Author Topic: Tek 2445A Repair, trace drift, CH1 out of spec, long startup  (Read 15239 times)

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Offline fivefish

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Hi all, I finally bought a Rigol DS1054Z, thanks to Dave's and y'all reviews and recommendations.  Like it very much, some little irritations, but for $375, can't really complain. 

Now I feel it's time to open up my Tek 2445A and fix it up.

The issues on my 2445A are:

1. I noticed it seemed to take longer and longer to "power up" and for all the lights/self-test to go through it's sequence.

2. Upon powering up, I had to let it warm up a bit before trace gets stable and not drift upward or downward.  Even then, sometimes the trace would just drift slowly up from the scope screen.  Adjusting it back to center line is futile as sometimes it will drift in the opposite direction.

3. Ch1 is out of calibration (voltage readings). Ch2 and others seem fine.

I've taken lots of pictures so I'll know how to assemble it back, posting some of them here.

I reckon I should start with the power supply section and replace all electrolytic capacitors with new ones? From the photos, it seems all the caps are still original?

Any Tek technicians here who can point me where to concentrate/look first regarding these problems? Thanks!
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #1 on: June 14, 2015, 12:10:13 am »
photo 1
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #2 on: June 14, 2015, 12:10:37 am »
photo 2
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #3 on: June 14, 2015, 12:11:45 am »
photo 3
 

Offline tautech

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #4 on: June 14, 2015, 12:23:12 am »
I reckon I should start with the power supply section and replace all electrolytic capacitors with new ones? From the photos, it seems all the caps are still original?
Yes PSU must be "spot on" first.
The Tek pdf in the first post in this board is a must for download and study.

Not sure about 2445, there maybe DC voltages listed in the schematics and they can be very useful at times but the scope should be setup a certain way for them to be relevant. RTFM lots.  ;)
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Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #5 on: June 14, 2015, 12:55:23 am »
I reckon I should start with the power supply section and replace all electrolytic capacitors with new ones? From the photos, it seems all the caps are still original?
Yes PSU must be "spot on" first.
The Tek pdf in the first post in this board is a must for download and study.

Just to be clear, that would be "yes" to start with the power supply as tautech suggests.  But NOT "yes" to start replacing capacitors just because they're old.  Let them be for now.

Check the power supply voltages with a DMM, and check that the voltages meet the ripple specs with your Rigol.

On problem #2, do both traces drift?  And if both, by the same amount?

On #3, how far is it out of calibration?  Is it off on several ranges by the same percentage?

No ideas yet on #1.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #6 on: June 14, 2015, 05:26:16 am »
Thanks tautech and MarkL for the reply.

I got hold of a service manual and performed some PSU measurements.

All the measured voltages are spot on (using Fluke 179 meter), and within tolerance per Service Manual.
My reading ----- Service Manual
+10.00V  --- 9.99 to 10.01
+87.00V   --- 85.26 to 88.74
+42.42  --- 41.55 to 43.25
+15.02V  --- 14.775 to 15.225
Digital 4.994V --- 4.85 to 5.15
Analog 4.98V --- 4.925 to 5.075
-4.992V  --- -4.965 to -5.035
-8.01V  --- -7.88 to -8.12
-14.95V --- -14.775 to -15.225



BUT measured Vpp Ripple reading for +/-15V seems marginal, and Digital/Analog +5V are on the high side (see below)
PSU ---- Vpp Service Manual ---- My reading
+15V --- 15mV  ---- ave 15.6, max 18mV
Digital +5V --- 150mV --- ave 207mV, max 220mV
Analog +5V --- 15mV ---- ave 63mV
-15V --- 10mV  --- ave 12mV
-5V --- 15mV ---- ave 14mV

The Analog +5V is really way off. 4X times as much.


Now I'm thinking replacing electrolytic caps on these sections.
For +5VDigital, I'm thinking C1110 and C1111 (both 250uf)
For +5V unreg, the C1115 (250uf)
For +15V, Im thinking C1116 (180uf)
For -15V, the C1113  (180uf)
For -5V, the C1114 (250uf)
Might as well replace C1112 (4.7u)



What do you guys think?

PS: The service manual I'm using is 2445, but my scope is 2445A. It seems the parts placement diagram look slightly different.
Does anyone have an actual 2445A service manual?
« Last Edit: June 14, 2015, 05:31:05 am by fivefish »
 

Offline tautech

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #7 on: June 14, 2015, 06:22:10 am »
As MarkL says wholesale cap replacement is not always necessary. I'd be checking them all first.
In this thread: https://www.eevblog.com/forum/repair/tektronix-468-repairrefurb/ member A2AT saw first hand how increased load on a PSU may bluff you into thinking the main caps might be stuffed, when just an "exceeding design" load can send ripple levels high.

