Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Zee on May 26, 2021, 06:07:36 pm
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Hi,
I recently bought a kld jcm 25 kit but I’m having a few problems. A resistor burned. I followed the instructions properly and rechecked the wiring again after replacing the burned resistor. It powers on fine but the minute I flip the standby switch the 2.2k ohm 5w resistor burns. Can anyone help me?
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Welcome!
Links are good.
product page (http://www.kldguitar.com/KLDguitar-English/KLD-amp-kits/KLDguitar-JCM25-amp-kits-guitar-amp-1.html)
Schematic (http://www.kldguitar.com/MANUAL/amp%20kits/KLD-JCM25/KLD%20JCM25%20Schematic%2020200327.pdf)
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..the minute I flip the standby switch the 2.2k ohm 5w resistor burns.
Literally "minute"? Or second? How long is it?
Think. What would blow that resistor? A heavy load on the downstream side. If a small added leak it would just run warm. If a dead-short, it could blow-up almost instantly. So time is an important clue.
Pull the power tubes. Flip the switch. Now does it blow up? If so I would re-re-re-check the wiring on the power tubes +AND+ that 'half power' switch. Not just wiring but stray lead scraps that may be hiding in the build.
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Within seconds. The build itself is a pcb so there isn’t any wiring I can change on the power tubes. Here is the link showing how the pcb is set up. http://www.kldguitar.com/MANUAL/amp%20kits/KLD-JCM25P/JCM25%20pcb%20Ver%20C%20connect%20%2020200730.pdf
I will replace the resistor again this weekend I will shrink tube the half power switch in that case as well.
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Verify that C16 polarity is correct.
Then measure resistance from B+2 to chassis. What have you?
Disconnect the violet wire from B+2 and remeasure resistance from B+2 to chassis. What have you?
Leave the violet wire disconnected and also disconnect the positive lead of C16 from B+2 and remeasure resistance from B+2 to chassis. What have you?
I want to see three resistance readings.
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I will be working on it again this weekend I will get the values and let you know then.
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Yes, power-off resistance readings make more sense than burning more resistors.
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The build itself is a pcb so there isn’t any wiring I can change
there are many places where you could have a solder bridge, do a REALLY good visual looking for an errant blob
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Hi,
I think I found the problem I had the “half power” violet wire connected to standby instead of B+2 and had the standby switch connected to B+2 instead of standby. Unfortunately most of the wires provided in this kit don’t match up with the colours on the diagram (neither written or coloured). Would this have caused the resistor to burn? Or is there some other problem?
Thanks
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Verify that C16 polarity is correct.
Then measure resistance from B+2 to chassis. What have you?
Disconnect the violet wire from B+2 and remeasure resistance from B+2 to chassis. What have you?
Leave the violet wire disconnected and also disconnect the positive lead of C16 from B+2 and remeasure resistance from B+2 to chassis. What have you?
I want to see three resistance readings.
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Sorry for the late reply.
C16 has the correct polarity.
B+2 to chassis started at -0.3M ohms and increased all the way up to more than 2M ohms (my multimeter only goes up to 2M)
With only violet wire disconnected the value was 2.23k ohms.
With the C16 and the violet wire disconnected it was 2.23k ohms
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With the C16 and the violet wire disconnected it was 2.23k ohms
That 2.23K reading is why that 2.2K resistor is burning. Your challenge is to find out why you have that 2.23K reading.
Leave C16 and the violet wire disconnected and also disconnect R30. Then measure resistance from B+2 to chassis. What have you?
Leave C16, violet wire, and R30 disconnected and also disconnect R31. At this point there will be nothing connected to B+2. Then measure resistance from B+2 to chassis. What have you?
I want two resistance readings. Please don't wait a week this time. :icon_biggrin:
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Yeah sorry about that I had a really busy few weeks with work and family.
For both of those measurements I got no response from my multimeter
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So, when you disconnected R31 from B+2 point, the 2.23K resistance disappeared from B+2 point? This means that there is a short circuit to ground on the other side of R31. Check the resistance from the "Standby" point on the board to chassis. Does it measure zero ohms?
I feel we're headed down a rabbit hole because a short at the Standby point could not possibly burn R31.
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I think I mentioned it before but I had the 2 violet wires in the wrong place before.
I have now fixed that but I get no responce between chassis and standby point for resistance
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I don’t mean zero ohms. I mean no response at all
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I don’t mean zero ohms. I mean no response at all
Yes, I knew that. Now measure the resistance from Standby point to chassis. I think we are about to find the culprit.
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It gives no response from standby to chassis
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It gives no response from standby to chassis
Do you still have 2.23K reading from B+2?
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No everything is still disconnected on B+2
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OK. Now reconnect R31 to point B+2. Measure resistance from B+2 to chassis. What have you?
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Ok after reconnecting R31 only I still have no response on B+2 to chassis and standby to chassis
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Ok after reconnecting R31 only I still have no response on B+2 to chassis and standby to chassis
Really! Earlier when we were chasing that 2.23K resistance, you had the reading on B+2 but when you disconnected R31 the resistance reading went away. Now you reconnect R31 but the reading is still gone. I suspect maybe something on the bottom side of the board was touching chassis? Go ahead and reconnect everything to B+2 EXCEPT C16 and recheck resistance between B+2 and chassis. What have you?
Can you post some hi-rez pics of the area of the board we are working on? Top and bottom side if possible.
Also, just to be sure we are on the same page I'm using this layout. You should also. I did notice there are two points labeled Standby on the layout. Earlier I was referring to the one that the right side of R31 is connected. Is that the same one you checked resistance?
http://www.kldguitar.com/MANUAL/amp%20kits/KLD-JCM25P/JCM25%20pcb%20Ver%20C%20connect%20%2020200730.pdf
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Hi I’m getting no response from B+2 with everything other than C16 connected.
I figured you would be talking about the standby that R31 was connected to so that was the one I measured.
I could only get the front picture because to get the back I would have to remove a whole mess of wires. If necessary I can do that but I would prefer not to :P
https://imgur.com/a/Fvomd4o
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Hi I’m getting no response from B+2 with everything other than C16 connected.
OK. May as well reconnect C16 and turn on the power. See if the resistor still burns.
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I will have to test it in the morning now have a few things to do before bed.
Should I test it with tubes in or without?
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I tested it again this morning with tubes. As soon as I flip the stand by switch it makes a buzzing noise but no guitar sound. Then there is a smell of burning but I turned it off pretty quickly so I couldn’t see what was burning this time.