[Tek 485] No intensity control


Ozan
 

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.
What I wrote in previous message still applies. +1V means one of two things: If you see a dot on the screen that means beam is already consuming 5uA so the circuit is decreasing intensity control (so you can't exceed 5uA). If there is no dot on the screen R1536 could be bad but my understanding is you see a dot. Either "GRID BIAS" setting is wrong or DC restorer is not working.

Ozan


Ozan
 

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.
What I wrote in previous message still applies. +1V means one of two things:
If you see a dot on the screen that means beam is already consuming 5uA so the
circuit is decreasing intensity control (so you can't exceed 5uA). If there is
no dot on the screen R1536 could be bad but my understanding is you see a dot.
Either "GRID BIAS" setting is wrong or DC restorer is not working.
By the way, do you see a very bright dot in XY mode or is it showing normal brightness?

Ozan


marcosjl31@...
 

Ozan

On DC RESTORER side : I changed all the diodes that are in the High Voltage zone (below the plastic shield) on the Horiz Amp Board as 3 of them were dead, and there have been some repair attempt on 3 other ones in the same DC Restorer area. This modification give me back some Intensity control at least for Time Div <20ms. For 50ms, .5s, .2s and .1s as well as the X-Y mode I do not even get a trace ! (and that's new).

I do not see any dot when in X-Y mode or in lowest Time/div positions => This seems to point out something wrong elsewhere.

R1660 modifies voltage at CR1660 cathode from ~+79V to +120V. Currently I set arbitrary 105V reading by adjusting R1660 => In normal mode it gives me a normal range of intensity control.

Jose


Ozan
 

On DC RESTORER side : I changed all the diodes that are in the High Voltage
Please see Tom's message for another DC restorer issue, these diodes need to have fast reverse recovery time in case you used a different diode than the original.

for Time Div <20ms. For 50ms, .5s, .2s and .1s as well as the X-Y mode I do
not even get a trace ! (and that's new).
Is it new meaning it happened last few days, or new meaning that new for this scope from the beginning?

I do not see any dot when in X-Y mode or in lowest Time/div positions => This
seems to point out something wrong elsewhere.
Are you still seeing ~ -61mV as before at collector of Q1548 with no dot on the screen? With no dot on the screen base of Q1544 should be closer to 6V. If base is still at ~1V please check R1536 for drift to >2.2M, CR1539 for leaky diode, Q1544 for low hFE.

Ozan


marcosjl31@...
 

On Wed, Mar 3, 2021 at 07:00 AM, Ozan wrote:

On DC RESTORER side : I changed all the diodes that are in the High Voltage
Please see Tom's message for another DC restorer issue, these diodes need to
have fast reverse recovery time in case you used a different diode than the
original.
I used - as someone (Mark ?) suggested - 1N4937.

for Time Div <20ms. For 50ms, .5s, .2s and .1s as well as the X-Y mode I do
not even get a trace ! (and that's new).
Is it new meaning it happened last few days, or new meaning that new for this
scope from the beginning?
It happened a few days ago.

I do not see any dot when in X-Y mode or in lowest Time/div positions =>
This
seems to point out something wrong elsewhere.
Are you still seeing ~ -61mV as before at collector of Q1548 with no dot on
the screen? With no dot on the screen base of Q1544 should be closer to 6V. If
base is still at ~1V please check R1536 for drift to >2.2M, CR1539 for leaky
diode, Q1544 for low hFE.
Will check later on this afternoon and keep you posted. I guess it makes no sense to check R1536 in circuit ?

Jose


Ozan
 


I used - as someone (Mark ?) suggested - 1N4937.
Looks OK. I will let DC restorer experts to chime in.


Are you still seeing ~ -61mV as before at collector of Q1548 with no dot on
the screen? With no dot on the screen base of Q1544 should be closer to 6V.
If
base is still at ~1V please check R1536 for drift to >2.2M, CR1539 for leaky
diode, Q1544 for low hFE.
Will check later on this afternoon and keep you posted. I guess it makes no
sense to check R1536 in circuit ?

Jose
This check is only needed if you are seeing ~1V instead of ~6V at the base of Q1544 with no beam. You need to lift one leg of R1536 to measure, 2.2M will be swamped out with any other connection.

With no beam R1536 should be able to pull base of Q1544 but it only supplies few uA so a small leakage at base node is all it takes to turn off the beam. It could be any of the components at base node of Q1544 (even Q1544 itself).

Outside XY or slow timebase modes R1534 is providing extra current.

Ozan


marcosjl31@...
 

On Wed, Mar 3, 2021 at 07:00 AM, Ozan wrote:
Are you still seeing ~ -61mV as before at collector of Q1548 with no dot on
the screen? With no dot on the screen base of Q1544 should be closer to 6V. If
base is still at ~1V please check R1536 for drift to >2.2M, CR1539 for leaky
diode, Q1544 for low hFE.
I see ~42mV at collector of Q1548, no dot on screen in XY Mode.
Base of Q1544 is at ~1V, HFE measured with a TC1 tester : 214
CR1539 is ok : ~.6V and 0L
R1536 measured in circuit gives : 300kohm and 100kohm (when inverting the test leads of DMM).


