Author Topic: Tektronix THS7xxx Scope Hack/Teardown/Discussion - WANTED FW1.16 PLEASE HELP US  (Read 57790 times)

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Online capt bullshot

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One thing to note is that in those two ranges the X10 attenuator kicks in while in other ranges either there is no attenuator or only the X100 attenuator is in the signal path.
Any suggestions?

Yes, should be quite obvious: There's an issue with the X10 attenuator. Maybe a capacitor gone bad. The usual scope attenuator is a parallel connection of a high impedance resistive divider and a capacitive divider of the same ratio. If DC is OK, I'd believe the resistors are still OK, but if you've got a deviation at AC, there's a mismatch in the capacitors. Older Tek scopes had the capacitors as trimmer caps, while in these ceramic hybrids the resistors get adjusted during the manufacturing process.
I guess you'll have to remove the ceramic hybrid from the PCB to gain access to the capacitors, either they are underneath the relays, or on the other side of the ceramic.

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Offline analogRF

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Thanks @Jim Narem for the detailed response. Yes, I can see either of the optocouplers can cause the offset problem
right now the offset is small and I can deal with that later but my major concern is the messed up frequency response of channel 1
it  seems the input capacitance changes to wrong value in two vertical ranges when the X10 attenuator kicks in
my suspicion is also the relay (the middle one) but I cannot quite figure out how/why the relay can cause
capacitance imbalance? I can see it might cause unwanted contact resistance but capacitance? how?

on the other hand, as you said, there is not much else on the attenuator board since the elements are all laser etched or something
It's also hard to imagine one of those integrated capacitors has changed its value over time
 

Offline analogRF

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One thing to note is that in those two ranges the X10 attenuator kicks in while in other ranges either there is no attenuator or only the X100 attenuator is in the signal path.
Any suggestions?

Yes, should be quite obvious: There's an issue with the X10 attenuator. Maybe a capacitor gone bad. The usual scope attenuator is a parallel connection of a high impedance resistive divider and a capacitive divider of the same ratio. If DC is OK, I'd believe the resistors are still OK, but if you've got a deviation at AC, there's a mismatch in the capacitors. Older Tek scopes had the capacitors as trimmer caps, while in these ceramic hybrids the resistors get adjusted during the manufacturing process.
I guess you'll have to remove the ceramic hybrid from the PCB to gain access to the capacitors, either they are underneath the relays, or on the other side of the ceramic.

don't you think the capacitors are also integrated inside the ceramic pcb? I mean like they are eatched into the circuit board?
if they are underneath the attenuator, there  is hope !

also, can it be due to the relay as @Jim Narem suggests? Although i cannot figure out how the relay can cause imbalance capacitance
 

Offline analogRF

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....I think this might be how it works: there is a series C (or RC) between the input and output of the relay. when the X10 atten is out, the relay is closed and that series RC is short circuited and when the X10 atten kicks in, the relay is open and the series RC (or C) between the I/O of relay comes in to the signal path (parallel to the series branch of the attenuator ). So, that capacitor is probably at fault here not the relay itself.

If that capacitor is a discrete component underneath the hybrid, i might have a chance but i don't know what the value should be.

this problem seems very similar the issue people have had with Siglent SDS1202X-E early models for which a remedy was also discussed extensively on the forum but I dont know how exactly they modified the circuit....i do have that scope myself and it has that issue but I didn't care to fix it. never affected my work really...
 

Offline zucca

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Online capt bullshot

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don't you think the capacitors are also integrated inside the ceramic pcb? I mean like they are eatched into the circuit board?
if they are underneath the attenuator, there  is hope !
I've never taken a look into the shielded attenuators of these THS scopes myself, so I just was guessing. One can see a few capacitors soldered to the ceramic, especially the big one for AC coupling as I suppose. I've replaced the relays in my TDS580 attenuators (found some pictures here: http://www.eevblog.com/forum/testgear/tektronix-tds520b-attenuator-swap/ ), here one clearly can see the large metallized areas - these are the capacitors for their attenuators, see the laser trimming. I don't know wether the THS ceramics are made the same way or they've soldered discrete capacitors. For sure, one can see laser trimmed resistors there (Edit: looked again at the high res picture, looks like a trimming scheme applied to whatever is hidden under the blue coating, can't tell if it's a capacitor or resistor).

