Neovibe help

[rhdwave]

Hey all, here's another Neovibe issue for me

First off, it was working perfectly and then I was all ready to box it up, I was going to do one more test before hand with the 500k volume pot which had been suggested and that day I put the power plug in and the bulb blew or something wasn't right.  It turned out that it was the bulb.  I have since replaced the bulb but then I ran into this problem with only the vibrato side working.  Well actually, i'm getting a signal from the chorus side, it's just an unaffected signal and it seems to have much more volume than the vibrato signal.  The other thing that is strange is that I did a little recording of me playing with just the vibrato side and I did this with I believe the ground wire disconnected from the volume pot and listening back with headphones on the subway, it sounded suspiciously like the chorus side.      I've checked the r36 resistor, and it has its full resistance intact.  I've checked the resistance on the trimpot and that all seems to be in order as well.  The other thing I did to try and debug was to disconnect the wire from the trimpot which normally goes to the chorus side of the spst switch and instead I connected a jumper wire just to try and trace the sound.  When I touched it to one side of r36, I could hear the faint pulsing of the lfo and thus the vibrato, however on the other side of r36 all I get is the unaffected signal.  This led me to believe that the problem was with r36 somehow or a solder trace etc... but I've checked the board countless times for continuity and everything seems right as well as the resistor being as it should. 

Sorry for the long description, I just wanted to bring all up to speed on everything I've done to try and figure this out.  A question I have is this: is it possible that the trimpot is burned out even though the resistances check out or for that matter the r36 resistor as well?

Oh and to clarify, the way i have it set up is as per R.G. used two 82k resistors for the mixing of the vibrato and unaffected signals, each going to one of the outside lugs of a 50k trimpot.  The center lug of the trimpot is connected to the chorus side of the spst switch. 

Anyway, any suggestions would be greatly appreciated.  I may just try and replace the trimpot to see if that works, but it's a bit of a pain with all the strange soldering to the lugs from the resistors, so i just thought someone here may have a different interpretation of the problem.

Much thanks!!!

[rhdwave]

Last chance! lol 

[R.G.]

It is always possible that R36 is broken or that the trimpot is broken.

I would put the two fixed resistors back in  and see if it works that way, however imperfectly. If it does, replace R36 and the trimpot.

[rhdwave]

Thanks R.G.! So, just to clarify...even if i have measured the correct resistance on r36 after desoldering one side and also the trimpot is still showing correct resistances from the outer lugs to the wiper...it's still possible that it is fried or just not working?

[R.G.]

Yep.

Just to clarify - there is a time when analytical procedures are absolutely necessary and you'll waste more time getting to a fix if you just try replacing something. However, there is a time when the evidence says "it's one of these two components". If the parts are cheap, spend the time replacing both of them. If one is very expensive, replace the cheap one and see if that fixed it. There is a time to just go for it too.

Unfortunately, it takes experience to know what that point is.

[rhdwave]

Thanks again R.G.! I'll do some replacing and let you know the results.

[rhdwave]

Ok, R.G. of course was correct! Much thanks amigo!!! I ran out of 50k trimpots, but i put in a new 82k resistor in place of r36, tried this with the original trimpot, no dice.  This morning i desoldered the r35 and r36 from the trimpot lugs and hooked up jumper wires to a regular 50k linear pot and walah! Working again.  I have to say though to my ears, based on the memory of the sound before (which i know is always a dicey subject) the regular pot sounds different...i would say almost a more metallic sound.  This could all be just in my head of course, but i thought i would share my observations.  The trimpot seemed a bit more mellow to me. 

I guess it's time to order some more trimpots or maybe i'll measure the resistances off the pot for the optimum vibe sound and get some metal film resistors as fixed r35 and r36, but i do like having the option to tweak the circuit in the future.  It was kind of cool just messing with the pot to see how the sound changes.  Was actually thinking of mounting it as a kind of a bias adjustment pot or something and then marking spots on the enclosure for different sounds.  You can definately dial the vibe down with this, though the same can probably be done by using the depth pot.

Anyway, i'm rambling a bit here.  Just wanted to thank R.G. for his help again and post some observations.