1967 Band Master

 

This was a repair for the original owner. He had owned it since he was 16 or 17 years old. He had lent it to a friend that had if for several years and when he got it back it was making crackling noises. He wanted the amp as a player and not a museum piece so it was decided to keep it as original as possible using currently available parts. Taking care of the issues outlined below brought it back to life. It sounded really good with the old tubes in but was low on power. New tubes (current manufacture) brought the power up but it lost that sweet tone. I put the originals back in and now we are looking for NOS tubes for the future. 

 Initial Findings  

Mechanical            

•  Amp is in good physical condition with minor wear and tear for amp of this age
• Some unknown liquid had gotten inside the amp. This caused minor corrosion of the cad plating making a
• white dust form on cad plated chassis parts. The chassis and cabinet were cleaned to remove the white dust.
•  The front panel on the far left above the Normal channel input jack was bent out and that was straightened.
•  One of the knobs is damaged and is missing its chrome center. This was not replaced.

 Electrical            

• Amp still has 2-coductor power cord and is frayed.
• The fuse had been removed and replaced by length of cloths hanger wire.
• Rectifier and bias rectifier boards looked to have been rebuilt as the diodes and the electrolytic cap are not 1967 vintage parts. This is not a problem but the electrolytic will be replaced.
•  One of the screen grid resistors on the power tubes had been replaced with the wrong value resistor
• (both wattage and resistance) and never re-soldered.
• All of the electrolytic capacitors are original and should be replaced. The high voltage filter caps show electrolyte bulges at the vent holes.

Summary of Repair            

• Two conductor power cord replaced with three conductor grounded power cord.
• Ground polarity switch disabled.
• Replaced fuse.
• Replaced pilot light.
• Replaced bias supply filter cap.
• Replaced all filter caps and voltage dropping string resistors on high voltage filter board.
• Replaced all preamp cathode bypass caps.
• Replaced power amp screen resistors.
• Replaced four tone control potentiometers.
• Cleaned and re-tensioned tube sockets.
• Checked operating voltages and reset bias at 70% plate dissipation.

Front view of chassis.

Bottom front view of chassis. 

Bottom Front view of chassis with filter cap cover removed.

High voltage filter section filter caps showing bulges on cap ends.

High voltage filter section with new Sprague Atom filter caps and all new voltage dropping resistors.

Bias rectifier with new Sprague Atom filter cap.

Original dual section cathode bypass caps in preamp circuits.

New Sprague Atom cathode bypass caps in preamp circuits. Note that there are two individual caps replacing the older dual section cap. Currently there are no dual section caps of the proper form factor available for this application.

Another close-up view of the new Sprague Atom cathode bypass cap. This one replaced a single section cap.

These are the screen grid resistors that were removed from the power tubes. The bottom resistor is the correct physical size (Size is an indicator of the wattage or power handling capabilities of the resistor). The bottom is a 470 ohm 1 watt and the top is 47 ohm ¼ watt. They are both supposed to 470 ohm 1 watt.

Here is one of the power tube sockets with the new 5 watt metal oxide resistors in place. Early amps failed quite often due to no resistors in this location. Later on they started using 1 watt devices here. You will find mostly 5 watt screen grid resistors in modern amps. These resistors protect the tube from shorting internally when played fully cranked by limiting the screen current to the tube.

Here is a photo of the components that were removed from the amp. All components were de-soldered rather than cut out from the circuits. This was done in case someone in the future wanted to restore the amp to be physically correct for collection value purposes. The amp owner will get all the removed parts with the repaired amp.