Tektronix THS710A Voltage offset and Batteries  - Page 1
Forum section: electronics / testgear

author: hpmaxim
author_rank: Regular Contributor
timestamp: March 04, 2015, 08:03:08 pm
content: 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).

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: March 04, 2015, 09:14:46 pm
content: 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.

author: z01z
author_rank: Regular Contributor
timestamp: March 05, 2015, 08:47:06 am
content: 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...

author: hpmaxim
author_rank: Regular Contributor
timestamp: March 05, 2015, 04:02:39 pm
content: 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.

author: BravoV
author_rank: Super Contributor
timestamp: March 05, 2015, 04:48:33 pm
content: 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.

author: hpmaxim
author_rank: Regular Contributor
timestamp: March 06, 2015, 12:16:07 am
content: 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?

author: z01z
author_rank: Regular Contributor
timestamp: March 06, 2015, 06:08:56 am
content: Signal path compensation is on page 48 inservice manual:Signal path compensation optimizes the oscilloscope accuracy for the currentambient 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 CalSignal Path —OK Compensate SignalPaths3. When the signal path compensation is complete, press CLEAR MENU to resume operation of the TekScope instrument.

author: hpmaxim
author_rank: Regular Contributor
timestamp: March 08, 2015, 03:33:56 am
content: 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?

author: soptea
author_rank: Newbie
timestamp: December 27, 2015, 09:49:00 am
content: 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.

author: TheSteve
author_rank: Unknown
timestamp: December 27, 2015, 09:54:34 am
content: 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.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 27, 2015, 02:41:58 pm
content: Quote from: hpmaxim on March 08, 2015, 03:33:56 amSignal 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.

author: Jim Narem
author_rank: Contributor
timestamp: January 05, 2016, 11:02:29 pm
content: There are actually two signal paths per channel, the high speed AC coupledsignal goes through an isolation transformer, a low pass filtered version ofthe signal is fed through an HP/Avago analog optocoupler. DC offsets arecalibrated out via a DAC. This process occurs after the attenuator so theseoffsets will vary with range settings. If uncorrectable offsets exist after runningthe calibration proceedure, the optocoupler has drifted and needs to be replaced.It's like a $5 part and easy to replace.Seehttp://w140.com/tekwiki/wiki/THS720for details and also seehttp://w140.com/tekwiki/wiki/THS700for service manuals and CLIP.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 05, 2016, 11:52:04 pm
content: Hi,Members of the forum will remember this thread were I explained how to change a TDS744A to the TDS784A:https://www.eevblog.com/forum/testgear/conversion-of-500mhz-tds744a-to-1ghz-tds784a/msg568463/#msg568463The 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.

author: hibone
author_rank: Regular Contributor
timestamp: January 06, 2016, 01:29:01 am
content: What about firmware upgrade??Is it possible? Is it worth the while?

author: Jim Narem
author_rank: Contributor
timestamp: January 06, 2016, 09:14:11 pm
content: I tried changing the ID resistors on my THS720std to that of a THS720P andit didn't seem to do anything. The power menus were not there. I suspectthat different firmware was used, or that the P version simply didn't exist whenmy "std" scope was made.

author: z01z
author_rank: Regular Contributor
timestamp: January 07, 2016, 07:53:13 am
content: 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.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: January 07, 2016, 08:34:00 am
content: 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_

author: Jim Narem
author_rank: Contributor
timestamp: January 07, 2016, 07:52:00 pm
content: 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 otherminor differences including the isolated power supply.Someone else will have to try converting a THS720A to a THS720P.

author: Kjo
author_rank: Regular Contributor
timestamp: February 03, 2017, 03:04:58 am
content: 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.phpI hope this helps some of you.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: February 03, 2017, 06:26:54 pm
content: My NiMH batteries are charging fine in this. I don't recommend leaving the charger plugged in 24/7 though...

author: Satbeginner
author_rank: Regular Contributor
timestamp: April 08, 2018, 09:35:38 pm
content: 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

author: wingerr
author_rank: Newbie
timestamp: April 26, 2018, 04:59:09 am
content: 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.

author: BravoV
author_rank: Super Contributor
timestamp: April 26, 2018, 06:11:59 am
content: Quote from: wingerr on April 26, 2018, 04:59:09 amThanks 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.

author: wingerr
author_rank: Newbie
timestamp: September 09, 2018, 12:58:43 pm
content: 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, my firmware version is old, v1.03 dated Sept.1995.