465B Horizontal Sweep help


Robert Simpson
 

The 465B I am working one has a horizontal sweep problem, that is, almost none. At the moment, I am working my way through theory of operation starting with the sweep gate page 3-12. The manual schematic shows a test point 58 on the collector of Q7256. All I see there at 5mV per division on my monitoring scope ( a 465M) is 2mV of fuzz. Its companion Q7254 has a pulse of 5mV. What is supposed to be there?

Bob


Ozan
 

On Mon, Aug 9, 2021 at 03:35 PM, Robert Simpson wrote:

The 465B I am working one has a horizontal sweep problem, that is, almost
none. At the moment, I am working my way through theory of operation starting
with the sweep gate page 3-12. The manual schematic shows a test point 58 on
the collector of Q7256. All I see there at 5mV per division on my monitoring
scope ( a 465M) is 2mV of fuzz. Its companion Q7254 has a pulse of 5mV. What
is supposed to be there?

Bob
--------------------
Bob,
I assume you already found A7 schematic which is in a separate PDF. Unfortunately some of the TP waveforms are missing. I don't have this scope but looking at the schematics:
Test point 58 is connected to test point 71 by CR7449. Test point 71 waveform is shown as a -4.6V to -2.5V pulse. Most likely expected waveform at TP58 is a pulse that goes from -8V (when Q7256 is off) to ~ -1.8V (one diode drop above -2.5V).

Signal at the base of Q7254 is the input to this diff pair, it is shown in TP47 waveform. Test point 59 is most likely a pulse from -4V (when Q7254 is off, voltage divider R7079 & R7175) to -1.7V (one VBE above the voltage marked at the emitter of Q7181 in the on position).

All of these waveforms will follow TP47 waveform, it is compared to the voltage at the base of Q7256. Base of Q7256 should be +0.2V when not in AUTO trigger mode, and -0.2V when in AUTO trigger mode according to the schematic.

Hope this helps.
Ozan


 

If you haven't already, you should check the video I made on 465B horizontal problems: https://youtu.be/Vl2dMT9ZK0c

--
Bob Haas


Robert Simpson
 

Thanks for the replies. I now have more to check.
Bob


Robert Simpson
 

Uh, I may not have phrased that well. I meant I now have more insights to check, a good thing.
Bob


Robert Simpson
 

Bob Haas,

I checked the resistors per your video, and they are not open like yours. However now I have a worse problem. Until mid day today, I would get a green 1/4 inch square trace unless I pressed beam finder when I would get a dot.
When I added an input using the one volt calibrator signal from my 465M monitoring scope, the trace would go vertical with a bright small quarter inch green square at each end of a vertical rectangle with a dim path between.

Then today the screen went completely dark.
I have checked all PS voltages and they are good. Intermediate high voltage has been and still is steady a 2446 volts. I checked the CRT and there is continuity across the filament and no short to the cathode. Powered up the voltages at the back of the CRT look good.
There is exactly 103.9 volts positive relative to ground on each horizontal wire going to the CRT with zero difference between them There is also about a 25mV saw-tooth like ripple on the output wires.

I hope my CRT voltage multiplier has not gone out, but I at the moment I don't know why there is now no trace even with beam finder.

Bob


Don Bitters
 

Bob,
An equal fixed voltage on each horizontal plate means no sweep. To get a sweep on the CRT you must have a ramp potential across the plates at whatever frequency (sweep time) you set. I would start troubleshooting the sweep ramp circuitry, including the front panel switches.
Don Bitters


Robert Simpson
 

Don,
thanks, yeah 25 milli-volts won't do much for sweep.
Bob


 

I had the sweep die on a 475 because a bypass cap (C1059) on the timing board spilled its guts. It was evident that the cap had failed from a visual inspection (one lead was green and corroded), and it was an easy replacement. I'm having trouble working out the similarities between the 475 and 465B timing boards, however. I'm also not sure if all the symptoms were the same as what you are seeing, as I knew a lot less about this stuff when that failure happened.

-- Jeff Dutky


Robert Simpson
 

As for my lost display, I may have found something. Pin 3 on the CRT goes from 2528V to 2512V varying the intensity control. the manual (page 3-22) indicates a control grid voltage of about 2465 volts. Looks like Pin 3 has too many volts and I am wondering if that could cut off the beam?

