April, 1989

Watch Out For The Unexpected!
The thermistor TH-1, in the 185 Series amplifiers was designed to limit initial turn on surge, improving the life span of the other components. This is a problem in some of the amps (editor note: most that I have serviced).

The vendor that Fender purchased these thermistors from purposely introduced small half bends in the leads during the manufacturing process. These semi-circle bends are there to stop the component from being inserted to far during stuffing of the P.C. board. Without these bends, the component leads may go in so far that the ceramic covering of the thermistor (and a 1/4 inch or more of the leads) may enter the hole and insulate the leads from the pad. These thermistors were damaged during the bending process causing the leads to crystalize and become very brittle.

Subsequent vibration from shipment added to the problem by causing the leads to fracture completely, resulting in a dead or intermittent amplifier. Replace the thermistor with a known good one and use a little RTV cement between the thermistor and the P.C. board after the solder cools (editor note: I have found that goop and electronics grade silicon work well for this application).

Thermistors vs. MOVs
On completeed warranty forms to Fender, thermistors are sometimes referred to as MOVs. This is understandable as both thermistors and MOVs are used in the primary wiring of transformer powered equipment, and they are virtually identical in physical appearance.

Take a close look and you will see definite differences in the circuit configuration. Thermistors are wired in one side of the A.C. line, in series with the primary winding of the power transformer (which is why the amp goes dead when a lead breaks open), and are used to limit the in-rush current when the amp is turned on cold. As the thermistor warms up, its internal resistance goes down gradually increasing current flow in the primary of the power transformer. Small changes of resistance in the 120VAC primary circuit are significant. Typically a thermistors value in ohms, ranges in the single numbers just as good surge resistors would. In the 185 series, TH-1 varies from about 2.5 ohms cold to about 0.06 ohms warm, and is rated at 8 amperes.

These thermistors are wired into the phase side rather than the neutral side of the A.C. line, and also provide a second benefit in reducing noise getting back onto the A.C. line from the amplifier. This can be a welcome benefit when you have lots of equipment connected to the A.C. line on stage, and you operate the 185 without without causing sudden transients over the high powered P.A. system.

Unlike thermistors, metal oxide varistors are wired in parallel, or across the primary winding (sometime the secondary) of the power transformer. Varistors are voltage dependent symetrical resistors which act in a manner similar to back-to-back zener diodes in circuit protection functions. When exposed to high energy voltage transients, the varistor impedance changes from a very high standby value to a very low conducting value, clamping hte transient voltage to a safe level.

January/February 1990

Proper Controls (Power Chorus)
The revised footswitch can not be used with the non-revised (early production) amplifiers, or vice versa. If you have a Power Chorus with problems in the switching circuit, it may be in the footswitch or the amp. For these situations you need the proper controls. One key to this control is the serial number cut-over used when the change went into production. The serial number cut-over is . . L099109, that is, the revision starts with serial number L099109. Serial number below L099109 do not have the upgrade. Most Power Chorus amps will have a higher serial number and will therfore have the upgrade.

A second key to maintaining proper control is to have a list of the revisions, or the proper schematic for the amp in service. the schematics distributed to dealers reflect the upgrade. Both revisions of the schematics for the footswitch and amp are shown below. Refer to the serial number and choose the proper schematic.

REF	PART#		TYPE	 	REF	PART#	TYPE	
CR1	006260		1N4448		CR1	006260	1N4448
CR2	006260		1N4448		CR2	006260	1N4448
CR3	027327		1N5234B		CR3	027327	1N5234B
LD1	028039		LED RED		CR4	031017	1N5223B
LD2	028039		LED RED		LD1	028039	LED RED
LD3	028039		LED RED		LD2	028039	LED RED
LD4	028039		LED RED		LD3	036588	LED ORANGE
LD5	9902202281	LED RED		LD4	036588	LED ORANGE

ONLY PARTS CHANGED FOR THE UPGRADE ARE LISTED:
(These are the parts used before the upgrade)

