ARRL

Secure Site Login

Forum Home - Rules - Help - Login - Forgot Password
Members can access, post and reply to the forums below. Before you do, please first read the RULES.

SGC SG 2000 repair

Oct 12th 2018, 16:11

KG2PM

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I am trying to repair an SG-2000 150 watt transceiver, manufactured by SGC. It dates from around 1995. Unfortunately, it has never worked properly. The problem is this. It seems to be putting out full power in cw. At the 50 watt setting, my MFJ VersaTuner II shows about 50 watts forward power with swr of about 1.4. My MFJ power supply draws about 15 amps. In the 150 watt setting, the tuner shows an swr of about 1.5, and the forward power reading is near 150 watts. The power supply draws about 28 amps at this setting. The radio's digital forward/swr display shows power out when transmitting a cw pulse, as expected. But the radio seems to be barely putting out any power in ssb. In ssb transmit, nothing shows on the forward power position of the radio's forward/swr display, nor on the versa tuner meter. The power supply meter shows only about 2.5 amps drawn in ssb transmit. Shouting into the mic (a replacement) barely makes the tuner meter move. These tests were done on 15 meters.

I inherited this radio almost new from a friend many years ago, but it had never been used. I made a few contacts, but had trouble right from the start. Soon after, the modulator chip failed, and I replaced it. But the radio still didn't work well, so I never used it except as a receiver.

Now I would really like to get it working right. I can hear the ssb voice signal in my other radios in the shack. Presumably the modulator chip is working. But no ssb signal, if any, is going out. Where should I look next for this problem?
Oct 13th 2018, 08:03

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I'd start at the beginning and make sure each stage is working properly. Does the mic amp have the proper gain and frequency response. Then move on to the modulator. Does it have the proper signal level at the output?

This is hardest stuff to fix--if it never worked to begin with. You don't know what could have gone wrong. Wrong parts, defective parts, parts that are missing entirely. It could even be a bad design.

Zak W1VT
ARRL Senior Lab Engineer

Oct 13th 2018, 09:59

KG2PM

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thanks, Zak. This radio was supposedly designed for military/commercial use primarily, amateur use secondarily, so the design should be excellent. I will check the mic amp and gain as a starting point once I figure out where it is and how to do it.
Oct 13th 2018, 15:36

KG2PM

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Well, I am definitely a radio AMATEUR, and do not have a lot of test equipment. I hope to find an elmer in the neighborhood who does, and see if I can get some hands-on help. One final question for now, please. I found the AGC circuit in the schematic and on the circuit board. Does this circuit control the mic gain? It is a Motorolla 8940, MC 1350P. This radio uses a lot of these. Or is the mic gain a different circuit.
Oct 13th 2018, 15:58

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
A bad design could include things like an audio amplifier oscillating at VHF under certain conditions. A spectrum analyzer will easily spot stuff like that, but not everyone is clever enough to do that. And some people don't have easy access to one when doing audio design work.

Zak W1VT
ARRL Senior Lab Engineer
Oct 14th 2018, 06:44

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The MC1350 is an IF gain chip. It would be unusual for this to be used for the mic gain.

The stages are typically located via signal tracing. Sometimes the documentation you have doesn't exactly depict the version of the circuitry you have in front of you.

Zak Lau W1VT
ARRL Senior Lab Engineer
Oct 14th 2018, 11:24

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Typically you start at the input, say the mic connector, and trace out what that is connected to.

Zak Lau W1VT
ARRL Senior Lab Engineer
Oct 14th 2018, 11:25

KG2PM

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Then by implication, the MC1350 is probably not the mic gain circuit. The schematic does label it as the AGC/ALC chip. So I'll keep scrutinizing the schematic. This radio manual has pages and pages of schematic. Again, thanks for the info.

Back to Top