Removing knob from switch on Roland JC77

[lukes]

Hi,
this is Luca, I recently bought a JC77 and I'm rather happy with it.

Reverb tank seems to have a broken output transducer, and most of the jacks are very oxidized inside. So I was thinking of replacing them with better parts, getting a new tank (8ab2d1a [8/2250Ohm] seems to be a good possibility, maybe?) and while I'm in there clean it all up a bit (it's all very dirty inside, and a portion of the main pcb is covered in a brown goo that doesn't look too good. As I have it dismantled I was thinking of probing a bit around with my scope to see if I can get the noise floor down a bit, it's a little hissier that I would prefer. Folks on the web don't seem to have a cure for it, but the schematic (for the 120's I've seen) doesn't seem too complicated.

Anyways, as I was dismantling it (or better: trying to dismantle it) I had trouble removing the knob from the chorus switch, and as I didn't want to break it I figured I'd ask for advice on how to proceed.

You can see the switch in this picture

http://upload.dazurn.com/jc-77.jpg

I tried pulling on the knob or trying to unscrew it, but neither seemed to work. I can't see a screw to old it in place, so I would really appreciate if you guys could share the trick to pull it off :-)

Also, as to small upgrades: do people replace the output transistors with better parts on this amp? (Say an MJL3281, for example) Another thing that perplexes me a bit is the feed/return for the reverb running right parallel to the speakers cables, wouldn't it be better for them to take paths as far as possible from each other? Wouldn't moving the reverb tank to the side or bottom of the cabinet help with mechanical insulation?

Thanks for reading
Luca

[DrGonz78]

I am betting that it is possible some previous owner thought it would be a wise idea to super glue that knob cap on the switch. My first thought is nail polish or something that has acetone. Put a few drops of that stuff right up at the back of the knob and see if it loosens up a bit. I am curious about this "brown goo" all over the board, pictures? I guess I just like dirty pictures.

I keep doing stuff to an old JC60 that I bought a few years back. With the reverb off completely and chorus off the amp is completely quiet. I turn up the reverb just a bit and there is a bit of a hum. Also, of course you get the waterfall white noise sound out of the chorus.

[allante666]

You beat me to it DrGonz, I was just abou tto say I bet someone glued it on as it kept falling off. They usually just pull off. If its plastic though acetone will probably melt it?

[DrGonz78]

Yeah acetone might have a negative effect on the plastic. I would only use it sparingly applying using a lightly dipped q-tip or something. Also, if you have some expendable old knobs lying around try it out there on the plastic first. There are probably other ideas to weaken the super glue. Possibly try dabbing a q-tip with alcohol or something first just to see if it is dirt/grime holding tightly in place.

Edit: Also heard of people using heat guns to loosen them up a bit, but I have never tried that approach.

[lukes]

Hi guys,
thanks a lot for the information, this is really useful!

Also heard of people using heat guns to loosen them up a bit, but I have never tried that approach.

Do you know at all how much the paint on the front panel will be affected by the heat from the heatgun?

I'll try the solvents, I have a number of things to try in that direction.

As to the gooey bits, I'll post a picture once it's open again, likely this weekend.

Many thanks for now,
Luca

[DrGonz78]

Just don't face the front of the heat gun directly to front chassis of the amp. Instead come from the top and back of the amp (chassis removed) and hit the spot as accurately as possible. Like I said I have not tried this sort of thing so that is just what I would be thinking if I was going to try it. What I think might work with heat is that the metal shaft expands then when it cools it constricts a bit. Perhaps that is what we want to have happen enough just to break the seal of the glue. Another thought would be to heat the shaft of the switch with a soldering iron to get it hot to achieve the same. None of these ideas seem ideal or safe for plastic or any part of an amp really. Message to the world: Don't super glue these damn things to the control shafts!!

Edit: Solvents...? I have used Goof Off on plastics and had good results. Got a pick guard that someone painted red square patterns all over and it worked great getting it back to black.

[Enzo]

Wet a towel and lay it across the panel, so any heat gun blast that goes that way won't get to the panel itself. Or take a piece of aluminum foil and make a collar around the shaft of the switch. Like one of those things they stick on a dog's neck to keep him from licking a wound.

Possibly heat the shaft directly by laying a soldering iron on it. is there room?

[lukes]

Well, what do you know? After a bit of wiggling to the sides I pulled on the knob a bite more firmly and it slowly started moving and it eventually slipped off.

At a quick inspect it seems to be held in place by means of a plastic insert on the knob side, I'd think it would melt very easily if one goes at it with a heat gun... I'll try t post a picture in the next few days.

Also, I'll try to characterize the noise I have a bit better, but I don't recall much hum, if any (once the grounding started working), I'm talking more of the white noise thing DrGonz calls waterfall noise. I thought it was dominated by the main volume, I'll take a better look and report back on it.

