Author Topic: HP 54602B oscilloscope problems repair (Read 12333 times)

bd139 · « **on:** February 09, 2018, 01:50:00 pm »

Another scope crossed my hands today. Few issues with it with resolutions for the interested

Display squashed

Usual C608/609 on CRT board replacement fixed that. vertical was rolling after so the CRT went through the calibration steps in the manual.

Before:

After:

Button 2 not working

Cal and ground ports are right next to the track here which was damaged the board. This has been repaired and buttons are all working. Fix was reroute track with some kynar:

channels 2 & 4 not working

TBD

Noisy fan

Needs replacing.

bd139 · « **Reply #1 on:** February 09, 2018, 03:29:04 pm »

Channels not working are interesting. No sign of any physical damage after extracting the main board. The bottom is dirty as anything though. There were some rubber mounts on the bottom which have disintegrated into slime. These are coincidentally underneath channel 2 and 4. Also the bridge that connects the shield and connectors together was dirty on channel 2 and 4. Hopefully related.

don.r · « **Reply #2 on:** February 09, 2018, 04:13:22 pm »

Very nice sleuthing. I have a 54645D that needs some small front panel work, how hard are these to get into?

bd139 · « **Reply #3 on:** February 09, 2018, 04:19:15 pm »

Really easy to get into. Two screws on the back and you're done. To get the panel off, undo the ribbon cable from the main board and then pull it back at the base and it'll unclip.

Not having any luck getting channel 3 and 4 up so far. I'm working from some 54600A schematics which I have. Seems to be the same front end.

don.r · « **Reply #4 on:** February 09, 2018, 04:34:43 pm »

Super! Thanks for that. Mine is a mechanical issue so hopefully won't need schematics. Good luck with the repair.

bd139 · « **Reply #5 on:** February 09, 2018, 04:40:05 pm »

Fingers crossed for you

I've narrowed this down to missing signal entering the programmable amplifier. Hopefully just a problem with the control circuit around that as it's all discrete components, mmbt3604, mmbt3906, mmbt4416 transistors and an LT1097 which are all really easy to get hold of.

bd139 · « **Reply #6 on:** February 09, 2018, 05:04:58 pm »

Found channel 4 problem. Nuked transistor due to a short on the board. This means that the vertical position is stuck near the bottom.

Made a right mess. This has burned 0.5mm into the board surface at least. Cleaning hole out. Was marked P43 which looks up as a P channel MOSFET.

Also found a broken trace that is killing the erase button.

This thing has taken a beating.

bd139 · « **Reply #7 on:** February 09, 2018, 05:17:43 pm »

oh this is not pretty. Has been dropped. There's multiple fractures and track breaks on the front panel board. Extensive repairs. Fun.

bd139 · « **Reply #8 on:** February 09, 2018, 06:00:49 pm »

Boards repaired. Now to play “guess what component that was”. All others are parked P and 43. They’re all SOT23.

Specmaster · « **Reply #9 on:** February 09, 2018, 06:46:54 pm »

Have you found the cause for that yet?

bd139 · « **Reply #10 on:** February 09, 2018, 06:59:37 pm »

Board cracked and shorted.

don.r · « **Reply #11 on:** February 09, 2018, 08:23:47 pm »

Must have been a heck of a shock to damage the board like that. Was there any noticeable traces of damage on the outside?

bd139 · « **Reply #12 on:** February 09, 2018, 08:48:46 pm »

Yeah the top right hand corner of the front panel is cracked. Didn’t notice that to be honest. Ah well that’s how it goes

bd139 · « **Reply #13 on:** February 09, 2018, 09:00:03 pm »

Looks like it was once a bas40 diode pair which makes sense. It’ll be being used as a switch.

I will pop one of the I assume same parts near it and test that as a dual diode.

Specmaster · « **Reply #14 on:** February 09, 2018, 09:33:23 pm »

Quote from: bd139 on February 09, 2018, 09:00:03 pm

Looks like it was once a bas40 diode pair which makes sense. It’ll be being used as a switch.

I will pop one of the I assume same parts near it and test that as a dual diode.

