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September 21, 2021, 01:10:53 PM

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1
The Newcomer's Forum / Re: Basic testing of non-functional equipment w/multimeter?
« Last post by SwamiRob on September 20, 2021, 03:45:28 AM »
Thanks for the help! It's been so long since I've really used one that I forgot that they can be used with an active circuit (of non-lethal power ratings that also won't destroy the thing obviously haha). The supply was dead so got a new one, simple enough. All the wiring & fuse seemed fine, so I can only assume that the actual transformer had died maybe? Way above my level of understanding to mess with that anyway.
2
Amplifier Discussion / Re: Roland Jazz Chorus 50 troubleshooting
« Last post by Enzo on September 17, 2021, 11:01:46 PM »
And without looking at a schematic, I'd ask:  are the power supplies right?  I expect the op amps run on a pair of 15v supplies or similar.  f they are lopsided or have poor filtration, you can get distortion.   And certainly FX circuits need quality power.  Within a channel, I will take the mere moments it requires to go down the ICs and check their output pins for unwanted DC offset.  it is possible your signal runs into that DC offset and clips extra early.  Like that.
3
Amplifier Discussion / Re: Roland Jazz Chorus 50 troubleshooting
« Last post by joecool85 on September 17, 2021, 09:00:25 PM »
Hi, guys.

It's been a few years since my last visit, and I'm happy to see the forum thriving.

A few weeks ago I was lucky enough to catch a guy in the act of throwing a pristine looking 1981 Roland Jazz Chorus 50 in the dumpster. I managed to stop him, and brought the amp home with me. However, not all is well inside this thing. The JC-series is known for their clean sound. This one will break into some nasty sounding distortion, even when playing soft with weak single coils. If I turn the guitar volume down to almost zero it does not distort, so it does react to dynamics. Also, the chorus effect and the reverb is not working. The Vibrato works as it should. Plucking the springs in the reverb pan comes thru loud and clear. All other controls seem to do what they are supposed to to.

The amp uses the same circuit as described in the second half of the service manual found here : https://www.synthxl.com/wp-content/uploads/2020/01/Roland-JC-50-SERVICE-NOTES-2nd.pdf

Here is a little video of the amp, showing the distorted sound.
https://photos.app.goo.gl/zvWB812VUS7bAtHz6

There is no sign of bad solder joints, burnt components or bulging caps. I will recap this thing, but I want it up and running first. I don't think the problems are caused by gone electrolytic caps.
As far as I can see from the schematic, there is no logic in that both the reverb and chorus not working. Can anybody take a look at the schematic and help me with where I should start troubleshooting?

Any update on this amp?  Curious if you got it sorted.

Also, welcome back!
4
The Newcomer's Forum / Re: Hello
« Last post by joecool85 on September 17, 2021, 08:58:03 PM »
Welcome aboard!  I'd love to see pictures or video of some of your past projects!
5
Amplifier Discussion / Re: Fender London Reverb
« Last post by GOR3 on September 17, 2021, 02:32:15 PM »
Yes and no.  By which I mean, that is the fault that caused me to open up the amp, but it's not the reason for my post.  The reason for my post was to see if the low supply voltages were "normal."  That is, were the voltages I measured the same as others who had had this amp on the bench.  I'm confident (perhaps foolishly) that I can remedy the intermittent channel.
6
The Newcomer's Forum / Re: Hello
« Last post by Jazz P Bass on September 15, 2021, 11:25:45 AM »
Welcome to the sight.
It sounds like you are on a roll!
7
The Newcomer's Forum / Hello
« Last post by Chong on September 15, 2021, 10:00:27 AM »
Hi to all,
after i built a couple of e- guitars i wanted to learn more about  electronics. I built a few pedals and became interested in Amps. I allways played Solid State Amps. My first one was from GDR ( east germany) . After 40 years it started to Sound terrible . I kept the Speaker but dumped the electronics. I regret it ...would be a nice practise piece. Well, i m still at a very basic level in terms of circuits so i allways look for some easy approaches.
What i builded until now was all kind of Lm386 circuits but i didnt like the clean tone ...i mean there wasnt any. So i went a step further...learned about mains and started to make experiments with TDA 2030 single and Split supply combined with different preamps. Either simple op-amp circuits or also Hybrid with a ECC82.
My newest build is a Blackface like preamp based on Fet s from redcircuits.com with a LM1875 in split supply.
You will probably read some from me in the future so see u later 8)
8
Amplifier Discussion / Re: Roland Jazz Chorus 50 troubleshooting
« Last post by alexsunny123 on September 14, 2021, 08:26:20 AM »
The Reverb not working could be a faulty wire or transducer in the receive side of the tank.

The Vibrato working means that the circuit for Chorus works.
What differentiates the two is the switch. (Chorus does not use the variable Depth and Rate)
I would suggest that you check that.

thanks for the awesome information.
9
Amplifier Discussion / Re: Fender London Reverb
« Last post by g1 on September 13, 2021, 12:25:09 PM »
I've seen other products with schematics that mis-label +/-12V  as 15V.
That could be the case here as well.

If ch.2 always works well with no issues, I think your supply levels are ok.

Can we just call the fault for now as 'ch.1 cuts in and out' and take it from there?
Is that an accurate description of the symptom?
10
Amplifier Discussion / Re: Fender London Reverb
« Last post by GOR3 on September 13, 2021, 10:56:30 AM »
Thanks, g1.  The transformer itself is marked 019252.  That matches the part number on the schematic I have.  The primary voltage is good at about 120VAC and the primaries are wired correctly (in parallel and using the 120V windings).  I too believe the 12-0-12 is low. 
The fault I'm finding is that when I try to switch channels, Ch. 1 often -- but not always -- is silent; Ch. 2 always works.  The fault occurs whether the channel-select push-button on the Ch. 2 volume control is used or the footswitch.  The LEDs that indicate which channel is selected always work properly, i.e., the Ch. 1 LED lights when Ch. 1 is selected; the Ch. 2 LED lights when Ch. 2 is selected.  So I have to believe the trouble spot is elsewhere.
George
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