Fender Hot Rod Deluxe Car Wreck

This seasoned road warrior was happily touring and now was called up for recording duty.  However, at the final live set, the unit became very warm and the audio dropped way off.  Could the Unbrokenstring Crew work some magic to return this guy to the studio?

01HRD the patient1This unit has been in the shop once in the last year for a defective plate load resistor in the phase inverter.

02HRD nameplateApparently Fender received a batch of defective plate load resistors. The vendor corrected the problem, but a few of the amplifiers built with that particular batch of resistors have been in the field for years.

03HRD take apartThe circuit boards need to come loose to access everything. It turns out, the heat is coming from several sources.

04HRD probingThe schematic is on the monitor.  Unbrokenstring is trying out the Paperless Office Experience.

05HRD the problemThis is one of the open-circuited plate resistors on the phase splitter tube.

06HRD FixedHere is an NOS carbon composition resistor from stock.  It looks right at home in this amp.

07HRD caps1The real issue today is that the capacitors have begun to vent.  We need to address this with a recap.

08HRD caps2These units are all on the same circuit and are all the same age.  So they are all to be replaced.

09HRD heat1A previous non-Unbrokenstring repair around the low voltage regulator circuits fixed some burned circuit boards.

10HRD heat2These large resistors limit the Zener current. They are too close to the printed circuit board.

11HRD temp rangeThe capacitors will be replaced with units that have a higher temperature rating.

12HRD new partsHere, the Zener diodes and power resistors are spaced above the circuit board for better ventilation.

13HRD teflon tubingSome amp techs touch each cap with a screwdriver to discharge them. The positive lead carries about 400vdc and represents a safety hazard.  Those who ‘know’ will use a discharge wand on the tube socket or choke terminals to discharge this high voltage.  For the screw driver’s sake, we’re installing Teflon tubing on the exposed high voltage points.

14HRD plate resistor1We are changing out the plate load resistors.  All of the preamp tubes use a 100k resistor in the plate circuit.  This resistor is also part of the audio path e.g. the amplified signal appears on the plate end of this resistor, so we will benefit from modern 21st century components for duty in this part of the circuit.   Again, they are spaced above the circuit board to keep the body temperature (and noise) low.

15HRD plate resistor2More plate load resistor goodness…

16HRD input jacks1The plastic input jacks are replaced with American-made Amphenol units which are bolted directly to the front panel.

17HRD first testThe circuit boards are placed back into position and the unit is placed under test.  After an hour and a half of a four hour burn-in, the amplitude dropped off again.

18HRD power1Bias current climbed a few milliamps and could not be put back to normal. Here is a temperature reading at the base of one of the output tubes.

19HRD power2This is the temperature of the other tube base.  Obviously, the circuit is unbalanced now because the other tube is a lot hotter.  We will check the tubes again, but something else is happening here.

20HRD preampHere is the temperature of one of the preamp tubes.

21HRD transformersSurprisingly, everything returns to normal when the amp cools.  The chassis is on the bench now to take a closer look.  Something looks wrong with the output transformer.  Running the unit on the bench while applying a heat gun to the output transformer duplicated the problem!

22HRD disassembly againA new output transformer is still available from Fender!  Let’s substitute the new one and restart the test.

23HRD wires1The red and brown leads are the ends of the output transformer primary winding that are driven by the output tubes.

24HRD wires2The red wire is the center tap, which is fed high voltage.  The black wires connect the power supply choke into the circuit.

25HRD wires3The output windings of the output transformer go here.  Pay attention to the striped, unstriped, and black wires.

26HRD wires4All of the wires to the output transformer go through this TieWrap.  We’ll snip this to get it out of the way for now.

27HRD wires5Here are the output transformer leads.  Now, we unscrew the transformer from the chassis.

28HRD xformer1I compared the two transformers.  The bases are lined up, but the top is misaligned.  What the…?

29HRD xformer2Yes, the feet and bottom of the frame is aligned.

30HRD xformer3But the top of the lamination stack is definitely displaced on the top.

