Author Topic: VAC and VDC Readings Not Right

March 08, 2021, 01:16:01 AM
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rob@roblanter.com

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Hi JP,

I have built two EQ501 units.

With unit 1 I am getting a reading of .227 VAC on TP2 when sending 2VAC 100hz signal, TP1 is ok at 1.3VAC

With unit 2 I am getting in the setup Step 4 DOA Check, I get  -.7 Millivolts on TP2. Is that ok?

Also with unit 2, in the "In case your EQP501 doesn't work as expectedI am getting some strange readings. When sending 2VAC 100hz signal I am getting .947VAC on TP2, not 1.15VAC. TP1 is ok at 1.3VAC on TP1.

What can the problem be?

Thanks,

Rob

March 08, 2021, 09:14:33 AM
Reply #1

JPK

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The AC voltage on TP2 must be measured with all the pots at 0.
Check again the usual suspects.


Quote
With unit 2 I am getting in the setup Step 4 DOA Check, I get  -.7 Millivolts on TP2. Is that ok?
Yes OK.


JPK

March 08, 2021, 11:01:05 AM
Reply #2

rob@roblanter.com

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When i take my readings of TP2, all the pots are set to 0. I am getting .227vac on TP2 ...that can't be right. What else can I check?

March 10, 2021, 12:03:12 PM
Reply #3

JPK

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We need to find out if the issue comes from the passive part or from the amplifier part.
Measure the AC level on the top pin (close to P1) of P5 (bass ATTEN). You can do that without the DOA inserted. You should get 0.11VAC. You can increase the input to 4VAC to make the read easier, 0.22VAC in this case.
JPK

March 10, 2021, 03:57:08 PM
Reply #4

rob@roblanter.com

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Hi JP,

Without the DOA inserted, at 4VAC I am getting .039VAC on the top pin of P5. What does that tell me?

Thanks again,

Rob

March 10, 2021, 06:29:13 PM
Reply #5

JPK

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The issue seems to be in the passive section. 
We also have the TP3 pin to check on the pot PCB. Measure the voltage on it. It should also be 0.11 VAC (or more).
Check the solder joint of the pots and connectors on the switches and pots PCB's. 
JPK

March 10, 2021, 10:49:08 PM
Reply #6

rob@roblanter.com

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Hi,

I re-soldered the joints on the pots and connectors on the switches. At 2VAC I am getting .010VAC on TP3. What else can I check?

On my other unit I am getting .09VAC at 2VAC on TP3. Is that ok?

Thanks,

Rob

March 11, 2021, 09:25:49 AM
Reply #7

JPK

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With the unit disconnected, measure the DC resistance between TP1 and TP3.
You should read about 10k with all the pots at 0 and 20k with the low freq attenuation at 10.
JPK

March 11, 2021, 02:52:15 PM
Reply #8

rob@roblanter.com

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Hi JP,

My readings were:
 
10.41k with pots at o
11.60k with low freq attenuation at 10.

Thanks,

Rob

March 12, 2021, 05:37:26 PM
Reply #9

rob@roblanter.com

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Hi JP,

I think I might have found the problem. On the Potentiometer circuit board R25 is the wrong resister. Its Red Red Black Black Brown with a reading of 181.1 ohms. I think it should be Brown Black Black Red Brown with a reading of 10K. If this is the problem, I should have checked the resistance with my DMM before I installed it. Having said that though, I think it was the last resister left in the build and I installed it. 

Anyway, can you send me the right resistor if this is indeed the issue.

Thanks,
 
Rob

March 16, 2021, 08:06:51 AM
Reply #10

JPK

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You might want to check first the SK25 DOA which has a 220R resistor in R9.
If OK then check all the other resistor as there is a fair chance that you swapped some.
JPK

March 16, 2021, 07:17:13 PM
Reply #11

rob@roblanter.com

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HI JP,

I think the problem is solved...I had mixed up R9 on the SK25 with R25 on the Potentiometer circuit board.
Everything seems to be workng.

I do have on e question...when I measure DC resistance between TP1 and TP3 on one of the units I get 11.06K at 0. Is that ok? The other unit is 10.41K. Is 11.06K ok?

Thanks for your help.

Rob

March 17, 2021, 08:52:48 AM
Reply #12

JPK

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Yes these values are both correct. They depend on the absolute values of the potentiometers which are specified at +/-20%.
JPK