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1big1
New member
does anyone here have a schematic for a jet city jca 2112rc combo? i e-mailed the company and asked for one.. but haven't heard back from them yet. any help would be appreciated. thanks in advance. 
MississippiMetal
New member
I would imagine it uses the same circuit as the JCA22 head version, which can be found here: http://www.guitarampboard.com/forum/vie ... f=5&t=3840
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1big1
New member
i talked to the tech who was working on mine and asked him if the schematic for a 20h would be helpful and he said no.. they aren't the same circuit because he'd already looked at it. i know the 2112 combo is based on the astroverb 16.. but i'm not sure if that schematic would be helpful either. i'm still waiting to hear from the folks at jet city. mike? doug? help! 
the guy who's working on mine said the schematic would be helpful because the scope says the problem is somewhere between the input and the phase inverter. it would be more helpful to have the schematic for doing the math rather than just start swapping out parts. that just runs the bench time bill up.
the guy who's working on mine said the schematic would be helpful because the scope says the problem is somewhere between the input and the phase inverter. it would be more helpful to have the schematic for doing the math rather than just start swapping out parts. that just runs the bench time bill up.
MississippiMetal
New member
1big1":1swqw2ln said:i talked to the tech who was working on mine and asked him if the schematic for a 20h would be helpful and he said no.. they aren't the same circuit because he'd already looked at it. i know the 2112 combo is based on the astroverb 16.. but i'm not sure if that schematic would be helpful either. i'm still waiting to hear from the folks at jet city. mike? doug? help!
the guy who's working on mine said the schematic would be helpful because the scope says the problem is somewhere between the input and the phase inverter. it would be more helpful to have the schematic for doing the math rather than just start swapping out parts. that just runs the bench time bill up.
So you track down the Astroverb schematic and use it to navigate the circuit. If you can't do that, id advise you to put the amp back together and take it to ANOTHER tech, as yours is questionable when he needs a scope to tell him the problem is between the input and the PI.
Or you can use this schematic because it can't be a world of difference between the head and combo circuit. These are budget amplifiers. It doesn't make business sense to have separate PCB's drawn up and manufactured.
http://starin.info/Product%20Info/Jet%2 ... ematic.pdf
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1big1
New member
thank you for the schematics. the guy was just trying to avoid having me drop a bunch of coin on it because they're budget amps. i'll see if i can find an astroverb schematic. thanks. he also said that he was getting an asymmetrical signal on his scope that he's never seen before. that was making things hard to pin down. the schematic would be helpful in sorting that out.
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mjtripper
Member
Here you go
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1big1
New member
you rock brutha. 
thank you. 
thank you.
MississippiMetal
New member
So with the exception of a simple reverb drive and recovery circuit, it's the same amp.
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mjtripper
Member
The 2112 is the same as the jca20h with added reverb so the board is a little different but the preamp setup is the same. The 2212 is the combo version of the jca22. The 2212 has a slightly different board layout then the 22 and is a little quieter.
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1big1
New member
my tech guy said he needed the schematic for signal tracing. he's pretty sure it's a signal cap.. but he needs to figure out which one.. rather than change them all. btw.. doug just sent me the schematics.
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mjtripper
Member
1big1":2mkrxl36 said:my tech guy said he needed the schematic for signal tracing. he's pretty sure it's a signal cap.. but he needs to figure out which one.. rather than change them all. btw.. doug just sent me the schematics.
They have a good support setup, he's busy but has always gotten back to me. You can also email the support group on the support page from the website. Their amps are well built and easy to work on.
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1big1
New member
their gear pretty much rips. i like it.. budget or not. my tech guy said he needed the schematic for signal tracing. hopefully i can pick it up early next week. doug did say he was sorry my amp failed. i told him it's not that big of a deal. when you're dealing with mass produced parts sometimes marginal parts slip through. it happens. thanks again for your help.
thanks again for your help.
MississippiMetal
New member
1big1":1oyvpp2d said:my tech guy said he needed the schematic for signal tracing. he's pretty sure it's a signal cap.. but he needs to figure out which one.. rather than change them all. btw.. doug just sent me the schematics.
I have a feeling you're going to get that amp back with a dozen parts swapped out and some vague explanation of what was wrong.
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1big1
New member
my tech said that there's a 1.8k resistor in the voltage circuit of the second triode of v-2. that causes an overvoltage to the second triode and makes the signal asymmetrical. the reverb driver is also tapped into that part of the circuit rather than a separate driver/recovery circuit. he piggybacked another 1.8k to the existing resistor to drop the voltage to a normal operating voltage. he figured the proper value for that resistor should be 1k. he'll check the waveform for symmetry and double check the voltage. if it looks right i can have it back.
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mjtripper
Member
Ummmm, maybe I'm way off here but that is supposed to be that way - it that's a shared cathode between the two stages which makes it an effective .9k for each individual stage. So if he added another 1.8k in parallel he made it .45k. Those help set the bias for the that stage, basically the lower the more gain so he added gain for that stage. What problems are you having with your amp anyway?
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1big1
New member
distortion and lack of response in the tone control section. it sounds distorted and compressed and the tone section lacks sweep. it reminds me of one of the old bakelite knobbed peaveys. you turn the tone controls and they don't do a whole bunch. he knows it's a shared stage. he's looking for the cause of the asymmetry. he said it reminds him a lot of an ac15.
he knows it's a shared stage. he's looking for the cause of the asymmetry. he said it reminds him a lot of an ac15.
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mjtripper
Member
But that's not normal for that amp so I would guess it isn't a wrong part but rather a broken part. Swapping values won't fix the problem, it might mask it but it won't fix it.
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1big1
New member
that's true. i'll let him dig around in the circuit a bit more and wait for him to get back to me. at least he's narrowed it down to something specific. if it's still asymmetrical he'll move on. i wouldn't doubt he gets back to me and says it's a cap.
if it's still asymmetrical he'll move on. i wouldn't doubt he gets back to me and says it's a cap.
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1big1
New member
one fried 500 pf cap.
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vandyoh
New member
Does anyone have a schematic for a Johnson JM60 Marquis amplifier? Having trouble; the power amp is fine, but the pre-amp either is not working or not communicating with the power amp. This amp is a hybrid of sorts. It does have a AX7 preamp tube and the rest of the electronics are closer to a computer. Thanks in advance..