Tring to fix a Bjfe dyna red and need a help

[walnutzh]

hello friends, recently I'm trying to fix a Bjfe dyna red pedal for my friend. I use a diy audio strobe to check the sound after IC, it seems ok(I'm not sure though) But after the transistor the volume is very low. I measured voltages of IC and transistor:
IC:
2: 3.9V
3: 3.78V
4: 0V
6: 3.9V
7: 8.2V
8: 4.3V

Transistor:
D: 0.12V
S: 1.74V
G: 8.28V

I really don't know where should I go next, I don't really understand how this pedal works. I assume the Drain of transistor should be above 8V...

Anyone can give me some directions? Thanks

[Elijah-Baley]

?m not sure about the IC voltage (pin 8 4.3?), but, the voltage at the transistor sounds a bit funny, indeed. The drain should get the full voltage (8.2v, like the pin 8 of the IC). Then, it seems a lot high at the gate, and maybe a bit low at the source.

May you take some pictures of the the pedal? Outside and inside, so we can see what version it is.

Here's just some information I found rushy.
http://revolutiondeux.blogspot.com/2009/05/bjf-dyna-red-distortion.html

[walnutzh]

thanks！
I just removed the transistor from circuit board and test it. The resistor should be a 2n5458, but I found that's no logo or any marks on the transistor. I think I mislabeled the drain and gate terminal. So the gate is getting 0.12v, and Drain getting 8.2V. According to my understanding, the gate which receives input signal should have larger voltage than Source, am I right?
Also the IC is CA3130EZ. Here's the coding of this chip:



I think pin 7 which is V+ should get full power, not sure what pin 8 does.
I also used your link as reference but there's no pin number on the pic so very confusing.

[antonis]

Although Source voltage (1.72V) should be considered low for an output buffer (it should lie about half-way of power supply), it's OK 'cause Gate input signal amplitude can't be higher than 1.3V_p-p (considering 1N4001 forward voltage drop 650mV..)

You can realize that Diode pair is the main responsible for low level buffer input which is further attenuated by JFET lower than unity gain..

e.g. for a 650mV Diode FVD and g_m of 4000 μmhos for JFET, output amplitude should be less than 450 mV_RMS (ignoring various losses due to shunt items after diode pair..)
( 0.65 X [0.004x10k/(1+0.004x10k)] X 0.707 = 448mV )


P.S.
You can easily turn JFET buffer into CS amp of gain X3 say by adding a Drain resistor and getting out put to Drain, if you like so..

[walnutzh]

thanks antonis, I use audio probe to test the gate and source, gate has quite good volume and source has significant less volume, does it mean something is wrong with this transistor?

[antonis]

IMHO, something, related to particular device pinch-off voltage, is wrong..
(or your measurements are wrong..)

Plz measure voltage differenece between Source and Gate..
(Red probe on Source and Black on Gate to get positive voltage difference..)

[walnutzh]

Vgs measured -1.65v
Vds 5.3v

[antonis]

Vgs measured -1.65v
Vds 5.3v

Those measurements are closer to reality 'cause Source is set at about 3V (8.28 - 5.3 = 2.98) calling for 300μA current..
Taking into account Gate signal swing (+/- 650mV) V_GS cant' go higher than -1V and lower than -2.3V..
Both V_GS voltage levels are within 2N5458 specs (lower than I_DSS and higher than cut-off levels, respectively)..
(of course, JFETs suffer from parameters wide spread but let's be specs believers..)

I can't see any reason for low output level (in relation with Gate signal level) other than output is somehow severly loaded..
(e.g. 5k Volume pot instead of 50k one..)

[walnutzh]

thanks antonis! After relearning some knowledge about JFet transistor, I think you're very right. The transistor voltages seems alright.

Another thing I noticed today was, when I pluged in the guitar into this pedal and then it goes to amp, with pedal off, the bypass tone was not right. It sounds a lot dull and lacks high frequency, like there's some hi cut going on. It's very strange, I checked the input and output jack, checked foot switch wiring, nothing wrong. When the pedal was on, I noticed the sound came from it sound like gated, the volume swing between low and high, the distortion sounds more like fuzz, or fart....

Then I use a pedal with builtin buffer before this dyna red pedal, the bypass tone became normal and right... were all thoes symptoms indicate something in this pedal was loading the guitar signal? Even when the pedal was off and true bypassing?

[walnutzh]

Remove input and output jacks, connect them directly, hot to hot and ground to ground, the tone also become dull as hell... maybe one of the jacks was faulty 

[walnutzh]

update:

Turns out the 1M resistor before IC and the 10K resistor after the transistor source are dead. Replacing them bring this pedal to life! Although I really wanna know how those two resistors caused the low output from IC and transistor 

[antonis]

1M resistor is a pull-down one (serves for input cap popping prevention - although 22nF can hardly cause a pop)..
10k Source resistor sets working current and output voltage level.. Without it, JFET works only with Gate signal swing (+/- 650mV max) using 4k7 resistor + Volume pot as Sourse resistor and providing output amplitude furthe attenuated according to pot setting..