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Post subject: Vibro King Issues
Posted: Fri Dec 14, 2007 8:47 am
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Joined: Fri Dec 14, 2007 8:42 am
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When I turn the amp up to 8 or higher, after a few seconds of play, the sound starts to worble much like the tremelo circuit would sound. Once that starts I can cut the volume on the guitar and there is a cyclic popping sound - psuedo motorboating. I have changed all the filter caps except C31 & 32 with orginal product, Ill. Cap. I did not change these because when I plug into the effects return jack, bypassing the preamp and reverb, the amp operates normally at any volume level, including the tremelo crcuit. Since the output is working correctly, I believe these (C31 & C32) are OK.

I removed V1 and the ticking was still there. Removing V2 causes the ticking to cease. I re-inserted V2 and started poking around with a chopstick. By applying pressure to pin 2 on V2 (at the base of R46) with the chopstick, it causes a loud low frequency (bassy) roar. The same happens when I apply pressure to the opposite end of the shielded cable (at C19). It stops the ticking briefly. No amount of lead dress altering affects it. I re-flowed the solder at both places.

I sprayed deoxit in the socket and worked a tube in and out several times then purged it. I checked the pin sockets with a strand of wire, all are very firm.

I checked R45 & R46 - 2.2M while out of the circuit and I also changed C19 & C2.

The voltage at V2 pin 1 is 363. The voltage at V2 pin 2 is 160 (dropped across the 2.2M), I checked R59 (10K) is also in spec. That V is higher than the schem shows so I checked R67 – 10K in P/S section. It looks to be in spec. I also used a “mini grabber” jumper to jump out the shielded cable (C19 to pin 2) to ensure good continuity. No change.


As an added note, the ticking changes patterns and interval frequency randomly. Also, if I cut the TMB stack pots to 1 the problem goes away, but so does the tone! The Mix (R2) control in the rev circuit also affects it so I changed C18, the coupling cap between R2 and V2 pin 3. This seems to be a gain related problem. I even tried a 12AU7 in V1 to lower the gain hitting V2. I tried a 12AY7 in V2 to lower the gain further… No change.

Any help you can offer would be greatly appreciated.

Are there any service bulletins avaialble for the Vibro King? My amp has the EL84 reverb Driver and I kno wit was upgraded at some point. Any insight into that?

Regards,

MIke Boone


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