31HRD xformer4Here, the laminations are parallel, but the straight edge shows that the old transformer is definitely parallelogrammed.

32HRD Broken Glass1A phone call to the owner reveals that this amp was in a serious car wreck about five years ago.

33HRD Broken Glass2That wreck explains why the bottom of this amp is filled with broken safety glass.  The force of the car wreck deformed the output transformer.  I’m guessing that as the deformed transformer heated up, the internal insulation broke down somewhere because the windings were stretched by the deformed transformer.

34HRD new xformer1Let’s bolt down the new transformer.  Here, the leads are fished through the grommet in the chassis.

35HRD new xformer2A replacement TieWrap is installed in place of the old one.  The small transformer is the power supply choke.

36HRD new xformer3More TieWraps are applied to dress the leads in a manner similar to the original.

37HRD cleanup panelI’m pretty confident that the unit will work fine now!  Let’s put everything together, including all those knobs!

38HRD input jacks2Here are the new jacks.

39HRD new finalsThe new output transformer deserves a new set of tubes.

40HRD new preampThese preamp tubes were ordered at the same time.

41HRD biasA one ohm resistor in series with the output plate circuit allows us to directly read the idle current of the output tubes.  60mV corresponds to 60mA of bias current, exactly what Fender specifies.

42HRD lower temp1Another temperature check is performed after everything warms up.

43HRD lower temp2This little thermometer came from Harbor Freight.  It works surprisingly well for a $20 infrared thermometer.

44HRD final final testWe are restarting the four hour test from the start.

45HRD happy ampIf you look closely, you can see a blue beam of electrons flying into the tube envelope through a small aperture in the plate.  These bottles are happy!

46HRD back homeThis road warrior has a new cover!

Listen to this amp in action at http://billybourbonmusic.com/ and support this artist!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE


Ampeg V4B Bass Amp Refurbishment

This wonderful old Selmer-era Ampeg bass head was pulled out of its retirement in the closet and put back into service.  But it had a few issues to address, so that it could reliably pump out the tones that is the Ampeg Experience.


01V4B the patientThis unit appears to be absolutely factory stock.  The Houston humidity has had an effect on the aluminum faceplate.

02V4V rear pnl1Taking a tour of the rear panel, we see a bracket upon which the line cord may be wrapped.

03V4B rear pnl2The convenience outlet is a three-prong unit, which is nice.  The hum balance control adjusts the bias current in the output tubes to be the same.

04V4B rear pnl3Magnavox owned both Selmer and Ampeg for a while, if I recall correctly.  Note the tube layout information.

05V4B rear pnl4Here is the other bracket for the cord, and the output jacks and impedance switching.

06V4B cordDude, are you still smoking?

07V4B internals1This is a nice intersection between hand wiring and the use of an etched circuit board.

08V4B internals2This cap and the bleeder resistors are slated for replacement.

09V4B cap1Yes, you can still get multi-section capacitors if you shop diligently.

10V4B cap2The prongs of the new capacitor need to fit in the slots in the chassis.

11V4B cap3There is plenty of height inside the chassis, but it doesn’t hurt to document what we have.

12V4B cap4Likewise, we’re documenting what we have.

13V4B cap1Here is the new multi-sectioned filter capacitor and the hole where it goes in the background.

14V4B cap2The outer can of all of the capacitors is isolated from the chassis, so these green fiberglass spacers are used under the capacitor.

15V4B cap3I think we’re done here!

16V4B cap4The new cap looks nice on the top of the chassis.

17V4B cap3The axial filter capacitor will be replaced with this part.  I am forming the leads to appear in a manner similar to the original part, seen above.  A little Teflon insulating tubing helps keep the electricity under control.

18V4B cap4Wires will be attached to the terminals, so the leads are formed into a loop to accept the wires.

19V4B cap5The wire bending is done with a hand-tool called “chain nose pliers.”

20V4B cap6The original part has a mounting ring around it.  We will need to recycle this mounting scheme to maintain originality.

