• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Galaxy Saturn no mirror board. Now what?

nomadradio

Analog Retentive
Apr 3, 2005
6,935
11,063
698
Louisville, KY
www.nomadradio.com
Here's an issue that surfaced when a member's Saturn base had problems with blown finals. Installed new ones. They blew out right away.

Oops. Bummer.

When time came to set bias current for the driver and finals, a problem emerged. The "mirror board" was missing. This is just a small rectangular cookie of circuit-board with the foil surface exposed. It serves as a jumper plug feeding power from the inboard 'fork' contact to the final transistors into the center contact. And then to the driver transistor through the outboard fork.

Pull this board and you can complete those two circuits through your current meter.


No other way to measure current. Have to break the circuit and pass the current through your meter.

Folks will ask why this measurement is so important. In this case, wattage was low with new finals. If one of the two finals can't be adjusted with the trimmer pot next to it, you know which transistor has a problem.

If the driver and finals pass this check, you can cross off a whole list of possible circuit faults.

And if part of this procedure/test flunks, you know where to look next. Either way, pass or fail it's a diagnostic result.

Back to the member who blew out his two new final transistors. This happened because of collateral damage from the finals that had blown out originally. The bias diode in the input circuit of the driver and each final controls the steady "idle" current through each transistor when there is no drive. This is only found on sideband radios. When a new final is installed with those parts still blown out, too much input current will reach the input of the transistor. This causes it to pull several Amps of steady current as soon as you key the mike. But not for long. The heat from the excess DC current will melt the silicon die inside and cause the transistor to fail as a dead short. Turns out he needed to replace more than just two finals. That being done he now he has a reasonable voltage reading at each base (input) lead of each final, below 2/3 of a Volt. But now he needs to set the idle-current bias trimpots. Told him to pull the mirror board and where to clip his meter leads.

Long story short, no mirror board. None. No jumper to remove. No "fork" contact in the middle. Just the two outside contacts.

This tells us that two wires have been added under the circuit board. One of them is wired across the two outer fork contacts. This feeds modulated power to the driver transistor only.

The other wire goes from the foil pad under the (now missing) middle fork, back to a steady source of main 13.8 VDC power. More than one of these to choose from in that radio, but that's what this wire does. It turns the final transistors into a linear amplifier in all modes, AM and sideband. Only the driver gets modulated in AM mode.
Hooking up a meter to do the bias adjust/check will require unsoldering the far end of the long wire now feeding the final transistors.

So here's the procedure.

Find the far end of the wire feeding the foil pad where the center fork was removed. Unsolder and bend it around to keep it from touching where it should not.

Find the wire that jumps from one to the other fork contact. Just remove it for now. Both ends. Leaving one end attached is just too big a hazard.

If you have a current-limited workbench-type regulated power supply, this is the "safe" smoke-free way to proceed. Set this to 14 Volts and set the current limit to a half Amp or less. Clip the power supply's negative lead to the negative side of the big filter capacitor near the transformer.

Now attach the positive lead of your current meter to the power supply's positive output.

Clip the meter's negative lead to the outermost fork contact. Set the radio to either USB or LSB. Won't matter which. Turn the mike gain to zero. Key the mike and see what current reading you get. Set the driver trimpot for 60 mA. I use the 10 Amp scale of our meter. It reads "0.060" set this way. I got tired of blowing small fuses in our meter when the negative gator clip would just brush against a grounded metal surface, or the transistor pulled too much current.

Now move the meter's negative lead to the far end of the wire leading to the final transistors. The trimpots should first be set at the end that shows a zero reading. This is the starting point. Now advance the trimpot on the center transistor until it reads 60 mA. Next advance the trimpot on the right-hand final transistor to increase this reading to 120 mA ( or 0.120 Amps).

If either final transistor won't turn up, that means there is still a fault in that transistors' base circuit. And if it won't turn down, your trimpot is bad.

And if it passes, put the wires back where you removed them. It's now safe to key the mike using the radio's power supply to run the final transistors.

Once the radio passes this test, at least you know that both finals are operating at some level. There are other faults that can reduce the power output, but those won't matter until these DC-current readings can be obtained first.

This procedure can be done without the safely current-limited external power supply. But it has the flavor of russian roulette. Instead of clipping the meter's positive lead to the positive side of the external supply, you clip it to the inboard-end fork contact.

You could rely on the fuse in your meter to protect the transistors during this test. Just stock up on spare meter fuses.

Yeah, yeah. I should have come up with some pics to go with the words. Been a long week, and it's not over yet. Best I can do for now.

73
 
Last edited:

dxChat
Help Users
  • No one is chatting at the moment.
  • dxBot:
    Tucker442 has left the room.
  • @ BJ radionut:
    LIVE 10:00 AM EST :cool:
  • @ Charles Edwards:
    I'm looking for factory settings 1 through 59 for a AT 5555 n2 or AT500 M2 I only wrote down half the values feel like a idiot I need help will be appreciated