I've got a Princeton Stereo Chorus on the bench and the schematic calls for a dummy plug inserted into input 1 for testing. I have no idea if it should be open to open the plug switch or closed (shorted). Any help would be appreciated.
Thanks!
Prinecton Chorus Dummy Plug
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Tone Snob Geezer
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Prinecton Chorus Dummy Plug
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Re: Prinecton Chorus Dummy Plug
No amp with the name Princeton should have a schematic that complex.
Do the simple thing. Experiment!
Do the simple thing. Experiment!
I build and repair tube amps. http://amps.monkeymatic.com
Re: Prinecton Chorus Dummy Plug
Agreed, that stopped being a Princeton long ago.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Prinecton Chorus Dummy Plug
IMHO, it doesn't make sense to insert an open plug into a shorting jack. Try it with the plug shorted. If you don't short the plug, the ground is floating.
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JazzGuitarGimp
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Re: Prinecton Chorus Dummy Plug
Just below the input jacks is R1. It has the net name INH at its top connection. I am guessing this stands for Inhibit. Without a plug inserted into either input, INH sits at +8V. When a plug is inserted into either input, INH switches to -8V. This is most likely used to mute the power amp chips when no instrument is plugged in. I don't see another reference to net INH in the schematic, but I have a feeling it connects to the top of C83, which is connected to the mute pin (pin 4) on the power amp chips. If all my crazy assumptions are correct, you will need to insert a plug into one of the two inputs to enable the power amps for the output power test - in which case, it doesn't matter if the dummy plug is shorted. My personal preference would be to short it, so that the input is not left floating. Or, if you use an open plug, just make sure the Volume control (R10) is turned all the way down.
Last edited by JazzGuitarGimp on Sat Jul 23, 2016 4:34 pm, edited 1 time in total.
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JazzGuitarGimp
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Re: Prinecton Chorus Dummy Plug
You can download the power amp IC datasheet here if you need it: http://www.datasheetcatalog.com/datashe ... 188H.shtml
Lou Rossi Designs
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Printed Circuit Design & Layout,
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Stevem
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H
I am sorry to hijack this thread some , but I think it's a good place to do so!
Earlier in the week I was working on a Bkackstart ht40 that would cut out, and it had two issues that would make the same cut out problem .
The main problem was found to be a bad cap in the bias control circuit that this and other model Blackstar amps have.
I already knew that this circuit kicks the bias up to -77 volts when the amp has nothing plugged into its input Jack and this cap was stopping the voltage from dropping down to -39 volts needed to get the output stage working , the dam thing I did not know was that the amp also needs a resistance load on the speakers jacks also to keep that -39 volts going !
This amp had a that bad cap and a bad speaker cable to boot!
Earlier in the week I was working on a Bkackstart ht40 that would cut out, and it had two issues that would make the same cut out problem .
The main problem was found to be a bad cap in the bias control circuit that this and other model Blackstar amps have.
I already knew that this circuit kicks the bias up to -77 volts when the amp has nothing plugged into its input Jack and this cap was stopping the voltage from dropping down to -39 volts needed to get the output stage working , the dam thing I did not know was that the amp also needs a resistance load on the speakers jacks also to keep that -39 volts going !
This amp had a that bad cap and a bad speaker cable to boot!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!