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Post subject: Blues Junior III - Help!
Posted: Sat Dec 19, 2015 1:31 pm
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So I picked up VERY MINT BJ III to do some repairs on and give as a Christmas Gift. Got it all dialed in but one last thing i cant figure out is:

1) Bass Control Pot

Works but as I increase volume drops a hint, and as I take it to FULL 10 the bass is not normal Muddy and the volume decreases.

Anyone one know which resistors / caps tie into this part of the circuit? :?: :?:


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Post subject: Re: Blues Junior III - Help!
Posted: Sat Dec 19, 2015 1:57 pm
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You mean ;

When you turn bass pot to increase bass , the volume of the amp drop ?

Check bass pot ,C6 , solder on these components / printed circuit


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Post subject: Re: Blues Junior III - Help!
Posted: Sat Dec 19, 2015 2:53 pm
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Pot replaced, c6, test normal, re-flowed solder.

Hmmm..


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Post subject: Re: Blues Junior III - Help!
Posted: Sun Dec 20, 2015 2:16 am
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Ryan41018 wrote:
Pot replaced, c6, test normal, re-flowed solder.

Hmmm..



Amp work or not ?

Bad was bad ?


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Post subject: Re: Blues Junior III - Help!
Posted: Sun Dec 20, 2015 7:57 am
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Amp is working yes. Powers up, plays perfect. Turning up the bass is when the tone deteriorates.

R


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Post subject: Re: Blues Junior III - Help!
Posted: Sun Dec 20, 2015 8:02 am
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Ryan41018 wrote:
Amp is working yes. Powers up, plays perfect. Turning up the bass is when the tone deteriorates.

R


I mean after new pot and solder reflowed , did the amp play better ?


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Post subject: Re: Blues Junior III - Help!
Posted: Sun Dec 20, 2015 8:21 am
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I'm sorry, no it did not. It's still the same. Tubes all new. I changed out the 100 ohm bias resistors too but still no improvement.


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Post subject: Re: Blues Junior III - Help!
Posted: Sun Dec 20, 2015 11:34 am
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Ryan41018 wrote:
I'm sorry, no it did not. It's still the same. Tubes all new. I changed out the 100 ohm bias resistors too but still no improvement.



See a qualified tech

Bias resistor have nothing to do with this issue
Pot ; ohms reading tell if pot is good or not , if good why put a new one?


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Post subject: Re: Blues Junior III - Help!
Posted: Sun Dec 20, 2015 11:41 am
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I'll figure it out in time, just thought by some chance another person may have had similar experience.

Bias resistor was necessary because it had been red platng, once I changed them it stopped red plating


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Post subject: Re: Blues Junior III - Help!
Posted: Sun Dec 20, 2015 12:32 pm
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Farting out like that can be caused by a number of problems. Anything from a bad ground to a torn speaker could be the culprit.
Now, to figure out where it's coming from, you need to break the daisy chain of circuits.
The easiest place to do this is at the reverb tank connections.
Set the reverb and master controls to minimum.
Unplug the reverb tank RCA connectors.
Turn on the amp and let it warm up.
Turn up the master and reverb controls to 1 or 2
Touch the center pin of the RCA connectors. one of them will produce a hum as you inject 60Hz into it. There are no dangerous voltages on either of these connectors.
Turn off the amp
Using adaptors, connect a guitar or other signal source, Ipod or anything with a headphone jack to the RCA connector that produced hum.
Turn on the amp and adjust the reverb and master controls for a decent volume. Keep your signal level low to avoid overloading the Reverb's input circuitry.
Evaluate the results. If the farting goes away, then its upstream in the preamp or tone stack. If it continues, its in the power stage, phase inverter or speaker. The amp won't sound normal, but you should be able to tell if the problem is gone

If I had to throw a blind dart at it, I'd hit on a leaky tone stack or coupling capacitor.
You can check for leaky caps by looking for DC voltage on the treble Master, and volume controls. Anything above a few millivolts DC is suspicious.
The DC voltage from the caps can leak into the preamp tube grids and lower the gain of the tube, causing early distortion. There should be zero volts on the V1/V2 grids.
If you meter the grids and they pop and your sound comes back for a few seconds, you know you have too much DC on a grid. By touching it with a meter probe, it discharges the DC from the grid and it operates until the DC slowly builds back up.


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Post subject: Re: Blues Junior III - Help!
Posted: Sun Dec 20, 2015 1:01 pm
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TimAudio have always a good answer.


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Post subject: Re: Blues Junior III - Help!
Posted: Sun Dec 20, 2015 1:27 pm
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I bet he's right about tone stack, coupling caps.


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