Discord time, EV related traffic is calming down, I can chime in now !
Been busy yesterday and today working on the 2467B PSU recap, been skimming TEA.
@Spec : will have a look at that video. Congrats on the miniscope... now that you have cables to split the boards, troubleshooting the second issue with the vertical amp should not take long...
@ TerraOperative : didn't know TI made their own special IS socket, will need to look into that, see how they are made, what's wrong with them...
So the recap was a chore, a roller coaster, did not go smoothly at all. Cap replacements not being 100% identical required adaptation and "oh well it's gonna stay like that no choice", mindset.
Putting new caps in was tedious and time wasting because I could not clean the holes before inserting them, because no fancy power desoldering tool here, only solder wick which did not work as the holes were usually too small.
Soldering did not go well either. My old Weller Magnastat is only, which did not cut it on many occasions. Every time a lead was connected to the ground plane, could barely melt the joint, double-sided at that. Damaged or ripped pads, lifted tracks, followed. Repairs had to be done.
The non-polarized 1uF 50V cap, redial, tiny, was replaced because no better choice, yet still mega expensive, by an axial 1uF 100V not 50V. I thought OK no worries I can just mount it vertically... but no, because somehow the cap turned out to be TEN times bigger than the old one !
Mounting it vertically would have been like erecting the empire state building on that board, just no way, if just for the risk of them interfering with something when I would put the PSU and scope back together. So I had to mount them (2 of them) horizontally...
Then I put the PSU and scope back together. Somehow it went pretty well. I mean I 50% remembered how to put it back together. Then for 30% of it I improvised successfully, and for the remaining 20% I was rescued by the pics I took when I took it apart. Always take pics when you take stuff apart....
At first I bitched at the golden "combs" pins that connect the two board back to back.. how to align them !
... then I realized you can remove the combs, there is a plastic clips for that, then you screw the two boards together, then you can just easily insert the combs back into place...
Then I went to replace the couple electrolytic caps on the CRT board.
Power up time... no explosions or smoke.. good !
Then I checked voltages, (quick before the ASICs start getting hot), following the service manual. First the 10V reference rail. It's very tight, tolerance only +/- 10mV ! i.e. 0.1%
So that's the 0.1% case where my Fluke 11 daily driver DMM and its 0.9% just won't cut it...
So had an excuse to use a Metrix MX56C instead, at 0.025% it was about good enough for the job. Just.
Cold, right after power up, it measures at 9.995V, which was already well within spec. It was climbing slowly. After 30s or a minute, say, it settled at... 10.000V ! That's one good scope, 30 year old and doesn't even need to touch up the reference rail !
Then I checked all the other rails, which were all wellll within spec.
I was happy with myself at this point... and allowed myself to have a look at the CRT to see how the scope was doing. Almost had a heart attack.. I got what you see below : a very, very sick puppy !
Nothing but a super fat blurry flat line, with a few big bright spots embedded in it.
First thought was to cry and shift it on Ebay for parts to try to get back some of the 300 Euros I paid for it almost 2 years ago.
Then I recovered my senses and tried to see what I could do. Tried diagnosing it.
1) When starting, I got the expected LED and relay dance, so looked like the CPU board was fine.
2) Could not focus the trace, so something wrong for sure on the CRT side of things.
3) Playing with the intensity controls, I noticed that I could make the bright blobs come and go using the readout intensity control.
4) vertical position control is unresponsive.
So... I thought hmm...maybe the problem is 100% CRT related, and the vertical deflection is toast. The bright blobs are probably just the various readout elements on the scree, being squished.
So I looked at the connector in the CRT area and found a lone wire with a Dupont connector at the end, lose, next to that small ceramic board mounted on the CRT neck. Reconnected it.. problem fixed, scope back to life ! Phew that was close !!
So then I could, for the first time since I bought that scope ages ago, give it a more in depth test ride.
Found some big issues....