Luckily the 468 manual described how to isolate parts of the scope from the PSU and help diagnose where the excessive PSU load might be. RTFM.
Try this one:
 http://www.schematicsunlimited.com/t/tektronix/tektronix-2465a-service-manual-may-87

So check caps, sure, but it may not be the magic bullet.  ;)
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Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #8 on: June 14, 2015, 08:41:56 pm »
What do you mean by "check caps" ? Visually or ESR meter? -- I don't have an ESR meter, might be a good excuse to get one now.

Question: Can you check ESR of caps in-circuit on the board? Assuming they're not power supply bypass caps, can I do ESR measurement with the caps still soldered in the board?
 

Offline tautech

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #9 on: June 14, 2015, 09:11:52 pm »
Check caps in several ways:
Visual: swolen, leaking
Capacitance:  failing values and IMHO value drift of ~10% downwards means replacement.
ESR:SMPS applications are more demanding on caps, usually low ESR is a prerequesite, while there is great importance on this for design and correct operation (cap life and output ripple), I generally place lesser importance on this for preliminary checks.
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Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #10 on: June 14, 2015, 09:52:15 pm »
...
PS: The service manual I'm using is 2445, but my scope is 2445A. It seems the parts placement diagram look slightly different.
Does anyone have an actual 2445A service manual?
I found this one:

  http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Tek/2445a.pdf

The schematics are a little crunchy.

If you want a top quality scan and OCR'd version of the 2445A user and service set (or almost any old manual), I'd suggest these guys:

  http://artekmanuals.com



 

Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #11 on: June 14, 2015, 10:23:22 pm »
I got hold of a service manual and performed some PSU measurements.

...

What do you guys think?
I agree the power supply voltages look fine, and at this stage of the game I wouldn't worry about the ripple readings.  They are not outrageously high, and it also depends on how you measured them.  It's not the power supply.

Can you describe your problem #2 and #3 in a little more detail?


Also, it seems the service manual link I just posted is missing the example waveforms.  There's also the same version here but with better download speed:

  http://w140.com/mmm/tek-2445a.pdf

But again, it's incomplete.  You should probably get the Artek version.

EDIT: I said above, "I wouldn't worry about the ripple readings." Meaning, I wouldn't worry that they are slightly out.  It's still important that you measured them.
« Last Edit: June 15, 2015, 12:31:21 am by MarkL »
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #12 on: June 15, 2015, 04:16:00 am »
Here's channel 1 trace drifting on it's own, downards.  Nothing connected.
https://www.youtube.com/watch?v=qKYQy2SaXoc&feature=youtu.be

And here's channel 4 doing the same thing... going upwards. Nothing connected.
https://www.youtube.com/watch?v=A5xkPuXpIG4&feature=youtu.be

These videos were taken within minutes of each other.
Ch2 will do the same thing also.
I didn't get the chance to test Ch3, but I think it's doing the same thing also.

These "drifting" will stop/stabilize after several minutes of the scope running.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #13 on: June 15, 2015, 05:28:42 am »
Discovered some other things.

Ch1 - 50ohm overload protection not working,  scope shows 5v @ 1v/div but actual volts input is only 3.2 volts.

Ch2 - overload protection working ok, voltage reading is also good...5 div @ 5v input

The drift problem affects all ch 1 to 4.
 

Offline PA4TIM

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #14 on: June 15, 2015, 07:22:24 am »
Do not buy an ESR meter but buy a decent LCR meter instead. One that can measure at least capacitance (Cs and Cp) and D at 1kHz. It will be better if it can measure at 100/120Hz for very big caps and 10kHz for small caps too. Modern ones often have a 100kHz range and have a Rac mode and that is ESR at 100kHz. If that is usefull ? It can be handy for quick checks on caps in situ. The datasheets of most caps gives ESR at 1kHz (most times in the form of D but 1/(2pifC) gives you the reactance and D times that value is the ESR) At 100kHz they give the impedance and because the reactance is near to nothing for bigger caps you are left with the ESR (but that can be a totally different value at the working frequency. ESR is not a constant and low ESR  is not even defined. http://www.pa4tim.nl/?s=ESR for more about ESR or the ESR site from EEVblog member Krypton.

