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Author Topic: 80's Era Roland JC50 blows internal fuses  (Read 814 times)

papa_bear75

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80's Era Roland JC50 blows internal fuses
« on: October 26, 2021, 03:34:25 PM »
I inherited the amp from my father who passed earlier this year and was excited to use it. However, I soon discovered a problem. The amp worked perfectly well until I plugged in a set of headphones. As soon as I did that, the audio in the headphones was distorted. After a couple seconds, the audio started fading. The LED on the power switch faded in sync with the audio until they were both completely off. I checked the main fuse and it was fine. I opened up the unit and found that both 4A fuses were burned. I picked up some replacements and installed them thinking the amp had a deeper issue. I powered it up and the amp worked perfectly. I proceded to plug the headphones back in and the fuses burned again. I replaced them again and tested the amp without headphones. It worked perfectly. I do live in a condo so having the ability to use headphones is key.

Is this something that anyone has encountered before?

Thanks!

Enzo

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Re: 80's Era Roland JC50 blows internal fuses
« Reply #1 on: October 26, 2021, 07:46:45 PM »
OK, so don't plug in the phones any more.  Open it up.  Is there DC voltage on the phones jack?  Is anything loose inside that an inserted phones plug could make touch something else?

Do yoi have different phones to try?

joecool85

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Re: 80's Era Roland JC50 blows internal fuses
« Reply #2 on: October 27, 2021, 02:03:37 PM »
Plugging in mono 1/4" plug into stereo headphone jack expecting stereo plug?  That could cause shorts.
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Tassieviking

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Re: 80's Era Roland JC50 blows internal fuses
« Reply #3 on: October 27, 2021, 10:04:02 PM »
Do you have the serial number ?
The circuit is different after a certain serial number.
A photo of the back of the Jack socket for headphones might help.

papa_bear75

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Re: 80's Era Roland JC50 blows internal fuses
« Reply #4 on: November 06, 2021, 02:29:04 PM »
I didn't realize anyone had replied to my post, since I didn't get email notifications. I appreciate everyone replying! I have to use headphones where I live, since I live in a condo complex and this thing is loud!

I've used the same headphones with another amp before so there is nothing wrong with them. And I'm using a 1/4" to 1/8" stereo headphone adapter for with my sony headphones.

Here are photos of the S/N plate and the back of the headphone jack. After examining the schematic, it looks like plugging in the headphones disconnects the speaker from the output. The output is then routed through two 330 Ohm resistors, one for the right channel and the other on the left. I checked the jack and there doesn't appear to be any foreign objects inside of it. And with headphones plugged in, there are no pins shorted to ground or to any other pins. I had ordered a jack from Syntaur but it turned out to be the other type of jack used on the amp. The one for the headphones is unique. I'm stupmed at this point. Preliminary poking around in the circuit leads hasn't caused me to find any obvious issues.

The amp works perfectly apart from this issue, which led me to believe the jack itself was faulty or had a piece broken off inside of it. I'm pretty sure my dad never used it with headphones.

Thoughts?

Enzo

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Re: 80's Era Roland JC50 blows internal fuses
« Reply #5 on: November 06, 2021, 05:31:56 PM »
SO the amp works seemingly fine on speaker, I don't recall you mentioning the adaptor. 

Adaptors come in two styles:  stereo to stereo, and stereo to mono.  Any chance the 1/4" male end is just tip and sleeve, and has no ring?   In other words if you plugged your little adaptor ONLY into the phones jack - no ear phones - does that cause fuses to blow too?

If so, then plugging it into a stereo phone jack will short one side to ground.

Can you come up with a pair of headphones with a real 1/4" TRS plug, jst to try?

g1

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Re: 80's Era Roland JC50 blows internal fuses
« Reply #6 on: November 07, 2021, 01:50:51 PM »
With that serial number, the schematic on pg.7 of the file posted above is correct.

The following is assuming there is no issue with headphones or their adaptor which was mentioned above.
The jack wiring is very busy (7 pins in use instead of 5), and my guess is it may have been mis-wired from the factory.
The info on the layout vs schematic does not seem to agree with each other.
(layout shows connection #19 going 2 places, schematic shows 1)

If your Dad never used headphones he would have never known.
Wiring it up like the later version with a simpler jack would be a fairly simple solution.
« Last Edit: November 07, 2021, 01:53:28 PM by g1 »

papa_bear75

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Re: 80's Era Roland JC50 blows internal fuses
« Reply #7 on: November 07, 2021, 03:00:04 PM »
I really appreciate all the suggestions from everyone. I popped over to a local vintage guitar shop in town on the off chance that they would have a similar headphone jack as a spare part. They had something very similar to I bought it for $7. As it turns out, the switching inside the jack is identical to the original. Even though I rang out all the connections on the old one, and everything seemed to be working, I went ahead and wired up the new jack and installed it. It ended up working perfectly. I'm guessing that perhaps the contacts internally were corroded, which may have loaded the circuit more than expected but that is a wild guess. Either way, I'm happy to say that the new jack worked.  :)

Enzo

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Re: 80's Era Roland JC50 blows internal fuses
« Reply #8 on: November 07, 2021, 03:59:03 PM »
Glad you got it.

Corroded?  Not likely.  Bent, way more likely.

joecool85

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Re: 80's Era Roland JC50 blows internal fuses
« Reply #9 on: November 07, 2021, 08:47:29 PM »
I really appreciate all the suggestions from everyone. I popped over to a local vintage guitar shop in town on the off chance that they would have a similar headphone jack as a spare part. They had something very similar to I bought it for $7. As it turns out, the switching inside the jack is identical to the original. Even though I rang out all the connections on the old one, and everything seemed to be working, I went ahead and wired up the new jack and installed it. It ended up working perfectly. I'm guessing that perhaps the contacts internally were corroded, which may have loaded the circuit more than expected but that is a wild guess. Either way, I'm happy to say that the new jack worked.  :)

Good job getting it fixed up!
Life is what you make it.
Still rockin' the Dean Markley K-20X
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