21V4B cap7The ring is off!  I was a little concerned that I would mess it up, but a little heat was all it took.

22V4B cap8Here is the original mounting ring applied to the new capacitor.

23V4B cap9The ring can slide around just a little bit to give us a nice-looking mounting solution.

24V4B cap10And here we are, all wired up and ready to go back to work.

25V4B good capsThese guys have been replaced recently, and they check out as new.  So, they will remain in service.

26V4B controlsSome of the front panel slide switches were dirty, so some cleaner and lubricant were sprayed into them.

27V4B top1Now that the caps are changed out, let’s look at the top of the chassis.  The output transformer and output tubes are on the left side of the chassis.

28V4B top2The preamp tubes and power transformer are at the right end of the chassis.

29V4B top2detail1The open areas around the tube sockets are a nice touch.  The chassis is steel and very stiff, even with the relief.

30V4B top2detail2More low and medium voltage goodness at the other end of the slot.  Nearly every schematic test point is accessible from the top of the unit without turning it over on the bench.

31V4B Magnavox 12DW7This 6K11 Compactron tube tests very good, with each of the sections closely matched to the others.  Good News!

32V4B inputjax1The input jacks were corroded, so these were changed out with new Amphenol units.

33V4B inputjax2Here is the inside-the-chassis view of the new jacks.

34V4B indicator1The neon indicator for the AC power was functional, but the indicator for the high voltage was not.

35V4B indicator2So, this neon indicator will take its place.  The mounting hole is the same size, but the new part is chrome.  What to do?

36V4B indicator3We scrubbed the chrome ring with steel wool, then applied several coats of black polyurethane paint to the bezel.

37V4B Indicator4Here are both indicators.  The high voltage is amber, and the AC indicator is red, as it was when the amp came from the factory.  The colors are a bit messed-up because of the jpeg processing in the camera…  looks good in Real Life!

38V4B Happy CustomerAnother happy customer picks up his finished bass head!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE

Digitech GSP1101 Effects Processor Repair

A frantic spouse called to explain that she was vacuuming and hit this ‘thing’ and broke this little piece off it.  Could I fix it before her husband found out?  The Unbrokenstring Crew to the rescue again!

01GSP brokenThe gain knob on this unit is a rotary encoder.  This component is available from several vendors.

02GSP what to doShe kept the knob, which is GREAT news, because although the rotary encoder is available, the knob is not.

03GSP salvage knobThe remains of the shaft of the rotary encoder was removed from the knob.  So far, so good.

04GSP opensesameLet’s get to work replacing the broken rotary encoder.

05GSP knobs offAs is the usual case, all the knobs come off first.

06GSP board looseThe circuit board lifts away easily.

07GSP spacer1These cool little plastic spacers set the geometry of the rest of the buttons and controls.

08GSP spacer2We will put this piece away somewhere safe while work elsewhere continues.

09GSP remove oldTo minimize the stress on the circuit board while removing the old component, the body of the broken rotary encoder is cut away from the component leads at an angle perpendicular to the circuit board.

10GSP part goneWith the component cut away from the circuit board, de-soldering the legs becomes trivial.

11GSP clear backThe component legs are gone.  This is a high-quality circuit board, double-sided copper and plated-thru holes.

12GSP clear frontBoth sides of the circuit board are cleaned up.

13GSP new partHere is the new part, ready to be soldered onto the circuit board.

14GSP reassemble1We are ready for reassembly.  That plastic spacer is re-installed as it was before.

15GSP reassemble2The cable carries all the signals back and forth to the front panel.  I like this bit of orgami.

16GSP reassemble3The circuit board is held in place by the bodies of the controls, which are in turn bolted to the front panel.

17GSP knobs onEverything is back together, ready for testing.

18GSP power onThis looks good!  While we were at it, we will do a factory reset.

19GSP final testNow, everything is as it once was.  And the husband was scolded for leaving his toys on the floor, where she could hit them with the vacuum.  Maybe I could sell him a rack case?

Thanks for reading all the way through!

CONTACT – David Latchaw EE