[BUILD] New FET/RACK Official Help Thread - Please read first post!

[Echo North]

Hello

I have built the REV A FET/Rack and am having trouble setting the QBias.

Working through the troubleshooting section I don't appear to have -10VDC at CR9... It in fact appears to be tied to the chassis (through continuity tester) along with the same rail as R66, R72, R76, R64...

From the DC voltage sheet, all the transistors appear to have the correct voltages apart from
Q12
E = 0.411
C = 26.7
B = 0.96

Q13
E = 0.391
C = 26.93
B = 0

This is obviously related to the discrepancy at CR9, but thought id provide as much info as possible!

Cheers
James

Pull CR9 and diode test it with your DMM.  Might be blown.

 

[Kevin Prochnow]

Hello!
The Input pot/input gain seem to be acting funny. There is a massive amount of compression as soon as the input knob dash hits the first dot mark (7o'clock), and as you keep turning it past the 7o'clock point, the compression/gain doesn't seem to change that much for the rest of the way.
So pretty much: Starting at 0 input, turning it slightly does nothing nothing until it hits 7o'clock and then all the compression hits it, and then nothing much else happens for the rest of the clockwise turn.

I pulled the T-pad out of circuit and the 2 outer gangs tested to be linear taper at 500ohms and the middle gang tested to be what looked like logarithmic taper at 1000ohms. Idk if that is what it is supposed to do, i don't really understand T-pads.

Let me know if this sounds familiar or if it makes any sense haha thanks!

Kevin

 

[saqeef]

What is your AC V reading at each orange wire of the transformer VS chassis ground now?

About 30.18 AC V and 31.19 AC V, give or take a little.

 

[Echo North]

About 30.18 AC V and 31.19 AC V, give or take a little.

1. Check your CR4-CR7 polarity.
2. What is your DC V on either side of R80?

 

[Echo North]

Hello!
The Input pot/input gain seem to be acting funny. There is a massive amount of compression as soon as the input knob dash hits the first dot mark (7o'clock), and as you keep turning it past the 7o'clock point, the compression/gain doesn't seem to change that much for the rest of the way.
So pretty much: Starting at 0 input, turning it slightly does nothing nothing until it hits 7o'clock and then all the compression hits it, and then nothing much else happens for the rest of the clockwise turn.

I pulled the T-pad out of circuit and the 2 outer gangs tested to be linear taper at 500ohms and the middle gang tested to be what looked like logarithmic taper at 1000ohms. Idk if that is what it is supposed to do, i don't really understand T-pads.

Let me know if this sounds familiar or if it makes any sense haha thanks!

Kevin

What revision do you have and what color is your TPad?

 

[saqeef]

1. Check your CR4-CR7 polarity.
2. What is your DC V on either side of R80?

1. CR4, CR6, and CR7 look correct for polarity, but CR5 is would be the 30V DC zener diode on the Rev F board, would it not?
2. There isn't an R80 on the Rev F board. :/

 

[Echo North]

1. CR4, CR6, and CR7 look correct for polarity, but CR5 is would be the 30V DC zener diode on the Rev F board, would it not?
2. There isn't an R80 on the Rev F board. :/

Oh sorry, for the F, check the orientation of CR7->CR10 and the DC V on either side of R84.

Mike

 

[saqeef]

Oh sorry, for the F, check the orientation of CR7->CR10 and the DC V on either side of R84.

Mike

CR7-CR10 all are good on orientation.
R84’s DC V on one side is 33.1V, and 12.0V on the other side.

 

[Kevin Prochnow]

What revision do you have and what color is your TPad?

I have Rev A and my t-pad is dark gray

 

[Echo North]

I have Rev A and my t-pad is dark gray

Shoot me an email.

 

[Echo North]

CR7-CR10 all are good on orientation.
R84’s DC V on one side is 33.1V, and 12.0V on the other side.

You're sure R84 is correct? 75Ω/3W?

