Among other issues with this 7603 were the Z axis operation, it seemed
like a blanking issue (beam is not blanked correctly), but on closer
inspection it is a bit more involved:

The intensity control works reversed (fully CCW is maximum intens, fully
CW blanks the beam). This is not a cabling issue.

Blanking signals are correctly processed by the Z axis logic, and the Z
axis amplifier produces the correct voltages on pin B (about 20 V with
P1171 open for a blanked beam, and about 90 V with maximum input current
for maximum intensity). I also cross checked this against a working 7623
just to be sure that I analyzed the circuitry correctly and understood
the circuit description.

So the inversion must happen somewhere else. I previously replaced all
HV caps in the HV unit, including the DC restorers. Now I also replaced
all diodes in the grid DC bias restorer. No change in behaviour.

The focus amplifier, preset and front panel control all work correctly.

I don't know what else I could check or what else could cause this.
Maybe someone here has another idea?

Cheers,
Marian

Unless the pot is hooked up backwards, the only way I can see that a signal can be improperly inverted is in an inverting gain stage that is damaged so that the input signal passes right through without inversion. This can happen in a common-emitter or opamp stage, where a failure or a missing part allows the signal to pass through the feedback loop rather than the active device.


Another possibility is that someone inadvertently changed the circuit during troubleshooting.


Ed

One more thing - another failure type can happen with certain opamps, where output phase reversal occurs if an input is driven beyond its normal dynamic range. I don't recall how the intensity signals are processed in the 7K scopes, but if opamps are involved, check for proper operating conditions on all pins.


Ed

The Z axis amplifier is kinda an op amp in the sense that it is a global feedback amplifier. It does work correctly, both with manually provided inputs and when connected to the Z axis logic.

The intensity pot isn't hooked up backwards.

In the mean time I also checked whether all connections between the tube socket and the pin header in the HV unit are correct, but, unfortunately, they all are correct.

Another possibility is that someone inadvertently changed the circuit during
troubleshooting.

This seems most likely at this point, however there are very few spots in the scope that have been reworked before. Some are on the HV unit (the transformer and the thick film network where exchanged or reworked), but not in that area. I also can't see any changes in parts, except that some European parts have been used in non-critical parts of the circuit (e.g. BZX series Zeners instead of whatever Tek Beaverton specified, I guess this is normal for a Heerenveen-built scope).

I should try to probe the grid and cathode voltages again and see if this is really an issue with the HV unit. Maybe it works correctly after all and there is some strange fault in the tube itself? --- I'd prefer a fault in the HV unit, though.

Hi Marian, do I understand you correctly if I think that the Z amplifier itself is working correctly but that the CRT gets brighter when the Z output at B gets lower (less positive)? In that case I have no idea what's going on, but then you should also "complain" that the Readout is shown as negative image.
If the Readout is correct then the logic board is suspect, for instance a faulty U99.

Albert

There are some ways you can narrow down the problem before tracing the
z-axis control circuits:

1. Does the readout intensity control also operate backwards?

Its signal goes directly to the readout board which then drives the
input to the z-axis amplifier in parallel with the normal intensity
signal so it bypasses all of the z-axis logic.

2. Plug-ins can also adjust the beam intensity. If you have a 7B92A
timebase, what does its intensity control do?

3. Is the leading edge unblanking working correctly when a fast pulse
edge is observed?

I checked my 7603 and with a 7B53A timebase, about 1/2 division at 50
ns/div (25 nanoseconds) is unblanked before a fast pulse edge. With a
faster timebase like a 7B92A, 1 full division at 50 ns/div (50
nanoseconds) is unblanked before a fast pulse edge. Oddly enough a
7B50A also unblanked 50 nanoseconds before the fast pulse edge so I
assume it shares more in common with the faster timebases than the
7B53A.

As far as tracing the circuit, the z-axis amplifier is straightforward
but the z-axis logic shown on schematic 2 gives me a headache. I will
have to break out Clipboard CAD to figure out exactly what is suppose
to be happening there. I do see one thing however:

The sweep gate rises at the base of U99B to redirect the current from
the intensity control through the emitter of U99B to the collector.
The intensity is controlled by this current. If the -15 volt
connection to the intensity potentiometer is open, then the current
would *increase* as it is turned counterclockwise lowering the
resistance between the wiper and ground reversing its operation. The
intensity control range would also be affected but this might not be
very noticeable if the grid bias was adjusted.