Bob


 

On Wed, Aug 11, 2021 at 10:33 PM, Robert Simpson wrote:


Pin 3 on the CRT goes from 2528V to 2512V varying the intensity control. the
manual (page 3-22) indicates a control grid voltage of about 2465 volts. Looks
like Pin 3 has too many volts and I am wondering if that could cut off the
beam?
Pin 3 is the beam current control grid. Unless it's at least several tens of volts more negative than the cathode (pin 2), beam current will be very high under normal conditions and you could expect a very bright spot or trace. The "grid bias" is used for static adjustment of minimum brightness. Beam cutoff is at about cathode -70V I guess. Playing with the grid bias setting (R1480) should quite visibly influence the voltage that you measure and allow setting full brightness and full blanking. It's easily reset to the right position later on, so some experimenting won't harm.
OTOH, pin 3 is a very high-impedance point and depending on your meter's input impedance, the voltage you see will be quite a bit off, IOW too high (less negative). I'd expect high brightness just connecting your meter, unless its input resistance is well above 10 - 11 MOhm.
To conclude, I don't think the voltage you see at pin 3 indicates a problem, on the contrary. Are neon lamps DS1425 and DS1426 off? They should be.

Raymond


Ozan
 

To add to Raymond's comments:
Amount of negative bias depends on the voltage swing at TP93. Higher the swing at TP93, dimmer is the beam. The swing at TP93 is set by two diode clamps: CR4111 and CR4112. Checking voltages at the cathode of CR4111 (schematic says ~ 100V expected) and TP91 (schematic says 15 to 95V expected depending on the intensity setting) can help you pinpoint the issue. It could be as simple as an accidental setting change of grid bias trimpot.

As a side note in the service manual I am looking at grid bias trimpot is R4109, and the neon bulbs are DS4124 and DS4125. Most HV probes would have input resistance much higher than 10M-ohm (the one I have is spec'ed at 1000M-ohm) so loading is probably OK.

Ozan


On Wed, Aug 11, 2021 at 10:33 PM, Robert Simpson wrote:


Pin 3 on the CRT goes from 2528V to 2512V varying the intensity control. the
manual (page 3-22) indicates a control grid voltage of about 2465 volts.
Looks
like Pin 3 has too many volts and I am wondering if that could cut off the
beam?
Pin 3 is the beam current control grid. Unless it's at least several tens of
volts more negative than the cathode (pin 2), beam current will be very high
under normal conditions and you could expect a very bright spot or trace. The
"grid bias" is used for static adjustment of minimum brightness. Beam cutoff
is at about cathode -70V I guess. Playing with the grid bias setting (R1480)
should quite visibly influence the voltage that you measure and allow setting
full brightness and full blanking. It's easily reset to the right position
later on, so some experimenting won't harm.
OTOH, pin 3 is a very high-impedance point and depending on your meter's input
impedance, the voltage you see will be quite a bit off, IOW too high (less
negative). I'd expect high brightness just connecting your meter, unless its
input resistance is well above 10 - 11 MOhm.
To conclude, I don't think the voltage you see at pin 3 indicates a problem,
on the contrary. Are neon lamps DS1425 and DS1426 off? They should be.

Raymond


 

On Wed, Aug 11, 2021 at 11:34 PM, Ozan wrote:


To add to Raymond's comments:
Amount of negative bias depends on the voltage swing at TP93. Higher the swing
at TP93, dimmer is the beam. The swing at TP93 is set by two diode clamps:
CR4111 and CR4112. Checking voltages at the cathode of CR4111 (schematic says
~ 100V expected) and TP91 (schematic says 15 to 95V expected depending on the
intensity setting) can help you pinpoint the issue. It could be as simple as
an accidental setting change of grid bias trimpot.

As a side note in the service manual I am looking at grid bias trimpot is
R4109, and the neon bulbs are DS4124 and DS4125. Most HV probes would have
input resistance much higher than 10M-ohm (the one I have is spec'ed at
1000M-ohm) so loading is probably OK.
Thanks for your comments, Ozan. My component references were for the 465 whereas Robert's is a 465B, sorry about that.

