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Tek 465: CH1 No signal, CH2 signal reads a fraction of input, V Pos distorts signal


Greenpitu@...
 

Hello everyone,

my name is Luis. I recently acquired a Tek 465 to complement my ham radio workbench. The scope wouldn't present a trace at first. Upon inspection, fuse F1419 was open, and a replacement was blowing straight away due to C1419 and C1416 tantalum capacitors.

Power rails looked alright and within acceptable thresholds:
* TP1558: +109
* TP1536: +55.01
* TP1548: +14.99
* TP1558: +5.09
* TP1568: -7.98

Both capacitors were replaced and, finally, a clear and focused trace came alive! That was good.

However, looks like fella is still demanding some TLC. Here some additional observations when connecting 10x probe to CH1 (or 2) on one side, and the onboard 300mV 1KHz calibrator on the other:

ON CHANEL 1
* Trace signal stays flat no matter what VOLTS/DIV setting. There appears to be no signal amplification at all
* Switching to different VOLTS/DIV settings makes the signal jump 2 or 3 sub-divisions in either direction i.e. around 1/2 division

ON CHANEL 2
* Trigger works fine and trace shows a clear square signal
* Horizontal sweeps at 0.5 and 1ms read 1KHz on display
* When V Pos is centred, the display reads 100mv only ie. 1/3 of the input signal
* When the V Pos control is moved all the way up to the top (or bottom), then the signal distorts and expands from 100 to 150mV i.e 1/2 of the total input

I also noticed:
* When on CH1, both lights behind VOLTS/DIV skirt are always off, no matter if probe set to 1x or 10x attenuation
* When on CH2, just the 1x light behind VOLTS/DIV skirt is always ON, no matter whether I set the probe to 1x or 10x attenuation
* When reading +55 and +110 power rails with DMM, on both CH1 and CH2 the trace flickers momentarily and the trigger light comes on - is this normal?

I appreciate I have a lot of questions (and problems!), but maybe someone here had similar problems. I would really love to get this 465 working completely. Any thoughts as to how to proceed next finding the source of the problems for CH2 is much appreciated!

Best regards
Luis


Greenpitu@...
 

Just to clarify, this observation happened with no probe connected at all to CH1 / CH2:

* When reading +55 and +110 power rails with DMM, on both CH1 and CH2 the trace flickers momentarily and the trigger light comes on - is this normal?


Chuck Harris
 

Depending on your DVM, the input may have significant
capacitance between the + and - probe. The input circuitry
on most is at least 10M, which will keep the capacitance
discharged until you connect to the power supply.

Because the 55V and 110V supplies are stiff, and pretty
low impedance, there is an inrush of current through the
DMM leads to charge the input capacitance, and this forms
a resonant circuit with the test leads, and will make a
small burst of RF.

The scope's trigger is quite good at high frequencies, and
the scope's front end circuitry is very sensitive, so it
can see the broadcast RF from connecting the probes and
trigger.

It is normal.

-Chuck Harris

Greenpitu@... wrote:

Just to clarify, this observation happened with no probe connected at all to CH1 / CH2:

* When reading +55 and +110 power rails with DMM, on both CH1 and CH2 the trace flickers momentarily and the trigger light comes on - is this normal?




Roger Evans
 

I don't have a 465 so this is hearsay, but I do have a 475 which has some broad similarities. You channel 1 problems sound like an issue with dirty contacts on the attenuator (V/div) and or the input coupling(AC/GND/DC) switches. The standard way to clean the attenuator switches with their cam operated contacts attached to the PCB is to move the switch to a position where the contact is open, slide into the contact gap a strip of paper soaked in IPA, rotate the switch to close the contact, remove the paper strip, repeat ... Search here for some advice on which types of paper are least likely to damage the gold flashing on the contacts. I believe physical access to all the attenuator contacts on a 465 is also a matter of some difficulty. Since the beam moves vertically for some positions it means the dirty contacts are likely to be close to the 'front' end of the attenuator nearest the BNC input and later segments of the attenuator are working mostly OK.

The channel 2 problems could also be poor contacts which sometimes are not 'ohmic' but beyond that they could be tricky to track down without a second scope to check where the distortion comes in. It is worth checking what happens when you pull the 'Invert' button and also what happens with 'Alt' and 'Add' selected on the vertical mode switch.

At this stage, I wouldn't worry greatly about the X1 and X10 lights, the circuitry shouldn't affect the signal integrity. You can at least check around the sensing and switching transistors with a DMM since it all DC/static operation.

Regards,

Roger


Greenpitu@...
 

Hello Roger,

I am glad to inform you that I have made good progress after following your advice with CH1. Thanks so much!

