Resistance check issues (Mainline)

[siggyc]

Hi folks,

recently just finished wiring my mainline.
When it comes to the resistance checks, iv followed the procedure and am reading all the correct values with the exception of:
Terminal 1 and Terminal 5 both reading values of 23.6 K-ohms instead of the 39.7K-ohms as per the manual.
I have checked that i used the correct resistors in the correct positions, and the solder joints look solid enough.


Let me know what you folks reckon is best to do.



cheers,
Siggy

[Paul Birkeland]

The position of the "Fine" control will influence that resistance value a bit.  Try turning it to -0dB and see if you're in the clear.

-PB

[siggyc]

Hey PB,

Thanks man for the help. But yeah, that was with both the attenuators set on -0db and the black test lead clipped onto terminal 7.
Will it still be possible to proceed to the next step- the Voltage Check?

Siggy

[Paul Birkeland]

OK, that's not quite what we want to see!

Can you possibly post photos of the attenuator build?  There's a lone resistor leaving terminals 1 and 5, you can check it's value even after the amp is built.  What value are you able to measure for this part?

With the fine switch turned all the way up and the coarse switch turned all the way up, what resistance values do you get for the two lower lugs on the coarse level control?

I wouldn't continue on to the voltage checks just yet, it would be frustrating for this to cause any additional problems down the road.

-PB

[siggyc]

Hi PB,

Sorry about the delay.
I have taken the resistance values of the lone resistors leaving terminals 1 and 5, they are 14.7 k-Ohms for both resistors.

With both attenuators turned up to 0db, when i bridge the two lower lugs on the coarse attenuator i am reading 17.74 k-Ohms.

[Paul Birkeland]

In those photos, your fine switch is not all the way up.  It's totally possible to not set the switch stops properly and end up with abnormalities like this, you'll need to double check that.

[siggyc]

Hey Paul,

Sorry, been away for work.

I'll desolder the wrongly set attenuator, unscrew it and ensure the stops are correct.


Will update soon.



Siggy

[Paul Birkeland]

Generally on the attenuator, you can reheat and disconnect the wires going from the attenuator to the selector switch, then the attenuator can come apart without and additional desoldering.  You'll have to remove the screws that hold everything together, then you can pop off the attenuator controls and reset the stops.

-PB

[siggyc]

Hey Paul,


Reset the attenuators and got the desired resistance values. THANKS!

Moved onto the Voltage testing and got the following values:

+275vDC on the power supply board: 281vDC
+6.3vDC on the power supply board: 6.285vDC
IA on the A side C4S Board: 282vDC
IB on the A side C4S Board: 282vDC
Breg (both sides): 220.1vDC and 219.1vDC
-reg (both sides): 0vDC, 0vDC
Kreg (both sides): 5.655vDC, 5.330vDC

Now, i am in Australia so im operating on 240v mains power, but are these values within the tolerances of the circuit?
Of note are the Kreg values which are clearly out of the stated 8-12v range in the manual.

Let me know your thoughts, cheers!

Siggy

[Paul Birkeland]

Those voltages look good.  The Kreg voltages are off, which I'll have to look into, but they are high enough for a functional circuit, so I'd go ahead and have a listen.

-PB

[siggyc]

Hey Paul,

Just got back from Christmas Holidays, hence the absence. I hope everyone on here had a good time off!

So, I moved on as you suggested to the Biasing stage. When connecting the test leads to Terminal 7 and 20, i got a reading of 218.5V DC and when i checked Terminal 7 and 30 i got a value of 217.8V DC. Is this okay to proceed?

Also when i attempted to make adjustments to both the trim pots, despite 2 turns in either direction, there was no change on both the readings of 218.5 and 217.8. Is this normal?

As an observation, I noticed that the LED's on the C4S boards are only lit up on the "A" side (as in the attached photo), is this normal?

Sorry to be a pain in the ass with all these questions.


Cheers,
Siggy

[siggyc]

I quickly plugged the mainline in and gave it a wee little listen, though even when both attenuators were turned up, the volume was rather quiet.
The headphones were Beyerdynamic DT770's with a 250 ohm impendence. Is this odd? Or perhaps my Kreg voltages were not high enough? Let me know your thoughts.

Siggy

[Paul Birkeland]

When an amp fails a voltage check, that means that it can potentially damage your headphones, source component, or body if you attempt to use it.  Please do not use the amplifier until it passes the voltage check.

Your 6C45's aren't drawing any current, as there is very likely a wiring mistake on the center PCB that is preventing proper operation.  Since both channels are having the same issue, it's likely that this mistake has been replicated which makes it a bit tougher to find.

You'll need to go over both the instructions for stuffing that center PC board, as well as the instructions for installing it to be sure that everything is where it should be.

-PB