R310. We expect signal at left side of R310 to have the opposite slope signal and all the steps.

If there is signal at pin 5 or pin 6 of U350B, R356 may have gone open circuit (perhaps R353 too).

If all above checks look good next step is to look at pin 7 of U350B while changing slope (relay K436-S1). U360 may be loading the output.

I believe you already checked pins 4 and 8 of U350B for proper voltages.?
Ozan

?
?

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Perhaps a systematic review might help.

The theory is that the signal progresses from generator to output without distortion (read, flat topping).?

So starting from the generator, identify the stage where the output is bad.? If there are multiple active components between good and bad, then they are all somewhat suspect.? You'd be treating the entire "block" of parts as a unit.

The theory says that an amplifier saturates (flat tops) for a number of reasons.

1) the current required to drive a load is beyond the amplifier's capability.

2) the amplifier swing is insufficient.? This is possible because of:

??? a) insufficient supply voltage

??? b) load resistance (in the amplifier)? is incorrect

??? c) load resistance in the actual load has changed (see #1)

??? d) the bias is wrong.

For (d), perhaps a bit of explanation is needed.? The common 741 op amp has an output swing of VCC+ - 1.5 volts and VCC- + 1.5 volts.? The amplifier clips at 1.5 volts less than the supply voltages.? Absent a transformer or any other kind of voltage step up device, the limit of any amplifier is the supply voltage (or both of them, if applicable).

If a supply voltage is off, or the supply voltage is *effectively* off (through a resistor/capacitor with bad components), then that makes a difference.

So I'd

1) with no steps or the control settings needed for the proper voltage and current steps, measure each voltage on the schematic.? You've checked the power supply voltages, of course, including the decoupled (R series, C to ground) ones.

2) then check to see which step voltage is bad.? This includes both the voltage swing and where the steps start and where they end.

3) analyze that circuit, trying to find the proper DC voltages and AC voltages.

Something is saturating, but what and where?

Apologies if you've done this already, but that's how I'd approach it from the start.

Harvey

Dear all,
Before measuring the signals with the scope as Ozan suggested, I decided to check for continuity and shorts with the unity off.
The resistance of all pins of U350, U305, U360 and U380 to ground was greater than 2.0K.
However, the resistance of the +12V to ground was only 18R in contrast of the -12V to ground that was 2.9K.
The +12V regulator is managing to keep the volatge at the nominal voltage, but I could not identify any section of the main board circuit that would explain this low value other than a leaky C297, but this cap has been replaced when I started the repair job.
So my questions are:
1 - Is this low resistance "normal"??
2 - Is it possible that this low resistance is causing the circuit to clip, despite the +12V rail voltage value is correct ?

I will measure the waveforms with the scope as soon as I can, and post the results on the photo album.
Thanks to all,
Roger
?

On Fri, Oct 21, 2022 at 08:47 AM, Rogerio O wrote:

....
However, the resistance of the +12V to ground was only 18R in contrast of the -12V to ground that was 2.9K.
The +12V regulator is managing to keep the volatge at the nominal voltage, but I could not identify any section of the main board circuit that would explain this low value other than a leaky C297, but this cap has been replaced when I started the repair job.
So my questions are:
1 - Is this low resistance "normal"??
2 - Is it possible that this low resistance is causing the circuit to clip, despite the +12V rail voltage value is correct ?

---
Hi Roger,
You may be seeing semiconductor junctions turning on. Does switching polarity of the ohm-meter leads change the measurement? Does changing the range of the ohm-meter change the measurement?

Observing ?+12V on channel 2 while looking at ramp signal on channel 1 can tell if supply sinking is the issue although it looks like clamping is happening at negative side.
Ozan

Hi folks,
Following Ozan's suggestion I have measured pin7 in on CH1 and pins 5 and 6 on CH2 and everything went as expected. 0V at pins 5 and 6 while having the staircase signal at pin 7.
Then I decided to replace Q310 with an originial tek 151-0192-00 that is used in the later models.
Since the problem was still there I measured the signals at the collector of Q310 (TP305) and its emitter (left side of R310) and, to my surprise the signal was "saturating" there.
I posted a picture of the signals st the photo album?/g/TekScopes2/album?id=279289.??
PLEASE NOTE THAT THE PROBE ON CH2 IS NON TEK SO THE REAL V/Div READING IS 2V/DIV!!!
So I checked the DC bias at Q310??base (using the settings of table 7-2)? and the mesured +0.6V as shown in schematics.
I made a "crude verification of signal loading" by measuring the resistance to ground of U350B pin 6? (13.4K) and pin7 (5K), but I don't know if these value may cause (or explain) the problem.
Any other suggestion?
Thanks
?