REF	PART#	TYPE			
C63	027278	CAPRF 63V, .1uF, 10%
C65	028459	CAPRE 50V, 2.2uF, 20%
CR21	031017	ZENER 2.7V, 1N5223B, 5%
CR23	006260	DIODE SIG., 1N4448
CR42	027327	ZENER 6.2V, 1N5234B, 5%
R132	015582	RESMF 10K, 1/4W, 1%
R133	036200	RESMF 9.31K, 1/4W, 1%
R138	036202	RESMF 118K, 1/4W, 1%
R139	036201	RESMF 34.8K, 1/4W, 1%
R141	024998	RESCF 120K, 1/4W, 5%

(These are the parts used after the upgrade)

REF	PART#	TYPE			
C63	027281	CAPRF 63V, .22uF, 10%
C65	028460	CAPRE 50V, 4.7uF, 20%
CR21	027329	ZENER 3.9V, 1N5228B, 5%
CR42	029898	ZENER 7.5V, 1N5236B, 5%
R132	024981	RESCF 10K, 1/4W, 5%
R133	024983	RESMF 12K, 1/4W, 5%
R138	036587	RESMF 158K, 1/4W, 1%
R139	036586	RESCF 52.3K, 1/4W, 1%
R141	025059	RESCF 220K, 1/4W, 5%

July/August, 1990

Channel Switching In The Princeton Chorus
After a warm-up period of 15 minutes or so, the channel switching function in some of these amps will not switch into the overdrive mode, even though the LED indicates that it has.

Look at I.C. U11. If it is a Samsung 4558, change it to a Texas Instrumantes or Motorola 4558. A TL0-72 will also work here.

Take A Close Look at Those Power Resistors
When you are servicing an amplifier, take a close look at the power resistors to see if the compound is badly craked or falling out. If so, replace the bad part(s) but do not use any with CB in the code. Resistors with the codes such as PW or CP are preferred over the CB coded resistors.

November/December, 1990

BXR Spectrum Crossover Changes
The change involves revising the printed circuit board bringing it up to revisoin "C". The drawings that follow show an earlier revision "B" printed circuit board (figure 1), and the latest revision "C" board (figure 2). The original P.C. board and the revision "A", are nearly the same as revision "B", so they are not shown here.

The change applicable to field service involves replacing the 126uF/100V capacitor with a 120uF/100V capacitor, and the 2.0mH/3amp inductor witha 2.2mH/5.5amp inductor.

Changing the capacitor does not require p.c. board modification, but changing the inductor does. Two new holes designated "H" in figure 3, with a diameter of 0.55 of an inch (1/16 inch is close enough) must be drilled 1-1/8 inches apart, slightly offset from the existing traces for the electrical leads of the inductor. Additionally, the mounting hole designated "M" in figure 3, must be enlarged to 0.175 of an inch (3/16 inch is close enough).

Note that the holes are drilled offset from the printed traces. DO NOT drill directly through the printed traces as this will remove to much of the current carrying capability of the trace material.

Use a 8-32 x 2 inch machine screw with an 8 x 1/2 inch neoprene washer, and 8-32 hex nut to mount the replacement inductor. the washer and nut should be mounted on top of the inductor. After mounting the inductor (L1), the leads must be bent over and soldered to the existing pads. Below are the Fender part numbers.

PART #	DESCRIPTION
028658	Inductor, L1, 2.2 millihenries, 5.5 amperes
028732	Capacitor, C3, 120 microfarads, 100 volts NP
029467	Screw, Machine, 8-32 x 32"
029468	Washer, Neoprene, 8-32 x 1/2"
022004	Nut, Hex, 8-32

The Twin
The Twin uses 1.0 ohm/1W resistors in the cathode circuit of each pair of 6L6GC output tubes to permit measuring and setting the bias from rear panel test points. Setting the bias control to produce 40 millivolts across the 1 ohm resistor equates to 40 milliamps of idle current, the proper biasing.

There are certain conditions of 6L6GC tube failure that can cause excessive current through these 1 ohm resistors resulting in heat build-up enough to char the p.c. board even though they are flameproof type resistors.

Adding diodes across the resistors will protect them in the event of tube failure. add a 1N4003 doide across R215 and one across R216. Refer to diodes D202 and D203 in figure 4:

Position the diodes so that the cathode of the diode connects to the ground side of the resistor, and the anode of the diode connects to the side of the resistor that connects to the cathode of th 6L6GC tube. The 1N4003 are Fender part number 064089.