Measure caps in situ only as a quick check, do not trust it 100%. (compare it to measuring resistors in situ, if a 10k measures 1M it will be bad but if it measures 1 ohm it still can be good, there is just something parallel to it )

In manuals the technical part is often divided in a few parts. most times including something like
- performance checks
- calibration
- explaining how the circuits work

The first one is the thing you have to start with. It will tell you the problems. sometimes people think there is a problem but it turns out to be a user fault. Following the manual is better because they tell you how to set up the scope and test equipment  for each test.
The calibration part often is doing the same tests but do not start adjusting things before you are sure the rest is OK.

Do not swap parts only because you assume or read somewhere they can be bad.  You can make the problems bigger or damage hard to replace parts or the pcb itself. Caps are well known for breaking but 95% of the time these are inside (cheap) consumer electronics. Most of my test equipment still has the original caps. Only Tek exception was a Tek 2710 SA and 2 caps in a 7704A but in its defense, those where the only two not original caps and from some vague brand.

Work systematic, follow the manual. Check the schematics if you are not sure what they want you to do. Read what instruments you need for each step. If you do not have them or something similar do not mess around hoping you can do it another way unless you are 100% sure that you realy know what you are doing.

Follow the steps in order because adjustments often influence other parts of the circuit. So adjusting A can influence the magnitude of B. If you first adjust B it will look like A is bad. And if you then adjust A and forget about B because you think you have done that right you can be in for a surprise because now A and B probably are way off. And some things influence a lot of circuits. For instance a reference  powerrail. Some rails are the "mother" and influence almost everything. If you adjust that just to make it perfect, you can make things overall worse.

So forget all problems until you are sure the PSU is 100% but always keep in mind a powerrail can look bad because of a fault elsewhere. That is why you first have to do the performance tests.

Repairing scopes like this is not easy. You often need lots of specialized test gear and a sound electronics knowledge.
But you can learn a lot from it.


 
www.pa4tim.nl my collection measurement gear and experiments Also lots of info about network analyse
www.schneiderelectronicsrepair.nl  repair of test and calibration equipment
https://www.youtube.com/user/pa4tim my youtube channel
 

Offline tautech

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #15 on: June 15, 2015, 07:38:59 am »
Great post PA4TIM.  :-+
Very good sound advice.
Yes we often assume members have a reasonable basic knowledge but that is not always the case.
Guidance is then required.  ;)
At least fivefish has another scope to help with this repair.
Reading the many posts of repair can give one a lot of knowledge of others problems and how they have managed to overcome them. It is my favorite board.  :popcorn:
Who doesn't like fixing stuff.  :-/O  :-DMM
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Offline PA4TIM

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #16 on: June 15, 2015, 08:47:19 am »
Yep, I like repairing so much, I ended up doing it as a living: http://schneiderelectronicsrepair.nl/
Jim Williams had the same opinion about the educational part of repair. He used it as a training aid while he worked at MIT. He even did some sort of repair contests with students.
www.pa4tim.nl my collection measurement gear and experiments Also lots of info about network analyse
www.schneiderelectronicsrepair.nl  repair of test and calibration equipment
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Offline siggi

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #17 on: June 15, 2015, 10:55:53 am »

Discovered some other things.

Ch1 - 50ohm overload protection not working,  scope shows 5v @ 1v/div but actual volts input is only 3.2 volts.

Ch2 - overload protection working ok, voltage reading is also good...5 div @ 5v input

The drift problem affects all ch 1 to 4.

If your scope has the SMD version of the A5 board, there are some electrolytics on there that tend to leak. When this happens they tend to take out components related to the DAC reference, including the 10k precision, low-tempco resistor that sets the ref current and the calibration trimmer.
This will cause all kinds of trouble in the scope, as most things (like trace position) are controlled with voltages derived from the DAC. The DAC is also used to measure e.g. the 50Ohm overload thermistors, by running them through a comparator against the DAC output. So far, all the symptoms you've listed are compatible with A5 problems. If so, this is good news, as the A5 board is very repairable, and it's all obtainable components...
There are 10V, 1.36V and -1.25V test points on the A5 board that are good to check first, then it gets more involved :).
My 2467 has the throughole version of the A5, and it suffered a shorted clamp zener on one of the reference voltages in the A5 board. This led to no traces, no on-screen display, and the 50Ohm overload sense always tripped.

The PSU ripple you've measured doesn't seem likely to be causing trouble, and I'd leave the PSU until you've sorted the other trouble.
Mind that when you measure the PSU ripple that the front panel is set as specified in the service manual, and that your 20MHz bandwidth limit is on. Otherwise your readings won't be comparable to the service manual specs.

