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[Tek 485] No intensity control


marcosjl31@...
 

Hi *,

I start this new thread on the new issue seen after fixing the PSU issue from now ended topic: https://groups.io/g/TekScopes/topic/79486087

;o)

Hands back on the 485and starting from some Service Manual reading! This one gives some hints on troubleshooting some issues (p.4-8) including "No intensity control / Poor focus control".

I checked all the DC restorer diodes (list from the description in p. 3-13) with my DMM Diode test function..

List is: CR1646/1651/1652/1656/1660/1663/1664/1666

It turns that CR1646 / CR1666 / CR1651 look open (0L) on both directions.

Probably the first thing to do is replace this 3 diodes. Service manual I have, points that these are NDP341. But trying to see the ref, I seem to read 282 marking on them (but not sure at all about that !!).

What would be the replacement diode ? CR1652, CR1656 and CR1660 were already changed to BAV21. So i guess this ref will do ? Will order some.

Are there some more checks I can do in the meantime ?

Jose

PS. My scope as zome TI IC marked "M 7519". This lead me to think the scope is from 75/76...


marcosjl31@...
 

Ok. As Mark Vincent suggested, I'll replace them with 1N4937.

One question : Mark sugested to check R1603-9 carbon resistors... I guess it is pointless to try to check them without desoldering one lead : right ?

Jose


 

Jose,

from the schematic for the CRT circuit <13> it looks like you could measure the value of R1603-R1609 by either lifting only one leg of either R1603 or R1604, or by lifting one leg of both R1605/R1606 and R1608/R1609. If you could easily disconnect the DD connections then there would be no need to lift any legs of any resistors.

-- Jeff Dutky


marcosjl31@...
 

On Fri, Jan 29, 2021 at 05:15 AM, <marcosjl31@gmail.com> wrote:

Probably the first thing to do is replace this 3 diodes.
Hi,
I was wondering if prevently replacing 15uF/20V tantalum caps as well on the Horizontal Amplifier Board could be a good idea ?

As per R. Dickinson list, It's 5 caps :
Orig inal Repla ce W/
Vc Vc On Schematic Board
C1099 15 20 15 50 20 R B Trig Gen Hor Amp
C1130 15 20 15 50 20 R Pwr Distribution Hor Amp
C1131 15 20 15 50 20 R Pwr Distribution Hor Amp
C1139 15 20 15 50 20 R Pwr Distribution Hor Amp
C1199 15 20 15 50 20 A Pwr Distribution Hor Amp

Second question : I have seen (EEVBLOG 485 Repair&Restoration thread) that some people changed the tantalum by electorlytic caps... Is this a good alternative (as tantalum are quite expensive)?

Thanx!
José


marcosjl31@...
 

On Tue, Feb 2, 2021 at 02:29 AM, <marcosjl31@gmail.com> wrote:
Second question : I have seen (EEVBLOG 485 Repair&Restoration thread) that
some people changed the tantalum by electorlytic caps... Is this a good
alternative (as tantalum are quite expensive)?
It surely depends on what board you're changing them : it is certainly possible for some caps on the PSU board... but for the Horiz Board?

José


marcosjl31@...
 

Hi guys,

some news... Replacing all the DC Restorer diodes as nicely solved the Poor Focus / No intensity control...

I'm happy with this first step in resurrecting the 485... But still many issues to deal with...

On the list :
=> Channel 1 :
1) x1/x10/x100 leds are not working
2) no 1MOhm/50Ohm light
3) No proper display of 1kHz/1MHz cal signal ?!? (that's new ! It worked well before I started the repair of the DC Restorer diodes... :( )

=> Chanel 2
1) no 1MOhm/50Ohm light
2) Variable volts/div not working (when the control is pushed away : no variation of voltage amplitude ?)

I mainly tried the CH1/CH2 with 1Mohm input selected... Not played with the 50 Ohm input yet.

A lot to go !
Jose


Tom Lee
 

Hi Jose,

Great progress!

The 1M/50 ohm indicator light is a tiny incandescent bulb that is a pain in the culo to replace. I still have several dead ones that I have left to some future date (which may never arrive) to replace. You have to remove the attenuator module entirely -- the bulb cannot replaced without that step. And to avoid ever having to do that again, replace with an LED (I've used surface-mount white ones, as well as some ancient orange-yellow ones in a rectangular package; the latter are a better color match to the original, but I prefer the white, actually).

The variable attenuation problem is likely to be simply a dirty switch. They often come back to life with repeated activation. The cal signal problem may also be a dirty switch (it's not clear if you were talking about the indicator light or the cal signal, but a switch problem can affect both).

So I think you are very close to a complete fix!

-- Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/24/2021 07:09, marcosjl31@gmail.com wrote:
Hi guys,

some news... Replacing all the DC Restorer diodes as nicely solved the Poor Focus / No intensity control...

