Looking for 50 Plus Sensitivity switch gut shots

20 Studio, 50+, 50PA, 50R, L100, B100, L120, L120PA, L120R, 120 SLAVE, B120, B150, L150, L200, B200, Concord, and SMF

Looking for 50 Plus Sensitivity switch gut shots

Postby bloodspoint on Sun Mar 12, 2017 2:57 pm

Hi,

Picked up a 50 Plus that had been "gone over" by a tech. Apparently one of the things he "fixed" was to remove the sensitivity switch. I don't know if he wired it permanently "on" position 2, or off at position 1. I think, based on looking at a schematic I found, that the switch disconnects a connection from pin 8 of V3 to ground in "1", and lets it connect in position "2". I have yet to open up the chassis and see what they actually did, but if someone has a shot that shows that switch and its connections, I'd love to see it.

Thanks!
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Re: Looking for 50 Plus Sensitivity switch gut shots

Postby Papa Dog on Wed Mar 15, 2017 5:47 pm

bloodspoint wrote:Hi,

Picked up a 50 Plus that had been "gone over" by a tech. Apparently one of the things he "fixed" was to remove the sensitivity switch. I don't know if he wired it permanently "on" position 2, or off at position 1. I think, based on looking at a schematic I found, that the switch disconnects a connection from pin 8 of V3 to ground in "1", and lets it connect in position "2". I have yet to open up the chassis and see what they actually did, but if someone has a shot that shows that switch and its connections, I'd love to see it.

Thanks!

in an "unhacked" amp that switch either engages a 22k variable resistor or bypasses to ground.
In other words, in one position the amp gains more resistance in the cathode circuit (becomes "colder") or is bypassed for "maximum gain".
Some of the Mk4 series actually had two 22k variable resistors, one in the by-pass position and another that would be added in series when switched to that side.
There should also be a 10k or 12k fixed resistor on pin 8, between the pin and the first variable resistor (if that amp has it).
Often the variable resistors go bad, and "back in the day" they were rather difficult to source for some folks in some locations.
I like to take the by-pass switch and variable resistors out and replace them with a 22k linear pot. Gives you more "granular" control. AND you can get such post with a built in switch, so rotating all the way to the left can switch it out of the circuit.
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Re: Looking for 50 Plus Sensitivity switch gut shots

Postby bloodspoint on Wed Mar 15, 2017 7:55 pm

Thanks. I'm not sure I follow all that, but what I see when I look at the guts is that on pin 8 of V3, where the schematic shows the sensitivity switch, I see a 22K resistor and a bypass .0001 cap which run to what I think is a ground strip. I can't tell if that's wired to the equivalent the "1" or "2" position.

What I'd really like to do is restore the original circuitry and put the switch back, but failing that, my plan B would be to wire it as if the switch were on "2" all the time.
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Re: Looking for 50 Plus Sensitivity switch gut shots

Postby bloodspoint on Thu Mar 16, 2017 12:56 pm

So wait. If the resistor and cap are there and tied between pin 8 and ground, am I right in interpreting that as being the same as if the switch were on "2", with the extra gain engaged all the time? That's what I would like to have.
This is the schematic I'm referencing: http://soundcitysite.com/sc_lb50_plus_schem.jpg
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Re: Looking for 50 Plus Sensitivity switch gut shots

Postby Papa Dog on Fri Mar 17, 2017 7:00 am

bloodspoint wrote:So wait. If the resistor and cap are there and tied between pin 8 and ground, am I right in interpreting that as being the same as if the switch were on "2", with the extra gain engaged all the time? That's what I would like to have.
This is the schematic I'm referencing: http://soundcitysite.com/sc_lb50_plus_schem.jpg

I honestly do not remember if position 1 or position 2 is "extra gain" or "full boost", and not the colder biased, lower gain setting.
IF the resistor is 22k then it is in the "colder bias" or "lower gain" position. According to that schematic, the "native" resistor should be 2k2 not 22k. Those already have a much colder bias.
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Re: Looking for 50 Plus Sensitivity switch gut shots

Postby bloodspoint on Fri Mar 17, 2017 12:43 pm

Hmm. I have a Concord with the switch still there. I may have to open it up and see if I can copy what's going on in there, or at least see what the switch does. To be clear, what they've done on this amp is to duplicate on pin 8 what's happening on pin 3. And sorry, 2.2K, not 22K.

What I'll probably do when I've got a few minutes to spend on it is to open up the Concord, see what that switch is doing and where, and then try to copy it on the L50. I have some good Carling switches I can use if it's just a SPST toggle.
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Re: Looking for 50 Plus Sensitivity switch gut shots

Postby Vucinic on Mon Aug 14, 2017 3:26 pm

http://s349.photobucket.com/user/vucini ... sort=3&o=0

Here is pic from my SC200+. Look for a gray coaxial wire.

Good luck.
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Re: Looking for 50 Plus Sensitivity switch gut shots

Postby bloodspoint on Thu Aug 17, 2017 2:29 am

Vucinic wrote:http://s349.photobucket.com/user/vucinic5/media/sensitivity%20switch%20SC200_zpshnvzwd1p.jpg.html?sort=3&o=0

Here is pic from my SC200+. Look for a gray coaxial wire.

Good luck.


Thanks for the pic! Coming out of that coax wire, is it just that red wire going up and left, and the green (?) looking wire off to the right a little that come out? If so, all it's doing is connecting or disconnecting the components those go to on the chassis, right?
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Re: Looking for 50 Plus Sensitivity switch gut shots

Postby Vucinic on Thu Aug 17, 2017 5:08 am

Yes, that 220K resistor. Red is after 22K, green is after 220K.
From pin 8 go to 22K, than in series 220K, than with blue wire to ground. It is only jumper who disconnect that 220K resistor. Ground wire: one side is attached to the chassie near sens.switch only for freq. protection.

At schematics for SC200 plus MKIV which i have, there is also one variable 22K resistor which i don't have.
I can send to you all schematics i have if you needed.
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Re: Looking for 50 Plus Sensitivity switch gut shots

Postby Vucinic on Sun Aug 20, 2017 4:31 pm

Hi, did you find different wiring of sensitivity switch? I'm just curious?
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