Also you may want to join TekScopes on yahoo. It's a great community where some serious Tek nerds hang out.


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Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #18 on: June 15, 2015, 03:51:32 pm »
On the drift, I'm not convinced yet it's a problem on the A5 board, but it's a good place to start.

siggi has a great hint on the DAC.  I would first take a look at the DAC reference voltage on U2101 between pins 14 and 15 with a DMM.  See if the voltage drifts in relation to the traces moving around.

If that looks stable, take a look at the trace position outputs for the four channels at W511 pins 10, 9, 8 and 7 to see if they're drifting.

This is all on schematic <diamond> 2.


On the calibration problem with Ch1, I would start with the attenuator since Ch2 is working fine.

One way to tackle a bad channel is to feed the same signal into both channels simultaneously and then start comparing the waveforms on the same corresponding test point between the two channels.

A good place to start comparing would be the attenuator outputs at test point 6 (Ch1) and test point 11 (Ch2).  And you should also try different combinations of vertical gain, 50ohm termination on/off, DC/AC input coupling, and a high impedance input (like a x10 probe on each channel).  With all those combinations, you can usually get any problems with contacts or burned out sections of the attenuator to surface.

Schematic <diamond> 4.


Not forgetting about the start-up delay, but I would try to fix the above two first.  The only idea I have for the delay is to run the start-up tests manually and compare the times with someone else who has a 2445A.  Manual control of the tests is described on page 6-9.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #19 on: June 15, 2015, 04:25:09 pm »
Thanks all.  This is my first attempt at scope repair, though I've bought and fixed defective amps and bench lab PSUs.

Upon powering up, I noticed both ch1 and ch4 give the same vertical offsets.  Even after I re-position them to center, turn off the scope, then back on, they always come back to these same positions. 

CH1 offset after power ON.


CH4 offset after power ON


Maybe the above 2 problems are related, maybe not. But I read the theory of operation and checked the schematics, and don't see anything common between ch1 and ch4. (The reference +10V is spot on at 10.000.)

I'll start checking test point waveforms and voltages per Service Manual.
 

Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #20 on: June 15, 2015, 04:52:59 pm »
I would suggest measuring the reference voltage going into the DAC for drift; the DMM across pins 14 and 15.

If you change any settings, like the position, the scope might not store it immediately.  I know the 2465 delays a few seconds before saving any front panel changes.  Not sure if the 2445A does that or not.  Just a thought.
 

Offline siggi

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #21 on: June 15, 2015, 05:34:25 pm »
I would suggest measuring the reference voltage going into the DAC for drift; the DMM across pins 14 and 15.

The voltage across pins 14 and 15 should be as close to zero as makes no difference, and both of those pins ought to be at ground potential. The reason being that those two pins are the inv/non-inv inputs on an op-amp in negative feedback. (I also believed that resistor network to be deriving reference voltages until recently, but that's just not so).
What matters to the DAC is the reference *current* that flows through the 10K resistor from the 10V rail, plus a small amount of trim current that goes through the trimpot. The nominal DAC reference current is 1mA, hence 10V/10K.

The 1.36 and -1.25 voltages are derived thorugh the DAC, so they ought to reflect trouble in the DACs reference.
 

Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #22 on: June 15, 2015, 06:31:06 pm »
Ah, you're right.  I should have looked at the AM6102 datasheet - thanks.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #23 on: June 16, 2015, 02:26:21 am »
I read the service manual and it says to enter Diagnostics, do the following:

Press and hold ?V and ?t simultaneously
Press Trigger Slope while holding down ?V and ?t
The display should now show:
DIAGNSTIC. PUSH A/B TRIG TO EXIT

On my scope, following these steps do nothing. I don't hear any relay clicking, and no change in the display.

DC calibration works though, as per Service Manual but I can't get it to go into Diagnostics mode using the above steps.

Am I missing something?
 

Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #24 on: June 16, 2015, 03:24:59 am »
That's strange.  Do the delta-V, delta-t, and SLOPE +/- switches work normally individually?

Another way to get into diag mode is to hold down any non-latching button while the scope is booting.  It will think it has a stuck switch and enter diag mode automatically.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #25 on: June 16, 2015, 04:05:42 am »
That's strange.  Do the delta-V, delta-t, and SLOPE +/- switches work normally individually?

Yes, they work fine in normal operation.

Quote
Another way to get into diag mode is to hold down any non-latching button while the scope is booting.  It will think it has a stuck switch and enter diag mode automatically.

