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Rhthym Channel low gain

Posted: Mon Mar 25, 2019 5:05 pm
by industrialarts
Hi

I have a Boogie MK III Red Stripe on my bench with low gain in the Rhthym mode. When I engage the Rhthym 2 the tone thins out and the gain drops some more.

I have fixed hundreds of guitar amps but am not so familiar with Boogies, I have fixed less of them than any other amp. However, I have been digging for a while and am stumped. The LDRs and relays are all working best as I can tell. When in Lead mode the amp can be driven to full power but the Lead Drive acts funny. When I bring up the gain it jumps to full then starts to attenuate a bit. The pot is working fine - I can read the resistance of the wiper and if I disconnect the wiper from the lead gain stage the signal acts properly.

The tube amp stage all have gain. The weird thing about the Boogie for me is the Rhythm channel coupling to V2, where there is a big gain drop and filtering.

I suspect both problems are caused by the same culprit but all my readings don't point me in any direction. I have yet to remove the pc board since it's such a PITA and I was thinking a failed component would show up by now. I don't know the underside of the board and am starting to suspect some sort of contamination but would love some input before I dis-assemble it more.

Thanks for any help

Re: Rhthym Channel low gain

Posted: Tue Mar 26, 2019 1:19 pm
by lovetoboogie
Hi there,

Welcome! ...and there are several peeps here that can help you troubleshoot. In the meantime a board member has completed a seriously thorough breakdown of each Stripe model. Here is the link...

viewtopic.php?t=83605

...follow down the thread to the Red Stripe schematic and board layout...the R2 drop out sounds cut and dry. Familiarize yourself with how R2 "relays" in and out the additional RC network. If you are sure the LDR's are engaging and your low voltage rail is correct, then start looking at plate voltages on V1-V3, etc...be methodical. The PCB's are hardy and can take a little corrosion and oxidation.

Also, do yourself a favor and clean very thoroughly the effects loop send and return jacks. Make sure they are seated closed when there is nothing plugged in. Lotta woes troubleshooting Mark III if you don't establish the little things first.

Hope this is a helpful start and let us know if you make any progress. I am sure someone else will chime in soon.

-j

Re: Rhthym Channel low gain

Posted: Wed Mar 27, 2019 2:23 pm
by industrialarts
Thanks for your input. Most of those things you mentioned I have already done, however the +15volt rail is low, around +11 to +13 when I switch some of the LDRs, so I will dig into that a little more. I have been studying the schematic for the Red Stripe and unless I am mistaken there should be a little more gain when R2 is switched due to the bypass cap that is engaged on V2. The manual describes it as having more grit and less chime but in my case it just sounds weaker than R1.

In any event, since I am kind of stumped any and all suggestions are welcome.


BTW - I think there might be an error in the schematics with tha way the +15 supply is drawn for the LDRs. All the hand drawn schematics I have seen show "D10" to be a +15 5W zener that goes to the junction of the 3300 & 470 for the LDR LEDs, which seems to jibe with the amp I am working on now. But double check me on that.

Thanks again for any and all help