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Re: Fender M-80 s

11/27/2005 5:18 PM
Peter	Re: Fender M-80 s There is some usefull information on Nat Cade's site re M80's among other info. Could be zener supply failure..... I'll paste it here (hopefully without offending anyone) http://members.tripod.com/NatCade/pages/tech_f2.html March, 1993 Overheating Solid State Guitar Amps In most cases the thermal switch will shut down the unit, and after cooling, it will turn on again. The problem is caused by poor heat transfer from the output transistors through the mounting bar to the diodes, leading to poor tracking of the bias circuit, and excessive bias current flow through the output transistors. In most cases the output transistors are not seated onto the mounting bar. Inspect the insulator used between the transsitors and the mounting bar. The overheating units all appear to be using Silicone insulators (Sill Pads). these are soft and flexible. June, 1993 M-80, M-80 Chorus Bias Modification A resistor change is required in the power amp section to reduce the DC Bias current through the output transistors. The change will minimize the possibility of overheating. M-80 : Remove R74 & R76 (1K, 1/4W), and replace them with 2.2K, 1/4W carbon film resistors. M-80 Chorus : Remove R127, R129, R154, R156 (1K, 2W), and replace them with 2.2K, 2W carbon film, or metal film resisotrs. M-80 Bass Power Supply Modofications A resistor change is required in the power supply section to reduce the amount of current through the 16 volt zener diodes CR49 & CR50. This will minimixe the possibilty of the zeners overheating. Remove R144 & R145 (270 ohm, 7W), and replace them with 390 ohm, 7W wire wound resistors. Make sure to glue the resistors to the p.c. board using an RTV silicone compound. A simple test will verify the overheating condition. Using a DC Volt Meter, attach the leads across one of the emitter resistors (.47 ohm/5watt). Turn the power on and let the amp idle with no input signal or output load. Obsrve the DC voltage drop across the emitter resistor. The measurement will climb up to a certain level and stabalize. This usually takes 3 to 5 minutes. An overheating amp will measure 80mVDC to 250mVDC. Modification procedure: Desolder and remove the transistors (TIP142/TIP147). Remove the Sill Pads (insulators). Install Mica Pads using a liberal amount of thermal grease to both side. Remount the transistors and tighten securely (6 to 8 lbs.), and solder the leads. Repeat the test. A good measurement is in the area of 40mVDC to 80mVDC The TIP142 and TIP147 Darlingtons have a wide gain range. In some cases the transistors may need to be replaced to obtain a lower idle (bias) current (editor note: It's been my experince that 50% of these amps will eat the replacement parts. Although more expensive, non-generic parts have worked well for me in keeping the call back ratio way down).

11/27/2005 5:18 PM

Peter

Re: Fender M-80 s

There is some usefull information on Nat Cade's
site re M80's among other info.
Could be zener supply failure.....
I'll paste it here (hopefully without offending anyone)

http://members.tripod.com/NatCade/pages/tech_f2.html
March, 1993
Overheating Solid State Guitar Amps
In most cases the thermal switch will shut down the unit, and after cooling, it will turn on again. The problem is caused by poor heat transfer from the output transistors through the mounting bar to the diodes, leading to poor tracking of the bias circuit, and excessive bias current flow through the output transistors. In most cases the output transistors are not seated onto the mounting bar.

Inspect the insulator used between the transsitors and the mounting bar. The overheating units all appear to be using Silicone insulators (Sill Pads). these are soft and flexible.
June, 1993

M-80, M-80 Chorus Bias Modification
A resistor change is required in the power amp section to reduce the DC Bias current through the output transistors. The change will minimize the possibility of overheating.

M-80 :
Remove R74 & R76 (1K, 1/4W), and replace them with 2.2K, 1/4W carbon film resistors.
M-80 Chorus :
Remove R127, R129, R154, R156 (1K, 2W), and replace them with 2.2K, 2W carbon film, or metal film resisotrs.
M-80 Bass Power Supply Modofications
A resistor change is required in the power supply section to reduce the amount of current through the 16 volt zener diodes CR49 & CR50. This will minimixe the possibilty of the zeners overheating.
Remove R144 & R145 (270 ohm, 7W), and replace them with 390 ohm, 7W wire wound resistors. Make sure to glue the resistors to the p.c. board using an RTV silicone compound.

A simple test will verify the overheating condition. Using a DC Volt Meter, attach the leads across one of the emitter resistors (.47 ohm/5watt). Turn the power on and let the amp idle with no input signal or output load. Obsrve the DC voltage drop across the emitter resistor. The measurement will climb up to a certain level and stabalize. This usually takes 3 to 5 minutes. An overheating amp will measure 80mVDC to 250mVDC.

Modification procedure:

Desolder and remove the transistors (TIP142/TIP147). Remove the Sill Pads (insulators).
Install Mica Pads using a liberal amount of thermal grease to both side.

Remount the transistors and tighten securely (6 to 8 lbs.), and solder the leads.

Repeat the test. A good measurement is in the area of 40mVDC to 80mVDC

The TIP142 and TIP147 Darlingtons have a wide gain range. In some cases the transistors may need to be replaced to obtain a lower idle (bias) current (editor note: It's been my experince that 50% of these amps will eat the replacement parts. Although more expensive, non-generic parts have worked well for me in keeping the call back ratio way down).