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Topics - DrewV

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Amplifier Discussion / Fender Stage Lead 2 grounding out
« on: December 12, 2013, 08:25:15 PM »
Hi All, So, I'm trying to bring this amp back to life. Initially it blew both fuses between the transformer and the rectifier diodes. After replacing the fuses and bringing the voltage up slowly (variac + lightbulb limiter) I was able to get some sound from a line in source. However, when I tried to increase the V the amp started sucking current like there was no tomorrow. All transistors read good on a transistor checker.After some checking I found that R78 was burning up.   I can't figure out what is causing R78 to burn. I replaced a number of carbon comp resistors, the worst out of spec was R78, which was going open at 200 ohms. Every single diode reads good. Lifting the cathode leg of D17 eliminates the short. The electrolytes all check out good. I put a 9vdc charge on the filter caps which came back at about 7.5vdc.
I was going to throw in the towel on this amp, but it just bugs me that I can't figure it out. All insights appreciated. thanks Drewv
Attached is all the info on this amp. A few of the values and/or part numbers are different than what I'm looking at, but nothing major.

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Schematics and Layouts / 1980's Fender Stage Lead / Studio Amps
« on: November 17, 2013, 05:56:43 PM »
Hi All, I am restoring a 1982 Fender Stage Lead II. I  found the schematic / parts list / layout for the amp I have at http://www.stratopastor.org.uk/strato/amps/twoseriessolidstatefenders/twoseriessolidstatefenders.html.
They have more Fender schematics listed. I have attached the pdf. Enjoy dv

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Amplifier Discussion / DISTORTED OUTPUT
« on: March 23, 2011, 05:59:22 PM »
Hello folks,
I have a Beta Bass amp that is distorting at the speaker output. Supply voltage is good. Can't find any bad transistors. Preamp out from the back panel is clean, but power amp in is distorted like the speaker output.
The amp sputters and cuts out. The power amp has plenty of output level but I have to crank the "Drive" control ( first gain stage) At this point it easily goes into self-sustaining mode. At low "drive" level the amp acts like the signal is being compressed and distorted at the same time. Has anyone seen these symptoms before? Thanks in advance DrewV

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Amplifier Discussion / Sunn Concert Bass Troubleshoot
« on: May 25, 2009, 10:56:03 AM »
Hello, I am restoring a Sunn Concert Bass amp. I have tested replaced, as necessary all resistors and capacitors including all five 2N3055 transistors, all electrolytics, all three rectifiers, and all emitter resistors.

Voltage readings on the power transformer (in circuit) are:
Lt Orange pair = 22.5vac & 22.7vac
Green pair = 19.9vac & 20.7vac
Dk Orange pair = 16.1vac & 18.5vac

Speaker terminal: acv = 0/  Dcv = 6.4mv
Collector to ground resistance:
In circuit Q1= reads like a capacitor / Q2= 0.5ohm/ Q3=465ohm/ Q4=465ohm/ Q5= 7ohm.  Q2 & Q5 pass continuity test. The 910ohm resistors associated with Q3 & Q4 also read 465ohms. With the transistors mounted but not electrically connected all collectors read OL.

The + rail voltage is 5 volts too positive (reads 30v when it should read 25v). The – rail voltage is 1volt too negative (reads –16v when it should read–15v).
               
Here are the symptoms:         
With the guitar plugged into either input the signal sounds clean if picked lightly at low amp volume but goes into massive ugly distortion if amp volume is turned up or guitar is played hard, drawing a good amount of current, thou not enough to burn or heat any components..

With the right distortion footswitch shorted tip to ground/ output level is reduced and the distortion control attenuates the signal as it is turned up with near total attenuation at “10”.  (something backwards?)

With the left distortion footswitch shorted tip to ground/ idle output level is increased by a substantial margin with a buzz/hum like quality. Rotating the distortion control from zero to “10” increases the overall level and causes the amp to go into massive ugly distortion drawing a good amount of current as described above.

With both the left & right distortion footswitches shorted tip to ground/ amp draws very little current and displays the same massive ugly distortion if amp volume is turned up or guitar is played hard. Rotating the distortion control to 10 produces good distortion at very reduced amp output levels, drawing very little current.
Thanks in advance
DrewV


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