I'm happy with this first step in resurrecting the 485... But still many issues to deal with...

On the list :
=> Channel 1 :
1) x1/x10/x100 leds are not working
2) no 1MOhm/50Ohm light
3) No proper display of 1kHz/1MHz cal signal ?!? (that's new ! It worked well before I started the repair of the DC Restorer diodes... :( )

=> Chanel 2
1) no 1MOhm/50Ohm light
2) Variable volts/div not working (when the control is pushed away : no variation of voltage amplitude ?)

I mainly tried the CH1/CH2 with 1Mohm input selected... Not played with the 50 Ohm input yet.

A lot to go !
Jose




marcosjl31@...
 

Ok guys

This thing is driving me nuts !
Today I power up the 485 and again the intensity of the beam is too high... I quickly checked the DC Restorer diodes : all ok.
I tried again the procedure - step 10 Adjust Grid bias - of manual page 5-10, and I have some question on that procedure :

-> Scope is in x-y mode
-> When both intensity and B intensity control are Conter ClockWise, I read something like 8.21V at Z OUT DC test point.
BUT !
no matter how much I turn the intensity control, the voltage read at Z OUT DC point does not change ?
I can actually get a voltage modification only if I put vert mode to CH1 (displaying grounded horizontal trace) but nothing happens when scope is in X-Y mode.

In X-Y mode the only way I manage to adjust somehow the intensity is adjusting R1660.

Am i doing something wrong here ?
Jose


Alain Mionnet
 

José,

I see that some big improvements....
For Calibrator, in the file 485733461.pdf given here page 163 ( "partial A10 sweep board)*, check that all power voltage with a filter are well here...
+50v, +15v ,-5v have a serial resistor who may have been blowed accidentaly.

Alain
France
*485 OSCILLOSCOPE REV. 6, AUG.1975 II93 CALIBRATOR. AND FAN CIRCUIT

I got a TEK P6015 20Kv probe.(dryed of oïl mode) 1000 ATN 100 MégOhms
Better a P6015A... but new 3000/4000 euro....
It may be find HV probe for TV CRT but this should be rare now....


Ozan
 

-> Scope is in x-y mode
-> When both intensity and B intensity control are Conter ClockWise, I read
something like 8.21V at Z OUT DC test point.
BUT !
no matter how much I turn the intensity control, the voltage read at Z OUT DC
point does not change ?
Could you confirm intensity on the screen is not changing as well in XY mode?

I can actually get a voltage modification only if I put vert mode to CH1
(displaying grounded horizontal trace) but nothing happens when scope is in
X-Y mode.
On 485 in X-Y mode, intensity does change the intensity as expected. It could be a logic fault on sheet <12> but it could be because of a dirty switch as well. If you exercise all of the six "VERT MODE" and all of the four "HORIZ MODE" switches few times does anything change?

Next step is to confirm ~ 1V at anode of CR1561 and > 1V at pin U1560 in XY mode.

If these are OK, next step is to measure all the pin voltages of U1560 in XY mode. Inside of U1560 is at:
https://w140.com/tekwiki/wiki/155-0012-00
We can try to figure out the state of U1560 in the faulty condition.
Ozan


marcosjl31@...
 

Hi Ozan

Intensity control works except in X-Y mode. In X-Y mode the spot remains displayed with the same intensity no mattter what is the intensity knob position.

Exercising the VERT MODE / HORIZ MODE switches doesn't improve X-Y mode intensity modification.

I set both Intensity & B Intensity knobs to 9 o'clock position.

At CR1561 anode, I get 1.12V
Output (p8) of U1560 is : 14.1V
Note that changing intensity knob position does not affect the voltage reading.

I also measured all the pins Voltage (all in V):

(1) .250 (9) 0
(2) 1.251 (10) 4.18
(3) GND (11) 15.02 - VCC
(4) .448 (12) 5.022
(5) 0 (13) 4.99 - VCC
(6) .832 (14) .097
(7) 1.055 (15) .905
(8) 14.11 (16) .246

Jose


marcosjl31@...
 

This is somehow what is expected if I read correctly the service manual : schematic 13 indicates that output of U1560 should measure 14.1V at base of Q1742.

As part of trying to find if some components had failed nearby test point Z OUT DC, I checked Q1772 and Q1752, as well as 1N4152 diodes - CR11775/1776/1777 -

The latter puzzled me as when using diode test function of my DMM, I measured ~0.6V in forward bias, and 1.55V in reverse. But as i'm doing the measurement on circuit, it may be normal.

Jose


Ozan
 

This is somehow what is expected if I read correctly the service manual :
schematic 13 indicates that output of U1560 should measure 14.1V at base of
Q1742.
Amplifier built around Q1742 is what is called a "transimpedance" amplifier. It tries to keep the voltage at that node ~ 14.1V. Input signal to this stage (output of U1560) is current rather than voltage. The amplifier drives "Z OUT DC" node such that current coming back through R1762 (and constant current from R1761) cancel the current coming out of U1560. The voltage at that node should be pretty much same at any intensity setting.

