Author Topic: Tektronix THS710A Voltage offset and Batteries  (Read 5959 times)

0 Members and 1 Guest are viewing this topic.

Offline hpmaxim

  • Regular Contributor
  • *
  • Posts: 56
Tektronix THS710A Voltage offset and Batteries
« on: March 05, 2015, 07:03:08 am »
I have a THS710A that I bought off eBay a while ago.  I've only used it previously with the 5V test signal to set some scope probes and just basic use to test the functionality.  I don't recall if I noticed this before or not.  There appears to be a DC offset on both channels (using both DC and AC coupling).

The offset changes based on vertical scale.

100+V/div : 200V (Ch1), -384V (Ch2)
10-50V/div: 20V (Ch1), -38.4V (Ch2)
1-5V/div: 1.96V (Ch1), -3.84V (Ch2)
50-500mV/div: 196mV (Ch1), -384mV (Ch2)

I tried two probes, and swapped between the channels.  The problem is not the probe, its the channel. 

Anyone know what's going on here and how to fix it (other than simply writing down the offset and calculating it out)?

PS:  If anyone needs a new battery, I found a very cheap solution...  Cut the shrinkwrap off the old battery, and remove the positive battery terminal (you can do this with a small screw driver to break the welds on top.  Then go to a decent battery store that has a welder (Batteries Plus did mine) and ask them to make you a battery pack out of 4 C-size NiMh cells, as a copy of what you have.  They'll probably charge you about  $10-15 for labor and materials and about $20 for the cells.  Only catch is you'll need a NiMH charger, I'd be scared to put that in the original NiCd charger (never charge NiMH batteries with a NiCd charger).
 

Offline SoundTech-LG

  • Frequent Contributor
  • **
  • Posts: 384
  • Country: us
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #1 on: March 05, 2015, 08:14:46 am »
Been' doin that for years with my THS-720A, but no welding, as the pressure from the battery cap is more than enough to make good contact. Also the Li Ion batteries tend to be a bit longer, so  sometimes need a bit of filing of the + terminal to fit a bit better. The original charger works fine. Just don't leave it powered on for more than a few hours after they charge up. A great upgrade. Even using the AA size Li Ions in "C" size adapters.
 

Offline z01z

  • Regular Contributor
  • *
  • Posts: 122
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #2 on: March 05, 2015, 07:47:06 pm »
Have you tried the signal path compensation or the self test? They're in the utility menu.

There is a service manual downloadable from Tek site, unfortunately it doesn't contain much that would help repair. At least these procedures are described...
 

Offline hpmaxim

  • Regular Contributor
  • *
  • Posts: 56
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #3 on: March 06, 2015, 03:02:39 am »
I'm not sure what the procedure is for this. I just tried hooking both probes together (i.e. both probes, both grounds all connected).  I didn't ground it to the scope ground.

It improved considerably, but it still seems pretty bad.  Channel 1 went from 1.96V to 1.52V (in 1V/div).  Channel 2 went from -3.84V to 320mV.  I tried running again with the probes disconnected from the scope and Channel 1 shifted to about -1.5V.

Did I do it correctly?  How much error should I expect?  I'd think <100mV at 1V/div is a reasonable number.  It's hard to understand how they have the ability to compensate for error, but the compensation is this bad.
 

Online BravoV

  • Super Contributor
  • ***
  • Posts: 5303
  • Country: 00
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #4 on: March 06, 2015, 03:48:33 am »
About the batteries, I use modded AA to D cell converters and high capacity eneloop pro AA cells.

They work like charm as I charge them externally using a dedicated Powerex MH-C9000 charger.

Of course I don't use the slow scope's built-in slow charging process as it will roast the nimh cells  :-\,  imo its not too troublesome just popping them out from the converters, and then put another freshly charged cells set.

« Last Edit: March 06, 2015, 03:53:01 am by BravoV »
 

Offline hpmaxim

  • Regular Contributor
  • *
  • Posts: 56
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #5 on: March 06, 2015, 11:16:07 am »
BravoV, Why would a slow charger roast NiMH batteries, usually what roasts them is a fast NiCd charger which is expecting a larger delta V than actually occurs on a NiMh battery?

Also, I think you meant a C adapter, not D (D is way too big, definitely wouldn't fit in the slot), and how do you get the positive terminal of the 4th battery down to the right spot?

Anyway, can anyone tell me how to properly do a signal path cal (if I didn't do it correctly) and what the voltages should be after cal --- and for that matter, if there's anything I can do to fix mine?
 

