Resistance low at terminal 1 and 5

[Rican]

Sorry for the repost, I tried to attach images and can no longer view the original post (get an http 500 error), so am re-posting.

I just completed my build, and when going through resistance tests, with course and fine controls all the way at 0db, I measure 19.7K ohm at terminal 1 and 5. However, the manual says I should read 39.7K ohm on each, so there is a big discrepancy.

All other terminals read 0 ohm (or close to) except terminal 15 and 25 which read 205 ohm, which I believe are in the correct range.

Resistance at terminal 1 and 5 go down when I lower the fine tuning. At -45db I read 9.9K ohm at terminal 1 and 5. Changing the course attenuator does not change the readings on terminal 1 and 5.

I get 17K ohm on the mid post of A and C with fine tune and course at 0db. This does drop as I rotate either, with both at -45 db I get 130 ohm on the mid post of A and C.

I'm testing with a Klein MM325, and also checked with an OWON HDS272S. they vary slightly but are close enough to know I am getting accurate readings.

I've retraced the wiring, and double-checked the resistors on the attenuators and can't seem to find the issue. Thoughts?

[Paul Birkeland]

I would definitely post build photos. 

Can you turn the fine control all the way up, then measure the resistance at each of the lower lugs on the fine control?

The -45dB setting would be the coarse control, you'd want to see the readings at 1 and 5 change as you move down the smaller set of steps.

Also I'd want to double check that nothing is plugged into the RCA jacks.  It's actually somewhat possible that there's an issue ahead of the attenuator that is causing some issues, but we can dig into that if necessary.

[Rican]

Thanks PB!

fine and course control all the way up:

front lower lug (fine control) to terminal 1: 11.89K
front lower lug (fine control) to ground (terminal 7): 17.08K
rear lower lug (fine control) to terminal 5: 11.89K
rear lower lug (fine control) to ground (terminal 7): 17.09K

Moving the course control does not change resistance on terminal 1 or 5 measured to ground, both remain at 19.77K. Course control does change readings at the center lug of tube A and C.

Nothing plugged in to the jacks.

[Rican]

Pictures below, did it this way because it gave me errors and locked me out of the post when I tried to attach inline.

Fine Tune pic:
https://drive.google.com/file/d/14zjSPzQ2L_b8EzeEY07N9GfrQhtEN4FC/view?usp=sharing

Course Tune Pic:
https://drive.google.com/file/d/1soo3pqN8dEiAN3PV5RYPxNEfOz9Ne0jw/view?usp=sharing

Mainline Overhead pic:
https://drive.google.com/file/d/1PQtFxa8FeBV970Cx7pPcsAaRRu0HgHL4/view?usp=sharing

Submissive Attenuator overhead pic:
https://drive.google.com/file/d/1PhJBTYEs_nmmP4dNMST8MMkMBHqlobuZ/view?usp=sharing

Submissive Attenuator front pic:
https://drive.google.com/file/d/1k91TPglaWBovSnk0zuElYvA2xm22zbIM/view?usp=sharing

Submissive Attenuator rear pic:
https://drive.google.com/file/d/18W1ezRBhqAhfOIpKNtkv5O1zUoKtQ45G/view?usp=sharing

[Rican]

BTW, I noticed I took the picture of the course attenuator when it was all the way down to -45db. I can confirm that I did my testing with it at 0db (I've tested at least 50 times by now...), and that the pins do connect to the 1 and 7 position when turned to 0db, so I believe the stops are all set correctly.

[Paul Birkeland]

Next you will want to turn the fine control all the way up, then check every resistor value with your meter.  The ones I can read in your pictures look to be in the same place, but your ~17K reading that is measuring the entire resistance of the coarse stack isn't correct unless the fine control is engaged (not set all the way up at 0).

I do note that the blue and blue/white stripe wires going to your selector switch look to be soldered to the same lug at the selector switch, or possibly they have melted together a bit.  This would throw the readings off on at least one of your channels.

[Rican]

I noticed the blue wires in the picture looked that way, but I did a continuity test to make sure they were not touching. Here is a better pic:

https://drive.google.com/file/d/1nUpcNiOiUuOG7iT6Z8G-OBeMMZIENUyB/view?usp=sharing

Fine tune and course at 0db:

Fine tune position 2: 48.2K
Fine tune position 8: 48.2K

Fine tune position 3: 22.6K
Fine tune position 9: 22.5K

Fine tune position 4: 13.6K
Fine tune position 10: 13.5K

Fine tune position 5: 9.2K
Fine tune position 11: 9.2K

Fine tune position 6: 6.6K
Fine tune position 12: 6.6K

Fine tune lower lug Front: 11.5K
Fine tune lower lug Rear: 11.8K

[Rican]

Course attenuator (at 0db):

1 to 2: 12.82K
7 to 8: 12.81K

2 to 3: 5.32K
8 to 9: 5.30K

3 to 4: 1.97K
9 to 10: 1.97K

4 to 5: 700
10 to 11: 700

5 to 6: 250
11 to 12: 250

6 to terminal 10 / ground: 130
12 to terminal 8/ground: 130

Lower lug front to ground: 17.07K
Lower Lug rear to ground:  17.07K

[Paul Birkeland]

1 to 2: 12.82K
7 to 8: 12.81K

If this is with the fine control all the way up, can you read the writing on the resistor to indicate the value?  They should say 1622.  Definitely don't remove these because they aren't measuring properly.

[Rican]

Yes, both read 1622F. Do not remove, or did you mean to say do remove because they are measuring wrong?

[Paul Birkeland]

Do not remove them.  I would strongly suspect that your meter is a little on the iffy side if it's not reading those values properly.  Could you post the model number of the meter you're using?  If it's not auto-ranging, then you may need to go to a lower range to get more accurate readings.

[Rican]

The previous measurements were taken using a Klein MM325 on the 20K scale.
https://www.kleintools.com/catalog/multimeters/digital-multimeter-manual-ranging-600v-0#Specifications

I re-tested with an OWON HDS272S.
https://www.owon.com.hk/products_owon_hds200_series_digital_oscilloscope

I get very similar readings from both tools:

Course attenuator 1 to 2 (1622F):
Klein = 12.82K
Owon = 12.931K
 
Course attenuator 6 to 9 (1622F):
Klein = 12.81K
Owon = 12.928K

I retested all other resistors with the owon, and they all test pretty close to the klein, about a 1% difference. Why those resistors are so far off, but both reading the same readings the same on two different tools is just confusing to me.

[Rican]

If i understand the circuit, those two resistors should be out of the circuit when the course is at 0db? So although those measure wrong, they wouldn't make terminal 1 and 5 wrong at 0db? I have two different problems?

[Paul Birkeland]

Can you disconnect the red and white wires at terminals 1 and 5 and recheck those values again? 

[Rican]

Thanks again PB for all the help!

Removed the white and red wire from terminal 1 & 5. 1 & 5 to ground is OL.

Fine tune Front lower lug to terminal 1 = 14.58K
Fine tune Rear lower lug to terminal 5 = 14.58K

Course 1 to 2: 16.09K
Course 7 to 8: 16.09K

So, looks like removing those wires brought the resistances into correct range? Does that mean the problem is at the switch or the input?