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Tektronix 475


tenareze32@...
 

I have a Tektronix 475 oscilloscope with no display. All voltages test correctly including the -2450V supply. On pressing the beam finder I see a very faint vertical stripe which can be moved from side to side by the horizontal deflection knob. I suspect a fault in the high voltage multiplier. I am wondering if there is a way of testing it without extensive disassembly. I was thinking of sticking a hypodermic needle through the HT insulation and connecting it to a neon bulb through a large resistor. Is this dangerous or is there an easier way?
Many thanks
Simon


tenareze32@...
 

I went ahead anyway and the HT was OK, it lit up a neon bulb through a 22MΩ resistor.
I can only think that the CRT is bad. I saw that the instrument must have been dropped on its rear end as the plastic case was cracked and the aluminum bowl protecting the CRT socket was dented. There was no damage to the socket, but I surmise that a shock like that might have damaged the CRT. I have taken it out, but cannot see anything obvious and it does not rattle. I am wondering if there is anything else I should check before buying another tube.
Simon


tenareze32@...
 

I took a photo of the tube (see album)
Simon


tenareze32@...
 

With the CRT removed, the two neon bulbs between the cathode and first grid still do not light and the potentials are the same on both grid and cathode (-2450V). I am not sure how the beam find works but I thought it re-biased the grid, maybe not. I think I will have to look more carefully at the grid bias circuitry.
Simon


Roger Evans
 

The two neons should not light in normal operation. They are to protect the CRT from damage in fault conditions.

If you have no horizontal trace but can move the faint line horizontally then the X amplifier is basically working but the time base is not running. I assume the mode and trigger controls are set correctly.

If the time base is not running you will not generate the unblanking pulse necessary to brighten up the beam in normal operation.

Regards,

Roger


tenareze32@...
 

Thanks for the information about the neons. I thought they might be for protection. I still do not understand how the beam finder works as it is a DC voltage, which is in specification on the test points, but it does not display any beam or spot. It looks as though the grid bias is obtained from grid leak through a 22 MΩ resistor and another winding on the HT transformer. I have not yet found that part on the circuit. I am still leaning towards the tube being bad.
Regards
Simon


Roger Evans
 

Have you obtained and read the manual. It should explain how the CRT grid supply is obtained by rectifying the output from the high voltage transformer, but the output 'sits on top' of the voltage generated by the Z axis amplifier. The Z axis output varies with the setting of the brightness control and also with a voltage generated by the timebase circuitry which turns the beam on during the sweep and blanks the beam during retrace. You say that when you press the beam finder you see a vertical line, if this is with an input to the Y amplifier then that is normal. If you can move the line horizontally with the X position control then that shows the X deflection voltage is getting to the X plates but the timebase is not running. The trigger mode should be 'auto' to make sure the timebase runs in the absence of a trigger signal.

You have no evidence the CRT is bad. You do appear to have evidence the timebase is not running.

Roger


tenareze32@...
 

I have about -20 V between the first grid and the cathode, so I cannot see why the beam finder does not display at least a spot. I only saw a very faint vertical stripe with no input on the Y amp. An absence of spot or trace generally indicates an HT fault, but all HT voltages are OK.
From your explanation a running time base is required for a display, but I thought the beam finder would override that. I will check the time base circuit with another scope.
Regards
Simon


Harvey White
 

The beam finder generally does two things:

1) unblanks the CRT by changing the bias voltage

2) changes the gain of the deflection circuits so that the maximum extent of any deflection is within the visible area of the CRT.

Some problems happen when the beam finder switch does work partially.  If, on the other hand, one plate is not being driven, I'm not sure that the beam finder would work properly.

Harvey

On 8/9/2020 7:29 AM, tenareze32@... wrote:
I have about -20 V between the first grid and the cathode, so I cannot see why the beam finder does not display at least a spot. I only saw a very faint vertical stripe with no input on the Y amp. An absence of spot or trace generally indicates an HT fault, but all HT voltages are OK.
From your explanation a running time base is required for a display, but I thought the beam finder would override that. I will check the time base circuit with another scope.
Regards
Simon



teamlarryohio
 

Appears to be offscreen vertically. To test, pull the blue and brown wires from the neck of the jug. Don't let them short to ground. It should give you a fat trace near center.
-ls-


greenboxmaven
 

There are a couple things to check. First of all, turn the scope on in a dimly lit or dark room. Turn the intensity up all the way, and turn the focus up and down while watching the screen. If you get a glow or haze of light, most likely on the top, bottom, or either side, the CRT is probably good. The glow is likely coming from electrons bouncing off the side walls of the tube because the beam is being deflected totally off the screen. if the glow fills the entire screen, check the focus voltage. If it is drastically off, or missing entirely, it can cause a weak glow across the entire screen. Turn the intensity back to the middle of it's range, push the trace finder button, and check the voltages on the deflection plates. They should be fairly close, if one of them is drastically different, find out why. In addition to a drastic difference in the voltages on the plates, a faulty connection to a plate pin or one with zero voltage on it can cause the beam to be deflected totally off screen.