Does anybody know how quiet all the jfets in these circuits are meant to be? Those SIP opamps sure look like they could use a more modern part, although in that form factor I don't know how much fun I'll have... I wonder if the make SIPtoDIP thingies...

Edit: re SIPtoDIP: browndog makes them, and also has them good to go with the opamp already soldered and all :-) I should have known...

Anyways, thanks a lot for now all!
L

[Jazz P Bass]

Don't think too hard on this.

If you have a specific problem, state it.

It's a guitar amp, not a piece of lab equipment.

[lukes]

Hi Jazz P Bass,
sorry I don't understand what you mean, to be honest.
I find that I want my instrument in good tune, my tone to be in place, my strings to respond as I want them to, why wouldn't I want the same from my amplifier?
To me it's the same as a poorly exposed or blurry photograph, unless you're after that effect for a reason, you want your tools to respond properly to you, no?

L

[Enzo]

The photo looks sharp and clear, but if we magnify it and look real close we might see small chromic aberrations. But only under that laboratory scrutiny. Anyone looking at the photo would not notice it. I think what Jazz meant - not speaking for him - was that it can be diminishing returns by sweating every last component. Paper improvements are not always auditory improvements. Just an opinion.

Does anybody know how quiet all the jfets in these circuits are meant to be? Those SIP opamps sure look like they could use a more modern part,

The amp, JFETs and all the other parts included, ought to be fairly quiet. Did we link a schematic? I don;t see one. Individual op amps and JFETs might be noisier than others. We can't extrapolate from one to all or from data sheet to individual parts. SIPs are the same parts inside as the same numbered DIPs. If we have reason to suspect one or more is not up to par, I imagine you could knock out a small board to edge mount a DIP or an SM part. A row of 8 or 9 pin header pins in 0.100 spacing to solder to the board, and a small perf board or similar to mount atop that with a DIP on it.

[The Dude]

A guitar signal is very low level and carried on an unbalanced cable (active/hotter pickups can help this). Because of this, we need lots of gain in order to be able to get it up to speaker level. It's nearly impossible to achieve that without also amplifying some noise along the way. (I believe) What Jazz is saying is that you'll never achieve silence, so don't overthink it or worry too much about a little noise.

[olddawg]

A guitar signal is very low level and carried on an unbalanced cable (active/hotter pickups can help this). Because of this, we need lots of gain in order to be able to get it up to speaker level. It's nearly impossible to achieve that without also amplifying some noise along the way. (I believe) What Jazz is saying is that you'll never achieve silence, so don't overthink it or worry too much about a little noise.

We always said a good noise floor covers a multitude of evils !

[Enzo]

Don't recall if it was here or another forum, but someone was showing response displays on a scope to demonstrate harmonics appeared over 5kHz and that could be improved upon, ignoring that they were already 120db down.

[lukes]

Well ok,
so I picked up my meter and took a quick sample. My room is relatively quiet, with no cables connected, all the knobs to 0 (volume, reverb, rate, depth) and chorus on off turning the amplifier on reveals about 4 dbA higher noise than the room background noise. Bringing the volume to 5 increases the noise by anothe 7dbA (to room+11, if you will).
The reverb knob mostly introduces some 50Hz hum, the chorus depth knob (with the switch on manual) has the effect I'd imagine.
By noise I mean some sort of white-ish spectrum signal.

With the volume on 5, I was moving the tone-control knobs, while the treble knob seems to let through more high, as one would expect, and the middle and bass knob have no macroscopic effect, the hi-treble knob seems to let through less noise on higher values (opposite as you would naively expect). I've had a hard time finding a JC77 schematic, but I do have a JC120 schematic (in a file saying it's a 77) which has about the right age (1984, matching what the Boss serial number finder says of the serial number on my amp). I'll spend some time putting together a schematic of my 77 now that I can actually take the board off... (and thanks again).

Anyways, I've built a number of amplifiers before (hifi), setup, rebuilt and fixed turntables, you know, got my hands into audio things for a few years now. I have owned a few guitar amps (and a numnber of hifi ones), and none of these are anywhere near as noisy as this one, that's all. I find it strange. I do understand high gain. I also know that balanced or not, amplifiers can be quiet, or at least, quieter than this.

And besides: I like learning things (a lot, I'm very curious) and I like tinkering with stuff and understanding how things work. There are a lot of very knowledgeable people on this forum, which is why I decided to subscribe (I have a rather minimal footprint on the web). I have learned a lot from you guys already, this is cool :-)

If I have time tomorrow I'll see if I can get a spectral analysis of this thing done to see what's up.

Thanks for now
L