So instead of checking to see if the intensity potentiometer is
connected backwards, check to see if the -15 volt supply is present at
the intensity potentiometer and that the wiper actually moves between
0 and -15 volts as the control is turned through its full travel
range. Note that -15 volts is suppose to produce the highest
intensity and 0 volts is suppose to produce the lowest intensity.

Hi David,

The sweep gate rises at the base of U99B to redirect the current from

the intensity control through the emitter of U99B to the collector.
The intensity is controlled by this current. If the -15 volt
connection to the intensity potentiometer is open, then the current
would *increase* as it is turned counterclockwise lowering the
resistance between the wiper and ground reversing its operation. The
intensity control range would also be affected but this might not be
very noticeable if the grid bias was adjusted.

Though it looks like this, I think that the CRT will simply remain dark. With the wiper at 0 V the CRT should definitively be dark. An extra resistance to 0 V will decrease the current to U99B and reduce the intensity which was already at dark. IIRC there are other scopes in which the wiper actively pulls a voltage or current higher and lower (and the current to the wiper changes direction). In that case the effect of an open connection can be different.

So instead of checking to see if the intensity potentiometer is

connected backwards, check to see if the -15 volt supply is present at
the intensity potentiometer and that the wiper actually moves between
0 and -15 volts as the control is turned through its full travel
range. Note that -15 volts is suppose to produce the highest
intensity and 0 volts is suppose to produce the lowest intensity.

Sure that's the first thing to check anyway. I thought this would have been done already as part of checking for a cabling faults.

Albert

On 23 Jan 2016 10:45:22 -0800, you wrote:

Hi David,

The sweep gate rises at the base of U99B to redirect the current from
the intensity control through the emitter of U99B to the collector.
The intensity is controlled by this current. If the -15 volt
connection to the intensity potentiometer is open, then the current
would *increase* as it is turned counterclockwise lowering the
resistance between the wiper and ground reversing its operation. The
intensity control range would also be affected but this might not be
very noticeable if the grid bias was adjusted.

Though it looks like this, I think that the CRT will simply remain dark. With the wiper at 0 V the CRT should definitively be dark. An extra resistance to 0 V will decrease the current to U99B and reduce the intensity which was already at dark. IIRC there are other scopes in which the wiper actively pulls a voltage or current higher and lower (and the current to the wiper changes direction). In that case the effect of an open connection can be different.

You're going to make me break out my Clipboard, aren't you?

One thing I wondered about the z-axis logic shown in the 7603
schematics is if the schematics were drawn wrong but I checked the
other 76xx series service manuals and they are all identical.

The gate signal is generated by an active pull-up from the horizontal
plug-in and its level is about +5 volts. That produces about 1.25
volts at the base of U99B and 0.6 volts at the emitter of U99B which
will be sufficient to turn U99B completely on and U99A completely off.
The voltages shown in the 76xx schematics are present when the gate is
off.

With -15 volts missing at the 5 kilohm intensity potentiometer, the
current through the emitter of U99B will vary from 0.6V/3k to 0.6V/8k
or 200uA to 75uA and increase as the intensity control is turned
counter clockwise.

Under normal conditions, the current will vary from 0.6V/3k to (0.6V +
15V)/8k or 200uA to 2.0mA and increase as the intensity control is
turned clockwise. That range of 1:10 had to be a deliberate design
criteria.

Is 200uA to 75uA enough to unblank the display and cause an adjustable
change in intensity? I did some of the calculations for the current
at the emitter of U99C and it looks to me like the intensity
adjustment will definitely be active and working from 200uA to 75uA if
the grid bias is adjusted appropriately.

So instead of checking to see if the intensity potentiometer is

connected backwards, check to see if the -15 volt supply is present at
the intensity potentiometer and that the wiper actually moves between
0 and -15 volts as the control is turned through its full travel
range. Note that -15 volts is suppose to produce the highest
intensity and 0 volts is suppose to produce the lowest intensity.

Sure that's the first thing to check anyway. I thought this would have been done already as part of checking for a cabling faults.

Albert

Public only stated that cabling was not an issue unless I missed
something and if the potentiometer was open between the -15 volt lead
and the element, then the only way to verify it would be to measure
the wiper voltage.