No go. It just displays TEST xx FAIL xx message (thinking a key is bad or stuck?)  It doesn't present me the menu option (according to service manual) I should get.


Another thing, I don't have a AM6012 DAC chip, and the layout of my control board almost looks like the diagram in the Service Manual, but different enough. The chip #s designations are different. I'm going bonkers here trying to correlate what the service manual says and what the actual PCB looks in front of me.

Heres some photo.  It says J651 and J652 for the left ribbon connectors, and that's what I have on my board too... so I know I'm looking at the right Control Board. But my IC is U2101 vs. U2134, U2201 vs U2234, etc.... every thing is different from the service manual. The service manual is 2445, while my unit is 2445A. I didn't know the difference is that huge, different part# designations, drastically different board layout.





I want to bang my head to the wall, because I'm studying the Service Manual, and then look at my board and "Where the heck is that chip? Put your meter on test point XXX, and I don't have test point XXX on my board." (maybe it's called differently on mine) . Very frustrating.
 

Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #26 on: June 16, 2015, 01:28:15 pm »
 

Offline siggi

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #27 on: June 16, 2015, 01:31:18 pm »
The service manual is 2445, while my unit is 2445A.

The 2445 and the 2445A are a generation apart. See <https://en.wikipedia.org/wiki/Tektronix_analog_oscilloscopes> - there's three years between them, and you really do need the right service manual.
IMHO it's well worth the money to buy a good quality scan of the service manual. All the Artek <http://artekmanuals.com/manuals/tektronix-manuals/> manuals I've bought have uniformly been of great quality, and they're OCR processed, so you can do (some) text searching in them. Artek has the 2445A - (the "A" is critical :), and has bundle pricing for all the manuals (no relation except as a satisfied customer).

On a tangent, one thing that can cause wonky deflection ratios is oxidized contacts in the attenuators.
If you set the channels to DC coupling, you can measure the input impedance with your DMM. Then scrub the channel through the different attenuation ratios (there are 2 10X attenuators in each attenuator hybrid, so you want to hear two relay clicks), and see whether the input impedance stays at 1MOhm. If you see readings like 1.8MOhm or open, it's a good bet that you have a "mechanical" issue with the attenuators - most likely oxidation.
 

Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #28 on: June 16, 2015, 01:48:33 pm »
"TEST xx FAIL xx" is what it should do, but then it should dump you into the diag monitor.

As far as not getting the "DIAGNSTIC. PUSH A/B TRIG TO EXIT" display, perhaps it's trying to display it but it's positioned off the screen.

None of your screen shots have the top status line on them either, which may be more evidence of a problem with positioning or the readout logic.  Try the beam finder to see if there's anything outside the edges of the screen.


Maybe there's a setting to disable diagnostic mode, but I'm not aware of it.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #29 on: June 16, 2015, 01:50:14 pm »
Thanks all. I didn't know about the non-A and A difference.

I bought the Artek manual, now just waiting for the emailed links.
 

Offline PA4TIM

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #30 on: June 16, 2015, 01:52:51 pm »
I bought a Tek 2710 manual from Artec and that was well wordth the money. I had a hard to read pdf from an other subversion of the 2710.  Daves copy was a very high quality and included the updates/changes/faults info. That made repair a whole lot more easy . I will not try to repair a scope without the right documentation.( And I'm "used" to repairing instruments without documentation)

Only use the right manual. The A is not sort of subversion, it is a complete different scope.

www.pa4tim.nl my collection measurement gear and experiments Also lots of info about network analyse
www.schneiderelectronicsrepair.nl  repair of test and calibration equipment
https://www.youtube.com/user/pa4tim my youtube channel
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #31 on: June 16, 2015, 04:21:21 pm »
On an earlier post, I mentioned I can't get my scope to go into Diagnostics mode (pressing delta V, delta T, slope).

Upon closer look, it seems my SLOPE button isn't working?  I don't think I've ever used this button during normal mucking around.

I shot a video of it, can someone confirm?  The LED lights are not switching from (+) to (-).
https://www.youtube.com/watch?v=BQNo6bC5MXc&feature=youtu.be
 

Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #32 on: June 16, 2015, 04:28:54 pm »
Yep, that's broken.
 

Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #33 on: June 16, 2015, 04:37:51 pm »
It would be worthwhile to play with all the buttons and switches to see what else isn't working.  Could be something in common.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #34 on: June 16, 2015, 04:40:08 pm »
Thanks for the confirmation Mark.

UPDATE: Confirmed... the Push-Button switch for SLOPE is broke, tested it with a DMM.  Gotta find a replacement. 

