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Plastic Stan
Copper Member
United Kingdom
14 Posts |
 Posted - 02/22/2009 : 00:59:58
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MIJ DD-3, sorry don't know the year as the serial number sticker is missing, but it is the version that is the same as the DD-2.
The problem is that their is no guitar signal being output on the mono output socket, but the direct output socket works okay. This happens whether the delay effect is on or off. The delay does seem to work, and responds to the feedback, time and level controls, and the LED switches on and off as normal. Using the DD-2/3 schematic, I've been doing some investigation with a multi-meter and audio probe, and think I have tracked down the fault to Q5 which is the JFET in the output stage that is used to mute the direct signal. Their is a signal at the input of Q5, but not at the inverting input of the 5218 op-amp. I checked the operation of transistor Q7 which controls the gate voltage of Q5 and that seems to be working fine. There is 9V at the collector of Q7 at all times except when the delay effect is on and there is a plug in the direct out socket, when it drops to 0V as you would expect. So this looks okay.
What is a good, easily available replacement for the 2sk30a-y JFET. So that I can replace it to see if this is the problem. I'd also be interested in other peoples thoughts on my diagnosis as this is the 1st pedal I've tried to fix, so I'm still a bit of a noob at this. 
Thanks. |
Edited by - Plastic Stan on 02/22/2009 01:04:52 |
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Plastic Stan
Copper Member
United Kingdom
14 Posts |
Posted - 02/24/2009 : 00:20:49
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I think someone has tried to fix this before because there was a 2sk118 where the 2sk30 should be.
Anyway I replaced Q5 with a good 2sk30 and tried the pedal and it worked okay for about 5 minutes. Then the sound started to get quieter and distorted, until there was no signal getting through just as before.
Also the delay effect does work but the delayed sound is about half the volume level of the direct sound even when the effect level control is turned all the way up.
So now I'm back to where it was. Has anyone got any ideas on what could cause the FET to fail after just a few minutes? |
Edited by - Plastic Stan on 02/24/2009 00:21:32 |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 02/26/2009 : 01:36:27
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A strange problem indeed. The JFET can fail if it is operated beyond its allowed IDSS value. The max IDSS for the 2SK30 is 6.5mA at a VDS of 10V.
I would try a JFET replacement with higher IDSS such as BF245B or BF245C and see if this solves the problem.
Could you also measure the electrode voltages of the JFET against ground when the pedal is on and off ?
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Plastic Stan
Copper Member
United Kingdom
14 Posts |
Posted - 02/26/2009 : 14:41:37
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Thanks for the reply cctsim.
When I replaced Q5 it had ~4.5V at both the source and drain. I never checked the gate voltage as I had tested the collector voltage of Q7 and never thought to check the gate of Q5. So I'll check that tonight.
When the JFET failed it was just after I had put a jack into the direct out to test that the dry signal was muted on the mono output, which it was. So I thought that it was fixed at this point. When I took the jack out of the direct out socket, the dry signal switched back on, but then after maybe 30 seconds started to fade away.
Is it possible if the diode on the gate of Q5 has failed short, that when the gate-channel junction is foward biased the JFET would fail. I assume that's why the diode is there?
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 02/28/2009 : 02:58:01
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If the diode on the gate of Q5 fails the gate potential will be floating, which is most unlikely to damage the JFET.
This assumes the failed diode will be an "open" circuit.
I have never seen a failed diode that is shorted, but I might be wrong. |
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Topic |
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Problem with MIJ DD-3.