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IF Q310 is used as a current amplifier (low impedance input) then the base voltage can be 0.6 volts even if the operating conditions are bad.? Is the base connected to multiple sources through resistors?? If so, it is likely a current amplifier.? Please check the voltages and signals at the other end of such resistors.? Check the voltage drops across the collector and emitter resistors to see if they are correct, likewise, too low a collector voltage supply can cause saturation.? Try these tests with no steps generated (if possible) to see the DC operating points.

Harvey

On Sun, Oct 23, 2022 at 01:34 PM, Rogerio O wrote:

Hi folks,
Following Ozan's suggestion I have measured pin7 in on CH1 and pins 5 and 6 on CH2 and everything went as expected. 0V at pins 5 and 6 while having the staircase signal at pin 7.
Then I decided to replace Q310 with an originial tek 151-0192-00 that is used in the later models.
Since the problem was still there I measured the signals at the collector of Q310 (TP305) and its emitter (left side of R310) and, to my surprise the signal was "saturating" there.
I posted a picture of the signals st the photo album?/g/TekScopes2/album?id=279289.??
PLEASE NOTE THAT THE PROBE ON CH2 IS NON TEK SO THE REAL V/Div READING IS 2V/DIV!!!
So I checked the DC bias at Q310??base (using the settings of table 7-2)? and the mesured +0.6V as shown in schematics.
I made a "crude verification of signal loading" by measuring the resistance to ground of U350B pin 6? (13.4K) and pin7 (5K), but I don't know if these value may cause (or explain) the problem.
Any other suggestion?
Thanks
?

———-

Hi Roger,
In normal operation base of Q310 should move with ramp. Schematic shows DC value in a specific condition, which is a great debug help, but may not tell the whole story.?

?

S429 (PULSED mode) switch should be pulling cathode of CR234 to ground in normal mode, which should pull base of Q234 below turn on voltage (Vbe below 0.5V). With Q234 off R234 should pull base of Q320 high. Base of Q310 should follow the ramp with a 0.7V offset.?

?

If you pull out Q234 do you see all the steps? If it works possible failure modes in the order of probability: dirty PULSED switch, open CR234, open or drifted R237, bad Q234, drifted R234.?

?

Since you already replaced Q310 we can assume it is good.?

Ozan

Thank you for your help, Ozan,
I have posted a picture of the signals at Q310 base & collector and Q310 base & emitter on the photo album?/g/TekScopes2/album?id=279289.
The base signal really follows the emitter, but the voltage difference seems to be 7.5V rather than 0.75V.
This time I am using two tek probes.
Am I reading the voltage difference with the cursors wrong?
If not, what may be causing this?

PROBLEM SOLVED!!!
The culprit was Q234, which was testing as two diodes.
I did not realized that the steps should be present at Q310 base, so I never checked this part of the circuit, especially because the DC bias was correct.
After replacing it by a 2N3565 the circuit began to work as expected.

Thank you very much to all, specially Ozan and Harvey for the continuous support.

Going back to calibration...

I will probably be back since as far as I remember I could not complete the Step Gen Output Z calibration?, but that is another story....

Roger

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Excellent, glad to see that it works now.

Harvey

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On Mon, Oct 24, 2022 at 12:24 PM, Rogerio O wrote:

PROBLEM SOLVED!!!
The culprit was Q234, which was testing as two diodes.
I did not realized that the steps should be present at Q310 base, so I never checked this part of the circuit, especially because the DC bias was correct.
After replacing it by a 2N3565 the circuit began to work as expected.

Thank you very much to all, specially Ozan and Harvey for the continuous support.

Going back to calibration...

I will probably be back since as far as I remember I could not complete the Step Gen Output Z calibration?, but that is another story....

Roger

----
Congratulations for the repair.?
Ozan