Fat Pants Jr - no sound after J201

[mikecost1031]

Hi I am having a problem with the Fat Pants JR build. I get no sound past the J201 which I replaced to no avail. The voltages are:

I 9.7
G 0

IC1 (LT1054)
1 1.40
2 5.24
3 0
4 .9mv
5 0
6 2.543
7 1.405
8 9.42

J201
D 3.3mv
S 10mv
G 0

As I noted the J201 is not receiving the correct voltages. The Trim Pot does in fact have 27.5 v at its 3 pins but the TP has a minimal 3mv.

I used a sound probe from the input thru the circuit. There is sound up to the J201 G pin but nothing after.

Is the C12 cap the problem or is it something else?

Thanks

Mike

[BrianS]

Post a picture of the board, botttom and top. 

Is the 201 in the correct way? Don't take that as a slight plse.  It wasn't intended to be.  And do you have another 201 to make sure it's not a bad transistor?  Also, where did you purchase it. It's strange that you have sound into the 201 but it won't come out.

Oh, do you have the switch hooked up or jumped.  It shouldn't make a difference but I've had builds that I tested thinking the switch wouldn't make a difference and as soon as I put it in it worked.

[Boba7]

Sounds like a bad J201 to me too... Had a bunch of bad ones a while ago, I remember trying to make them work in a Fat Pants Jr... no luck. Now a good J201 is hard to come by.

[reddesert]

I've never made a Fat Pants, but looking at the schematic: the J201 should be allowing some current to pass through from the +27V supply, through the trimpot to approx +18V at pin 3, which is also the drain of the JFET. Current passes through the JFET and then R3, the 3.3K resistor, to ground. The source pin of the JFET should be at some small voltage above ground due to the current flowing through R3.

If your voltage readings are that the trimpot pin 3 is 27V, the JFET drain is 0 (3.3mV is effectively 0), and the JFET source is 0, then (a) trimpot pin 3 and the JFET drain aren't connected, since they should be at the same voltage, and (b) no current is flowing through the trimpot or the JFET.  (b) could just be due to (a).

[mikecost1031]

Hi thanks for the feedback. I did replace the first J201 I had in and I am pretty sure it is in the right way. The switch is hooked up. I bought the J201 from skeyescraper on ebay I have one left what is the best way to test it? I have heard there are bad ones out there I have some on B/O from Mouser.

The curious thing is I get nothing at the TP which would be before the J201 and after the trim pot right? Still baffled.

[reddesert]

Please check for continuity between the trimpot leg and the J201 drain. I don't understand how you could have different voltages at those two points if they are correctly connected.

[mikecost1031]

Reddesert There is continuity between the TP and the Drain, however does not appear to be between the tripot and TP. I will take a deeper look tomorrow. thx

[mikecost1031]

Pictures of build

[BrianS]

I would reflow all your joints and try to clean up some of the extra solder. It looks like you've got a bridge between Diode two and pin 1 of the IC on the top of the board. Check and make sure you do not not have continuity there.  There are areas on the back that look close. Also your in, out and ground wires have a lot of solder around them. 

[mikecost1031]

Thx Brian I'll check it out. These boards are small...

[Boba7]

Yes, some of the joints look a bit suspicious.

It's a good idea also to socket key components, here the transistor and IC.

Good luck, hope you get it to work!!

[mikecost1031]

Thanks I have sockets for 8 and 16 pin ICs now and triangle trans but not in lines which I think small bear has. You always need more parts eh?

[BrianS]

Get some of these and just cut them for the transistors. You can also use this for the ones that aren't in line.  Doesn't look pretty but it beats the hell out of paying what they want for the offset sockets.

https://www.taydaelectronics.com/connectors-sockets/sockets/30-pin-dip-sip-ic-sockets-adaptor-solder-type.html

[mikecost1031]

Thanks Brian for the Tayda lead. So these in lines will fit on the MBP boards it is hard to tell by the picture much cheaper than SB for sure.

[BrianS]

Yes they will. Not sure if you used Tayda but they have loads of stuff.  Some of the guys use them for most of their builds.  I buy pots, sockets, enclosures, resistors if I've ran out, film/MLCC/box caps, some of the diodes and leds.  I've had a couple of of their ICs and electrolytic caps go wonkie so I don't buy them there.  For the most part I've been satisfied with their products.