Hi David,

Sorry, it was not my intention to force you to do "homework" calculations!
Having read the discussion again and again, I think I see now why we seemingly disagree about the effect of an open -15V end. You wrote "The intensity control range would also be affected but this might not be
very noticeable if the grid bias was adjusted." and now you wrote "working from 200uA to 75uA if
the grid bias is adjusted appropriately." I was thinking of a properly adjusted scope in which the fault occurs at some instant. But what you meant and mean must be a re-adjustment of grid bias *after* the fault occurred.
BTW A 125 uA change would give about 2.2 V change in voltage across R1159 and in grid-kathode voltage. In my experience such a change, just above cut-off, is well visible but not enough to make you think the intensity range is normal.

Albert

On 01/22/2016 10:11 PM, aodiversen@... [TekScopes] wrote:

Hi Marian, do I understand you correctly if I think that the Z amplifier
itself is working correctly but that the CRT gets brighter when the Z
output at B gets lower (less positive)? In that case I have no idea
what's going on, but then you should also "complain" that the Readout is
shown as negative image.
If the Readout is correct then the logic board is suspect, for instance
a faulty U99.

Albert

Albert, yes this is what I am thinking. If I remove all inputs to the Z
axis amplifier it outputs about 20 V, which should blank the beam - but
doesn't. Shorting the input to ground, i.e. full input current, produces
approx. 90 V at the output (pin B), which should produce full intensity
- instead it blanks the beam.

On 01/22/2016 10:11 PM, aodiversen@... [TekScopes] wrote:

In that case I have no idea
what's going on, but then you should also "complain" that the Readout
is shown as negative image.

I don't think it would work this way, since the readout uses vectors to
display the text, fiddling with the Z axis would only change the beam
current, while the beam still traces the same characters.

On 01/23/2016 06:41 AM, David @DWH [TekScopes] wrote:

1. Does the readout intensity control also operate backwards?

The readout board in this unit was shot (1 simple ASIC missing, at least
2 ROMs dead), I replaced it by a known-working board yesterday. Readout
works just fine, but like the other intensity control the readout
intensity control works backwards.

3. Is the leading edge unblanking working correctly when a fast pulse
edge is observed?

If I adjust intensity so that I can see the beam it is never blanked.
When retrace / wait for sweep occurs the Z axis logic produces a low
current (high voltage, turning Q1107 off), which leads to ~20 V from the
Z axis amplifier output, which should blank the beam, but instead the
beam turns all the way up.

On 01/23/2016 06:41 AM, David @DWH [TekScopes] wrote:

So instead of checking to see if the intensity potentiometer is
connected backwards, check to see if the -15 volt supply is present at
the intensity potentiometer and that the wiper actually moves between
0 and -15 volts as the control is turned through its full travel
range. Note that -15 volts is suppose to produce the highest
intensity and 0 volts is suppose to produce the lowest intensity.

Yes, I measure this correctly at logic board pin ES and see the expected
effect in Z axis amplifire output as described above.

-

I tried to probe the grid-cathode voltage yesterday with my homebrew
"HV" "probe", but it's just to bulky and doesn't stay in place. I
suspect at this point that one of the original caps in the DC restorer
was failed, either open or leaky, and that one of the replacement caps
has the same or a similar problem. I'll buy another round of HV caps and
see if that changes anything.

Cheers,
Marian

Good info Marian, this makes all discussions about intensity pot and Z axis logic superfluous.
An easy explanation would be that the polarities of both diodes CR1269 and CR1270 are reversed. That doesn't do any harm to that circuit but produces the effect you describe.

Albert



---In TekScopes@..., <public@...> wrote :

On 01/22/2016 10:11 PM, aodiversen@... mailto:aodiversen@... [TekScopes] wrote:
> Hi Marian, do I understand you correctly if I think that the Z amplifier
> itself is working correctly but that the CRT gets brighter when the Z
> output at B gets lower (less positive)? In that case I have no idea
> what's going on, but then you should also "complain" that the Readout is
> shown as negative image.
> If the Readout is correct then the logic board is suspect, for instance
> a faulty U99.
>
> Albert

Albert, yes this is what I am thinking. If I remove all inputs to the Z
axis amplifier it outputs about 20 V, which should blank the beam - but
doesn't. Shorting the input to ground, i.e. full input current, produces
approx. 90 V at the output (pin B), which should produce full intensity
- instead it blanks the beam.

