FS [EU]: TEKTRONIX A6909 Isolation amplifier for oscilloscope SOLD - Page 1
Forum section: electronics / general

author: manganin
author_rank: Regular Contributor
timestamp: April 20, 2023, 10:14:02 am
content: Galvanically isolated "floating" inputs for a regular ground referenced oscilloscope.Very high degree of isolation. There seems to be an optical barrier between the input and the output.TESTING:Passes the power-up self test. No error codes.Goes through the 30 second calibration routine, started by pushing the CAL-button. No error codes. (There seems to be some sort of internal generator and level meter which connect to the signal path by relays and adjust the gain and offset constants.)An external audio generator was connected (via an oscilloscope probe) to the A6909 inputs and an oscilloscope to the outputs. Both channels produced a nice clean sine wave on the oscilloscope screen, no visible distortion. Also no difference when switching between the DC and the AC coupling modes.This instrument is beyond my expertise. Selling it as-is for someone who knows more than I do.Cosmetically very clean. Purchased in the late 90s.180 euros. EU only.Shipping estimate 15 to 35 euros depending on your location. Please ask. The actual postage only, nothing extra.The buyer must be familiar with electrical safety regulations and know how to safely operate and repair professional test equipment.

author: jfphp
author_rank: Regular Contributor
timestamp: April 20, 2023, 09:52:29 pm
content: Where are the dedicated probes ? Useless without.

author: bdunham7
author_rank: Super Contributor
timestamp: April 20, 2023, 10:09:22 pm
content: Well, this one won't need the dedicated probes because it has been modified with BNC connectors, uninsulated ones at that.And I have no idea what the two BNCs on the right are for, those are blank on an unmodified A6909.The instrument may work fine this way with standard 10X probes, but it will not strictly meet safety standards for isolation because any exposed area of the BNC may be live. Also, just being modified may be an issue, it would be nice to see the work inside. All that said, it is a handy instrument with very good isolation and CMRR.Edit: I see the outputs have also been moved to the front. The work looks neat from the outside anyway.

author: super7800
author_rank: Regular Contributor
timestamp: April 20, 2023, 10:11:25 pm
content: Idk how "useless" it is without the probes. It shows it comes with 50ohm bnc feed through Adapters, so can still isolate signals without the proprietary probes. Your right though, it should/needs the special probes to do alot of what it was designed to do, but it will still work. I'd be interested if they'd ship to the usa.

author: bdunham7
author_rank: Super Contributor
timestamp: April 20, 2023, 10:35:41 pm
content: Quote from: super7800 on April 20, 2023, 10:11:25 pmThe proprietary probes are just 10X probes with a special plastic push-in connector and a simple common-mode choke built into the little box. The choke is just a toroid with the probe coax looped around it a dozen times or so.

author: manganin
author_rank: Regular Contributor
timestamp: April 21, 2023, 06:45:09 am
content: Quote from: bdunham7 on April 20, 2023, 10:09:22 pmThe input BNC looks like it was factory made. Or at least far from what I am used to see when hearing the word "modified". I didn't even realize that it was a non standard version.The output side is not bad either.Generally looks quite boring because all the magic is inside the black boxes which, for obvious reasons, I prefer not to disassemble.

author: bdunham7
author_rank: Super Contributor
timestamp: April 21, 2023, 01:38:58 pm
content: I agree, it looks like very nice work and not hacked at all. The input BNCs in particular make me wonder if this isn't a factory one-off or special model.

author: Hydron
author_rank: Super Contributor
timestamp: April 21, 2023, 03:44:48 pm
content: Quote from: bdunham7 on April 20, 2023, 10:35:41 pmPretty sure probes designed for 10M input R are not normal 10X probes! Do you know if they are 90M input R, or are more normal probes with 1.11M in parallel at the BNC?Manganin: how much to post to Spain, for arrival by the 8th of May? Am normally UK based but heading to the EU (Spain) next week until the 9th. I also need to look at how big/heavy this thing is but am interested.

author: bdunham7
author_rank: Super Contributor
timestamp: April 21, 2023, 04:10:43 pm
content: Quote from: Hydron on April 21, 2023, 03:44:48 pmWell, you're right, sort of, but 10X probes should still work OK since the instrument can be calibrated. The probes measure infinite (open) from the tip to the ground clip when disconnected. The same measurement is 10.16M when plugged in to the instrument. I would have expected this to mean the probe is 9M and the instrument is 1M, but it isn't. The instrument has an input resistance of 500k, the probe when measured from the tip to the center conductor of the connector is 9.66M. So electrically, it works as a 20X probe. However, a regular 10X probe with a 9M resistance will work as a 19X probe, which should be well within range of the front panel gain calibration IIRC. A decent 10X probe with a completely insulated BNC cover would be a pretty decent solution and mostly cover the metal BNC inputs.To answer your other question, the unit is about the size of a 2200-series Tek scope and isn't very heavy. I had it set up with a 2445 stacked on top of it and the two outputs going to CH3 and CH4, since the oscilloscope has to be set to 0.1V/div and those limited channels have that. You do need 50R terminators on the A6909 outputs.

author: manganin
author_rank: Regular Contributor
timestamp: April 21, 2023, 04:31:59 pm
content: Quote from: Hydron on April 21, 2023, 03:44:48 pmI think the 10 Mohm printed on the front panel could bewiththe probe. Originally the user was not meant to have access to the connector.(Based on this drawing taken from the manual.)

author: bdunham7
author_rank: Super Contributor
timestamp: April 21, 2023, 06:02:53 pm
content: Quote from: manganin on April 21, 2023, 04:31:59 pmExactly so, and mine has the same printing on the front as yours.

author: manganin
author_rank: Regular Contributor
timestamp: April 21, 2023, 09:20:23 pm
content: Sold.

author: Louiebluey
author_rank: Contributor
timestamp: April 26, 2023, 03:53:08 pm
content: Thank you for the pics! I think this might be a very neatly done home brew mod, but possibly a good example at that! The nicks in the nut on the moved output BNCs do not look factory, not a concern, but probably an indication that this was a mod and not a Tek offered option.I picked up one of these many years back, and held out for the special probes, which never showed at a reasonable price. Looks like a good mod to change the input to BNC and then re-calibrate.Here are a few more pics. I had to break that gray input assembly to get the end of the cable free. That darker insert just slides out, but the lighter grey retainer plastic part was glued in. In hindsight, probably could have unsoldered the lead at the input PCB, but I doubt I would ever need to go back to that odd input connector.

author: Louiebluey
author_rank: Contributor
timestamp: April 26, 2023, 03:56:02 pm
content: another pic, was too large for the first post.

author: manganin
author_rank: Regular Contributor
timestamp: April 26, 2023, 04:49:46 pm
content: Interesting photos, never had the courage to look inside the black box. Seems to be a standard input module of some Tek scope plus some additional circuitry on the PCB.Shipped to Denmark today...