RT 246 power supply needed

[Cav Trooper]

I/ we ( a very knowledgable military radio/Ham guy) are trying to get my RT 246 functional. We found a bad switching power supply and got one from American Milspec. Now the radio will receive but will not transmit. We checked the D connector on the chassis and got the +25 vdc on the appropriate pins but, we can not get any voltage at the test point on the power supply module A9000. We have no idea why since we appear to get continuity from the D connector on the bottom of the A9000 to the test point. We've swapped out mics and all of the obvious easy tested culprits. Still scratching out heas on that one. I would like to try a new A9000 power supply but I'm told that the RT 246 is a later A model and I have to find the correct one for it to function. American Milspec has a refurb but we don't think it will work in the RT 246A. We are still trying to figure it all out. We have the TM's so, working through it is just slow going. Any ideas or sources for the correct parts?

Thanks,

CT

 

[Wile E. Coyote]

There are a bunch of different test points on A9000. One problem is that the A9400 module (module with all the transistors screwed into the right side of the radio as you're looking at the faceplate) and the A9000 module have to be compatible. There were two variations of A9400: the original style using Germanium transistors; the second type (A9400A I think) using Silicon transistors. If you're not developing high voltage at the test points on A9000 it could be either incompatibility or just a bad A9400/ A9400A. I've had both the A9000 and A9400s go bad a fair bit on RT-524s and RT-246s in the past.

Just as an aside - check to see the circuit breakers mounted to the inside of the front panel are properly reset, as sometimes the HIGH-LOW-RESET switch doesn't reset them properly in the RESET position, which usually gives you RX audio and squelch that won't quiet but not much else. Had a bunch of radios with that problem.

If you retest and you have all your voltages at the A9000 test points (here's where the manual comes in handy) then you likely have other problems.

 

[Cav Trooper]

Rt246

Here is what we've done. Radio was dead, checked the A9400A and determined it had two bad transistors. Replaced the A9400 with a new one from American Milspec. Next problem, radio receives but doesn't transmit. Checked the A9000A test points and don't have 25.5+ vdc at the test point, pull the A9000A and we have 25.5 at the appropriate 3 pins according the schematics on the chassis plug. It looks like the A9000 tp is a straight link through to the 25.5 vdc from the chassis plug which has us puzzled. That's why we suspect the A9000A. I will have to check the A9400 to be sure it's an A model. If not then, I guess we will have to repair the old one or find the correct one for it. That's part of the challenge, finding out what is and isn't compatable between the old and newer A models. We have gotten a clear answer from anyone about the compatability as of yet. What time should I call? I'm on EST time.

 

[Wile E. Coyote]

The most up-to-date manuals for those are found here:

Manuals Page

There are lots of earlier revs out there but they're all from the pre-silicon days and won't help much in that regard. The basic compatibility of A9000s and A9400s is spelled out in the manual, but here's the short form:

1-11. DIFFERENCES BETWEEN MODELS/SILICON VERSIONS. (CONT)
INTERCHANGEABILITY OF MODULES

Germanium and silicon modules may be used together in the same radio with the following exceptions:

Power Supply Rectifier A9000 and Power Supply Oscillator A9400. The A9400 power supply oscillator assembly exists in the following three versions:
1. A9400 germanium
2. A9400A silicon, 2 kHz
3. A9400B silicon, 4kHz
The A9400 germanium version is compatible only with the A9000 germanium assembly. The A9400A silicon version may also be used with the A9000 germanium assembly. The A9400B contains a 4 kHz high power oscillator, and therefore, may be used only with an A9000A.

These modules may only be paired as follows: (DO NOT USE ANY OTHER COMBINATIONS)
1. A9000/A9400
2. A9000/A9400A
3. A9000A/A9400B

-- back to me...so there you have it.

 

[Cav Trooper]

Rt246

This is great, it cleared all of my questions.

Thanks,

CT

 

[Cav Trooper]

RT246 repair saga

Well, after installing the new A9400 switching pwr supply, and putting the HV pwr supply back in, put pwr to the radio and instant smoke. Pwr off immediately and looked for source, found lead on one transistor on the new A9400 was so long that it was grounding to the chassis, luckily the resistor side and no damage. Took it out and trimmed the lead, reinstalled and pwr up, checked the A9000 and the 700 v tp was reading 1400 v and the 275 v was reading 600. +25.5 was +27. Put another pwr supply in with the same results. Decided to pull the A9400 out again and found one transistor shorting 2 leads together, 2 other component leads touching ground and one transistor with a loose crimped solder lug. Fixed all but the loose lug, crimped as tight as possilbe but still wiggles, I'll fix that later. Reinstalled the A9400 and the A9000 pwr supply 700 v tp reads 850 approx and the 275 v tp is around 325 v, much better. Radio transmits on high freq high pwr and puts out 60 watts, low pwr, high freq 10 watts, low freg does not work and has noise. Will replace the A6000 module next. Both high and low freq receives so, we are 95% there. Should have inspected the A9400 closer after finding the long solder lead the first time. Another lesson learned the hard way. Concerned about the output and still higher than spec voltages, may put in another A9400 module even though the components check ok. Don't want to fry something else with too much voltage.

 

[Wile E. Coyote]

Yeah those readings are high-high. If I didn't know better I'd say the radio's getting fed much higher than 25VDC, or there's still a problem with the A9400. Weird you had all that grounding/ long leads. I have *never* seen that before.

That main filter just after the antenna microswitch fails a lot too and I *think* it's the low freqs it attenuates into nothingness (no low out) but you can really only check via substitution, or check the input side with a wattmeter if you can fab the cables (I did.) Probably they either fail due to drop or just old age and treachery...but I've had...woah...probably four-five go on me.

 

[Cav Trooper]

The A/C fed power supply is putting out 27vdc. I'm leaning toward the A9400 with that loose lead connection on the transistor. I may tear it back down and cut a notch with a dremel and use a heat sink to solder that lug onto the transistor lead. Thanks for the tip on the filter.

 

[Cav Trooper]

RT246 repair saga

It's finally over. The RT246 is running well. We finally went through and cleaned all of the contacts and checked all of the jumper wire plugs and reseated boards etc. With the radio on a real antenna instead of a dummy load we've got 5-6 watts on low and 35 watts on high power, transmission sound is great. It will be a great radion installed, just have to do the finally install and tune to the antenna. It will look good with my GRR5 and maybe thinking about a Vic1 and a R442 later. I need to find covers for the RT and the GRR now. Don't want to pay $100 for one on ebay though. Thanks for all of you input, it helped greatly.

 

[Wile E. Coyote]

No problem...glad it worked out. I spent hours upon hours sorting those things out a dozen years ago and still fix probably two-three a year for collectors when I can get the parts and my Motorola R2600 feels like turning on properly. I love those sets. No coincidence that they served us for 40-plus years.

 

[Cav Trooper]

I know what you mean. I remember them from Germany and Nam. I don't know if I will ever use it but, I like stuff to work and it looks great in the truck.

 

[Cav Trooper]

Any idea what these rigs cost new?