EA Tremolo - doesn't trem, Q2 J201 pinout correct

[grolschie]

Hi there,

I have searched this forum, and I am aware that this is a FAQ with the solution being usually that Q2's pins are incorrect. I have the D going to the negative terminal on the cap, S going to ground, and G going to centre lug of depth pot. Seems correct. I have tried a different FET also.

I am fixing a buddy of mines failed build project. I have in my hand a schematic called "EA Tremolo - Layout by Paul Marossy - 10/25/03". This is not the "improved" schematic. It only has 2 transistors and a jfet, and no secondary pulsing LED.  I have re-wired much of it so that now the 3DPT and LED work correctly. Bypass works, but when engaged the is a nice clean boost but no tremolo action.

I am getting voltage readings on both Q1 and Q3, but pretty much nothing on Q2 (usual I guess).

I have checked for shorts. Armed with a DMM, where is a good place to start looking?

Many thanks in advance.
grol

[Processaurus]

To diagnose whether its the LFO or FET section that is buggy, this will seperate them: Set the speed to the slowest, and the depth at max, and see if you measure an alternating voltage on the gate of the FET, that will let you know if the LFO is working.

[mac]

Try a different fet. When I breadborded it, according to my notes, a 2SK117 which is lower Vgsoff than the 201 and has more gain, did not worked. It worked adding a resistor from source to ground. A BF245A worked fine.

mac

[grolschie]

To diagnose whether its the LFO or FET section that is buggy, this will seperate them: Set the speed to the slowest, and the depth at max, and see if you measure an alternating voltage on the gate of the FET, that will let you know if the LFO is working.

Hmmm..... when depth pot is full, the reading varies from 0.2mV to 0.6mV only when I turn the speed knob from one side to the other. There is no alternating voltage at any fixed setting. 

[grolschie]

Try a different fet. When I breadborded it, according to my notes, a 2SK117 which is lower Vgsoff than the 201 and has more gain, did not worked. It worked adding a resistor from source to ground. A BF245A worked fine.

mac

The one that was in it when I took over had "F2454" written on it IIRC. It's now gone walk-about, so cannot tell. I have a bunch of MPF102's lying around though.

[mac]

I forgot to say that I had the same problem with the mpf102. Adding a resitor from source to gnd worked.

mac

[Processaurus]

Hi, since you aren't seeing an LFO at the gate of the FET, then there is a problem in the LFO generation section, or on its way to the FET.  I'd check for shorts, pinout of the BJT in that section, that it has power, etc.

[grolschie]

Hi, since you aren't seeing an LFO at the gate of the FET, then there is a problem in the LFO generation section, or on its way to the FET.  I'd check for shorts, pinout of the BJT in that section, that it has power, etc.

According to http://diyguitarist.com/PDF_Files/EA-TremoloPCB.pdf and http://www.datasheetcatalog.com/datasheets_pdf/2/N/3/9/2N3904.shtml, the pins outs on the other transistors are the correct. The only thing I have noticed is that all the pots are wired in reverse.