SBE Sidebander III

[cadillaccharlie]

Hi all!

I recently got a hold of a SBE Sidebander III which is in really nice condition. I also have a Pal VCO A-4 that I will use with this radio. The radio receives very well on all channels and works well with the VCO on receive. This is a sideband only radio so it cannot be tested with a dead key for output. The radio does not transmit. I tried 3 different SBE mics with same results, no output at all on watt meter. I am waiting+ for a Sams Photofacts book to come in but I do have the owner's manual and schematic for it. I am testing it with a 13.8 VDC 10 AMP power supply which has an analog voltage & amp meter on it. When I key the mic without even speaking into it the amp meter kicks to around 1 1/2 amps. Where would be the best section to start looking? Could the final be shorted or a related Cap? Any help will be appreciated! I'm more an old school tube rig guy but I do like some solid-state crystal rigs. Thanks for looking!


Dave

[2600]

A SSB-only radio presents a dilemma if it won't transmit. Is the fault in a RF circuit? Or is mike audio? Either one will kill the transmit power.

If you have another radio handy, tune it to the same channel as the SBE, put a foot long wire into the receiving radio's antenna socket and key the SBE. If it's an audio problem, you will hear a little bit of carrier, and probably some noise when it's keyed, and nothing else. This would suggest audio trouble. On the other hand, if you can hear a weak transmit signal and mike audio, this suggests that the tracks are being blocked in a RF circuit.

That line of radios was prone to receive and transmit audio trouble caused by 1uf and 10uf electrolytic capacitors. A 'scope would reveal how far the mike audio is getting, following it from the mike jack to the sideband modulator circuit. And if that's what you find, you should consider replacing all the electrolytic capacitors. They were not engineered to last this long to begin with.

73

[cadillaccharlie]

Thanks much for your troubleshooting advice! I do have good audio but no rf whatsoever testing it as you suggested. Appears it is definitely a rf issue. Possibly blown final? 

73s!

Dave

[2600]

Apr 6, 2024 18:56:49 GMT -5 cadillaccharlie said:

Possibly blown final? 

Possibly. Seems to me they used a metal-can TO-5 style final transistor with a metal "saddle" bolted to the heat sink. They were kinda fragile if the SWR got too high.

That, and the radio has no provision to adjust the sideband bias current. If it runs too high, the final overheats. An oscilloscope is your best friend pinning down what to blame when this happens.

73

[cadillaccharlie]

Thanks so much for your help!! I checked all the Sams voltage points and they check okay! I have a small RF sniffer and I can get a RF reading on the input to the driver and also on the final by touching the input leads on each transistor when I key & whistle into the mic. I get a RF output on the driver but very very little RF output on the Final. I'm thinking my culprit is the final.  What are your thoughts?


Dave

[2600]

Pretty sure this one has the metal-can "TO-5" final transistor with a metal mount flange held by two screws. Those don't tolerate a high SWR very well, and may very well be blown.

Or might not.

Sounds like you have more going into the final transistor than you have coming out of it. A transistor tester would tell you if the final is blown. You only need to unsolder two of the transistor's three leads to clip onto a transistor and test it.

Making sure that DC power is reaching the final transistor's collector (outer case) is the one other thing that would cause what you're seeing.

A modern metal-tab plastic final transistor can be adapter to fit this radio, but it's a bit of a kluge, and you'll need an insulating shoulder washer for the mount screw and a mica washer between the metal face of the transistor's tab and the heat sink.

73

[cadillaccharlie]

Thanks for the info! I did a double diode test on both the driver & final. The driver checks okay Conductivity one way no conductivity the other way on both diodes. The final checks fried. Conductivity both ways on each diode.

Dave