Unfortunately, also broke one knob in half while in the process of removing them... didn't see there was a hex set screw. Bought a replacement on eBay.
« Last Edit: June 16, 2015, 08:15:22 pm by fivefish »
 

Offline siggi

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #35 on: June 16, 2015, 10:54:16 pm »
Thanks for the confirmation Mark.

UPDATE: Confirmed... the Push-Button switch for SLOPE is broke, tested it with a DMM.  Gotta find a replacement. 

If you have some (70-99%) isopropyl alchohol - or better some DeOxit - it's worth giving the button a squirt and a bit of a workout. Sometimes this is just contact oxidation and you can bring them back. It sure beats waiting for a replacement :).
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #36 on: June 16, 2015, 11:01:35 pm »
Thanks for the tip siggi... worth giving it a shot.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #37 on: June 17, 2015, 01:21:26 am »
Alcohol cleaning didn't work. I think it's a mechanical problem.

I don't have an exact Tek push button switch, but I do have a low-profile, short travel push button switch from my parts collection. The legs do not align, the Tek switch has a wider pitch... so bent the legs of my switch slightly, to go into the original holes. Made sure the switch is level and soldered it.



Next problem, the push button caps don't work with my switch. It doesn't have the right "profile" for the switch cap to snap in. So I had to break the legs of the switch caps so the switch caps will sit directly on top of plunger, and be able to actuate the switch.

Screwed together the front panel switch and pot board, making sure I can still hear/feel the clicking of all the switches while I align the switch caps with the panel. At this point, feeling good that the replacement switch  is working.

Installed front panel assembly back to the scope, connected the 2 ribbon cables and powered it up.... pressed the SLOPE switch and now it works, and changes from (+) to (-) and vice versa.




Now, the real goal.... go into Diagnostics mode. Press Delta-V, Delta-T, and Slope.... YES! Scope goes into Diagnostics mode.  Did the All Test, and everything passes test. 



Now I can continue following the rest of the service manual Calibration procedures.  :)

Since all TESTS ran successfully, I'm thinking maybe the scope just badly needs calibration?
So that's my next to do item.
« Last Edit: June 17, 2015, 01:23:01 am by fivefish »
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #38 on: June 17, 2015, 02:05:00 am »
The 2.82V marking you see on the scope is the actual Vpp of that sine waveform. I double-checked and verified that Vpp value with my Rigol scope.

As you can see, there is a problem around the BNC jack of Channel 1 causing it to display bigger/taller waveforms.... thus making it appear that it's more than 2.82Volts.   

Frequency value is correct though.

https://www.youtube.com/watch?v=-KFMzFOb9XU&feature=youtu.be
 

Offline siggi

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #39 on: June 17, 2015, 12:30:29 pm »
As you can see, there is a problem around the BNC jack of Channel 1 causing it to display bigger/taller waveforms.... thus making it appear that it's more than 2.82Volts.   

That's ... interesting ... Looks to be a mechanical problem of some sort. I replaced a worn-out BNC on my 2467, which caused intermittent connections. The amplitude would tend to drop from nominal, though, not increase.
It's easier to get at the attenuators than it looks like at first glance.
Remove the front panel bezel and a metal bar running across CH1/2, then the various screws fastening them. There are a couple of screws you get at from below the mainboard, inside the hollow left by the front panel assembly. You need to de-solder a couple of connections, and then they just pull out - they're socketed.

From your list of symptoms, it seems to me the channel 1/4 trace drift is still unexplained. This isn't something that'd be affected by calibration. Also I think you'll find that the calibration procedure is a PITA to get through unless you have the equipment specified, as the software routines are somewhat intimate with the specified calibration generators.
 

Offline MarkL

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #40 on: June 17, 2015, 01:16:32 pm »
From your list of symptoms, it seems to me the channel 1/4 trace drift is still unexplained. This isn't something that'd be affected by calibration. Also I think you'll find that the calibration procedure is a PITA to get through unless you have the equipment specified, as the software routines are somewhat intimate with the specified calibration generators.
Plus, I wouldn't attempt the calibration without fixing or at least explaining the drift problem.  If the problem is an unstable offset, you might not even be able to get through the calibration.

 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #41 on: June 17, 2015, 01:41:52 pm »
I looked more deeply in the calibration steps and I think it will be impossible for me to do.
I don't have a time mark generator. I only have a signal generator.