I am still, however, facing some challenges. Armed with ISO and strips of paper, I followed the instructions to clean-up all possible levers I could possibly reach. The task is cumbersome but (strangely) fulfiling when you pull the paper out and feel that bit of resistance against the gold plated contacts. In addition, I pulled out the 10X and 100X attenuators from CH1 attenuator boards, and cleaned all their pins, before plugging them back onto the board (few times).

I then turned the power back on, but CH1 was still deaf. In addition, I now realized CH2 was deaf, too! I wasn't expecting this. Got me thinking for a while as I was certain I didn't touch CH2 circuit at all. I left CH1 for a minute and focused back on CH2. I was upset I would be losing that one, even though we know it wasn't 100% working either when it broke.

In any case, after some pulling and pushing and whatnot, I realized that, if I pushed CH2 BNC adapter - where the probe meets the female connector - just at the right angle with the right pressure, then the 1/3 of my input signal came back to life again. There was hope! That was a lucky and easy one. It appears that C53 and R53 soldered in series with the BNC pin have a soft spot. Pushing the connection around a bit got the beam showing the calibrators 1KHz signal once again. I sense I need to solder the contacts once again, but I park that one for now. Still, back to 1/3 of the input signal.

I went back to CH1 and pushed around the equivalent C3 and R3 in series with CH1 BNC pin, in the hope it would get the channel back to life. And it did, but temperamentally so. The beam showed up as a heavily under-compensated signal on the screen. I noticed the problem was more noticeable on the .2mV position when a particular cam-operated contact was closed. I cleaned that contact once again with more ISO and voila.. stable square signal on CH1!

Now we got to this point where both, CH1 and CH2, show the 300mV 1KHz square signal, but just 100mV i.e 1/3 of its amplitude! Plus, the bean on both channels now equally distorts and expands from 100mV to 150mV when the V Pos controls are moved all the way up (or down)... there is consistency!

Any thoughts about what else to try next, please? Does it appear as if the signal is not being amplified as it should?

Regarding ADD, ALT and INVERT, all three appear to be in working order: I hooked two probes to the onboard calibrator, and then one each on CH1 and CH2. The ADD adds them, the ALT alternates between the two (albeit per V Pos above they distort greatly when moving off centre) and the ADD + INVERT on CH2 substracts both signals, leaving me with a nice, flat, focused beam at the centre of the screen.....

I am afraid, I won't be able to get hold of a second oscilloscope at least within the next 3 or 4 weeks :(

Any further suggestions much appreciated!

Best
Luis


Greenpitu@...
 

Thanks Chuck for the detailed explanation!
Much appreciated.

Regards
Luis


Roger Evans
 

Luis,

Since both channels are equally affected the fault must be either in the beam switching circuitry (not very likely) or in the Y amplifier stages following the beam switching.

There is a fairly simple test on the beam switching that you can do with just a DMM. If you locate test points TP364 and TP 374 on schematic <3> and the example waveform shown in <21> and <22>, you should be able to replicate the DC values ( +1V and -1V) by setting the horizontal mode to ALT and the timebase to its slowest setting so the voltage flips every half second or so. Set both traces to somewhere near the middle of the screen and trigger to AUTO. This is not a definitive test but if these voltages are wrong the diodes may not switch correctly.

I only just now noticed you have Trig View option on the 465. Try feeding the calibrator signal to Trig In, set Trig Source to Ext or Ext/10. Push the trig view and you can move the trace up and down with the Trig Level control. Does the vertical height of the displayed trigger signal change with position in the same way as do the Ch1 and Ch2 signals?

If Q322 and Q324 are socketed then remove and test them. The Service manual schematic <3> shows various voltages around Q322 and Q324, set up the scope as best you can to the conditions defined in the manual at the start of the Schematics section under the heading 'DC Voltages'. See how well your measurements match the figures in the manual.

Farther down the Y amplifier chain there was a change from discrete transistors to an IC based Y amplifier. Which one do you have?

Regards,

Roger


Greenpitu@...
 

Hi Roger,

I have the IC based Y amplifier.

I will proceed today with your instructions.

Thanks a lot!
Luis


Greenpitu@...
 

Hello Roger,

I am back with some measurements.

** Vertical Switching **
Both measurements taken in ALT mode, no probe attached, and lowest TIME/DIV setting.

TP364 (-2V and -0.102V)
TP374 (-2V and -0.071V)

** Trig View **
With calibrator signal feeding Trig In, Trig Source set to Ext

Trigger Level Centered: when the beam is at the centre of the screen, the beam width is just under 1x sub-division
Trigger Level +++: when the beam is at the highest pos, that is +2.5 divisions above centre, the beam is just over 2 sub-divisions in width
Trigger Level ---: when the beam is at the lowest pos, that is -1.0 divisions below centre, the beam is also just over 2 sub-divisions in width

When at the center, there is a fuzzy glimpse of the square signal. However, when on +++ or --- positions then beam becomes entirely flat.