Ozan
 

I see ~42mV at collector of Q1548, no dot on screen in XY Mode.
Base of Q1544 is at ~1V, HFE measured with a TC1 tester : 214
Look OK.

CR1539 is ok : ~.6V and 0L
Looks OK.

R1536 measured in circuit gives : 300kohm and 100kohm (when inverting the test
leads of DMM).
This is a very large resistor, you won't be able to get a good read without removing a leg. I recommend checking C1537 first, see below.

I just noticed C1537 is an electrolytic cap although its value is small. It could have developed a small leakage. If you remove it do you see any difference?
Ozan


marcosjl31@...
 

Ozan,

Remove C1537. It measures ,27uF with an ESR of 9.1 ohms. Scope does not power up if the cap is removed.

Checked R1536. It was above specs : 2.8Mohms (ref is 2.2 10%). Replaced it.

But still no trace in X-Y mode or 50ms and slower time div.

Jose


Ozan
 

Hi Jose,

Remove C1537. It measures ,27uF with an ESR of 9.1 ohms. Scope does not power
up if the cap is removed.
We are looking for few uA of leakage, you can think of this undesired leakage as about 1M-ohm resistor in parallel with the component. This value is so high compared to the reactance of the capacitor at the measurement frequency that ESR won't show leakage.

Checked R1536. It was above specs : 2.8Mohms (ref is 2.2 10%). Replaced it.
This should help in the right direction.

From your previous message:
CR1539 is ok : ~.6V and 0L
Same comment applies. Diode mode of most DVMs would not show a problem with few uA of leakage. If you want to measure leakage you need to apply reverse bias and measure with an ammeter that can measure <1uA.


But still no trace in X-Y mode or 50ms and slower time div.
I still think one of those components (CR1539, CR1534, C1537, Q1544) is leaky. ESR test and diode tests were not accurate enough to identify. As a last resort you can replace them all. You can start with diodes and cap, if it doesn't fix anything, replace transistor. Cross references are in:
https://w140.com/tek_common_design_parts_catalog_transistors_diodes_and_misc_may_1988.pdf
Note that the diodes have very low reverse leakage spec so a random diode won't work.

If replacing the components doesn't fix the issue probably one of the diodes inside U1600 (HV multiplier at the top left of sheet <13>) is leaky and not providing the right "LIM" signal. I don't have any recommendations for fixing the HV multiplier section, it is not something I have done before.

Ozan


marcosjl31@...
 

On Sat, Mar 6, 2021 at 07:48 PM, Ozan wrote:
I still think one of those components (CR1539, CR1534, C1537, Q1544) is leaky.
ESR test and diode tests were not accurate enough to identify. As a last
resort you can replace them all. You can start with diodes and cap, if it
doesn't fix anything, replace transistor. Cross references are in:
https://w140.com/tek_common_design_parts_catalog_transistors_diodes_and_misc_may_1988.pdf
Note that the diodes have very low reverse leakage spec so a random diode
won't work.
Thanx Ozan,
Will replace those components - I need to make some purchase so probably back on the air after a couple of weeks...

Jose


marcosjl31@...
 

One question on how to properly use the Cross Ref document...

I'm looking for Q1544 replacement, in tek485 service manual, tek ref is :
Q1544 151-0341-00 TRANS ISTOR : SILICON , NPN 07263 S040065

If i search '151-0341-00' in https://w140.com/tek_common_design_parts_catalog_transistors_diodes_and_misc_may_1988.pdf , I identify the replacement part to be a 2N3565 transistor.

Is this correct ?

Jose


Ozan
 

If i search '151-0341-00' in
https://w140.com/tek_common_design_parts_catalog_transistors_diodes_and_misc_may_1988.pdf
, I identify the replacement part to be a 2N3565 transistor.

Is this correct ?
Yes, I also see 2N3565 as the cross reference. This transistor may be difficult to find, you can start replacing diodes and the capacitor first. If it still doesn't fix the issue, and you can't find 2N3565 page 14 of the same manual gives the parameters of this transistor for searching an equivalent. Here you need a transistor with high hFE at low currents.
Ozan


SCMenasian
 

Actually, I see 2N3565s for sale on line at several places, including Jameco. However, they may be new "improved"
parts with different specs.

MPS6520 looks like an excellent replacement. Min hfe is better. Many other specs are better. MPS6521 is even better. These should be widely available. Only max emitter-base breakdown looks worse.


marcosjl31@...
 

I'm waiing a delivery from Mouser including 2N3565 transistors, 1N4152 diodes and capacitors.

Will get back when I'll receive it and change CR1539, CR1534, C1537, Q1544.

Jose