I think the un-centered trace *offsets* I see upon powering up is from old/non-valid cal data anymore stored in the EPROM? Because it seems consistent, though different for ch 1 to 3 traces. Only ch4 is perfectly centered upon power up. After I manually  centered the traces, the Volts peak to peak and frequency readings are correct (except for that CH1 flakiness)... the trace just starts uncentered from power-up. -- I can live with that.

And even if I do have a time-mark generator, I do need to fix that Ch1 BNC problem first. For now, this is the most serious problem. And my next to-do list.  Thanks siggi for that disassembly instructions.

Re: Trace drifting up or down.... I think this is another serious problem than the power-up uncentered offsets.  I admit I haven't seen this problem again in the last few days. Maybe because the unit has been left powered more often and for longer periods of time? As I said earlier, this trace drift disappears when the unit gets warm and running for maybe half an hour. Still something I'd like to troubleshoot.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #42 on: June 17, 2015, 04:52:09 pm »
I'm going through the TEST/EXER/CAL menu (thanks to my now working Slope button) and I cleared all the saved settings in the scope. I think these setups were there originally since I bought the scope almost 6 years ago. 

After clearing everything in memory, I adjusted all the 4 channels to be centered and in my preferred initial time/volt settings I want, and SAVED that new setup to #1. Then going to EXER 06, made the scope recall setup #1 during power up. 

Now everything is centered during scope power on! 

https://www.youtube.com/watch?v=giZzZTKgn88&feature=youtu.be

Bottonline: I think the scope was just recalling a bad stored setup during power up. Problem fixed.  Two down, two more to go.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #43 on: June 18, 2015, 02:44:03 am »
HA! Found the problem why my Ch1 is out of spec. It looks like the problem is mechanically related, specifically in the attenuator section.

So earlier I noticed if I wiggle my BNC hard, I can make the Ch1 reading come out correct. Also noticed that if I bumped the unit, it will cause disturbance in the Ch1 waveform reading. Hmmmmm.... a clue....

I repeated the experiment on Ch2 and this vibration problem doesn't happen with Ch2, no matter how hard I thump/bang on the unit. But on Ch1, I can see slight mechanical vibrations affect the signal I'm seeing. It will go from showing the correct Vpp reading to something out of whack.

The attenuator section have (4) relays inside them. And it looks like my problem is one of these mechanical relays not working properly.... stuck closed or open in it's position. Thumping and whacking the unit, sometimes dislodges that relay and then I get a correct reading.

This may also explain why my earlier 50-ohm overload test didn't work for Ch1.  The relay connected to that 50-ohm resistor didn't move, and at this point, possibly already burned up that internal 50-ohm resistor too.





Here's a video showing my "hammer test" method :). 500mVpp is the correct reading for that waveform, but thumping the attenuator with the palm of my hand, or my finger, would cause it to give wrong readings (super high). Then I had to whack it again to make it show the correct reading.

https://www.youtube.com/watch?v=cJdmQHiXiFs&feature=youtu.be

I can buy these attenuators from eBay for $35 to $45 apiece. And this should fix my Ch1 out of specs problem.

So now, the only problem left is the trace drift...
 

Offline tautech

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #44 on: June 18, 2015, 03:58:13 am »
i've seen other Tek scopes where the BNC solder joints have cracked from physical stresses.
Check for this first.

I'm not sure if 2445 BNC's solder directly to the PCB, others will know.  ;)
Avid Rabid Hobbyist
 

Offline siggi

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #45 on: June 19, 2015, 10:56:46 pm »
HA! Found the problem why my Ch1 is out of spec. It looks like the problem is mechanically related, specifically in the attenuator section.

So earlier I noticed if I wiggle my BNC hard, I can make the Ch1 reading come out correct. Also noticed that if I bumped the unit, it will cause disturbance in the Ch1 waveform reading. Hmmmmm.... a clue....

The attenuators are quite repairable - I cracked them open for cleaning on my 2430 and replaced the BNCs on my 2467.
I posted some pictures of the attenuator guts from my 2430 in that repair thread. See e.g. https://www.eevblog.com/forum/repair/attempting-to-repair-a-tektronix-2430/msg431422/#msg431422. The 2400-series all uses the same attenuators AFAIK.

However, I can't think of a fault in the attenuators that'd lead to larger readings than nominal, except perhaps if the substrate under the 1M film resistor was cracked, or maybe if the attenuator grounding was compromised.

So now, the only problem left is the trace drift...

Can you reproduce the drift?

This might be nothing more than contact oxidation on one of the hybrids. My 2430 had all kinds of glitching initially, but with some cleaning and subsequent use it all leveled out.