Quote

also, can it be due to the relay as @Jim Narem suggests? Although i cannot figure out how the relay can cause imbalance capacitance

From my opinion / experience, a weak relay should cause DC errors. Within the TDS 5xx / 7xx attenuators, weak relays cause SPC errors, and/or complete loss of signal for some attenuation settings. Exercising the relays can cure that for a while.
« Last Edit: September 11, 2018, 11:18:56 pm by capt bullshot »
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Offline analogRF

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You know we have the schematics, don't you?

http://www.eevblog.com/forum/testgear/tektronix-ths700-tekscope-component-level-information-package-and-schematic/

of course. But my frequency response problem is inside the attenuator hybrid (marked as AT1 on schematics) so the schematics don't help ....see the pictures posted by @Jim Narem  at http://w140.com/tekwiki/wiki/THS720
 

Offline analogRF

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@capt bullshot: I also believe the caps are etched into the ceramic. I highly doubt there is anything underneath that board.
SPC passes on my scope with no problem and DC is perfectly fine on all vertical ranges. Only when I apply a square wave with long enough period I can see on those two ranges the pulses have a exponential riding on top of them with overshoot and then settling down to the correct value.
Scope calibration didnt help either as most of the process only uses DC voltages.

I wonder what might have happened to a ceramic etched capacitor to go bad?
 

Online capt bullshot

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@capt bullshot: I also believe the caps are etched into the ceramic. I highly doubt there is anything underneath that board.
SPC passes on my scope with no problem and DC is perfectly fine on all vertical ranges. Only when I apply a square wave with long enough period I can see on those two ranges the pulses have a exponential riding on top of them with overshoot and then settling down to the correct value.
Scope calibration didnt help either as most of the process only uses DC voltages.

I wonder what might have happened to a ceramic etched capacitor to go bad?

Overvoltage? Environment (humidity)?
One of the laser cut links may have gone short again for whatever reason?

I've had some defective ceramic caps in an old 7 series plug-in: http://wunderkis.de/tek-7/ Don't know what caused them to go bad either.

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Offline Ice-Tea

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Joining the club: I got 6!  :clap:

First job: batteries!
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eBay shop with all the gear you need!
FS: TTi TSX1820P, Agilent Infinium 54815A, 54825A, R&S CMU 200  Tektronix CSA8000B, HP 8594E, 8595E, 8662A, 8663A, R&S SMHU, IRF 2051, Marconi 6201B 8GHz
 

Offline HighVoltage

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Wow, are all 6 of them THS720P?
Are they supposedly broken or in working condition?


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Offline zucca

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Holly Molly... welcome to the club.

I suggest you to build some DIY LI ION 2S battery packs, as discussed before here. If not I can sell mine NiMh pack used 2 hours in total.
« Last Edit: October 09, 2018, 10:38:47 pm by zucca »
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Offline Ice-Tea

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Yes, all 720P. Should be all perfectly fine, clearly labelled "excess", not broken or anything. And not a scratch on them. Just ordered a stack of 4500NiMh's  :clap:
An engineer never has a problem. He just needs more time.
eBay shop with all the gear you need!
FS: TTi TSX1820P, Agilent Infinium 54815A, 54825A, R&S CMU 200  Tektronix CSA8000B, HP 8594E, 8595E, 8662A, 8663A, R&S SMHU, IRF 2051, Marconi 6201B 8GHz
 

Offline zucca

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Offline Ice-Tea

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Works like a charm!

An engineer never has a problem. He just needs more time.
eBay shop with all the gear you need!
FS: TTi TSX1820P, Agilent Infinium 54815A, 54825A, R&S CMU 200  Tektronix CSA8000B, HP 8594E, 8595E, 8662A, 8663A, R&S SMHU, IRF 2051, Marconi 6201B 8GHz
 
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Offline Ice-Tea

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First unit on the Bay. This one includes 2 P5102's and a funky Fluke RF probe!  :popcorn:
An engineer never has a problem. He just needs more time.
eBay shop with all the gear you need!
FS: TTi TSX1820P, Agilent Infinium 54815A, 54825A, R&S CMU 200  Tektronix CSA8000B, HP 8594E, 8595E, 8662A, 8663A, R&S SMHU, IRF 2051, Marconi 6201B 8GHz
 

Offline Maxis

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Gentlemen,

I've got TEK THS720 without A. I wonder whether by a simple (nor not so simple) FW change it can be converted to THS720A. By looking at the main board is seems like the main components are the same (even down to the ID resistors in question). Have anyone done anything like this?

Thank you!

Maxim
 

Offline hardcoreman

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I've got TEK THS720 without A. I wonder whether by a simple (nor not so simple) FW change it can be converted to THS720A. By looking at the main board is seems like the main components are the same (even down to the ID resistors in question). Have anyone done anything like this?


same idea... but no free time to test.... i think software will run.
 


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