Well you're making good progress with it, should be working tomorrow with a tailwind.

bd139 · « **Reply #15 on:** February 09, 2018, 09:41:15 pm »

Fingers crossed. Although if I'm honest I'm dreading looking at the channel problems. There's just no signal on either 2 or 4. The plan is to compare the voltages on channel 1 and 2 when in the same state and see if there are any differences anywhere. There are large defined objects such as opamps which I can check voltages on. If not, it's old fashioned signal tracing with a generator plugged into the channel and work back from the front end to the ADC and see what happens. I've got the schematics for an earlier revision of the device so I can trace it roughly and go from there.

On a positive note, I didn't have any bas40's or any schottky diodes. I probably do somewhere but I couldn't find them. So I just jammed two 1n4148's across the burned hole where the bas40 was.

Bingo. Whole panel works now

Shitty but it works:

Fortunately with these, apart from a couple of usually reliable ICs, they're chock full of standard parts. All SMD but easy to work with as they're quite large parts. Smallest is 0805.

Specmaster · « **Reply #16 on:** February 09, 2018, 09:45:39 pm »

Neat, the board also look pretty clean as if its not seen much use, maybe the damage happened a few years ago and its been stuck in a repair queue and then the owner decided to get rid of it?

bd139 · « **Reply #17 on:** February 09, 2018, 09:48:01 pm »

That's only because I cleaned it

. It was covered in shit and the other half still is. The outside has been cleaned before he sold it. Seller was these guys: http://www.tics.co.uk/

Specmaster · « **Reply #18 on:** February 09, 2018, 09:58:33 pm »

Nice, but it would nice to them put some prices against things rather than call us for a price all the time. Surely you want to know if the price is within your budget before wasting time and money on phone calls?

bd139 · « **Reply #19 on:** February 09, 2018, 10:08:55 pm »

Yes indeed. I was having exactly that discussion earlier with a colleague about software as well. Certain types of software require you to call someone to set up a trial or get access to it to install locally. I have a rule of if you can't download it, then you can fuck off!

Think I might have isolated the channel issue at least in theory looking at the schematic. There's two hold ICs in the analogue signal path. Each of those has two analogue inputs and one output. A digital line called ch_bank_sel switches between the inputs on both ICs so it'll digitise pairs of channels 1,3 and 2,4. This flag is controlled by a 74act574 tristate DFF off the CPU bus. If the output of this is hosed or the resistors in the level converter between it and the hold ICs are hosed then it could be feasibly why alternative channels are working i.e. channels 1 and 3. I will check that tomorrow.

Hopefully the hold ICs aren't hosed as they are unobtainium and also bastards to get off the board.

Specmaster · « **Reply #20 on:** February 09, 2018, 10:12:49 pm »

Fingers crossed they are OK and its something else then.

bd139 · « **Reply #21 on:** February 10, 2018, 02:27:28 pm »

Well it wasn’t that. Confirmed hold ICs and ADCs are good and the entire front end. Phew. The problem lies in a CA3246 transistor array between the hold and the ADC This appears to be used as a summing amplifier that sums a voltage from an DAC and the reference voltage with the input voltage. Now there’s a big offset on channel 1 as well so investigation is now heading towards voltage reference and DAC. Channel 2’s offset pushes the signal out of the range that the ADC requires so there’s no data coming out hence no signal. It’s pegged off the screen.

Will resume again in a few days. Have other things to do at the moment.

Specmaster · « **Reply #22 on:** February 10, 2018, 02:30:51 pm »

Good work, looks like I might be getting my meter bits and bobs tomorrow now, assuming he see's my reply to his message asking him to confirm that tomorrow is OK and also for a contact number just in case....

bd139 · « **Reply #23 on:** February 10, 2018, 04:09:45 pm »

Fingers crossed for you

rob.manderson · « **Reply #24 on:** February 10, 2018, 06:34:12 pm »

Quote from: bd139 on February 09, 2018, 10:08:55 pm

I have a rule of if you can't download it, then you can fuck off!


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Author Topic: HP 54602B oscilloscope problems repair (Read 12333 times)

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