Rolland Jazz Chorus 77

today we got a cool amp in, the JC 77,  distorted fizz is the complaint, saw a lot of sawtooth on the pwr sup, bad 3300/50, replace it,

same thing happening, isolate to one channel being distorted like it is running off the drivers,

2 problems for the price of one, maybe the bad cap cooked the channel, maybe not,

gut the chassis like a 6 point buck,

what is a D716 transistor?  internal p/n?

no, it is shorthand for a 2SD716, people cutting back on ink we suppose,

replace the header cables with real wires,

hunt for schematic, a few out there, none with these transistors, ask Rolland?  good luck, time to trace the darn thing, quicker than searching 3 days on Goggle, or whatever it is,

there is no joy without suffering first, that's the game,  ;D

here is a schemo for one half of the pwr section,

.022 cap is puzzling, does not appear on the actual circuit board, only shown on schemo we found online which is in the next post,

gonna put in some voltages next, we have a good channel to steal voltages from, because this is a stereo amp,

here is the online schemo showing the .022 cap in the pwr circuit, this a jc 120 jc 160 circuit,

The 0.022u cap is utterly optional.

The Q33 Q32 bias source isn't stiff enough to yank-out stored charge in the output stages. When driven hard at the top of the audio band, there are glitches. Some designers will use much more than 0.02u here. Others ignore it; customers probably won't notice.

All Japanese transistors start "2S". In Japanese documentation (and marking), if it is "obvious" (to who?) that this is a transistor, they omit the "2S". Maybe a little like the way we say '3055, '2222, (2N3055, 2N2222) except the Japanese seem a little quicker to drop the prefix.

Of course you know "distorted fizz" could be as simple as a dirty switching jack. Or a royal pain to track down.

I'd put 0.02u (up to 1u) on the bias if a cap were handy, not if it were not. It might make me feel better. It is unlikely to be the cause of the customer's complaint. (Unless you only have fizz with a specific digital-based effect which may be throwing supersonics.)

yeah that ext spk jack is pretty sketch, i guess it does not matter if a solid state amp loses it's load while cranking out the watts, unlike a tube amp which kills the OPT, that jack will be the first thing to look at,

we will experiment with that .022 while scoping this thing out, should be pretty easy as the one channel is really weak and distorted,  and these amps all have a rep for hiss via the chorus circuit so we dont worry about that,

i thought that the .022  would short out the audio, need to read up on direct coupled amps, 

that 220K in parallel with the 15K looks to be a trimmer, maybe to dial in the offset, (R146, 147) in first drawing) we will see,

ordered some Toshiba output trasistors from China, hope they are the real deal, ebay is now dominated by offshore stuff when it comes to semiconductors, those drivers are not easy to come by either but probably are easy to cross,

output transistors test good, but three have an hfe of 600ma Ic/3.8ma Ib= 157, while one in the bad channel checks out at 450ma/3.8ma = 121 so they might have been stressed by the open filter cap on one of the rails, so we bought four, s/b able to get a match, that old stock has limited bandwidth which helps keep the original sound,

got a JC 120 in the shop, just needed a reverb pan,

couldn't find the chassis bolts, burried ,

no smoking allowed,

started scratchin, so we gave it the flea and tick spray, then some English Leather to drown out the tick spray,

i wonder if Ted recorded catch Scratch Fever with this amp,

then a complete truck mount shampoo job of course>

CJ said:

got a JC 120 in the shop, just needed a reverb pan,

couldn't find the chassis bolts, burried ,

no smoking allowed,

started scratchin, so we gave it the flea and tick spray, then some English Leather to drown out the tick spray,

i wonder if Ted recorded catch Scratch Fever with this amp,

then a complete truck mount shampoo job of course>

Click to expand...

Nice one CJ!!!

I Guess if the amp sounds cold at least try to give it a bit of Africa to warm it up!

bit of Africa? alright,

Procaro putin down some backbeat>

https://www.youtube.com/watch?v=qMbc5746HUo

Lololol

I think that amp would be amazing on that stage!