And some days you get the bear. Had a President Lincoln II+ arrive with the hint that high SWR caused it to fail. Plugged it in, showed a Watt and a little more on AM and then tripped the fuse when I switched to sideband.
Checked the modulator/switch transistor Q55 and it showed shorted from both base and emitter to collector. That, and the output trace from Q55's collector showed a short to ground. Sure makes it sound like failed IRF520 driver and finals. So I removed Q55. Sure enough the tester said it was two resistors.
But now the short from Q55's collector trace to ground is gone.
Huh? It's a good thing Uniden chose to lap the driver and final transistors leads over the edge of the circuit board, and not poke them through plate-through holes like they did for Q55. Took just the gate and drain loose from each one. Just ground one lead of your tester, it won't know the difference. The tester says all three driver and two finals are perfectly okay.
But where did that short to ground go when Q55 was removed?
The plot got thicker. When a new TIP36 was installed, the short from collector to ground came back. Didn't know what else to do but remove the new transistor and test it. Yes, it's perfectly okay. And the short to ground is gone now.
Again.
No short to ground on the top side foil pad for Q55 collector.
Gotta figure this radio has a three-layer circuit board. Double-sided boards have been around for a while, along with the plate-through holes that line each hole with a metal sleeve that bonds the top-side pad to the bottom-side pad at each hole.
The third "center" layer is traditionally used for power and ground circuits. My theory is that a ground foil in the board's center layer is shorting to the transistor lead where it passes through the hole. My hack is to insulate the transistor's collector lead where it passes through the hole. The brown insulation is a high-temp plastic. Not teflon, but close enough.
Clearly this hole does not have a plate-through sleeve in it, but there is a foil trace on the top side of the board, and it was clearly soldered to the collector lead. All I have to do is insulate the transistor's collector from the inside rim of the hole.
This wire will complete that circuit.
Had to punch a new mounting hole for Q55 to raise it farther above the board and make room for the hack.
Here it is all put together:
And it's ALIYEEEEV!
I have no proof that this circuit board has three layers of copper, but that level of info is not available from President Europe.
Just the same, today I got the bear. And a (largely) surface-mount radio goes home working.
73
Checked the modulator/switch transistor Q55 and it showed shorted from both base and emitter to collector. That, and the output trace from Q55's collector showed a short to ground. Sure makes it sound like failed IRF520 driver and finals. So I removed Q55. Sure enough the tester said it was two resistors.
But now the short from Q55's collector trace to ground is gone.
Huh? It's a good thing Uniden chose to lap the driver and final transistors leads over the edge of the circuit board, and not poke them through plate-through holes like they did for Q55. Took just the gate and drain loose from each one. Just ground one lead of your tester, it won't know the difference. The tester says all three driver and two finals are perfectly okay.
But where did that short to ground go when Q55 was removed?
The plot got thicker. When a new TIP36 was installed, the short from collector to ground came back. Didn't know what else to do but remove the new transistor and test it. Yes, it's perfectly okay. And the short to ground is gone now.
Again.
No short to ground on the top side foil pad for Q55 collector.
Gotta figure this radio has a three-layer circuit board. Double-sided boards have been around for a while, along with the plate-through holes that line each hole with a metal sleeve that bonds the top-side pad to the bottom-side pad at each hole.
The third "center" layer is traditionally used for power and ground circuits. My theory is that a ground foil in the board's center layer is shorting to the transistor lead where it passes through the hole. My hack is to insulate the transistor's collector lead where it passes through the hole. The brown insulation is a high-temp plastic. Not teflon, but close enough.
Clearly this hole does not have a plate-through sleeve in it, but there is a foil trace on the top side of the board, and it was clearly soldered to the collector lead. All I have to do is insulate the transistor's collector from the inside rim of the hole.
This wire will complete that circuit.
Had to punch a new mounting hole for Q55 to raise it farther above the board and make room for the hack.
Here it is all put together:
And it's ALIYEEEEV!
I have no proof that this circuit board has three layers of copper, but that level of info is not available from President Europe.
Just the same, today I got the bear. And a (largely) surface-mount radio goes home working.
73
