RG's TS10 Mods - Harsh sound after mod..?

[dpresley58]

Was doing some work on the TS-10 Classic. It has the JRC4558 chip, which I socketed. Hooked things up and it worked. Had the same sound as before. All was right in the heavens.

Got ambitious (at least for a newbie) and decided to try RG's retro-fit down to an 808. The resistors were changed on the output buffer to the suggested 100 ohm for R21 and the 10k for R22. I give the board labelling for those playing along on the home edition. The replacement resistor values were verified prior to installation.

Found the 1k resistor coming off pin 3 of the op-amp, which was R12. Here's where I had some doubt. The instructions said "short out the 1k resistor"... I pulled it and bridged the contacts with a short bit of resistor lead. Is that what was intended?

Regardless, I hooked it up again. The tone was _very_ tinny and harsh. Noise (a hiss) was progressively introduced the higher the drive was cranked up.

I replaced  the resistors mounted vertically, emulating what I had just pulled out. The new ones didn't have a coating on the leads like the old ones did, and I used 1% tolerance instead of the 5% I found in the unit.

For the initial hookup, I used a power supply in my pedalboard. The following readings were taken off the op-amp with a tested 9v battery :

1  4.3                
2  4.3            
3  4.3          
4  0            
5  3.6
6  4.5
7  4.3
8  8.5

Does anyone have an idea why the tone would be so harsh and where the noise is coming from? Is there more information required for an answer?

Life can be frustrating when you know enough to know that you don't know enough...

I also stumbled across Lenny Stearns' page concerning mods. His approach seems much more extensive. Any insight into why?
http://home.earthlink.net/~lens/p11-amp.htm

[petemoore]

YEahh...you don't have the feedback [across the diodes] cap in right?
 You got tricked...lol.
 Something's changed with that TS, sounds like.
 OA Pin voltages look just dandy !!
 Did you measure the resistance of each resistor before installing it ?
 There aughtta be a list.
 A list of things that could possibly go wrong, and often do. that way I wouldn't have to actually type stuff like
>>>>>>>>>>COLD SOLDER <<<<<<<<<<<<<<<<<<<<
 ...sorry I just HAD to type it that way...one time...no offense.
 Try debugging page for a nice long list of things that you can check for, one or two of them might be applicable to fix your effect.
 More directly, try an audio probe and see if you can trace through the signal path of the scheamtic and amplified board points there of, to find at what point the noise is getting in the signal path.

[dpresley58]

YEahh...you don't have the feedback [across the diodes] cap in right?

The cap wasn't removed. Was this something to have been added?

Yes, the resistors were checked before installation. And rechecked. And the colorbands eyeballed after installation. Almost to the point of paranoia..  :lol:

A list of things that could possibly go wrong, and often do. that way I wouldn't have to actually type stuff like
>>>>>>>>>>COLD SOLDER <<<<<<<<<<<<<<<<<<<<
 ...sorry I just HAD to type it that way...one time...no offense.

None taken. The solder joints flowed nicely and damn near matched the production joints. What with the mod involving 3 resistors, I figured there's not much that could've gone south with it. Who knew...?

I'll take a look at the debugging pages as suggested.

More directly, try an audio probe and see if you can trace through the signal path of the scheamtic and amplified board points there of, to find at what point the noise is getting in the signal path.

Well, that should keep me busy for a while. Thanks for the suggestions. Was kind've hoping someone else had this problem and might know the symptoms..

[petemoore]

Yupp with those pin voltages, it's 'look other places' time...
 Hopefully the audio probe'll shed some light.
 I don't think raising gain a little...or whatever effect those mods are intended to have would upset the balance of rolloff/noise in the feedback cap [diodes cap]...but if that caps not 'right' the symptoms you described took place in my builds...probably has nothing to do with it, especially since the circuit surely 'sees' it almost the same way as before, as it's not been 'directly' altered, no leads lifted there...
 Alternate list of 'not likelys' but possible:
 Bad cap
 I wish it were a longer list, I'm sure something else could be at fault...have you tried another OA ?