Bruce Gentry, KA2IVY

On 8/9/20 9:23 AM, Harvey White wrote:
The beam finder generally does two things:

1) unblanks the CRT by changing the bias voltage

2) changes the gain of the deflection circuits so that the maximum extent of any deflection is within the visible area of the CRT.

Some problems happen when the beam finder switch does work partially. If, on the other hand, one plate is not being driven, I'm not sure that the beam finder would work properly.

Harvey


On 8/9/2020 7:29 AM, tenareze32@... wrote:
I have about -20 V between the first grid and the cathode, so I cannot see why the beam finder does not display at least a spot. I only saw a very faint vertical stripe with no input on the Y amp. An absence of spot or trace generally indicates an HT fault, but all HT voltages are OK.
From your explanation a running time base is required for a display,
but I thought the beam finder would override that. I will check the time base circuit with another scope.
Regards
Simon





tenareze32@...
 

I connected a scope to the X deflection leads (no CRT) and both went to +50V when I pulled the power switch. The beam finder had no effect, and the timebase does not appear to be working. I will investigate this further before replacing the tube.
Many thanks
Simon


tenareze32@...
 

On both the Y deflection leads I measured +10V with no indication of any signal on either channel. Doesn’t look promising as a repair project.
Simon


Harvey White
 

All that means is that the vertical amplifier is dead at some point.  Feed it a vertical signal and go tracing through with another scope.  the "top" and "bottom" halves of the amplifier should be more or less mirror images of each other.  The channels are independent up to the scope channel switch electronics, and then after that switch, run a common amplifier.  I'd put the scope in channel 1 mode, apply a signal (sine waves are nice for showing distortion) and then do a quick trace at points.  If you apply the same signal as is in the manual, you should have scope traces and voltages showing what ought to be there.

Your problem could be as simple as one or two transistors not functioning.

Harvey

On 8/9/2020 11:38 AM, tenareze32@... wrote:
On both the Y deflection leads I measured +10V with no indication of any signal on either channel. Doesn’t look promising as a repair project.
Simon



tenareze32@...
 

Thanks for your encouragement as I was thinking along the same lines. The problem at the moment is the heat and I can only work comfortably in my workshop in the mornings. I was going to see first if the sweep generator is working. Without a display I could not see the extent of the problems.
Simon


Stephen
 

On Sun, Aug 9, 2020 at 05:42 AM, <tenareze32@...> wrote:


Thanks for your encouragement as I was thinking along the same lines. The
problem at the moment is the heat and I can only work comfortably in my
workshop in the mornings.
Over here it’s already 30deg C at 9:00am.
It’s an oven. Impossible to do anything if you don’t have AC.


tenareze32@...
 

Here in SW France it cools a bit at night. We have AC in the house, but not in the workshop. 37°C at 18:00 today.
Simon


Colin Herbert
 

Just a thought: the slowest rate of the time-base might just be slow enough to register on a multi-meter. An analogue one would be best, but a digital one might well give you some idea of the swing. You can investigate this safely from outside the casing by using the "A-Gate" BNC at the rear. OK, it gives a pulse, not a ramp, but if the time-base is running this should show up. It might even be worth using the X/Y mode with two different signals to verify if the X-deflection circuitry is doing what it should be doing.

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of tenareze32@...
Sent: 09 August 2020 17:42
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tektronix 475

Thanks for your encouragement as I was thinking along the same lines. The problem at the moment is the heat and I can only work comfortably in my workshop in the mornings. I was going to see first if the sweep generator is working. Without a display I could not see the extent of the problems.
Simon


 

On Sun, Aug 9, 2020 at 02:45 AM, <tenareze32@...> wrote:


I am wondering if there is a way of testing it without extensive disassembly.

I went ahead anyway and the HT was OK, it lit up a neon bulb through a 22MΩ resistor.
Are you talking about the HV (-2450V) or the HV multiplier (PDA) voltage output?
Checking either (HV and PDA voltage) the way you did it is not a good indication of functionality: Since a neon bulb needs only a few hundred volts to light and less than a mA or so to stay lit, that is not an indicative test.
There are two easy tests for the PDA:
1. Often, with relatively low air humidity (!), a very slight crackling may be heard immediately after switching the 'scope on after it's been off for a while and approaching the screen immediately after switching on sometimes causes the same sound, unless your 'scope has the EMC screen in front of the CRT.
2. Switch the 'scope off and pull the two parts of the high voltage cable apart at the connector in the pink wire. If all is well, this is not without risk of shock; use gloves if you can. Be especially careful to keep the ends away from any electronics or you'll zap them, again, if all is well... Approach the chassis with the protruding metal end of the CRT cable. You should see and hear a discharge with a spark length of at least 3 mm. Be aware that usually, not all charge is gone after the one spark.

Raymond


 

On Sun, Aug 9, 2020 at 01:29 PM, <tenareze32@...> wrote:


An absence of spot or trace generally indicates an HT fault, but all HT
voltages are OK.
Depending on the voltage between grid and cathode, HV and PDA may be ok but still no light visible.

Raymond