No Volume When in Final Testing of JMP 50 Build

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waggclan
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No Volume When in Final Testing of JMP 50 Build

Post by waggclan » Wed May 16, 2012 4:20 pm

With a lot of help, I corrected the bad rectifier in my JMP 50 build and was coasting to tone heaven when trouble struck again!

I began the first phase of testing and everything went well. Values were as expected. I began the 2nd phase of testing (Metro Instructions, page 27 "D", http://www.metroamp.com/downloads/50_WA ... CTIONS.pdf. I got to the part where all tone controls are set at 5 and volume controls are set at zero. The instructions are to turn up each volume knob separately and listen for either a normal hiss of the amplifier or an unwanted hum; I got nothing. No sound whatsoever. I figured I just built the quietest plexi on the planet or have another glitch; I believe it's the latter...

When I plugged in a guitar I began to hear some over modulated/terribly distorted sound when the volume knob was at 5 but completely silent before that. As I turned the volume knob up the volume increased but not as it should. I stopped increasing the volume when I reached 7 on the knob.

Just to be sure, I checked every value of the installed components, checked continuity from the board to the wire's destination. All are values are correct and properly connected to their destination point. I pulled the tube in V1 with the same results. The tubes are heating up; just no sound.

As you can see I am building a Germino style pot configuration using PEC and CTS pots. The ground from the board nearest the input plugs is grounded along with the input plugs to a separate ground lug I installed on the chassis. The two center ground wires from the board are also grounded this way to their own separate chassis ground lug, as are the PEC pots and the ground wire from the board that is on the far left (looking at the pots side of the chassis). Grounding should be sufficient but I'm considering grounding the two volume pots together and then to the middle chassis ground lug I installed.

Looks like I'm not quite done debunking this build after all.

Here's a link to some hi resolution pictures of the build:

http://s690.photobucket.com/albums/vv27 ... 0JMP%2050/

Thanks guys.

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neikeel
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Location: Suffolk, England

Re: No Volume When in Final Testing of JMP 50 Build

Post by neikeel » Wed May 16, 2012 4:58 pm

On first pass all I can see is that I am not sure the common tie point of OT centre tap, choke and HT feed turret is adequately soldered (seems empty).

You probably have a high impedence joint somewhere between the pre amp and power amp.
Neil

waggclan
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Joined: Thu Jul 15, 2010 8:05 pm
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Re: No Volume When in Final Testing of JMP 50 Build

Post by waggclan » Wed May 16, 2012 6:51 pm

I'll double check it. Thanks.

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