(continued)
Too quick conclusion since that DC restorer circuit has to supply the full grid-kathode voltage. A reversal would mean that the grid becomes positive w.r.t. the kathode.
Albert



---In TekScopes@..., <aodiversen@...> wrote :

Good info Marian, this makes all discussions about intensity pot and Z axis logic superfluous.
An easy explanation would be that the polarities of both diodes CR1269 and CR1270 are reversed. That doesn't do any harm to that circuit but produces the effect you describe.

Albert

On 24 Jan 2016 02:25:49 -0800, you wrote:

Sorry, it was not my intention to force you to do "homework" calculations!

I have been wanting to tear into the 7603 z-axis logic for a while
because it is so inscrutable at first glance. At least now I can look
at it and follow how it works.

Having read the discussion again and again, I think I see now why we seemingly disagree about the effect of an open -15V end. You wrote "The intensity control range would also be affected but this might not be
very noticeable if the grid bias was adjusted." and now you wrote "working from 200uA to 75uA if
the grid bias is adjusted appropriately." I was thinking of a properly adjusted scope in which the fault occurs at some instant. But what you meant and mean must be a re-adjustment of grid bias *after* the fault occurred.

I was thinking about a situation where no trace is present so the
operator goes through the z-axis calibration procedures and/or adjusts
the grid bias so even the low current from a missing -15 volts
intensity control becomes visible.

BTW A 125 uA change would give about 2.2 V change in voltage across R1159 and in grid-kathode voltage. In my experience such a change, just above cut-off, is well visible but not enough to make you think the intensity range is normal.

I would not expect it to look normal but with an apparently broken
oscilloscope, just seeing something might be such a relief that the
implications of a low control range gets missed.

Anyway, further posts look to rule out the z-axis logic as the
problem.

On Sun, 24 Jan 2016 13:18:26 +0100, you wrote:

...

On 01/23/2016 06:41 AM, David @DWH [TekScopes] wrote:

1. Does the readout intensity control also operate backwards?

The readout board in this unit was shot (1 simple ASIC missing, at least
2 ROMs dead), I replaced it by a known-working board yesterday. Readout
works just fine, but like the other intensity control the readout
intensity control works backwards.

So there is unlikely to be a problem with the z-axis logic. Progress
is being made.

3. Is the leading edge unblanking working correctly when a fast pulse
edge is observed?

If I adjust intensity so that I can see the beam it is never blanked.
When retrace / wait for sweep occurs the Z axis logic produces a low
current (high voltage, turning Q1107 off), which leads to ~20 V from the
Z axis amplifier output, which should blank the beam, but instead the
beam turns all the way up.

With no input at Q1107, I calculate a 21 volt output at R1157 which
agrees with your measurement and should blank the beam. That points
to a problem in the DC restorer.

You could measure the output of the z-axis amplifier with a voltmeter
and verify that the output *rises* when the intensity control is
turned clockwise.

I tried to probe the grid-cathode voltage yesterday with my homebrew
"HV" "probe", but it's just to bulky and doesn't stay in place. I
suspect at this point that one of the original caps in the DC restorer
was failed, either open or leaky, and that one of the replacement caps
has the same or a similar problem. I'll buy another round of HV caps and
see if that changes anything.

You need a high voltage probe to measure the grid or cathode voltage
to ground but technically you do *not* need one to measure the voltage
between the grid and cathode which should only be in the 10s of volts.
If you connect a floating voltmeter, presumably a handheld battery
powered one, between the cathode and grid, then it can safely make a
floating measurement. The meter and test leads need to be well
insulated from the surrounding environment including yourself so if
you try this, setup the meter with the oscilloscope powered down and
do not touch it while the oscilloscope is operating.

I suggest changing the high voltage capacitors *and* the high voltage
diodes in the DC restorer. It might be worth buying a duplicate set
of high voltage parts for the focus DC restorer.

Good info Marian, this makes all discussions about intensity pot and Z axis logic superfluous.
An easy explanation would be that the polarities of both diodes CR1269 and CR1270 are reversed. That doesn't do any harm to that circuit but produces the effect you describe.

(continued)

Too quick conclusion since that DC restorer circuit has to supply the full grid-kathode voltage. A reversal would mean that the grid becomes positive w.r.t. the kathode.