** Q322 and Q324 **
Using tester got the following values:

Q322: NPN, Gain: 75, Vf: 707mV
Q323: NPN, Gain: 71, Vf: 701mV

I should say I got (I think) bad news. I started hearing concerning cracking sounds for the first time while I was running the sweep at its slowest speed. At first, I heard like 5-7 in rapid succession. I powered off the scope and waited a little while. The scope cover was removed at the time, so I didn't get too close to the circuitry just in case. These sounds weren't too friendly. I increased the sweep speed before I turned it back on. Unfortunately, the crackling sound still persist - it doesn't seem to be related to the speed at all. In fact, it seems to be coming from the front of the cathode ray tube, but I can't be certain.

I made a short video where I captured 2 or 3 of the crackling sounds. My phone was capping out the percussive effect somewhat; the real sound is higher in pitch and volume.

https://youtu.be/DgHwnqH9xJs

Is this a concern?


Roger Evans
 

A crackling sound from the front of the CRT is not good. If you take the scope into a dark room with the cover off you may see some flashes from the region where the high voltage lead attaches near the front of the CRT. In any case you need to clean that area with IPA, but wait for maybe 30 minutes after the scope is powered off to allow the EHT to discharge.

Can you do some measurements on TP322 and TP324? With the scope displaying Ch1, input selector to ground, and the trace on the vertical centre line, these two test points should measure close to zero volts. If not can you adjust the vertical position to get TP322 and TP324 to read the same value, and then where is the beam? Also the Ch1 vertical position control should be somewhere near the mid point of its travel.

If these tests come out OK then the fault is most likely in the vertical amplifier not being biassed correctly. The manual I have doesn't give any voltage readings around U440 but you could measure the resistances of R446, R447 and R448 and the voltages on each end of R446 and R447.

The bias on the output transistors is adjustable with R455 but don't move it yet as it affects the vertical calibration and frequency response. Measure the voltages on each end of R457 and R452 to make sure that the transistors are drawing some base current and the voltage across R454 to ensure that the variable resistor R455 has not gone open circuit.

It is also worth removing U440, cleaning the pins and reinserting. If the mounting allows, I would apply some IPA around the pin/socket area and leave for an hour or so to penetrate before trying to remove the IC - they can be very stubborn. Check that there is not a nut on the other side of the PCB holding it in!

At this point I am running well beyond my competence and if anyone else would like to add anything I would be very grateful.

Regards,

Roger


Greenpitu@...
 

Hello Roger - thanks once again!

Went into a dark room and spotted the spark straight away. It's coming from the top right corner of the CRT. I can see the spark lighting through a very tiny opening that is available in the chassis.

Please see short video with the exact location: https://youtu.be/7CxqL82_3VI

Now, I have no idea how to get into that spot... Any thoughts?

I will get on with the other measurements and revert back. I think I can carefully and safely work my way through these checks.

Best regards
Luis


 

You need to get the service manual - it tells you how to remove the CRT.

The problem is likely the silicone sealant where the EHT is connected has failed.

You'll need to clean off the old and re-seal. Please don't use an acetic acid cure silicone - get a neutral cure type.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Greenpitu@...
Sent: 20 August 2020 12:40
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tek 465: CH1 No signal, CH2 signal reads a fraction of input, V Pos distorts signal

Hello Roger - thanks once again!

Went into a dark room and spotted the spark straight away. It's coming from the top right corner of the CRT. I can see the spark lighting through a very tiny opening that is available in the chassis.

Please see short video with the exact location: https://youtu.be/7CxqL82_3VI

Now, I have no idea how to get into that spot... Any thoughts?

I will get on with the other measurements and revert back. I think I can carefully and safely work my way through these checks.

Best regards
Luis


tenareze32@...
 

