Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Effects => Topic started by: Jbassfunk775 on October 18, 2019, 09:40:04 pm
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Hey! Im hoping some of you out there with experience in old late 1970’s early 80’s; Vox/Thomas Organ Wah circuits would bestow your wisdom on a frustrated fellow guitarist/gear lover..
-My wah died and I discovered a bad transistor during troubleshooting. I replaced it and it worked but was awful.. No wah more like muffled Warbling.. :think1:
-I specifically have an old Thomas Organ/Dunlop transition era Crybaby.. (late 79’-81’) With the two “4117” marked transistors & the “Stack O’ Dimes” inductor. It’s non tru-bypass of course with a small pc board for mounting the resistors, caps, trannys, & inductor.. The switch, jacks, Pot & battery are wired directly.. No ribbon cable junctions or board mounted jacks like the 90’s-present Crybaby. (See the included schematic & photos)
**Bottom Line; It up and died on me pretty much after I decided to replace both transistors w/MPSA18’s.. Everything else appears to check out ok. After I replaced both transistors with MPSA18’s, I’m getting signal bypassed but no signal at all when engaged.. Can anybody recommend a a possible Transistor? I was told NPN 2N3904’s w/EBC? pinout & 200-350 hfe was the closest modern equivalent to the Thomas Organ part #4117 trannys. Also if you have any thoughts, suggestions at all I’d appreciate it greatly! I’d really love to hear from any other owners with same or similar Crybaby Wah’s.. Thanks!!
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Quick note on the schematic I posted; This isn’t the same circuit that’s used in my wah.. However it’s the closest thing I can find
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I don't know for sure, but due to things sounding 'off' it could be one of a few things:
1. wrong type of transistor. I fo und the schematic for it here on el34world.com:
https://el34world.com/Forum/index.php?topic=9670.0
Scroll down a bit. They used 5117 transistors. I googled for a replacement and found this thread:
https://www.diystompboxes.com/smfforum/index.php?topic=41000.0
on DIY stompboxes forum, they recommended a replacement of a 2N3904
Here is at least one for sale a 44c in the us at mouser:
https://www.mouser.com/ProductDetail/Central-Semiconductor/2N3904?qs=sGAEpiMZZMutncetXCRkfoW1QkrbVyPn
2. the capacitors in the circuit could have also taken damage and for a really old pedal like that, as a matter of course it makes sense to replace any electrolytic caps. I think I only see that one 4.7uF but that should at a minimum be replaced as it's likely gone bad. check the output or input jacks for DC voltage.
3. the resistors may also have gone out, or the choke etc. Some part of the tone shaping circuit which is comprised of the pot, inductor and capacitor network between the two transistors may have gone out of spec or is leaking DC etc.
~Phil
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**Bottom Line; It up and died on me pretty much after I decided to replace both transistors w/MPSA18’s..
Put the original transistors back in.
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**Bottom Line; It up and died on me pretty much after I decided to replace both transistors w/MPSA18’s..
Put the original transistors back in.
Yeah my first thought was maybe the transistors aren't bad, but something else is. It's super hard to kill low voltage transistors. Unless you pump high voltage through them. I'd guess some other component is dead.
Since you have them out, it's usually pretty easy to test a transistor, they test like diodes one way and no continuity the other,
This video explains it pretty quickly and easily>
If you get a typical diode failure, then the transistor is likely dead as well.
~Phil