This is going to be super hard since you didn't test the supply initially.  Maybe CR5 is weird, but it also could be any of the semis on the rail pulling too much current. Q8 and Q9 are in the correct spots?  Correct orientation.  Also check CR1 and CR2.

Any transistors getting hot?

Mike

 

[SunkenCity]

Got a little over zealous and accidentally separated Q12, Q13  mixed them with the rest of the 2N3707's and soldered them all in.

What symptoms will the unit show/what happens if they aren't matched?( I was building two units at the same time and one is as it should be and the other has the unmatched pair, I think I remember which is which) 

What Hfe should a matched set be?  Will a off the shelf substitute for the 2n3707 work?   

Cheers & Thanks

 

[Echo North]

Got a little over zealous and accidentally separated Q12, Q13  mixed them with the rest of the 2N3707's and soldered them all in.

What symptoms will the unit show/what happens if they aren't matched?( I was building two units at the same time and one is as it should be and the other has the unmatched pair, I think I remember which is which) 

What Hfe should a matched set be?  Will a off the shelf substitute for the 2n3707 work?   

Cheers & Thanks

Keep them in there.  If you can calibrate your meter, you're fine.

 

[SunkenCity]

Keep them in there.  If you can calibrate your meter, you're fine.

So I tried (on unit 2 I believe to have put matched 2n3707s in) to do the q bias with the  test signal and wasn't getting anywhere close to +11/2.75 Vac at the output. It was showing up at around 0.050-0.090 Vac acting sporadically output adjust and all other controls have no effect. 

Unit one is shorting out at the cn4 connector lighting it up I took it out of the chassis and checked for solder bridges and found nothing looking in between the board and switches. checked the connector as well .  checked the off part of the switch and there is continuity between every single leg no matter if its depressed or not while all connectors are disconnected.  Bad switch?

 

[SunkenCity]

Disconnected all connectors from the meter pcb except c4 plugged it back in and it shorts and sparks immediately. I've gone over it again and again looking for a solder bridge and can't find anything.

 

[SunkenCity]

Swapped the meter board assembly with the other unit and it didn't spark attempted cal with no luck still no output at the xlr. Checked cr8 and cr9  got about 20.00 vdc and 9.70 vdc checked the other unit after putting it back together and got the same readings.  looked at cr8 on both units and it looks like it bubbled out a bit. 

 

[Echo North]

Swapped the meter board assembly with the other unit and it didn't spark attempted cal with no luck still no output at the xlr. Checked cr8 and cr9  got about 20.00 vdc and 9.70 vdc checked the other unit after putting it back together and got the same readings.  looked at cr8 on both units and it looks like it bubbled out a bit.

No even sure where to start.  So you have yet to get proper rail voltages on either unit (-10VDC/+30VDC)?

Mike

 

[SunkenCity]

when I built the power supplies I got the right voltage I thought.

I took cr8 out and tested it with the diode setting on the fluke and got .5 one way and OL the other. Got continuity both ways.

Started going thru the power supply and the voltage drops from 32. vdc( at cn1) to 20 vdc after r81. I double checked r81 (un soldering one leg) with the meter its the 1k.  While I had one side out I was still getting 20 vdc at the positive of c24 and r80. 

 

[Echo North]

when I built the power supplies I got the right voltage I thought.

I took cr8 out and tested it with the diode setting on the fluke and got .5 one way and OL the other. Got continuity both ways.

Started going thru the power supply and the voltage drops from 32. vdc( at cn1) to 20 vdc after r81. I double checked r81 (un soldering one leg) with the meter its the 1k.  While I had one side out I was still getting 20 vdc at the positive of c24 and r80.

You're unit is pulling too much current causing the drop to be bigger than it should be (ohms law).  So is CR8 getting continuity both ways right now?

 

[SunkenCity]

In the continuity setting on the meter with positive lead on the cathode and negative on the anode it goes from .5m and continues up.  the other way around it go 170k and continues up. 

in the diode setting with the positive lead on the anode I get .563 vdc and reversing the leads shows overload.