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Messages - Jazz P Bass

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31
Amplifier Discussion / Re: FENDER MONTREUX
« on: August 16, 2019, 01:00:05 PM »
Before adjusting the 'bias' of the output section, I would suggest that you measure where it is set at the moment.

Measure the voltage (in MV's) across either R14 or R15 (the ballast resistors).

There is not a Fender specified bias level on the schematic.

Adjusting P2-B potentiometer will either increase or decrease the current draw of the output section at idle.

Being a Class A/B output I would go for the lowest setting that just gets it out of crossover (Class B) distortion.


32
Amplifier Discussion / Re: Vox TB35C2 repair advice
« on: July 28, 2019, 11:08:02 AM »
Before condemning the power transformer, try this.

Take the power cord and put an ohmmeter across the male tines.
Turn the power switch on & off.
With the switch On, you should be reading the power transformer primary coil.
If no reading is shown, you need to isolate that issue.

There are five components.
The cord itself
The mains fuse (110V/ 5 A__220V/ 2.5A)
Two in series thermistors
The transformer primary

If the reading is good, then you need to disconnect the power transformer secondaries and measure the winding resistance.
Mark down where they go!

P16_P15_P14 is the high voltage winding
P19-P17 is the heater winding

If all of this checks out and with the leads still unconnected, you should get the secondary voltages required.
If the voltages check good, then it's Not the transformer and it's time to move on to the circuit board.



33
Amplifier Discussion / Re: Gallien Krueger 800RB
« on: July 23, 2019, 11:36:01 AM »
The amp may be unstable & is oscillating at a high frequency (above hearing)
A scope should show that.

34
Kind of difficult to help without a schematic.

IF the gain control is in the feedback loop of an op amp, you may be able to tone it down with a larger potentiometer value.

35
Amplifier Discussion / Re: Peavey Session 400 1976 troubleshooting
« on: July 13, 2019, 10:14:27 AM »
The first reverb circuit transistor, on the left, (3904) has weird dc voltages.
The collector and the emitter are the same voltage.
It may be shorted.

This circuit is an amplifier.
It should be able to drive a speaker.
That said, the -7Vdc on the tank coil is not a good sign.
I would expect zero volts dc.

36
Amplifier Discussion / Re: Lab Series L3
« on: July 12, 2019, 12:51:42 PM »
Nice find on the L3 schematic, PE.

It appears that R9 is a variable resistor for idle current adjustment.

Measuring the voltage drop across R16, 0.22 ohm ballast resistor, will give you your bias voltage.

37
Amplifier Discussion / Re: Lab Series L3
« on: July 11, 2019, 11:18:09 AM »
I can't seem to put my finger on the L3 schematic.

If it is anything like the L5/L7, then the bias is fixed.
3mv across any output ballast resistor.
http://www.rru.com/~meo/Guitar/Amps/Lab/schem.html

38
Amplifier Discussion / Re: Peavey Session 400 1976 troubleshooting
« on: July 10, 2019, 12:59:47 AM »
Please post the schematic.

Those 'sandbox resistors' are most probably the output ballast resistors.
The speaker output is taken off of them.
Therefore there should be zero dc voltage on them.

39
Quote:"lower than it should be for a 100W amp. "

Can you quantify this statement?

@ Max volume, what is the AC voltage at the (speaker) when running a 100 mv test signal through the amp.

(preferably you would want to run the amp output into an 8 ohm dummy load for this test, instead of the speaker. To preserve your hearing.)

100 watts will equate to roughly 28 Vac output.

40
Awesome!

41
That amp was made when Wegner bought out the original owners. (1985?)
Very little info on the web about the amp itself let alone a schematic.:(

Maybe you could try calling Acoustic.

"We offer phone support and can be reached at (855) 272-1090 from Monday – Friday 8:00 a.m. – 5:00 p.m. MST."

42
Amplifier Discussion / Re: Fuse or resistor?
« on: June 20, 2019, 12:25:34 AM »
My take on it is it is a 'rod thermistor'.

43
Amplifier Discussion / Re: Fuse or resistor?
« on: June 18, 2019, 11:43:45 AM »
Here is the schematic for the RG50T.

It shows a fuse on the AC mains inlet.

You will have to trace out the path of where the component goes.

44
Tubes and Hybrids / Re: ENGL Fireball E625 Head white noise issue
« on: May 31, 2019, 10:35:29 AM »
The "Ultra" circuit simply lifts the tone control circuit (V2b) out of the signal path.
My feeling is that the tone control is mitigating the hiss.
If that is the case then the issue (if any) is in V1a/b and or V2a.


45
Amplifier Discussion / Re: Peavey Stereo Chorus 212
« on: May 20, 2019, 11:01:41 PM »
150 mv AC (depending on the frequency) sounds like hum to me.

Or the output is oscillating.
Again, depends on what the frequency is.

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