1) Vertical amps for Ch1 and Ch2 have huge offsets ! I measured them all, see below. At worst, offset makes the trace disappear off the screen !
I centered the trace on the 5V / div caliber, then cranked the attenuators and noted the offset.
Calibre (V) | Ch1 (Div)| Ch2 (Div)
-------------------------------------
5 | OK | OK
2 | -0.1 | -0.75
1 | -0.2 | -2
0.5 | OK | OK
0.2 | -0.1 | -0.75
0.1 | -0.2 | -2
50m | OK | OK
20m | -0.1 | -0.75
10m | -0.2 | -2
5m | -0.9 | OFF screen
2m | -2.5 | OFF screenLooking at the table summarizing everything, there is clearly a pattern here. Apart from the most trow sensitive settings, 2 and 5mV, whic are much worse than the other, all the other settings follow a repeatable pattern / sequence.
Looks like the attenuator is made of two section in series. One that handles the decades, and another that does the 1/2/5 steps thing.
Looks like the decade section is working fine, but the 1/25 is not. Two fo the steps generate an offset, whic get "copy/pasted" as is, in every decade.
Offset is constant no matter the decade. Ch2 has worse offset levels than Ch1, but does follow the exact same pattern.
Hmmm.... that's suspicious now I think of it ! The problem must therefore lie in some circuitry common to both channels...
I will check the service manual to see if there are adjustments I can make. If not, well I guess I have dead amplifier ASICs, great.
1) Time base problem :Now for the second problem... I have not spent considerable time looking into this one, but enough time to witness it and confirm it... there is definitely something wrong here...
It happened when I briefly played with the second/delayed time base. I would work the big sweep speed knob, the CPU/readout would change/update the sweep speed accordingly, but... the trace on the screen would NOT budge ! It would not ACTUALLY modify the sweep speed ! .. but if I keep cranking the knob, it eventually does so. Weird.
Moving back to the main time base now, I didn't experience this problem.
sub-problem : once I was in Time base B mode, after a minute, the signal trace suddenly becomes extremely unstable, showing huge amount of horizontal "jitter", like, a few minor divisions. Scary.
Again, problem disappeared as soon as I switched back to the main time base.
So again, maybe another dead ASIC, great....
As for the already known, minor issues :
A) dead statu SLED : it's not dead, my recollection was wrong. The problem is simply that it's dimmer than its neighbours. But it does light up. Good news is, it's the status LED for enabling Ch2.
That means, the CPU will need to control this LED individually, hence unlike 5% of the LEDs on the front panel, this one will NOT share it's series limiting resistor with other LEDs. So, I can play with the resistor to adjust brightness when I replace the LED, to try and match it with the other LEDs.
B) The readout on the screen is still very "noisy" and a bit giggling, unstable. however the signal trace is perfectly clean and stable.
So it's not a PSU or CRT problem, I would think. Since the readout is generated on the CPU board, maybe that board needs a recap as well... there are quite a few in there, including dipped tantalums...
However since my soldering iron is way underpowered to handle this kind of board, and I don't have desoldering gun... I will NOT touch this board until I have better gear !!!!
I guess the readout signal path also traverse the main analog board, so maybe the noise and instability are introduced there instead. This board does have electrolytic caps as well.
C) Backup battery. I looked at how much space was available between the CPU board and the scope cover. LOTS to my surprise ! Eyeballing it, a replacement battery can be well over twice the thickness of the OEM " flat " shaped cell, yet still clear the cover. So my 14.5mm axial leaded " half AA " Varta Battery will fit juuuust fine, zero worries there, good !
So that's it... if I do have 2 dead ASIC then I guess the scope is good for scrap anyway, but for now I will leave it at that. Will replace the battery and recap the CPU board in a few months when I hopefully can afford some better soldering and desoldering gear, and take it from there. Maybe by then I will have collected information about these issues and be able to tackle them... if there is hope.
So to sum it up, recap went well, but scope might still be scrap later this year