Crate G20 XL SCG Circuit Problem

[gump]

Hey all, found this site while trying to get some information on a 90's Crate G20XL amp I picked up off the local Craigslist for $5.

The clean channel works perfect. The SCG circuit, not so good.

When I push the SCG button nothing happens, no overdrive. BUT the 3 knobs on the SCG side are coloring the sound. I can get distortion though, when I max the volume up on the clean channel, but I then use the Level control on the SCG side for overall volume. Seems bassackwards to me.

It seems to me that somehow the functions of these 2 knobs 1. Level on the SCG side & 2. Volume on the clean (EQ) side got swapped somehow.

I know a bit of electronics, but not much. I put in 8 pin sockets in place of the RC4558 chips so I could swap the chips around to see if it would make a difference. Nope. I popped in some TL072 chips and that didn't work either.

I'm hoping that someone might have the schematics or direct me where else to look in the circuit because I'm just aimlessly poking around right now, hoping to get lucky.

Thanks in advance for any help!

Mark

[gump]

No bites yet?

Hopefully this will help. I managed to get a copy of the schematic.

Thanks in advance.

[Enzo]

What is "SCG" anyway?

The high, mid, and low controls should work no matter what. The volume control should work no matter what. Gain, shape, and level should disappear when SCG is off.

If they still function, plug something into the footswitch. That should turn it off. If you have a footswitch, try it, the tip should work the SCG, and it overrides the panel switch. If this makes a difference, check the cutout contacts on the fotswitch jack, and its solder. Make sure the panel switch is OK too. Grounding the tip contact turns on the SCG. With nothing plugged into the FS jack, measure resistance from th tip contact to ground. does the panel switch ground and unground it? It should.

Always check the solder under controls and check them for a cracked wafer.

[cobychas]

plain jane g20

does anyone have a schematic for a plain g20 (not xl)..I have one i came across...worked fine until the reverb housing fell down and shorted across the spk terminals and blew the output...

only diff i can spot is that it only uses 1 IC (tda2040)..still has all the same effects and controls of the xl as far as I can tell..

is it possible that the IC is the only thing that got smoked?

any comment thoroughly appreciated..

not the best electronics guy here..

[teemuk]

does anyone have a schematic for a plain g20 (not xl)

On the last page...
http://www.crateamps.com/pdf/manuals/G20_OM.pdf

[52 Bill]

"Sequential Cascading Gain", ooh how sexy!

Anyway, yes the output chip is probably the only problem with the amp, but when it went, it might have taken something else with it, so check the associated resistors, etc. before powering up the amp after chip replacement.

Replacement of the output chip by a novice is not impossible, but not the easiest job for a beginner. If you do try this one, it may help to have a more experienced friend available for consultation.

[cobychas]

g20 plain jane

Thanks a million Teemuk..

Just for kicks I'm looking for 2 other schematics if anyone happens to have a lead...

fender rumble 15 bass amp..(probably same bug as the g20)

and

Sears Sivertone mod.1482 (harp amp /tube type)..this one i've been searching for quite sometime. Short somewhere keeps burning the rectifier circuit open.

I'd love to get my hands on either of these..(the later one especially) and maybe get 1 or 2 decent amps working for home use.

Electronic organ repair guy here...don't have many inst amp prints but have lots of older/vintage organ schematics I'd be glad to share if any interest.

Cheers..

[cobychas]

Thanks for the advice 52Bill

No prob soldering in a chip here..and i'll be sure and check his neighbors
before pluggin it in.

I'd heard: (S)omekindof (C)runchysounding (G)ain

[52 Bill]

fender rumble 15 bass amp..(probably same bug as the g20)

Try e-mailing Fender

Sears Sivertone mod.1482

This one's available at Schematic Heaven under "Bargin Bin" amps.

[cobychas]

1482

thanks again bill52.. didn't think of looking in the bargain bin section.

[dbraun]

30 yr Silicon Valley HW engineer here. Old post but solution to this problem is the ground. Just that simple. Crate did some sort of wash or solvent on the terminals of the ground nut and bolt and the corrosion results in a highly parasitic contact to ground which gets worse over time(which probably explains why the amps worked for a time then slowly stopped working). Cleaning this and having a good ground is critical for these mostly analog designs. Mine is working once again after sitting about 12 yrs.
The the rest of the board is of good quality. Good solder work and the components are high quality. High volume ICs like these from TI are often designed by a team with established practice and QA so unlikely to be the problem.. Boards often just a few individuals with little verification and I can even see an ECO on the revB board of the G20 I have.
The pots are dirty but easily cleaned. The switches worked and are fairly easy to test and clean. Hope this helps other Crate amps that have similar issues as it would not surprise me them using the same practices.
Darren

[dbraun]

Wow, this is just amazing. Been playing this amp a lot this past year and it started to fade a bit here in the last couple of weeks.. kind of intermittent on the SCG/max distortion operation. So pulled it apart and the corrosion is just unbelievable. Every nut and bolt on the power transistors and heat sink mounts, the transformer mount. I recently bought an ultra sonic cleaner so cleaned everything in a vinegar solution and put it back together and sure enough its working much better. The metal enclosure has some rust too.. probably a lot of these Crates met their demise due to this corrosion. too bad