Siglent TPA10 - I want to get off Mr. Bone's wild ride

[FlexibleMammoth]

Dear all,

after spending way too much time to find out that the first Siglent TPA10 TekProbe adapter I received was defective (read all about that here [1]), I purchased a new one from a german supplier. That was a 5 week process, but when I finally received it - it was dead, too. The voltage rails were completely unstable and the trace would shift wildly. So after yet another return and a third unit, I am the proud owner of a possibly fully functional adapter.

I say possibly, because I cannot get the offset to work with my Tektronix P6247. The power rails are stable as evident by the stable and correct traces. However, turning the offset knob only shifts the trace marginally, and in the wrong direction. If e.g. I set an offset of +3V, the trace will shift by a few hundred mV to negative values.

Measuring at the offset pad (Pin 5) of the adapter with no probe connected reveals +-200mV, which is correct according to Siglent (even though both their specification and that of the original Tektronix 1103 say +-1V should be present). However, the last adapter would also give correct values for all pads until you connected a probe, then it would go haywire.

I would greatly appreciate any pointers to how I can determine if the problem is in the adapter or probe - I only have the P6247 and the ADA400A, whereby the latter does not use the offset anyway. I could buy another probe, but how do I verify that one is functional?

Obligatory    because this has been going on since February.

Maybe this is interesting to @tautech @TiN @alm .

Thanks,
Andreas

[1] https://www.eevblog.com/forum/testgear/help-needed-siglent-tpa10-tektronix-tekprobe-offset-not-working/msg4013356/#msg4013356

[alm]

The adapter outputting +/- 200 mV while the specificationsl is +/ 1 V sounds like the adapter not meeting spec to me.

The obvious test would be to test the P6247 with an original Tektronix scope (with TekProbe interface) or 1103, and check if the offset control works there.

As for why the offset voltage goes haywire when you connect the probe, my guess would be either the probe is more current than the TPA10 can deliver, or the resistance / EEPROM as sensed by the TPA10 confuses it.

In my opinion, if the probes works correctly on a Tek scope but not on the adapter, then clearly the adapter is at fault. I might have said something similar in the previous topic on the same subject 😅

[FlexibleMammoth]

Thanks as always alm

The 200mV are odd indeed, but according to siglent support the way it should be (even though the spec says otherwise?):

1. Remove and reconnect the adapter without the probe connected;
2. Measure the test points in the figure above, and adjust the offset from maximum to minimum. The voltage at the test point is about - 0.2 ~ 0.2V;
If the voltage still does not change as the offset being adjusted, then the TPA10 should be defective.
Regards

Unfortunately, I already roamed the entire university, and nobody has another Tek scope, 1103, or probe. So for every verification, I would have to buy it myself. Hence, I hope someone can enlighten me if there are some testpoints in the probe that i can measure or other trick.

> or the resistance / EEPROM as sensed by the TPA10 confuses it.
According to Siglent, TPA10 does not have any probe attenuation sensing:

Es werden nur Tek probes mit TekProbe interface level II unterstützt. Aber leider ist die automatische Erkennung nicht möglich.
-> Only Tek probes with TekProbe lvl 2 interfaces are supported. But unfortunately there is no automatic detection [of the attenuation factor].

[alm]

Could you measure the current flowing into the offset pin, for example by putting an insulator with copper and wires on both sides between the offset pin and pad, and putting a DMM measuring current between the two wires, so the DMM is between the pin and the pad?

I don't think Tek specified the max current into / minimum impedance of the offset pin, but studying the 1103 schematics might give you a clue on the output impedance. Is it a high value pot, a low value pot or a pot followed by a voltage follower?

A short in the offset sensing circuit is about the only thing I can think of that would cause this. Otherwise I'd look for either a different brand of scope or probe.

[alm]

How about buying a used Tek 1103, like this ebay listing which looks like it would be an easy repair: https://www.ebay.com/itm/255646186402

It has a separate offset control, so you can control offset and vertical position separately, and should perform fine until up to the hundreds of MHz.

[wasedadoc]

How about buying a used Tek 1103, like this ebay listing which looks like it would be an easy repair: https://www.ebay.com/itm/255646186402

It has a separate offset control, so you can control offset and vertical position separately, and should perform fine until up to the hundreds of MHz.

That item does appear to be missing the (plastic?) skirt around the CH1 probe BNC.  Perhaps only cosmetic and does not affect functionality.

[FlexibleMammoth]

Damn I forgot to turn on notifications on this thread and missed the listing.

I have a search running on ebay.de for a 1103 v2 for about a year, but this unit did not show up since the seller did not include Germany in the shipping destinations. I thought I knew my way around eBay, but it seems like I need to start watching ebay.com as well.

BR Andreas