As regards the -2500V: Of course, you're right, hardly any standard DVM (e.g. with 10 MOhm input resistance) would be suitable for measuring above 1 kV or so. I often use a 1:100 probe with 66.67 MOhm input resistance if I want to use a 'scope, or a P6015, with 100 MOhm input impedance at DC. For DC, a Fluke 80K-40 with 1 GOhm does a nice job but is "a bit" big.

Raymond


Robert Simpson
 

The HV meter I am using is a Fluke 27 kit that came with a 6KV HV (80k-6) probe adapter. According to the Fluke website the probe has 1000:1 75Mohm resistance for use with a 10Mohm meter. I didn't see a meter resistance statement but since this probe is one of the options, I would expect the meter to be 10Mohm
Bob


 

On Thu, Aug 12, 2021 at 01:24 AM, Robert Simpson wrote:


The HV meter I am using is a Fluke 27 kit that came with a 6KV HV (80k-6)
probe adapter. According to the Fluke website the probe has 1000:1 75Mohm
resistance for use with a 10Mohm meter. I didn't see a meter resistance
statement but since this probe is one of the options, I would expect the meter
to be 10Mohm
The resistance between the probe tip and the common connection is what counts, apparently 75 MOhm in your setup.

Raymond


Ozan
 

On Wed, Aug 11, 2021 at 04:24 PM, Robert Simpson wrote:

The HV meter I am using is a Fluke 27 kit that came with a 6KV HV (80k-6)
probe adapter. According to the Fluke website the probe has 1000:1 75Mohm
resistance for use with a 10Mohm meter. I didn't see a meter resistance
statement but since this probe is one of the options, I would expect the meter
to be 10Mohm
Bob
------------
Hi Bob,
C4010 (47pF) in series with R4112 (390k) replenishes the charge on C4118 (connected to grid) at 50kHz rate. Extra 75M-ohm load should be fine.

TP93 should be a clipped sine wave clamped at top by CR4111 (~100V) and clamped at the bottom by CR4112 (~ 15V to 95V depending on intensity setting). What do you observe on a scope?
Ozan


 

On Thu, Aug 12, 2021 at 02:12 AM, Ozan wrote:


TP93 should be a clipped sine wave clamped at top by CR4111 (~100V) and
clamped at the bottom by CR4112 (~ 15V to 95V depending on intensity setting).
What do you observe on a scope?
Also, compare the waveshape and voltages at point 91 (TP4217) with the waveform as shown in the diagram. Horizontal settings as under "AC Waveforms" above the image of point 91.

Raymond


 

If both deflection plates are stuck at 100V+, the problem is not likely to be the sweep, rather, the horizontal amplifier itself. A sweep problem would leave one plate high and the other low.
--
Bob Haas


Robert Simpson
 

Strange,
After some testing, the Trace is back on for some reason. Changes: I cleaned the dirt around in the HV area. Also lifted one end of R4124 to measure it and it was fine. Re-soldered with sliver bearing electrical solder. Then I noticed a loose wire in the HV area near the PCB edge where C4016, C4020 and C4025 are connected to ground. One end of the wire is soldered to the common ground of those capacitors. The other end of the wire was connected to the neighboring PCB, the Vertical Pre-amplifier board. I re-soldered the broken connection which is also a ground on the Pre-amplfier board. Since both boards are already grounded, why this extra connection? Possibly ground out stray signals of some sort or maybe prevent a ground loop? And why is the trace now back on?
Just in case i affected some other intermittent connection, I poked around the HV area with a nylon RF tuning screwdriver, and didn't find anything untoward.
Ah well, at least the trace is back on.
Bob


Robert Simpson
 

Back to the sweep problem. See photo at https://groups.io/g/TekScopes/album?id=267142 Tektronix 465B Horizontal Sweep Problem.
This is a photo of test point 71, the A sweep generator. The signal from my 465B does not look like what is from the manual. I superimposed the manual signal on my photo for easier comparison. looks to me like this is part of the horizontal sweep problem. So, More testing needed. If you have any ides, your insights would be appreciated.
Bob