Albert

The output from the z-axis amplifier is AC coupled to the grid via
C1269 so nothing is going to reverse its operation at high
frequencies. I wonder if C1269 is open and we are seeing the effects
of stray capacitance and leakage coupling the output of the z-axis
amplifier through the DC restorer.

DC restorers give me a headache also. I will have to study it in
detail to find a failure which would cause this behavior. I hope the
problem is not a damaged CRT.

Hi David,

This is nasty, you comment on a response by Marian but that response does not appear at the Tekscopes website.
Some minor remarks here on your last comments:

>With no input at Q1107, I calculate a 21 volt output at R1157 which agrees with your measurement and should blank the beam. That points to a problem in the DC restorer.

I calculate it at 16 V, never mind.

>>
>>Too quick conclusion since that DC restorer circuit has to supply the full grid-kathode voltage. A reversal would mean that the grid becomes positive w.r.t. the kathode.
>>
>>Albert

>The output from the z-axis amplifier is AC coupled to the grid via
C1269 so nothing is going to reverse its operation at high
frequencies. I wonder if C1269 is open and we are seeing the effects
of stray capacitance and leakage coupling the output of the z-axis
amplifier through the DC restorer.

Marian now says that the readout display itself is normal but that the readout intensity control is reversed. This suggest that C1269 is fine I think

Albert

On 24 Jan 2016 11:46:17 -0800, you wrote:

Hi David,

This is nasty, you comment on a response by Marian but that response does not appear at the Tekscopes website.
Some minor remarks here on your last comments:

I am not surprised the web interface has issues. I sometimes check to
see if email post are congruent with what is shown through the web
interface. I regret not having a viable alternative to Yahoo.

With no input at Q1107, I calculate a 21 volt output at R1157 which agrees with your measurement and should blank the beam. That points to a problem in the DC restorer.

I calculate it at 16 V, never mind.

I did not find a way to get 16 volts but my work can be checked:

With Q1107 disconnected there are three currents into or out of the
base of Q1156:

1. R1141 is -1.7mA
2. R1121 + R1122 is -1.15mA
3. R1120 is +2.3mA

The later two connect at the top of VR1148 but affect the base of
Q1107 the same way except for an offset of 11 volts.

2.3mA - 1.15mA - 1.7mA = -0.55mA

The amplifier is inverting and note that it does not care whether the
currents are applied to the top or bottom of VR1148 as long as VR1148
is conducting which R1141 guarantees so 0.55mA * 17.4k = 9.6V

The output is offset by VR1148 and the Vbe of Q1156 so:

9.6V + 11V + 0.6V = 21.2V

I do not know why the automatic focus circuit and the z-axis circuit
are interconnected as shown. Maybe they were just being thrifty with
the R1120, R1121, and R1122 voltage divider.

The 7603, 7613, and 7623 use the same z-axis amplifier design with the
interconnected focus amplifier but the 7623A and 7633 (and 7623B?)
share a slightly different design where they are completely separate
and I do not see any reason that it needed to be changed other than it
is misleading.

Too quick conclusion since that DC restorer circuit has to supply the full grid-kathode voltage. A reversal would mean that the grid becomes positive w.r.t. the kathode.

Albert

The output from the z-axis amplifier is AC coupled to the grid via
C1269 so nothing is going to reverse its operation at high
frequencies. I wonder if C1269 is open and we are seeing the effects
of stray capacitance and leakage coupling the output of the z-axis
amplifier through the DC restorer.

Marian now says that the readout display itself is normal but that the readout intensity control is reversed. This suggest that C1269 is fine I think

Albert

If it is working correctly up to the output of the z-axis amplifier,
and this is something he can check easily enough by directly measuring
the output of the z-axis amplifier with a normal voltmeter, then I
think C1269 has to be open because otherwise it would conduct high
frequencies directly to the grid and thing would work normally at
least immediately after unblanking. He has not described the
intensity changing along the sweep which would indicate that C1269 is
charging and discharging.

Ever see the movie Dark Star?

Doolittle: That's good. That's very good. But how do you know that
anything else exists?
Bomb #20: My sensory apparatus reveals it to me. This is fun.

