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HP 54502A scope with defective input channel: help needed

Question asked by silbermann on Feb 1, 2008
Latest reply on Feb 15, 2008 by silbermann

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hi experts!

i just acquired a HP 54502A scope with one input attenuator missing. When the remaining attenuator is in Ch1 the scope seems to work, however i am not sure whether the ac-square wave signal looks a bit too noisy and the following self-tests fail:

'Analog Trigger': (error code 0000 0000 0000 0001)

'D/A Converter': (error code 0000 0000 0000 0001)

'A/D Converter': (error code 0000 0000 0011 1101)

i have no idea what these codes mean, but i hope they are merely due to the missing attenuator assembly in Ch2.
Now, if i swap the attenuator assembly to Ch2 the same self-tests do fail. when i connect the ac-calibration signal to Ch2 i can see the signal but when i decrease the vertical sensitivity starting from 5V/div with the FINE key selected, the displayed amplitude of the signal does not change continuously but only jumps up exactly when i can hear the attenuator's solenoids switching (this takes place between 1.20 and 1.15V/div, 235 and 230mV/div and 47 and 46mV/div).

I am not exactly an expert in electronics so any help with this problem would be greatly appreciated!

Thanks in advance
joerg

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ko2054 Feb 5, 2008 3:39 PM
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You sure purchased a very old scope. Usually a self cal will reduce noise on a channel. But if the AD is bad, that will also add noise. Usually missing AD codes is the failure mechanism.

'Analog Trigger': (error code 0000 0000 0000 0001)
The above code just means that there is a problem with the trigger on channel 1. Notice that last digit is a 1. If it was a 2, then the problem would be with channel 2.

'D/A Converter': (error code 0000 0000 0000 0001)
Again, you have a D/A converter problem on channel 1.

'A/D Converter': (error code 0000 0000 0011 1101)
The last digits should all be ones. Yours says 1101 which means that the A/D is bad.

You bought a scope without an attenuator on channel 2. It looks like the AD on channel 1 is bad.

If you move the attenuator to channel 2, you will still get the trigger and DA errors. However, you might try moving the channel 2 AD over to channel 1. On early units the ADs were not soldered in. There were finger contacts under neath the AD. If your channel 2 AD is not soldered in, you could move it to channel 1. You have to be careful because the finger contacts were easy to damage with too much pressure and then the AD pins would loose connection. The screws holding the AD have to be torqued down evenly to about 3 in lbs. If you over tighten the screws you will damage the gold finger contacts and the AD is lost. We got away from the finger contacts and went to soldering the ADs in place. The reliability went went up, but removing the 40 pin ADs is almost impossible. If the AD is soldered in, it is very improbable that you could desolder the AD without damaging the small traces or through hole eyelets. You can try it, but I doubt you will be sucessful. We had to use special tools here at the factory to remove the ADs. (No we do not have those tools any longer.)

You can also check the AD to make sure there are no missing AD codes. Apply a pure low frequency sine wave and carefully observe the sine for small stair steps. The small stair steps would indicate ADs codes are missing. If there are missing AD codes, that would explain why the channel is noisy.

We do not have the parts to support that old of a scope. If you cannot fix it yourself or live with the performance, there is not much more we can do for you. The bottom line is that I hope you did not pay too much for that scope. If the noise on channel 1 is not so bad that it impacks your work, you could still use the scope to just see if signals are present at pins. But I would not use the scope for critical measurements.

I hope this information helps you out.
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silbermann Feb 14, 2008 8:05 AM
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Hi, ko2054!
Thank you very much for your elucidations! now that i know what these error codes mean, i realize however, that my readings were somewhat misleading. The error codes that i stated were displayed after performing the self-tests with the attenuator assembly present in Ch2, not in Ch1 (sorry, but due to my lack of knowledge i thought this would be irrelevant).

Fortunately i was able to get a spare for the missing attenuator and since installation things really improved: the device now passes all self-tests, when connecting both inputs to the ac-cal output everything looks reasonable and the noise that i had before vanished.

But two problems still remain, that you hopefully can help me with:

First is, when i try to self-calibrate Ch1 (after loading default calibration factors) i get the following errors:
- in "0.) vertical cal": FAILED A/D Cal Ch1 0000 0000 0001 0000
- in "1.) delay & repetitive": FAILED Gain (rep) Cal Ch1 0000 0000 0001 0000
and a subsequent service cal will abort saying "Ch1 cal not complete".
The second problem concerns switching of the input impedance of Ch1. When i again connect both inputs (set to 1MR input impedance) to the ac-cal output i get nice square-waves with ~800mVpp for both channels. If i then set the input impedance of Ch1 to 50R i can't hear a click and there is no change in amplitude of the wave of Ch1 (while in the display now 50R is being highlighted).

Now it becomes funny: first of all i reset Ch1 to 1MR. if i then change the input impedance of Ch2 from 1MR to 50R i can hear clicking and the displays for Ch1 and Ch2 are BOTH affected resulting in square-waves with ~400mVpp.

