Show posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

Show posts Menu

Messages - vin97

I completely removed R55 (R60 in my schematic) for now and the level is good.
My plan is to replace it with a trim pot for variable adjustment.
The setting of the power amp trim pots seems to be very critical for the amount of distortion and noise so I want to experiment with this a bit more while keeping the line level constant.

In addition to the noisy chorus (which is expected) I noticed a strange thing where a distorted version of the dry CH1 signal seems to make its way to CH2.
I suspect it has something to do with the signal loopback for the muter circuit. I checked the signal at C121 and it is exactly the distorted signal that I can faintly hear in the background on CH2.
It is very obvious when listening to the chorus signal at C93 with the chorus off (SW2 in position 2), which should normally not give any signal at all.

Is this behaviour expected or is something wrong? I'd guess it was always like this on my amp but I am only noticing it now after listening to CH2 completely isolated through headphones.
Wouldn't there be a danger of burning out the 47K resistor when running without speakers as it would have to withstand the full load of the power amp?
So more volume but higher impedance?
Any chance of hurting the amp?

Because higher level on the line out would be pretty nice. At the moment it is extremely low, basically instrument level. I have to route it through an external headphone amp to get usable levels.
Nice, thanks!
Will have to see if I have a 100 ohm resistor around.
What would happen if I leave it out?

I have a 76 JC120 and it only has a mono line out, where both outputs are merged at the jack.
Can I just solder them to separate jacks or could this cause problems for the power amps?
Cool, one thing less to worry about.

I am planning on using my JC120 as a preamp to record electric bass. Obviously this means I will have to disconnect the speakers.
I am wondering what kind of resistors I should use as a dummy load, if necessary at all?
Ok, thanks for the info!
Finally got hold of an old ('76) JC-120 for a reasonable price, which will arrive in a few days.
The seller said the reverb doesn't work. Could I replace the original one by a longer Accutronics, like the 9DB2C1B?
Also, where can I find schematics for this early model?
Amplifier Discussion / Re: The ultimate JC-120 thread
January 16, 2017, 06:50:22 PM
Ok but I can assume that it is one of the old versions since the chorus works but the vibrato doesn't?

Apart from that it has a bright switch and plastic corners.
Amplifier Discussion / Re: The ultimate JC-120 thread
January 14, 2017, 12:32:15 PM
I've spotted an JC-120 that may be one of the older models but the seller doesn't have the label with the serial anymore.
He says that the vibrato effect doesn't work but the chorus does.
How difficult/expensive is it to repair the vibrato?
Amplifier Discussion / Re: The ultimate JC-120 thread
November 12, 2016, 01:36:28 PM
Ok, slightly different question: What is the maximum delay time for the chorus and could it be increased by simply adding more memory?
Amplifier Discussion / Re: The ultimate JC-120 thread
November 08, 2016, 08:03:32 PM
Ah damn.
That makes getting one of the old models online very difficult.
Amplifier Discussion / Re: The ultimate JC-120 thread
November 07, 2016, 11:11:22 AM
Is there any way of knowing whether it is the old Chorus/Vibrato circuit or the new one apart from getting a guitar and testing it (even then I'm not really sure how you would know)?
There are a lot of '81 and '82 models for sale online that may or may not have the old circuit and I don't know for sure.