Hi David,

It seems that you forgot about R1155?
4.8 V across R1155 gives 0.48 mA. That 0.48 mA reduces the zener current and reduces the current via R1159 by the same amount.
I think you took 130 V across R1120 while it is about 128.4 V, this gives 0.21 mA less via R1120 and 0.21 mA more via R1159.
Together this yields 0.27 mA less via R1159 or 4.7 V lower output voltage. Then our results are the same.

Albert


---In TekScopes@..., <davidwhess@...> wrote :

On 24 Jan 2016 11:46:17 -0800, you wrote:

>Hi David,
>>With no input at Q1107, I calculate a 21 volt output at R1157 which agrees with your measurement and should blank the beam. That points to a problem in the DC restorer.
>
>I calculate it at 16 V, never mind.

I did not find a way to get 16 volts but my work can be checked:

With Q1107 disconnected there are three currents into or out of the
base of Q1156:

1. R1141 is -1.7mA
2. R1121 + R1122 is -1.15mA
3. R1120 is +2.3mA

The later two connect at the top of VR1148 but affect the base of
Q1107 the same way except for an offset of 11 volts.

2.3mA - 1.15mA - 1.7mA = -0.55mA

The amplifier is inverting and note that it does not care whether the
currents are applied to the top or bottom of VR1148 as long as VR1148
is conducting which R1141 guarantees so 0.55mA * 17.4k = 9.6V

The output is offset by VR1148 and the Vbe of Q1156 so:

9.6V + 11V + 0.6V = 21.2V

Typo in "I think you took 130 V across R1120 while it is about 128.4 V". Should read 130 V - 11.6 V = 118.4 V, not 128.4 V.
Albert

On 24 Jan 2016 13:45:16 -0800, you wrote:

Typo in "I think you took 130 V across R1120 while it is about 128.4 V". Should read 130 V - 11.6 V = 118.4 V, not 128.4 V.

Albert

I have been going through your post and figured that was a typo. I
used 11.6 volts at the top of VR1148.

On 24 Jan 2016 13:40:48 -0800, you wrote:

Hi David,

It seems that you forgot about R1155?

Yep, I missed it. I might have designed it with an AC coupling
capacitor in series with R1155 but technically that is not needed. I
thought R1155 controlled the gain of Q1156 but the effect would be
very small. Who designed this thing?

Hmm, the later design used in the 7623A and 7633 lacks R1155 among
other minor changes and I had all of the schematics open when I did my
first analysis although I only scribbled on the correct schematic.

4.8 V across R1155 gives 0.48 mA. That 0.48 mA reduces the zener current and reduces the current via R1159 by the same amount.
I think you took 130 V across R1120 while it is about 128.4 V, this gives 0.21 mA less via R1120 and 0.21 mA more via R1159.
Together this yields 0.27 mA less via R1159 or 4.7 V lower output voltage. Then our results are the same.

Albert

4.8V / 10k [R1155] = +0.48mA
-15.6V / 9.1k [R1141] = -1.7mA
(130V - 11.6V) / 56k [R1120] = +2.1mA
-11.6V / 10.1k [R1121 and R1122] = -1.15mA
2.48mA - 2.85mA = -0.27mA
0.27mA * 17.4k [R1159] = 4.7V
4.7V + 11V = 15.7V

With Q1107 (a slow 2N3904???) removed on my 7603, the output at point
B is 14.8 volts which closely agrees with our current calculation so I
wonder if Marian's <public@...> measurement of 20 volts at the
output of the z-axis amplifier is significant and indicates a problem.
I get the same 14.8 volt output with Q1107 installed, the readout off,
and the intensity control turned to minimum.

I also measured the output at point B and as the intensity control is
turned fully clockwise, the output increases to about 28 volts
monotonically.

Hi David,

From 15 V to 28 V seems a rather small change compared to the data in the calibration section. Maybe you set the time base for a slow speed that activates the Intensity Limit?
Albert

============

With Q1107 (a slow 2N3904???) removed on my 7603, the output at point
B is 14.8 volts which closely agrees with our current calculation so I
wonder if Marian's <public@... mailto:public@...> measurement of 20 volts at the
output of the z-axis amplifier is significant and indicates a problem.
I get the same 14.8 volt output with Q1107 installed, the readout off,
and the intensity control turned to minimum.

I also measured the output at point B and as the intensity control is
turned fully clockwise, the output increases to about 28 volts
monotonically.