Finally!
Got me a guinea pig. A Palomar 300A guinea pig. Got a good deal, considering it lacks the power transformer. Not a problem. This amplifier will serve as the guinea pig for two design ideas. An outright replacement for the plug-in relay circuit board used in early production amplifiers, and a universal power transformer that will work with either of the two high-voltage rectifier setups used in this model.
First the thing has to get rehabilitated. Pretty sure this rubber-stamped number bears the month, day and year.
1977 seems about right. And the same number shows up here, inside on the chassis deck.
Not too dirty, and only shows bad oxidation on things that didn't get properly protected, like these spring clips on the tubes' plate caps.
This HV filter cap has a 7-digit code that's three digits for the manufacturer's EIA member ID, followed by two digits for the year, and two for the week 01-52. Makes this capacitor a GE product (188) made the third week of December 1976.
The fan's shaft was pretty stiff. Would turn, but not spin. The cure is easier than it looks. Just cleaning the schmoo from inside the bushings and off the shaft surface works best with a solvent that's an aggresive degreaser that leaves no residue. First, unbolt the rear pillow block and pull it off.
Next, push the shaft down into the fan blade's hub. You can pull the rotor out and free from the fan blade.
This way you won't have to unbolt the fan's frame from the chassis.
Once it's cleaned up, lubricant has to be replaced. It's stored in a felt doughnut that surrounds the porous bronze "oilite" bushings.
Saturate the felt, and the capillary attraction will pull it through the pores in the sintered bushing. Until it all evaporates years down the road.
Thought I had heard of everybody that was in the business of making small fans back then. Can't remember seeing this name before.
Here's the old circuit board with the amplifier's "unobtanium" relays on it.
Definitely some oxidation schmoo on the board's edge-finger contacts. No problem, since this board gets tossed.
It helps that the spring contacts in the socket where this board plugs in are gold plated. They look perfect.
The driver tubes do not have the "G2" screen grids grounded. They have 12 Volts DC fed to them, borrowing the relay's power supply. The 10k resistor is just my superstition about having a screen grid with no current path to ground.
Connecting that grid of the two driver tubes serves to "goose" the peak power they'll deliver, but it creates a hazard for the amplifier's low-voltage circuits if a driver tube develops an internal short or a dose of gas.
The diode will turn off and block high voltages if they appear on that pin of either driver tube.
The bare wires attached to the rusty final-stage plate clips used to each have a tiny ferrite bead slipped over it. They get hot, crack, fall off and nobody ever misses them. Generations of geeks have grown up thinking that those bare wires on the plate caps were the original design. Not so. This one gets a set of conventional parasitic-suppression chokes. And shiny new plate-cap clips.
There is more to do, but dinner was calling. It will get finished soon, and I can find out of my plug-in relay board prototype will fly or crash and burn.
As the old Johnny Carson segue cards used to say "More to come".
73
Got me a guinea pig. A Palomar 300A guinea pig. Got a good deal, considering it lacks the power transformer. Not a problem. This amplifier will serve as the guinea pig for two design ideas. An outright replacement for the plug-in relay circuit board used in early production amplifiers, and a universal power transformer that will work with either of the two high-voltage rectifier setups used in this model.
First the thing has to get rehabilitated. Pretty sure this rubber-stamped number bears the month, day and year.
1977 seems about right. And the same number shows up here, inside on the chassis deck.
Not too dirty, and only shows bad oxidation on things that didn't get properly protected, like these spring clips on the tubes' plate caps.
This HV filter cap has a 7-digit code that's three digits for the manufacturer's EIA member ID, followed by two digits for the year, and two for the week 01-52. Makes this capacitor a GE product (188) made the third week of December 1976.
The fan's shaft was pretty stiff. Would turn, but not spin. The cure is easier than it looks. Just cleaning the schmoo from inside the bushings and off the shaft surface works best with a solvent that's an aggresive degreaser that leaves no residue. First, unbolt the rear pillow block and pull it off.
Next, push the shaft down into the fan blade's hub. You can pull the rotor out and free from the fan blade.
This way you won't have to unbolt the fan's frame from the chassis.
Once it's cleaned up, lubricant has to be replaced. It's stored in a felt doughnut that surrounds the porous bronze "oilite" bushings.
Saturate the felt, and the capillary attraction will pull it through the pores in the sintered bushing. Until it all evaporates years down the road.
Thought I had heard of everybody that was in the business of making small fans back then. Can't remember seeing this name before.
Here's the old circuit board with the amplifier's "unobtanium" relays on it.
Definitely some oxidation schmoo on the board's edge-finger contacts. No problem, since this board gets tossed.
It helps that the spring contacts in the socket where this board plugs in are gold plated. They look perfect.
The driver tubes do not have the "G2" screen grids grounded. They have 12 Volts DC fed to them, borrowing the relay's power supply. The 10k resistor is just my superstition about having a screen grid with no current path to ground.
Connecting that grid of the two driver tubes serves to "goose" the peak power they'll deliver, but it creates a hazard for the amplifier's low-voltage circuits if a driver tube develops an internal short or a dose of gas.
The diode will turn off and block high voltages if they appear on that pin of either driver tube.
The bare wires attached to the rusty final-stage plate clips used to each have a tiny ferrite bead slipped over it. They get hot, crack, fall off and nobody ever misses them. Generations of geeks have grown up thinking that those bare wires on the plate caps were the original design. Not so. This one gets a set of conventional parasitic-suppression chokes. And shiny new plate-cap clips.
There is more to do, but dinner was calling. It will get finished soon, and I can find out of my plug-in relay board prototype will fly or crash and burn.
As the old Johnny Carson segue cards used to say "More to come".
73
