The "like a box of chocolates" and the "Franken-whatsit" memes are getting a bit worn. '
Just the same, the older the DX300 amplifiers get the rougher they become around the edges. This one has had the small transformer replaced, the one that powers the tube's heater, the lights, relays and the tube's screen and grid-bias voltages. This transformer draws no more than 1 Amp, but the main fuse is rated at ten Amps. Any overload on the small transformer will croak it before a 10-Amp fuse can trip out.
The one on the right is just way bigger than stock.
The tiny one in the middle is the correct transformer. Never mind where I got it. And no, I don't have any of them to sell you.
At least the substitute transformer has an ID number on it.
The number "SR-237" will be recognized by anyone who has had his/her fingers in late-production D&A amplifiers like the Maverick, Phantom or MDX/PDX. It's surely more than big enough for the job, but there's the problem. It puts out 680 Volts, but the correct transformer puts out closer to 500. May not sound like a lot, but it definitely ramped up the stress level on the circuit board it feeds into.
In all fairness, it's routine for the 10-Watt power resistors on the factory pc board to darken the laminate from heat, especially after four-plus decades. But this one is over the top. The resin has burned away, exposing the glass fabric. Putting half-again more than normal voltage on these parts got them really hot.
I'll give credit where credit is due. Someone figured out the original 5-Watt zener is too small to regulate the tube's bias voltage. Looks like they just kept making it bigger until it stopped breaking down. Pretty sure this zener is a fifty-Watt unit.
The nastiness doesn't stop at the wrong transformer and collateral damage from the excess voltage. This tie strip is where the 120 Volts from the line cord gets distributed to two transformers and a fan motor.
Did. Not now.
Fortunately I had one on hand. And that was all I needed for this job. More of them are coming from Mouser next week.
And it's gonna need a new power switch. This one looks way too tired.
The comparisons to a taxicab in Havana will get old too, by and by.
But it's only money, after all. With any luck we'll find and fix all the gremlins before it gets keyed up the first time when we're done.
73
Just the same, the older the DX300 amplifiers get the rougher they become around the edges. This one has had the small transformer replaced, the one that powers the tube's heater, the lights, relays and the tube's screen and grid-bias voltages. This transformer draws no more than 1 Amp, but the main fuse is rated at ten Amps. Any overload on the small transformer will croak it before a 10-Amp fuse can trip out.
The one on the right is just way bigger than stock.
The tiny one in the middle is the correct transformer. Never mind where I got it. And no, I don't have any of them to sell you.
At least the substitute transformer has an ID number on it.
The number "SR-237" will be recognized by anyone who has had his/her fingers in late-production D&A amplifiers like the Maverick, Phantom or MDX/PDX. It's surely more than big enough for the job, but there's the problem. It puts out 680 Volts, but the correct transformer puts out closer to 500. May not sound like a lot, but it definitely ramped up the stress level on the circuit board it feeds into.
In all fairness, it's routine for the 10-Watt power resistors on the factory pc board to darken the laminate from heat, especially after four-plus decades. But this one is over the top. The resin has burned away, exposing the glass fabric. Putting half-again more than normal voltage on these parts got them really hot.
I'll give credit where credit is due. Someone figured out the original 5-Watt zener is too small to regulate the tube's bias voltage. Looks like they just kept making it bigger until it stopped breaking down. Pretty sure this zener is a fifty-Watt unit.
The nastiness doesn't stop at the wrong transformer and collateral damage from the excess voltage. This tie strip is where the 120 Volts from the line cord gets distributed to two transformers and a fan motor.
Did. Not now.
Fortunately I had one on hand. And that was all I needed for this job. More of them are coming from Mouser next week.
And it's gonna need a new power switch. This one looks way too tired.
The comparisons to a taxicab in Havana will get old too, by and by.
But it's only money, after all. With any luck we'll find and fix all the gremlins before it gets keyed up the first time when we're done.
73
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