Attempting to repair a Tektronix 2430

[siggi]

The attenuators are in a bad way though. They both click with likely sounds at likely times, but channel 1 seems to be frozen in one configuration. The nominally 400mV P2P calbrator signal is reading 1.4V P2P on the 500mV/div vertical setting - I can't figure out what configuration of the relays would cause that?

It turns out this is what happens when you lose the ground connection to one of the 10X attenuators. Apparently the resistance of the attenuator will then fall in-line with your signal path and attenuate the signal somewhat against the downstream (1M?) resistor. The lower half of the attenuator's resistive divider will however float, instead of going to ground.
So, for anyone reading this thread, if you get really wonky attenuation levels, this may be your problem. You should also be able to measure a change in the scope's input impedance with an Ohm meter when the wonked attenuator switches. Sadly I didn't have the presence of mind to try that.

[David Hess]

The same switch which connects the 50 ohm termination also disconnects the AC coupled path so only the relatively high value precharge resistor connects the vertical input to the first attenuator stage.

Ah, that's what that resistor on the schematic is for. Thanks, somehow I couldn't put that together myself .

So as I read it, the upshot is that without the 50Ohm  resistor, the GND coupling puts the AC decouping cap and the precharge resistor in-line with the 10X attenuators.
A fair amount of signal will evidently bleed through. This messes up my self-cal randomly if I leave a signal on the inputs during calibration.

With the 50Ohm resistor in the picture, it makes the bottom half of a voltage divider against the decoupling cap and the precharge resistor (aka effective short, I assume?). I get no visible calibrator signal bleeding through, and my self-cal succeeds even with a signal on the input.

Yes.  The voltage division is so high that the input is effectively disconnected.  Many earlier Tektronix input coupling circuit designs operate in a similar manner.  Note that the parasitic capacitance of the relays allows some of the AC signal to couple through as well.  The precharge resistor is not the only leakage path.  Large inputs can produce a measurable result.

[Matrixx]

Siggi, how do you managed to remove the attenuators..? I have a 2430A with Ch2 glitchy. I would like to clean the latch contacts like you did to fix it.

What steps and things do I need to remove in order to remove the attenuator..?

[Matrixx]

Ok, after reading in more detail (my english is not good) I found the hidden screws on top of the bezel. I sucesfully take apart the front panel and remove the CH2 attenuator. Alcohol and a toothpick help to scrub gently the pcb contacts; I bent a little bit the cooper metals and this was all it needs. Also I chenge the Li-ion battery due to fail to pass the FPP.

Finally did a self-cal and it looks fine now, both channels are operational.

[David Hess]

Ok, after reading in more detail (my english is not good) I found the hidden screws on top of the bezel. I sucesfully take apart the front panel and remove the CH2 attenuator. Alcohol and a toothpick help to scrub gently the pcb contacts; I bent a little bit the cooper metals and this was all it needs. Also I chenge the Li-ion battery due to fail to pass the FPP.

Finally did a self-cal and it looks fine now, both channels are operational.

It is actually a lithium thionyl chloride primary battery.

If the battery was dead then the external calibration data would have been lost.  The oscilloscope will be fully operational without the calibration data but will not meet its accuracy specifications.

[oldway]

To be remembered for all of those who want to attempt to repair a 2430:
(Service Manual p. 5.3)
Note
Operation (for more than a few minutes) of the 2430 without its cabinet installed requires that cooling be provided for the components on the Main board.

[Matrixx]

Yes you right about the battery composition. And yes, the external calib is uncalibrated now. The only thing I can do for now is the "DC generator" method  calibration as I do not have formal calibrator. I have seen somewere the procedure to inject some 0.2V, 2V, etc well filtered. But as you say is fully functional now; and  I'm glad with that.

The only thing I don't like at all is on grounding the input via external or with the ac/dc/gnd button shows me some -0.40mV on distal measurement. I expect 0.0 as is grounded. moving a little bit the vertical position I get the 0.0 reading but the small mark at the trace beginning is not exactly aligned with the "zero" graticle line.

I need to read more about this.

But in general terms, is working nice.

[Trax]

I also found a Tektronix 2430A that spills out a lot of errors,
Mostly significant SYS-RAM Error 3700 3800 manual says IC A12U664
Is that also related to battery or an other issue?
further down I also get nvram error and many more

SYS-ROM: PASS
REG: PASS
SYS-RAM: FAIL
FPP: FAIL
WP: PASS
CKSUM-VNRAM*: FAIL
CCD: FAIL
PA: FAIL
TRIGS: FAIL

When I remove the j156 or so jumper and run extended calibration attn trig and the 3rd one all fail :/

All internal voltages on the side PCB seams ok (although i looked only with a dmm not a scope)

Do you think the unit is worth repairing in this state? And if so where would I get replacement chips they are all quite old

[David Hess]

Yes you right about the battery composition. And yes, the external calib is uncalibrated now. The only thing I can do for now is the "DC generator" method  calibration as I do not have formal calibrator. I have seen somewere the procedure to inject some 0.2V, 2V, etc well filtered. But as you say is fully functional now; and  I'm glad with that.

The only thing I don't like at all is on grounding the input via external or with the ac/dc/gnd button shows me some -0.40mV on distal measurement. I expect 0.0 as is grounded. moving a little bit the vertical position I get the 0.0 reading but the small mark at the trace beginning is not exactly aligned with the "zero" graticle line.

I have done the "DC generator" method of external calibration on a 2440 and it was not too difficult.  All it takes is a relatively low noise voltage source and voltmeter.  A good linear power supply will certainly work.

Voltages of 20.00, 2.000, and 0.200 volts are needed for the vertical calibration but the trigger calibration also requires 0.2 millivolts (see below) and 0.5 volts.  With a power supply as the source, it may be advantageous to use an external resistive attenuator to generate the lower voltages because most power supplies lack the kind of resolution necessary to produce an accurate 0.200 volts out of a full scale range of 20+ volts.  I used a Tektronix PS503 which has both coarse and fine voltage controls and it was not good enough without an external attenuator.

I think the 0.2 millivolt calibration of the trigger can just be a short to ground and the service manual implies this.  I think the reason they list it as 0.2 millivolts instead of 0.0 millivolts is that the recommended calibration generator has a minimum output of 0.2 millivolts and the difference between that and ground is insignificant as far as the calibration.

When I first did this, I had the voltages reversed so they were all negative as far as the oscilloscope was concerned but positive on my voltmeter so of course calibration failed but the oscilloscope did not say why and I wasted time doing it again and then backtracking to figure out the problem.

[grtyvr]

did you use the probes to do the external calibration or did you use a purpose built cable?  What polarity should be used?  Tip on pos, ground on neg?

[Neomys Sapiens]

Turns out it's all digital display, although the details are still pretty fascinating to me. Using those CCDs for capture and storage, then digitizing at a much lower rate seems like a pretty scrappy thing to do.

Hey, those are harsh words for the dream scope of my youth!

Have 2430A Ser.B022530, could cross check something, if needed.

[David Hess]

Turns out it's all digital display, although the details are still pretty fascinating to me. Using those CCDs for capture and storage, then digitizing at a much lower rate seems like a pretty scrappy thing to do.

Hey, those are harsh words for the dream scope of my youth!

The vector display on the CRT would qualify as a "retina" display today.

Using CCDs for sampling and acquisition record storage was very competitive with a fast ADC followed by fast RAM through the 1990s and beyond.  The Tektronix DSOs with the highest sampling rates used CCDs while the DSOs with long record lengths and DPO operation used interleaved ADCs and fast RAM.