SERVICE BULLETIN No.2 4/18/75
I TRANSMITTER REGENERATION - SELF OSCILLATION I
These notes are to help identify and cure a potential feedback problem which may occur in some installations of the Atlas transceiver. It is a self oscillating condition in transmit mode which may be triggered when the transceiver is very close to the antenna field, such as in mobile installations, or when there is R.F. current on the outside of the coax. antenna cable, and it couples back into the transceiver. This may be caused by high SWR, or feeding a balanced antenna without use of a proper balun.
The problem is one that has shown up at times on 160, 80, and 40 meters, and on a few occasions on 20 meters. It can be easily recognized by the fact that the transmitter will not modulate, and the panel reads several amperes in transmit mode instead of the normal resting current. Mic. Gain has no effect on the condition.
MODIFICATION No.1. This modification will cure the primary source of the feedback condition. It is not a difficult mod., and will save the cost and inconvenience of shipping the set back to the factory. Production sets with serial numbers greater than 2300 have this modification.
There is a small diam., white colored coaxial cable which connects the transmit input tuning circuits on PC-900 to the transmit pre-amp. on the heat sink. PC-900 is the circuit board next to plug-in PC-100, and has a number of mica trimmers. The coax. runs from PC-900 to the underside of the chassis, and then back through a hole to the topside, and thence through a grommet into the P.A. compartment. The problem occurs at this point, where the coax. comes in close proximity with the low pass ou tput filters of the transmitter. On model 180's the end filter is for 160 meters, and the problem thus was most likely to appear on that band, although sometimes on 80 meters. On model 210's and 215's the end filter is 80 meters, so the problem is more likely to occur on 80 meters, and sometimes on 40 meters.
If the self oscillating condition is encountered, it can usually be c'ured by dressing the coax. away from the filter components. There should be no excess cable looping around in this area, but instead any excess should be pulleJ down under the chassis. If this cures the problem, no further action is necessary.
If a tendency toward self oscillation still persists, the following change is recommended.
1. Remove the top two screws in the P.A. heat sink, and swivel the amplifier to open position. Locate the coax. input cable and unsolder both the shield and center conductor. The insulation is Teflon, so the heat of soldering will not damage the coax. Be careful with the center conductor since it is quite small and fragile.
2. Drill two holes, about 1/8 in diam., as illustrated. Exact location of holes is not critical. Measure, mark, and center punch. Then drill. Use caution when drilling the hole through the chassis so as to avoid damaging other wiring. Pull the wiring away while drilling. Clean the hole edges with a countersink of larger drill to remove burrs.
3. Reroute the coax. by going out through the rear chassis hole and up into the P.A. compartment, as illustrated. Reconnect and solder the coax. to the pre-amp. input terminals.
4. Notch bottom cover with file or cutters so its rear lip will clear the newly routed coax.
This modification will sharply reduce the possibility of self oscillation, even with highly reactive loads. If you have any concern about the performing the mod., you might ask a technician friend with more experience for a hand, or ask your dealer's service department. Don't hesitate to tackle the job, since you can always box it up and ship it to us, in which case we guarantee to make it right. Our point is that if you can cure the problem yourself it will spare the cost and inconvenience of shipping it back to us. But, it is your inconvenience we are concerned about, so if you'd rather not get your hands into the rig, and you have encountered this problem or any other, by all means ship it to us and we'll take care of it. That's what we're here for.
MODIFICATION No.2. There is another possible cause for self oscillation which has been encountered on a few sets with serial numbers lower than 2300 series.
The amplifier chain on the heat sink has 4 stages, and if the overall gain from input terminal to output is greater than what is necessary to achieve full power, an unstable condition may arise, particularly with reactive of mismatched loads.
Production sets are now being adjusted so that overall gain in the 4 stages is adequate for full drive and full output, but not more than is required. This adjustment is being accomplished by changing the value C505 on the 1st pre-amp stage. Earlier sets used a .001 MF Disc type capacitor which can be located on the narrow stand-up circuit board containing the 1st and 2nd stages of the amplifier chain. It will be found between the two transistors on this narrow board. There are two other disc capacitors in this area, but they are .01 MF. C505 is the one marked as .001 MF.
If you encounter an unstable oscillating condition in transmit mode on one of the lower bands, and Modification No.1 did not cure the problem, remove C505, the .001 disc, and replace it with a 470 pF disc or silver mica. Test for the unstable condition. If it is still present, then replace the 470 pF with a 270 pF.
The capacity value at this point reduces gain on the lowest band considerably, but has little effect on the highest band, and only a moderate effect on the middle bands. In other words, the purpose of the capacitor is to balance the overall gain across the frequency range. Originally .001 MF appeared to be optimum, but experience has proven that the value should be tailored to each individual set.
If adjusting C505 to lower values does not cure the problem, contact the factory, or return the set for factory service.
We want to make it clear that the self oscillating problem is not a prevalent one, but has cropped up in perhaps 30 to 40 installations out of some 2000 transceivers in the field. This led to an investigation by our engineering department, with solutions as described in these notes. By incorporating the modifications in production sets, the Atlas transceivers have been further refmed as part of our program to constantly improve reliability, and ensure trouble free operation. If you have any questions regarding the mods., contact our customer service de p artment. If we can help you take care of a field problem yourseIf we'll be pleased to do so, and save you the inconvenience of shipping the set to us. We encourage you to give it a try, and as a last resort send it in, in which case we guarantee to make it right.
Atlas Radio Customer Service Department
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