FET Reassurance for COVERT Build

[debrad]

At the risk of exposing my electrical ignorance, I just want to check on something that has me a little worried:

Today I soldered five SMD versions of the MPF4393 (MMBF4393) transistors into my COVERT Overdrive build...something that is relatively new to me. As a check, I measured for continuity between the SMD pads and their associated "thu-hole" pads to make sure the pins were soldered down nicely. I also checked the continuity between the Source and Drain pads, the Source and Gate, and the Drain and Gate to make sure I didn't have any solder bridges.

To my surprise, I appear to have continuity between the Source and Drain pads on all 5 transistors so, after checking the schematic to see if that made sense, I started scouring the web to determine whether this was a sign of damaged parts...I was VERY careful soldering but wouldn't put it past myself to overheat things; however, having said that, I find it hard to believe that I could kill all five!

Anyway, knowing I might not get an accurate reading from transistors soldered into the circuit, I found that each transistor measured between 0.4 and 0.9 V from D-S in "diode mode" as outlined on one site (my DMM indicates ~0.67 V), and approximately 500k from D-S in "resistor mode", but probing the D and S pads together made my DMM beep in "continuity mode". Reading another site, I understand that the resistance between Source and Drain changes depending on whether the transistor is "on" or "off" but I'm just hoping that more knowledgeable folks here in the forum can tell me if what I'm seeing indicates a problem or not.

Thanks!

- brad -

 

[Robert]

They're most likely fine.

N-channel JFETs have low resistance from Drain / Source until you turn them off by applying a negative voltage to the Gate.

 

[debrad]

Do you think it might be low enough to let my DMM "beep" in continuity mode? Doing some more reading...and crossing my fingers! ?

 

[Mcknib]

If it's in the ohms value yes according to the datasheet it's only 100 ohms

If you continuity check resistors in the ohms region it'll beep I can't remember how high they'll go but they're more or less like a piece of wire internally as I found out debugging

I'd removed several resistors thinking they were faulty until it finally dawned on me

 

[debrad]

Thanks. I think I feel my heart rate slowing...at least a little bit, anyway!

 

[Chuck D. Bones]

Fire it up, Man! If you want a quick health check, measure the source voltages on Q2, Q4 & Q5. They should all be around +6V or so. It's a great pedal. There are a couple of mods for this guy posted in the mods forum. Not mandatory, but they do fix two minor issues.

 

[debrad]

Thanks Chuck. I'm close to finished...and I've used sockets in a few places (C18, C19, R4, R11) to test drive the mods before finalizing my component selections. Looking forward to the final results (provided I haven't killed the 4393 of course!).

 

[debrad]

Finally got my Covert Overdrive soldered up to the point I could give it a quick test drive (still not boxed yet). Just as many of you predicted, the SMD 4393's appear to be fine...the circuit is alive and sounding pretty darn awesome! Thanks everyone!

Now I just have to decide which cap to use for C19 and whether or not to go stock or mod with R4 and R11 (I already opted to use 22n for C18)...

 

[debrad]

Just to close the loop on this one: finally boxed up my completed Covert Overdrive build and I think it might take the top spot in my collection of "Marshall-In-A-Box" pedals.

As I mentioned, I opted for 22n in the C18 position (and modified R3 and R10 to what most folks consider their "proper values...1k and 680R respectively) but left C19, R4 and R11 stock.

Huge thanks to everyone for helping me out and especially to @PedalPCB for offering up this great board. Soldering the components was straight forward, the circuit sounds great, and...unlike a couple other MIAB effects in my collection...the final result is VERY quiet.

 

[Jovi Bon Kenobi]

Excellent design choices! Glad it came together smoothly.

 

[music6000]

Neat Graphic & Tidy Build!

 

[dlazzarini]

Killer job on the graphics and knobs

 

[Barry]

Cool graphic and a tidy build!

 

[Chuck D. Bones]

Looks great! A fun pedal. Have you tried it on +18V?

 

[debrad]

Thanks everyone!

I haven't had a chance to try the Covert at +18v but I will have to do that.

Something interesting that I discovered last night: With the bass and treble on "0" and the middle on "max", there is no output from the pedal...I get sound as soon as I reduce the middle or if I increase either of the bass or treble but nothing at that specific setting. Does this make sense? Does the original DLS do the same thing?

 

[Dreamlands]

The graphic design and knob choices are perfecto. I want this on my pedal board.

 

[Chuck D. Bones]

Something interesting that I discovered last night: With the bass and treble on "0" and the middle on "max", there is no output from the pedal...I get sound as soon as I reduce the middle or if I increase either of the bass or treble but nothing at that specific setting. Does this make sense? Does the original DLS do the same thing?

Something is wrong. In a properly working Covert, there is no combination of BASS , MID and TREBLE settings that result in zero volume. If you'd like some help troubleshooting this, then please open a topic in the Troubleshooting forum.

 

[debrad]

Thanks Chuck. Everything else seems to function properly and it's not a setting I can see myself using so I'm not too worried about it but I suppose it could be a sign of other issues. At this point I'm suspecting the potentiometer itself but maybe I should investigate a little further...

 

[caiofilipini]

I'm sorry to hijack the thread, but what would be a good through-hole alternative to the MPF4393s? 2N5457s?

 

[debrad]

I had a few MPF4393 alternatives lined up (including 2N5457s, MPF102s, and J201s) but Catalinbread seems pretty adamant that the circuit needs the right component so I just went with the SMD version. Despite my panicked post above, it wasn't that hard to solder them in.

I also managed to resolve my issue with the "Mid" control...turns out lug#3 was touching the enclosure!