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Old 21st September 2004, 06:54 AM   #11
Eva is offline Eva  Spain
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In worst case the circuit should produce some sound even with the wrong component values or types [provided the error is not huge, ie: 100k replaced by 100 ohm ]

For 100k resistors : Have you tried measuring them without placing your fingers in contact with the measurement tips of your multimeter or the legs of the resistors?

Be aware that our body is conductive enough to thrash high-value resistor measurement and low-value capacitor measurement

Check if U1 is receiving and outputing any signal, without excess offset. Check same thing for U2

Also check if both U1 and U2 receive the right supply voltages
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Old 21st September 2004, 07:38 AM   #12
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Thanks Tube Dude but then why do all the other resistors measure OK?

George
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Old 21st September 2004, 07:52 AM   #13
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Quote:
Originally posted by gchrist
Thanks Tube Dude but then why do all the other resistors measure OK?

George
If the resistors that measure OK are lower value resistors ,is because then your fingers resistance in parallel have less influence.

Another possibility ...your mulltimeter can have more accuracy in certain ranges than in others.

But i agree with Eva...this difference in the resistors is no reason for your pre do not work.
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Old 21st September 2004, 07:56 AM   #14
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George

What op amp have you used?

The power supply is dual rail (+,0.- ) or is single rail ( +,0 )?
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Old 21st September 2004, 09:38 AM   #15
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I have used the 2134 dual opamp. Their voltage is ok 15volts DC.
At the moment I am at work , this afternoon I will check again.

Thanks
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Old 22nd September 2004, 05:06 AM   #16
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Tube Dude was correct. I was wrong. When measuring the spare resistors I touched them with my fingers and that altered the final measurement result. All resistors are OK. On the underside I can see no errors. Everything looks very tidy on the bottom with no bridges between any tracks. The power supply (P05) is working fine and gives me +-15V Dc.

I have connected the - voltage from the ps to the - voltage point on the riaa pcb. The same with the + and ground points. I beleive that on the riaa pcb all the outer connection pins are GND pins and all the inner connection pins are signal pins. That is how I have connected my power and signal wires.

The ICs are ok, they get the correct voltage and have the correct orientation. I do not know where to start and what to measure. Hoe do I measure the input signal from my TT? One point from the voltometer goes to the hot rca inner connector. Where do I put the other point of the voltometer. Do I put it to the GND point of the RIAA pcb at the input location or the power GND on the riaa pcb on the other side of the pcb? I cannot measure the input signal and trace it afterwards on the pcb. I turn the scale on my voltometer to 200mV and I get a reading of 00.3! Is this 3mv or 0.3mV?
What is my ground reference point for measurements?

When I plug the TT signal to another input on my preamp (not thr riaa one) I get signal at the output of the preamplifier which is strong but without riia demodulation. When I pass the TT signal through the riaa module I get silence. You can faintly hear the music in the background.

Finally some resistors when measured on the riaa board give a different measurement from the spare ones that I measure off the board. Other just measure fine on or off the board. Is this ok?

I will look at the capacitors values this afternoon. Maybe a capacitor somewhere has a wrong value. WHo knows I may be lucky!

George
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Old 22nd September 2004, 05:25 PM   #17
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I think I have traced the problem with an active loudspeaker probing the pcb to see where the signal stops. It stops at the legs of capacitor c5 just before the output pin. This capacitor is used to block any DC from the output signal.
One leg has signal ok while the output leg of the capacitor does not. I changed the cap with another one but to no avail.
Should this cap be 1uF or some other value?
Can I bypass this cap?


Regards
George
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Old 22nd September 2004, 08:17 PM   #18
Eva is offline Eva  Spain
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Check : broken PCB track, bad solder joint, output shorted to ground, capacitor marking misunderstood [maybe 1nF instead of 1uF]....
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Old 22nd September 2004, 11:33 PM   #19
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Quote:
Originally posted by gchrist
I think I have traced the problem with an active loudspeaker probing the pcb to see where the signal stops. It stops at the legs of capacitor c5 just before the output pin. This capacitor is used to block any DC from the output signal.
One leg has signal ok while the output leg of the capacitor does not. I changed the cap with another one but to no avail.
Should this cap be 1uF or some other value?
Can I bypass this cap?


Regards
George
And the signal stopping at c5; Is this true on both channels (L and R) ?

What value are you using for R9?
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Old 23rd September 2004, 04:57 AM   #20
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Yes it stops at c5 for both channels. R9=100k

George
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