JCM800 2204 build Troubleshooting

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sts800
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JCM800 2204 build Troubleshooting

Post by sts800 » Wed Jun 08, 2016 2:05 pm

Hello There. I am new to this forum and I have found a ton of great info here. Thanks to all that contribute. I was wondering if someone with more experience than myself could take a look at my JCM800 Build. This is my second build and I am loving learning about amps. I have to admit, I am still pretty new to this though.

The amp sounds pretty good right now but I am having 2 issues I cant seem to fix.

1. I am getting a very slight bit of oscillation when I strike a chord and hold it...
2. When I turn the amp off standby, I get a loud siren type sound for about 2 seconds. Then when I turn the power switch off, I get the same siren / squeal sound for 2-3 seconds. Its pretty loud and annoying.

Here are a few pics. The layout I used was a stock JCM800 2204 from Triode. Any help would be greatly appreciated. Ive moved wires around with a chopstick and researched quite a bit and have yet to come up with the solution.

https://drive.google.com/file/d/0B4qWT_ ... sp=sharing
https://drive.google.com/file/d/0B4qWT_ ... sp=sharing
https://drive.google.com/file/d/0B4qWT_ ... sp=sharing

danman
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Re: JCM800 2204 build Troubleshooting

Post by danman » Wed Jun 08, 2016 5:08 pm

One common issue in new builds is the OT's primary leads being reversed which will cause the "siren blast" that you mentioned. It usually occurs all of the time when the amp is on but it is a possibility that it is your problem. One quick way to check is to unsolder the purple NFB wire from the output impedance selector switch, tape the end of the wire so that it does not touch anything and then play the amp. If your siren blast is now gone, you need to swap the OT leads at pin 3 of each power tube socket. Just place each lead on the opposite socket, solder them back in and then reconnect the NFB wire that you removed for testing.

sts800
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Re: JCM800 2204 build Troubleshooting

Post by sts800 » Sun Jun 12, 2016 2:02 am

danman wrote:One common issue in new builds is the OT's primary leads being reversed which will cause the "siren blast" that you mentioned. It usually occurs all of the time when the amp is on but it is a possibility that it is your problem. One quick way to check is to unsolder the purple NFB wire from the output impedance selector switch, tape the end of the wire so that it does not touch anything and then play the amp. If your siren blast is now gone, you need to swap the OT leads at pin 3 of each power tube socket. Just place each lead on the opposite socket, solder them back in and then reconnect the NFB wire that you removed for testing.
Ill give that a try! Thanks!
Scot

sts800
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Re: JCM800 2204 build Troubleshooting

Post by sts800 » Wed Jun 22, 2016 5:49 pm

Well I swapped the secondary wires from the OT and the amp sounded worse. So I put them back. I still get the siren sound for about 5 seconds when turning off standby. then again when I turn off power. I also noticed another problem once I put my faceplate on. The High input jack has less gain and less volume than the low input...I've checked the wiring and it seems to be wired correctly. Any other ideas?

Haze13
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Re: JCM800 2204 build Troubleshooting

Post by Haze13 » Thu Jun 23, 2016 7:54 am

What schematic did you use? Are the standby switch in the correct place?
Seems to me, that this oscillation happens only when your filter caps are being discharged...

Haze13
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Re: JCM800 2204 build Troubleshooting

Post by Haze13 » Thu Jun 23, 2016 8:06 am

I see there are some mods... Like a Cap on the first pre socket, 68k instead of 470k||500pF, no Bright Cap across the gain pot. 1MEG is on the Inputs?
Make it stock model, than trouble shoot the problem. If every thing is ok, make mods, one by one with listening tests in between.

sts800
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Re: JCM800 2204 build Troubleshooting

Post by sts800 » Wed Aug 03, 2016 12:51 pm

for future reference, I didn't ground the speaker output jacks....Once I did this, the siren went away..

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