Can you (or somebody else) tell what might be the reason for this behavior? Is it possible that the self-cal for Ch1 fails due to a problem with switching the input impedances? any ideas how to fix?

Best regards and many thanks for your help,

joerg
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ko2054 Feb 14, 2008 9:03 AM
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Hello Silbermann,
You have a bad attenuator on channel 1. You should be able to measure the 1Mohm and 50 ohm inputs with the scope turned on. When you select 50 ohms on channel 1, you should hear an attenuator click which switchs from the 1Mohm resistor to the 50 Ohm resistor.
The first calibration error is because the scope is trying to calibrate the 50 Ohm path and it cannto because the 50 Ohm resistor does not switching in. The second calibation error is related. Because the 50 Ohm path is broken, the signal for the delay cal is overdriving the waveform off screen and the scope is seeing the wrong slope on the delay path cal.
When the scope inputs are set to 1Mohm, you generator is driving a high impedance instead of 50 Ohm. If the 50 Ohm path on both channels worked, then switching either channel to 50 ohms would terminate the generator into 50 ohms and you would see the proper signal display. When you switch channel 2 to 50 0hms, the generator sees the proper termination. Because the channel 1 50 ohm path is broken, the generator is still terminated into a high impedance hence the voltage levels are higher than you expected. I hope this answers your questions.
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silbermann Feb 14, 2008 10:09 AM
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Hi ko2054!
thanks for the quick answer and for your patience! as i wrote in my initial post i am far from being an expert in electronics but now i am really a bit ashamed due to my foolish question: it sounds pretty reasonable that Ch1 will show 400mVpp (even if it is stuck to 1Mohm) when there is Ch2 with 50ohm connected at the same time and hence pulling down the voltage level.

now the question arises why the attenuator in Ch1 is not able to switch to 50ohm? do you think this is caused by a mechanical problem (i saw these solenoids when i installed the attenuators) or maybe just a broken connection to one of the solenoids that might be repairable (or is it more probable a malfunction of the integrated circuit on the attenuator ***. board)? if yes, can you say which of the solenoids should be the broken one? can i test the functionality of that solenoid by applying some voltage to one of the pins which connect to the main board?

all the best,
joerg
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ko2054 Feb 14, 2008 12:16 PM
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Hello Jeorg,
The relays are soldered to the attenuator substrate. We never tried to replace them here at the divison. The pads are very small and easily damaged when you apply heat. One relay switches in the 50 ohm resistor, the other two relays are divide by 10 and divide by 20. The input amplifier has different gain stages built into the amplifer. We combine different amounts of gain and division to get the right amount of attenuation. Yes you should be able to test activate a relay by applying a voltage, but I have to be honest and admit I forget what the voltage level should be. We used a special silver nitrate conductive epoxy to epoxy the relays to the attenuator substrate. I see no way for you to remove a relay without damaging the attenuator substrate. You best bet would be to find a working attenuator and replace the channel 1 attenuator. But it is only the 50 ohm path that is defective and if you are using passive probes, then you only need the 1Mohm input path anyway. The 50 ohm path is used for active probes, low impedance passive probes, and connecting to 50 ohm sources like a pulse generator. There were two typical failure mechanisms. The first was the relay coil would open up. The second was the two contact pins inside the relay would oxidize over time so the the gold spring would not make electrical contact with the gold pins. The clicking sound you noramlly hear with the relays is the gold spring clicking against either of the two gold contact pins.
I hope this answers your questions. Ken.
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silbermann Feb 15, 2008 9:00 AM
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Hello Ken!
You really convinced me to keep my fingers away from this attenuator. I am pretty happy with the device as it is and i am glad that the mainboard with all its A/Ds, D/As and so on is obviously fine. so as i will not need the 50ohms, there is indeed no point to botch everything while trying to fix the problem.
Let me thank you very much for your great help and for giving all these detailed explanations.

greetings from Berlin
joerg
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ko2054 Feb 15, 2008 9:13 AM
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Hello Jeorg,

Because the self tests failed the 50 ohm path, I do not know if the software completed and kept the 1Mohm path cal factors. I suggest your run the performance tests from the service manual for time and vertical measurement accuracy. If the performance tests pass, that will give you confidence in your measurement results. Otherwise inaccurate measurements could lead you down the wrong path. Ken
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ko2054 Feb 15, 2008 9:18 AM
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Joerg, sorry that I typed your name wrong in my last response. After I pressed submit, I realized that I had mistyped your name. My humblest appologies to you. Ken
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silbermann Feb 15, 2008 1:04 PM
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You're welcome Ken - don't worry!
If only colorado springs wasn't that far i'd invite you for a beer, honestly!

Thank you for that hint. I already wondered whether cal factors for Ch1 will be saved at least for those cals that succeeded but i did not fully understand (due to my shortcomings in the language) what was written in the service manual concerning that issue. I will try to make the performance tests you mentioned. If they fail i can still keep in mind that i should always be sceptic to some extent about what Ch1 tells.

jÃ¶rg
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HP 54502A scope with defective input channel: help needed

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