[SOLVED] Required: HP 54600B Oscilloscope circuit diagram...

[omelette]

Hi. Channel 1 inexplicably stopped working on this 'scope - in use for hours, working perfectly, turned off & switched on the following day, no channel 1!

Channel 2 still work fine in all respects when connected to the test-waveform, whereas channel 1 displays either a flat trace, or a blank screen as if voltage is off-scale (depends on voltage setting selected). I also notice that I can't adjust the trace's base-position on screen - the trace does not follow the 'GND' symbol when it is moved up/down, it's vertical position on screen remaining the same.

I have had it apart and although both channels have identical circuitry, the voltages on I.C pins on the faulty channel are all over the place - for instance, one SMD AT&T chip (UB1202AM) has almost -12V on several pins whereas the working channel's chip voltages are mere fractions of a volt! Despite this, these chip temperatures are within a degree Celsius of one another, so no apparent overheating. Finally, a continuity-check on what appear to be supply decoupling capacitors on the other side of the PCB but directly under this chip buzz through as being almost shorts. I have even removed the I.C. to see if it was the culprit - it wasn't!

Seeing as this PCB has at least 3-layers, without a circuit diagram, this is an exercise in futility! Worse, I have failed to even find a pin-out of the chip I mentioned, which most list as "obsolete stock", let alone a complete circuit diagram. I have found HP's "User & Service manual" which is a bit of a joke as there's no circuit diagram and the cure for every system-board fault listed is to replace the system-board!!!

Excuse the long-winded post, but can anyone either provide me with a circuit diagram for this scope or else suggest a possible cause of the problem. Thanks.

 

[ckshivaram]

**broken link removed**

 

[omelette]

This is the User Guide & Service Manual I mentioned I had already found. Repair-tips along the lines of "if the power supply isn't working, replace the power supply..." makes this 'repair information' essentially worthless...

A circuit diagram of some kind - even a block-diagram - is needed here...

 

[omelette]

Well, things aren't going according to plan - I just killed my one working channel, junked the scope basically!

Working almost completely in the dark, I figured from voltages reading etc. that the little 8-pin I.C. pictured below was a voltage regulator of some kind - again, another part number that just laughs at Google's attempts to provide any info about it - so had the not-so-bright idea of interchanging the working channel's for the non-working one and vice-versa - bad move 'cos then neither channel worked and switching them back didn't restore it either...

I have quickly grown to detest HP scopes. Great to use, great to fault-find with but seemingly no info at all is available to help fault-find problems with the scopes themselves and it seems that are filled with obscure components that you can neither find information on or/and are impossible to source...

I included the pic. below just in case anyone could tell me what the little 8-pin does, or by some miracle, order from somewhere! The UB1202AM is also pictured. Despite spending hours searching the net, I could not even find a description of what it does...

A new(ish) scope is needed methinks and it sure as hell won't be a HP.............
**broken link removed**

 

[sdumas]

Hi,

The 8 pin IC is a LT1097 from Linear Technology,
a precision Op Amp.

The AT&T don´t know...

But to repair this oscilloscope,you must have the schematics,
you have no alternative...

I have a lot of SM from HP (Agilent) but this one,no.

Good luck...
SDumas

 

[omelette]

Wow, I've been thrown a bone, thanks for that!!! Now at least I've got something to go on and I should be able to check if these op-amps are the problem. I can also order them if necessary, which is great too!

Things are looking up a bit as well. I spent and hour or so on it again today and Channel 2 has some life in it after all. When it's left on for a while, it detects the signal, slowly 'drifting' up and down. Certainly not ideal but it's actually usable! I may just close it up and make do with it as it is.

I'm more and more convinced that it's just a missing voltage supply. But you're absolutely right, without a schematic, it's pretty hopeless - and HP seems to have those locked down pretty solidly...

Still, hope springs eternal as they say! Thanks again.

 

[omelette]

Just a little update which may be of interest to some. The genesis of the fault appears to have been a 100nF decoupling capacitor that 'kinda' went short. 'Kinda' 'cos although I removed it from circuit, checked the resistance across it, which was about 100 Ohms, and a capacitance test showed it as out-of-range, so I thought "Sorted!" - until the next day when I by chance measured the resistance across it again, this time it was about 400 ohms! Examining the side that had been towards the PCB, I noticed reddish stuff on it - a close examination of the board revealed that only a very few capacitors had this, only those that had copper tracks running between their legs, so it's purpose was apparently to provide electrical isolation. For whatever reason, it had become a conductor 'cos scraping it off resulted in a perfect 100nF capacitor!!! Never came across that before...

But that was just the root cause. This short had killed one of the LT1097 amps & I damaged the other from the 'working' channel when I swapped the op-amps before discovering the short. So I bought 2 LT1097's online, soldered them in, result; my 'still working' channel now works perfectly again, all the negative voltages that were present in the faulty channel circuit were gone, adjusting the trace-position control resulted in a perfect varying biasing voltage being present, I had a trace on some voltage settings which showed a minuscule test-waveform on-screen , but still no control over its positioning.

Well several more hours later, I've discovered the problem - the faulty channel's UB1202AM was also damaged, removed from the PCB, 2 adjacent pins measure a virtual short when compared with the still-working one.

So I need to source these somehow. I have found one Chinese supplier that sells them for a very reasonable $10 each, but requires a minimum order of 5 i.c's. Plus courier fees no doubt will see it approach $100 - and I only need one of them! Cheaper than a new scope though - presuming there's nothing else fried! - and though I'm reluctant to go this route, I may not have another alternative.

 

[sdumas]

Hi omelette,

look carefully at this schematic,page 3,i think the UB1202AM is not what you
are to buy (sorry for my english...).

I have compared the IC PA300 (???) U2 and i think is the same as UB1202AM.

I make a google search of UB1202AM and they tell this is a RS232 IC (???).

Look the circuit of both and compare...

Good luck

SDumas

Edit:
Complete Component Level Information Package here:
**broken link removed**

 

[omelette]

Hi sdumas. I don't believe it, a proper schematic, board-layout and all - I had long since given up any hope of finding this!!!:shock: Thank you ever so much! :grin: :grin: :grin:

Regarding the UB1202AM, pin-out wise, the schematic seems identical to the actual circuit-board layout, I think the "PA300" label was an internal HP reference - so unless I'm missing something here, I'm pretty sure that it's UB1202AM's I still need to get. I saw the RS232 description you mentioned - these jokers seem to be just copying & pasting random stuff into the i.c. information fields in many cases! I pulled up a few other chips they had as well and like the RS232 rubbish, you get descriptions starting mid-sentence, with info probably having nothing to do with the device in question. This is definitely the case with the UB1202AM's anyway - it's so nice to actually know what its function is now!

So, at the moment I'm waiting to hear back from at least one other company that sells UB1202AM's. Hopefully I won't have to order multiple chips that I'll never use. Worse case scenario, I get them & try selling the unwanted ones on eBay. Either way, I'll post an update here.

Thanks again.