You will need to pull the CRT forward to reveal the rubber connection to the tube and seal appropriately. I don’t think it is an immediate worry, but should be fixed in due course. You might try inserting some plastic film through the gap between the top right corner of the CRT and the frame.
I am not sure if the CRT on the 465 is the same as the 475, but removing the tube is not difficult although fiddly.
Look on the left side and you may see four wires going to pins on the tube. These are for the Y plates. If the same as the 475 , the X plate pins are on the bottom of the tube and you must desolder the wires on the X amplifier that go underneath the CRT (one red, one green). Solder at least a 1 ft length of pull through wire to these joined wires so that you can pull the wires back when you need to put the tube back, otherwise it is impossible. The socket at the end of the tube can be prized off fairly easily using symmetrical leverage. You can push the tube forward from the rear and then ease it out from the front when you have some edges to grip. If you don’t need to get the tube out far, there is probably enough slack in the HT lead, if not you will have to pull the HT lead out from the socket at the bottom of the tube enclosure. It will come out with some wiggling. I suggest a little dielectric grease on it when putting it back. It will make it easier next time!
Putting the connectors back on the Y plate pins is fiddly and you must avoid bending these pins so use light force under a good light (Maglite, penlight).
Simon


Greenpitu@...
 

Hello Roger,

here the additional measurements.

There are some non-zero voltages detected in TP322 and TP324 but not sure if close enough to zero e.g. -80mV
Seems like U440 transistors are indeed drawing some base currents.

*** Measurements on TP322 and TP324 ***
On CH1, no probe, input selector to ground, volts/div set to 0.1V

A) When the beam is centred on screen…

T Point Voltage V Pos Travel
TP322 -80 mV 10 o'clock
TP324 20 mV 10 o'clock

B) When V Pos is at the centre of its travel…

T Point Voltage Beam position
TP322 3.1 mV +3 sub-divisions
TP324 -80 mV +4 sub-divisions

C) When TPs have closest values…

T Point Voltage V Pos Travel Beam position
TP322 -33 mV 11 o'clock +2 sub-dvisions
TP324 -31 mV 11 o'clock +2 sub-dvisions

*** Voltage and resistance readings around U440 ***
On CH1, no probe, input selector to AC and volts/div set to 0.1V

T Point R (Tek) R (Actual) V1 V2
R446 52.3 53.5 -1.32 V -0.82 V
R447 52.3 53.1 -1.32 V -0.77 V
R448 334 334.2 n/a n/a
R457 200 199.9 14.96 V 14.66 V
R452 100 100.7 9.24 V 9.23 V
R454 499 430 11.5 V 9.23 V

For R457, the 14.66 reading is from the side facing U440 pin 13 (14.66/199.9=74.8mA)
For R452, the 9.24 reading is from the side facing U440 pin10 (9.23/100.7=91.7mA)
For R454, the 11.5 reading is from the side facing variable resistor R455

Best regards


Greenpitu@...
 

Thanks David for your message and recommendations!

I now found a place where to source neutral cure type silicon from.

Best regards


Greenpitu@...
 

Thank you, Simon! Much appreciated.

Trying to get my head around this one: "Solder at least a 1 ft length of pull through wire to these joined wires so that you can pull the wires back when you need to put the tube back, otherwise it is impossible." What do you mean by pull through wire sorry. Is that one of those twisted steel cables with a coating on top or similar?

Best regards


tenareze32@...
 

Any kind of hook-up wire will do provided it is fairly flexible. String would do if you make a good knot on the red and green wires. I don’t know if the 465 CRT has X deflection pins on the bottom of the tube, but you can trace the wires from the X amplifier to see where they go.
Simon


Greenpitu@...
 

Understood. Thanks!


tenareze32@...
 

On looking at a picture of a 465 CRT I see it is different from that of the 475. The Y deflection wires are on the left side and look difficult to remove and refit as the delay line is in the way. There are also wires coming through the top left of the metal screen through a grommet. You might have to feed them through the screen to extract the CRT. I think you will need better instructions than mine. It looks as though you have access to the HT lead boot on the CRT without removing the tube.
Simon


Greenpitu@...
 

In addition, I had another look at U440 this evening, to try and remove the IC and clean its leads etc. It seems to me in this instance the U440 leads are actually soldered to the PCB board. Could that be? I cleaned around the pins nevertheless with a soft brush soaked in IPA.

Unfortunately, I accidentally broke the small cable that connects the output board to one of the CRT vertical deflection plates pins via LR471. I was being very careful, so that connection must have been hanging by its teeth, by a very wobbly and frail cable strand it would seem. The cable connecting the other deflection plate via LR461 doesn't inspire much confidence either to be honest.

I made a short video showing U440 leads and the broken deflection plate connection.

https://youtu.be/-JlGm6hQLU4

As shown on the video, two pins protrude from the side of the CRT. On closer inspection, both pins appear to have an internal cavity through which the cable was inserted, and then either clamped or soldered maybe.

Trying to figure out how to repair this one! I tried pulling the pin, hoping it is pluggable itself, but no luck (I didn't try too hard just in case). Does anyone know how I should proceed to fix this one? Can I simply solder the cable to the pin itself? Or do I need to get it through the pin somehow? The pin itself seems to be hollow on the inside.

Regards