As part of trying to find if some components had failed nearby test point Z
OUT DC, I checked Q1772 and Q1752, as well as 1N4152 diodes -
CR11775/1776/1777 -
My guess is schematic in sheet <13> is OK. You are able to control the intensity in normal sweep.

Let me look at the voltages you measured and reply.

Ozan


Ozan
 

One more question: Does B intensity work? Set "HORIZ DISPLAY" to B, now "B Intensity" knob (below Intensity knob) should change waveform intensity.

Ozan



This is somehow what is expected if I read correctly the service manual :
schematic 13 indicates that output of U1560 should measure 14.1V at base of
Q1742.
Amplifier built around Q1742 is what is called a "transimpedance" amplifier.
It tries to keep the voltage at that node ~ 14.1V. Input signal to this stage
(output of U1560) is current rather than voltage. The amplifier drives "Z OUT
DC" node such that current coming back through R1762 (and constant current
from R1761) cancel the current coming out of U1560. The voltage at that node
should be pretty much same at any intensity setting.

As part of trying to find if some components had failed nearby test point Z
OUT DC, I checked Q1772 and Q1752, as well as 1N4152 diodes -
CR11775/1776/1777 -
My guess is schematic in sheet <13> is OK. You are able to control the
intensity in normal sweep.

Let me look at the voltages you measured and reply.

Ozan


marcosjl31@...
 

On Mon, Mar 1, 2021 at 08:33 AM, Ozan wrote:

One more question: Does B intensity work? Set "HORIZ DISPLAY" to B, now "B
Intensity" knob (below Intensity knob) should change waveform intensity.

Ozan
Yes. Changing HORIZ DISPLAY to B allow intensity control through B intensity knob.


Ozan
 

U1560 voltages look normal, I will look at it in more detail but there is a possibility the circuit around Q1546 and Q1548 is not behaving. These blocks provide the reference voltage for the intensity pot. In XY mode cathode of CR1536 is pulled to ground to change intensity max, this is the same connection in 0.1/0.2/0.5 second (slowest timebase settings).

Can you control intensity in 0.5s time base?

What is the voltage at the collector of Q1548 in normal mode and XY mode. This node is the same node as pin 1 of connector P1570 if it is easier. My 485 is not open so I don't remember how easy it is to access.

Ozan


On Mon, Mar 1, 2021 at 08:33 AM, Ozan wrote:

One more question: Does B intensity work? Set "HORIZ DISPLAY" to B, now "B
Intensity" knob (below Intensity knob) should change waveform intensity.

Ozan
Yes. Changing HORIZ DISPLAY to B allow intensity control through B intensity
knob.


marcosjl31@...
 

Ozan

I definitely cannot control intensity below 20ms time base.
Voltage at collector of Q1548 is -15.01V in Normal mode and -61mV in X-Y mode

Jose


marcosjl31@...
 

I meant above (sorry !) : 20ms is the max time base I can control intensity; above it does not work.


marcosjl31@...
 

Ok as per description in page 3-14 (Beam current limit), I was expecting to measure something like +6V but I only get +1V.

When in Normal mode, I measure ~+6V at base of Q1544.


Ozan
 

Hi José,

I definitely cannot control intensity below 20ms time base.
Voltage at collector of Q1548 is -15.01V in Normal mode and -61mV in X-Y mode
I have a guess of what is happening: The circuit around Q1544/Q1546/Q1548 limit beam current to ~ 5uA in XY mode and in slower than 20ms sweep time, otherwise limit is ~ 25uA. In your case it looks like even with minimum intensity beam current is at ~ 5uA. Then intensity control doesn't work because it can't go above 5uA. However, in other modes limit is 25uA so intensity control can make a change.

This could simply be an issue with wrong "GRID BIAS" setting. Please confirm whether you see a dot in XY mode at minimum intensity and you can't turn off even with "GRID BIAS". If you can't turn off the dot the issue is in DC restorer of grid bias. If you are able to turn off the dot with "GRID BIAS" you should gain control of the intensity and collector of Q1548 go to ~ -15V.

To debug grid bias (if needed): When you set "GRID BIAS" adjustment to max (schematic doesn't say which direction) do you see +120V at cathode of CR1660? With +120V at cathode of CR1660 and +8V at "Z OUT DC", grid bias DC restorer should put enough negative grid bias (~ -100V) to turn off the beam. If you have a high voltage probe you should measure grid voltage (top of DS1669) ~ 100V more negative than the cathode voltage (bottom of DS1668) with max "GRID BIAS" setting and min intensity.

I read that you debugged an issue with grid bias DC restorer. There could still be an issue remaining.
Ozan