Offline z01z

  • Regular Contributor
  • *
  • Posts: 122
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #6 on: March 06, 2015, 05:08:56 pm »
Signal path compensation is on page 48 in service manual:
Signal path compensation optimizes the oscilloscope accuracy for the current
ambient temperature. For maximum accuracy, recompensate the signal path if the ambient temperature changes by 5° C or more.
To compensate the signal path, perform the following steps:
1. Disconnect any probes or cables from the channel 1 and channel 2 input BNC connectors.
2. Execute the signal path compensation with the following steps:
SCOPE UTILITY System Cal
Signal Path —
OK Compensate Signal
Paths
3. When the signal path compensation is complete, press CLEAR MENU to resume operation of the TekScope instrument.
 

Offline hpmaxim

  • Regular Contributor
  • *
  • Posts: 56
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #7 on: March 08, 2015, 02:33:56 pm »
Yep, I tried that.  Still getting about 1.52V error on CH1 and 320mV on CH2.  Trying to understand how signal path compensation could work partially, but not completely.  Very odd.  Any ideas?
 

Offline soptea

  • Contributor
  • Posts: 7
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #8 on: December 27, 2015, 08:49:00 pm »
I have same problem (1.8 volts instead of 1.5 volts ) which is annoying as I use scope to measure voltage .I am not sure but I think the problem occur AFTER a SPC  .To make things worst now I have a permanent noise/offset when no signal at input about 40 mV on 50 mV scale ,which don t remember to happen before. |O
 

Online TheSteve

  • Supporter
  • ****
  • Posts: 2542
  • Country: ca
  • GHz or bust
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #9 on: December 27, 2015, 08:54:34 pm »
This seems to be a common issue. A full calibration fixed it on a friends THS730a. The scope portion of the calibration doesn't need anything too exotic and can be done without disturbing the DMM portion.
VE7FM
 

Offline MagicSmoker

  • Frequent Contributor
  • **
  • Posts: 752
  • Country: us
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #10 on: December 28, 2015, 01:41:58 am »
Yep, I tried that.  Still getting about 1.52V error on CH1 and 320mV on CH2.  Trying to understand how signal path compensation could work partially, but not completely.  Very odd.  Any ideas?

Signal path compensation most likely has a limited ability to correct offset/drift through the vertical amplifier pathway (input BNC -> attenuator -> isolator -> ADC). Given that you have a lot of offset present in both channels, and that the offset scales with vertical sensitivity, that suggests the fault is at/within the ADC.

 

Offline Jim Narem

  • Contributor
  • Posts: 13
  • Country: us
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #11 on: January 06, 2016, 10:02:29 am »
There are actually two signal paths per channel, the high speed AC coupled
signal goes through an isolation transformer, a low pass filtered version of
the signal is fed through an HP/Avago analog optocoupler.  DC offsets are
calibrated out via a DAC.  This process occurs after the attenuator so these
offsets will vary with range settings.  If uncorrectable offsets exist after running
the calibration proceedure, the optocoupler has drifted and needs to be replaced.
It's like a $5 part and easy to replace.

See http://w140.com/tekwiki/wiki/THS720 for details and also see
http://w140.com/tekwiki/wiki/THS700 for service manuals and CLIP.
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1667
  • Country: ca
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #12 on: January 06, 2016, 10:52:04 am »
Hi,

Members of the forum will remember this thread were I explained how to change a TDS744A to the TDS784A:

http://www.eevblog.com/forum/testgear/conversion-of-500mhz-tds744a-to-1ghz-tds784a/msg568463/#msg568463

The CLIP and schematics contain some interesting information regarding the differences between the various models of the THS700A family.

First we have some jumpers on the CPU Board (RS232 sheet):



Then we have the input schematic:



The input schematic shows that different attenuators were used on the THS730A model (200MHz).

However, the CLIP for the THS720A NV, shows:



Which is the higher bandwidth attenuator.

It looks like it is possible to do some upgrades.



 

Offline hibone

  • Regular Contributor
  • *
  • Posts: 54
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #13 on: January 06, 2016, 12:29:01 pm »
What about firmware upgrade??

Is it possible? Is it worth the while?
 

Offline Jim Narem

  • Contributor
  • Posts: 13
  • Country: us
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #14 on: January 07, 2016, 08:14:11 am »
I tried changing the ID resistors on my THS720std to that of a THS720P and
it didn't seem to do anything.  The power menus were not there.  I suspect
that different firmware was used, or that the P version simply didn't exist when
my "std" scope was made.
 

Offline z01z

  • Regular Contributor
  • *
  • Posts: 122
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #15 on: January 07, 2016, 06:53:13 pm »
What about your fw version? There are two THS720Ps on ebay that both have version 1.16.
I suppose your std is older, if I'm not mistaken A and P was released together.
 

Offline R_G_B_

  • Frequent Contributor
  • **
  • Posts: 368
  • Country: gb
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #16 on: January 07, 2016, 07:34:00 pm »
The 710A, 720A and 720P are the next line of scopes so I guess these share the same firmware. The 720STD may have different firmware used. I think someone mentioned difference's in the NVRAM in this forum.


