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Old 12th October 2012, 06:07 PM   #531
Itsmee is offline Itsmee  England
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With the power off and the PSU discharged, check with a multimeter that P1 and P2 are functioning correctly, reset them to maximum resistance.

Power up, check that there is a voltage across P1 and P2, this shows that the JFETs are conducting current.

Apply a signal to the input, check with a scope (or a multimeter set for ac) that you have a signal at the junction of Q1/P1 and Q2/P2.

If you have no signal, the JFETS are probably short cct (24 volts across P1 or P2 would indicate this), power off and order some new (matched) ones, ensure that the PSU is fully discharged before replacing, it is important that you observe the static precautions to prevent damaging them.

If everything is good so far, power off and put in an order for new output devices, they are cheap enough if you are only using a single pair, they do not need matching.

Again, ensure that the PSU is fully discharged before replacing the output devices, and observe the static precautions.

Last edited by Itsmee; 12th October 2012 at 06:17 PM.
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Old 13th October 2012, 08:47 AM   #532
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Itsmee View Post
With the power off and the PSU discharged, check with a multimeter that P1 and P2 are functioning correctly, reset them to maximum resistance.
Is this correct?
I thought the bias setting pots/variable resistors were set to zero resistance for the set up condition.
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Old 13th October 2012, 08:28 PM   #533
Itsmee is offline Itsmee  England
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Quote:
Originally Posted by AndrewT View Post
Is this correct?
I thought the bias setting pots/variable resistors were set to zero resistance for the set up condition.
You are correct Andrew, good catch, my mistake, apologies
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Old 14th October 2012, 01:11 PM   #534
Itsmee is offline Itsmee  England
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Quote:
Originally Posted by AndrewT View Post
Is this correct?
I thought the bias setting pots/variable resistors were set to zero resistance for the set up condition.
The test process I posted earlier is valid if you put a short across the gate and source of Q3 and Q4.
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Old 16th October 2012, 11:08 AM   #535
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Hi all.

I'm having a problem with one chanel:
adjusting P2 has no affect (p1 does work), I changed the 5K trim pot and still no good.

Any advice- please
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Old 16th October 2012, 11:31 AM   #536
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check the board and see if the whiper(middle pin) of the pot have contact with one of the outher pins.
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Old 16th October 2012, 12:28 PM   #537
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Thank you

I'll be checking tonight
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Old 17th October 2012, 05:21 PM   #538
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Hey again Audio san

Checked connectors and all seems allright
Anything elso I can check? Can I check if trip pot is okay while its connected to board?
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Old 17th October 2012, 06:00 PM   #539
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massure resistance between midle pin and the outher pin that is not conected to the center pin. and what is the resistance over the resistor in paralell with the pot? do this resistance change when you turn the pot?
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Old 17th October 2012, 06:25 PM   #540
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When I'm mesuring the trimpot I get the same R betweem pins 1 & 3 and 1 & 2 (wiper)
(resistance is changing when i turn pot)
between pins 2 & 3 resistance is always 0

Is this allright?
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