October, 1991

PX2108, PX2112, PX2116 Powered Mixing Consoles
To improve reliabilty and cooling of the power amp, a higher speed fan is required. After changing the fan, the low speed mode requires a modification to operate properly. R130 must be changed from a 100 ohm/2W metal oxide resistor to a 39 ohm/2W metal oxide resistor. THis resistor must also be mounted above the circuit board using 1/2 inch long fiberglass tubing over the leads. This allows air circulation for improved heat dissipation.

The old fan may be identified by the green label attahed to the blade. The newer higher speed fan has a silver label. In either case, the label is visible from outside the unit looking through the grill. The part number is the same but the green labeled fan is obsolete. Part numbers:

1	036177	Fan, 24VDC, 3-1/8" Sq., 44CFM
1	037177	Resistor, 39 ohm/2W, metal oxide (R130)
1	029962	Tubing, Fiberglass #12, 1/2" long

January/February, 1992

Fan Starting Torque
A problem with the starting tiorque of the cooloing fan on an SPL7250 power amplifier. This 24VDC fan is set up to operate at two speeds, with a normally open thermal switch sensing temperature. When the temperature rises above 60 degrees Celsius, the switch closes, shorting across a 2 watt resistor. This resistor is in series with the fan motor, and shorting across it causes the fan to run at high speed.

Originally the specified value for the resistor was 300 ohms, used in both the SPL7250 and the PX2100 series mixers. This has been changed, the new value for use in the SPL7250 is 39 ohms, 2 Watts. In the PX2100, the value is 100 ohms, 2 watts. Refer to R310 below:

October, 1992

SKX-35R, SKX-65R, SKX-100R Bias Modification
These solid state amps require two component changes.

SKX-35R

SKX-65R/100R

The resistor change reduces the amount of idle current (bias) that flows through the output transistors.

Replacing hte diode with a jumper will allow the bias transistor, mounted on the heatsink, to track the heatsink temperature more accurately.

Novenber, 1992

Reissue '63 Vibroverb Bias modification
During the summer of 1992, Fender began shipping all production tube amplifiers with 5881 output tubes as original equipment. The 5881 has slightly different specs than the standard 6L6GC. This combined with the lower plate vlotage of 435VDC used in the '63 Vibroverb, causes an increase in crossover distortion. Decreasing the negative bias voltage turns the tube on harder, reducing the crossover distortion.

To perform the modification:

Remove the knobs and nuts on the "SPEED" and "Intensity" controls.

Gently remove the small circuit board.

Locate R58 (1K, 1/2W, 5%) and desloder.

Install a 3.9K, 1/2W, 5% carbon film resistor.

With the above modification, 6L6GC and 5881 tubes will be biased correctly.

February, 1993

Crossover Modifications
SUNN - 1211, 1228, 1291C, SPL1225, SPL1226
FENDER - 2825
first determine if the crossover is the old version. The differences between the old and the new versions involve:

the number of standoffs used to mount the circuit board
the amperage rating of the inductor used at "L1"
the capacitors used at "C1, C3", and the addition of "C5".

OLD					New
6 mounting stand-offs			8 mounting stand-offs
L1 Inductor 1800uH 5.5Amp (028657)	L1 Inductor 1800uH 7.2Amp (033612)
C1 Capacitor 33uF 100V    (028724)	C1,C3,C5 Cap 15uF 100V Ployfilm (040272)
C3 Capacitor 18uF 100V    (028721)	NOTE: C5 is parallel with C1

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.

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).

April, 1993

Reissue '65 Twin - Intermittent Operation
A problem with the control pots can cause intermittent operation. There are three leads which protrude from the bottom of the pots. One of the leads appears to have not been riveted securely to the phenolic material on the back of the pot. this problem was noarrowed down to the date code of (9208).

To inspect the pots, remoce the circuit board they are mounted to, being careful not to damaga the flex jumpers. Inspect the back side of the pots. The date code is printed on the phenolic backing, just above the lead mounting rivets. the fender part number is printed above.

remove the pot with the date code in question and attempt to wiggle the leads. A poorly secured lead will wiggle easily. Replace all pots as necessary.

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.

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.