R_G_B_
« Last Edit: January 07, 2016, 07:40:03 pm by R_G_B_ »
R_G_B
 

Offline Jim Narem

  • Contributor
  • Posts: 13
  • Country: us
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #17 on: January 08, 2016, 06:52:00 am »
Firmware on my THS720std is V1.03 dated Fri Sep 1 10:15:56 PDT 1995.
I suspect a later firmware might not work given the hardware differences,
most notably the ability to trigger from the DMM input.  There are other
minor differences including the isolated power supply.

Someone else will have to try converting a THS720A to a THS720P.
 

Offline Kjo

  • Contributor
  • Posts: 6
  • Country: us
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #18 on: February 03, 2017, 02:04:58 pm »
While this thread is a year old, anyone with a Tek 720 series will eventually be looking for a battery replacement. There has been a lot of good info in this thread. But I've something more to add. The internal charger on these scopes tops up (trickle charges) at 50mA. That's just fine for a C size NiCD or NiMH battery. Modern 3000mAh NiMH batteries can tolerate 50mA trickle without evolving free oxygen (it's reabsorbed). You don't even need to weld or solder the battery terminals for a 720 series battery pack. There is more than ample pressure in the scope housing to provide good continuity.

I wrote a blog article on re-building a 720 battery pack a while back. The key is using a 8" x 1" piece of heat-shrink tubing to hold the 4 C-NiMH batteries. And DONT use batteries with flush Plus ends designed for tab welding. Use regular C batteries with protruding plus ends. Oh, and you still need to allow 10-15 hrs to recharge a battery since you are not changing the charging characteristics. You can find the article on my WEB at:

http://hollywoodcontrols.com/php2/TekScopeBattery.php

I hope this helps some of you.
 

Offline SoundTech-LG

  • Frequent Contributor
  • **
  • Posts: 384
  • Country: us
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #19 on: February 04, 2017, 05:26:54 am »
My NiMH batteries are charging fine in this. I don't recommend leaving the charger plugged in 24/7 though...
 

Offline Satbeginner

  • Regular Contributor
  • *
  • Posts: 108
  • Country: es
  • Dutch, early retired, living in Spain
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #20 on: April 09, 2018, 07:35:38 am »
Hi all,

I know this thread is old, but I recently got my THS710 and based on this thread I build a new NiMh based battery pack.
I do not have the external charger, and after a night in the scope the battery is lukewarm, so it should be ok.

I have a question about the scope calibration l like to perform because there is a small offset in ch1.

This what I did so far:

Power down,
Open the device,
Remove the jumper,
Perform calibration steps,
They end with 'pass',
Power down the unit,
Open the unit,
Replace the jumper,
Close the unit,
Power back on.

However, in the utility menu the scope item does not show 'pass'.

What did I do wrong?

To be honest, I now realize I left the battery in...

Hoping for some feedback,

Leo

You need a scope to repair a scope, and you need many multimeters to repair another multimeter!
*Tek 2465B, Tek 2465B, Tek 485, Keithley 175A, Keithley 2000's, HP 3468B, HP 3457A, HP 34401A, PM 6671, PM 5716, Fluke 45, Fluke 75, Fluke 77, Fluke 79, AFX 9660BL, KPS 605D, etc. *
 

Offline wingerr

  • Newbie
  • Posts: 4
  • Country: us
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #21 on: April 26, 2018, 02:59:09 pm »
When you ran the calibration, it got through all the steps and said it was writing the new calibration values?
I went through the cal several times because it would kick out of the routine prematurely at one point or another, maybe because the signals were out of the expected range.
It needs a fast rise time 300mV pk-pk 1kHz signal with 50 ohm termination for the first part, then a grounded input with termination, and then unterminated DC cal signals of +/-7V, +/-700mV, and +/-70mV.   It got through it finally after I tweaked the output for the 1kHz square wave to the right amplitude with the load termination.
After that, it indicated the passing scope cal status, and the small offset was gone.
 
The following users thanked this post: BravoV

Online BravoV

  • Super Contributor
  • ***
  • Posts: 5303
  • Country: 00
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #22 on: April 26, 2018, 04:11:59 pm »
.... and the small offset was gone.

Thanks for the tips. How big was the offset ? Was that on both or just one channel ?

Btw, whats your firmware version ?

Any chance you contribute to this ? -> The hunt for the Tektronix THS7xxx 1.16 firmware:P

 

Offline wingerr

  • Newbie
  • Posts: 4
  • Country: us
Re: Tektronix THS710A Voltage offset and Batteries
« Reply #23 on: September 09, 2018, 10:58:43 pm »
It was on one channel, and the cal corrected it.
I have some intermittent issue now, however, and one of the channels is at the rails, may be an isolator component.
I'll have to see if I can fix it, as it is, I've got only a single channel scope now..
Didn't see this come up until I happened to get back on, I'll look at the firmware thread too-
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf