Studio 22+, using eq footswitch 'pop'

[bamyasi]

Hi,

Does anyone use the EQ footswitch successfully (without a popping sound) on their 22+?

I am using an after-market, 1-button latch switch with mono input and no LED.

It seems as though the pop occurs when engaging eq but not when disengaging. There is no pop sound when using the rocker switch for the EQ on the front of the amp, just with the footswitch.

I am using the eq ftswtch jack at the back of the amp. I have a patch cable going from one effects loop jack to the other to avoid the well known signal drop problem on this amp.. but apart from that I am running the amp normally

Thanks in advance for any advice 8)

 

[bigbean]

22+ schematic http://www.schematicx.com/view-schematic/mesa-boogie-studio-22-caliber-plus-schematic/

Here is some chatter on eq switch issues from another board:

Join Date: Dec 2009
Location: Durham, NC
Posts: 52
Help with Mesa/Boogie Studio .22+
Hey everyone, I'm trying to see if I can get some help narrowing down some issues I'm having with my Mesa Studio .22+.

The first thing that has been going on a while is that EQ switch on the amp does not work properly. It is a 3-position EQ switch (Auto, OFF, and IN) and when set to the IN position, the EQ is supposed to be engaged on the clean channel and bypass the EQ footswitch. The EQ switch on the amp doesn't work at all, and the EQ footswitch works for the clean channel even when it shouldn't. Not sure where the problem could be other tan a bad switch.

Second, the channel footswitch on the amp no longer works. I usually don't play in the lead channel, but I think this problem has been going on a while too. I don't know if this problem is related to the EQ switches not working correctly. Since the amp has no onboard channel switch, I don't know if the problem is in the switch or the lead channel tube circuit or what.

The 3rd problem is that after about 15 minutes of playing, the volume kind of sputters and drops out. It's done this several times, and the first time the volume was noticeably lower than usual (had to turn it up to 5 in practice, where I usually play around 2). I suspect this is a power tube failing, but given the other problems, it could be something else that fails when the amp heats up perhaps.

Any help narrowing down the problem or tips for isolating the bad components are welcome, I'm feeling my way through some amp repair basics like biasing and 2-3 prong conversions, etc. but channel switching and EQs are over my head at this point.
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#2
Old 04-03-2010, 11:24 AM
Jerry Glass Jerry Glass is offline
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I would guess that the problem is in the footswitch power supply section. Check for correct voltages in both modes (3 volts and 8 volts). If these voltages are not correct the LDRs can hover in a semi-on state that will produce exactly the problems you are experiencing.
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#3
Old 04-03-2010, 05:36 PM
taherbert taherbert is offline
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Join Date: Dec 2009
Location: Durham, NC
Posts: 52
I love TGP! Sorry, just had to get that out of the way.

I would have to desolder the board to get it out and look on the component side, but based on probing the underside of the PCB, I found something interesting.

I checked the Footswitch supply voltages at D1, R100, C9/R115 and R131 on the schematic (page 2). The incoming voltage to the diode is about 80 VAC (schem says it should be 50 VAC) then on the other side 40 VAC/33VDC to R131 and drops to nothing on the other side of R131 and the rest of the way. I measured the resistance across R131 and it was in the MOhm range (supposed to be 1K). I'm guessing this resistor is fried or there's a bad connection.

So I guess that's the problem with the footswitches. I have to order a 1K/2W resistor to replace that one and hopefully it will work.

Thanks a lot for the help. Do you think that will also fix the volume dropping out or is that a separate (power tube) issue?
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Suhr Custom Classic T / Fender Lonestar Strat (Original USA version)

Mesa/Boogie Maverick 2X12 / 1971 Fender Vibro Champ --> 1968 Dual Showman Cab / Mesa/Boogie Studio 0.22+ / Peavey Classic 30
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#4
Old 04-03-2010, 09:29 PM
jamme61 jamme61 is offline
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did you check the effex loop? sometimes that will cause a volume drop. Just bypass it with a patch cord and see if the volume comes back. Also I think there's LDR's in there that could be bad. Cool amp, I have the 50 cal head, which I like a lot.
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#5
Old 04-18-2010, 12:12 PM
taherbert taherbert is offline
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Join Date: Dec 2009
Location: Durham, NC
Posts: 52
I replaced the 1K/2W resistor, and sure enough it was fried. I put in a Vishay 1K 2W wirewound, and low and behold, the amp is now 100% functional. Thanks for the help guys, I would never have figured this one out on my own.


I don't see the EQ footswitch jack on the schematic. I don't feel like disassembling my amp to check out where it ties in. I'm guessing that the LDR power supply has gone wobbly and that is where the snap is coming from but it's just a guess.

Best of Luck!

 

[bamyasi]

Many thanks for the post and the link bigbean,

Don't think I could do this myself so better take it to a tech

Hopefully the servicing will cost less than the price of a boost pedal