If you can reproduce, you can measure your way to a diagnosis for sure, or you can just release and re-seat the hybrids - perhaps cleaning the contacts while you have them out with a bit of IPA. The latter only takes 10 minutes.
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #46 on: June 20, 2015, 12:05:53 am »
siggi, thanks for the reply.

Right now, there's a couple of "related problems" with Channel 1. So my hunch is it's the relays in the attenuator that's bad, or stuck?

I'm thinking if it's just a bad trace in the BNC path, then the signal would either be there or not.  But in this case, the signal goes from "correct" reading to "going ridiculously large"... i.e. 500mVpp becomes a little bit above 2.5Vpp shown by the scope.  Tap and whack the attenuator, and the reading going back to what it should be, in my example, 500mVpp.

Also, the 50-ohm overload test as outlined in the Service Manual doesn't work as expected with my scope. I don't hear any relays clicking. So my hunch is there's a problem with the relays.... and maybe even possibly burned out the 50-ohm resistor.


Can you reproduce the drift?

No not yet... want to fix Ch1 first before I tackle this.

My observation on the drift are: if unit is left powered on for long periods of time, and I turn off/on the unit, it doesn't exhibit drift problem. (Maybe because unit is still warm? or all caps still charged? )  The drift happens *usuallly* the first few minutes the scope is powered on (after being unpowered for some time).

Also... sometimes the scope takes a long time to turn on (i.e. displaying a flat trace). More likely to happen if the scope hasn't been powered for a long time. So first time you turn it on, it either takes a long time before it shows a trace on screen, or it "hangs up" and never actually show a trace on screen.  Turning scope off then on again, fixes this "slow bootup" time. The second power on, everything starts fast and trace shows up on screen. Maybe there's some bad/failing caps in the PSU (or some other board) I'm thinking.

Also, I bought a temperature sensor gun (the one with the laser pointer) and pointing it at the various caps, on the PSU low-voltage board shows 2 caps that seems to be reading a couple of degrees higher than their neighboring capacitors of the same kind/size. One of these caps also had some slight residue on the top. It's not bulging, but you can tell some electrolyte did leak out ever slightly..
 

Offline tautech

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #47 on: June 20, 2015, 01:09:04 am »
Of course Ch1 gets most of the work on any scope and most of the abuse too, this is what I guess is your problem, the input has been overlaoded at some point. Who knows what a previous owner did to it.  :scared:

So that should point to the dividers, fets etc, sure an attenuator module should sort it, but then where's the fun in fix gone?  :-//
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Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #48 on: June 28, 2015, 03:38:41 am »
SUCCESS!  Cost me $22+shipping (cost of Channel 1 attenuator module) to fix my scope!



Replaced Channel1 attenuator.  And scope is now working properly, giving good and correct readings for Channel 1.



After replacing the attenuator and powering up, I got a TEST 04 FAIL 10. -- oh oh, what did I screw up?
Reading the manual, it seems I need to re-calibrate the scope... or replace the EAROM.



Problem is I don't have a pulse generator, so I cannot calibrate this scope properly.
So what I did is I just performed CAL01, and just continued to press NEXT (up Coupling button) without
adjusting anything.  Exit, power off, power on and the FAIL message is gone.

I then checked if voltage and frequency readings via the cursors are correct and they are. Fine by me!


But I need to buy some replacement knobs that I broke.
Also need to find the cause of that Channel 2 drift problem. It seems only Channel 2 is drifting from my close observation.
That will be for another day.
« Last Edit: June 28, 2015, 03:46:21 am by fivefish »
 

Offline fivefish

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Re: Tek 2445A Repair, trace drift, CH1 out of spec, long startup
« Reply #49 on: August 08, 2015, 08:20:16 pm »
100% fixed.

The problem with the "long startup" and trace drift (and also volts/time cursor drift) is related to the Power Supply capacitors.

I figure I have nothing to lose so I decided to replace the PSU caps.

That's when I discovered several 180uf/40V, and 250uf/20V caps were leaking electrolyte from the bottom. The tops weren't bulging, so it wasn't obvious. When I removed them, that's when I discovered the leak. Replaced them all with 330uf/50V caps 105C.

Also replaced other caps like the 100uf/25V, and 47uf/25v, 100uf/100V... I already did all the hard work of disassembling and removing the PSU boards so I might as well replace them.

Now the scope is stable, no more long startup times, and the traces don't drift upwards or downwards. Everything is now stable.

Photo showing all the caps that have been replaced in the HV PSU and Regulator board sections.
« Last Edit: August 08, 2015, 08:34:06 pm by fivefish »
 


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