Mods for Atlas Radios
Atlas Bulletin No.2
This mod is to reduce the likelihood of oscillation in the PA. The input cable is re-routed outside the main compartment. Easily spotted as you don't even have to open the rig to see if it's been done. Click on the picture above for a big version.
The full text of the bulletin is reproduced here.
Replacing the PA transistors for more output.
If you have to replace blow PA transistors you may as well uprate to an 80W 12V HF transistor such as SD1405 / MRF454. They will be more robust straight away but some mods are necessary to get the most out of them. You may find that the holes in the board aren't quite big enough if your Atlas has the original, smaller, devices still in situ. You will have to open them up slightly. (RF Parts still have 50W CD2545s available at a reasonable price if you want to keep it original).
A straight replacement with 80W devices will work but it won't yield much more ouput. You need to change the ratio of the output transformer to 1:4 , that's four secondary turns. If you are keen on 160 or 80 you'd better throw the whole o/p transformer away and replace it with bigger ferrites. Large EMC cores work fine.
You'll also need more drive for more output. I couldn't find a better device than the weird RCA thing fitted to my early model but I had to adjust the bias a bit by altering the base resistors and increase the gain by decoupling the emitter resistor with 100nF disk ceramic. It now does 170W output with 14V supply.
The final bias circuit is not the greatest either. I thermally coupled the diodes to one of the output transistors and used the circuit here which is stiffer than the original and less liable to drift. The 470R resistor can be altered to allow max bias current of 1A.
AGC mods. (To improve AGC action)
On PC200 card, remove R218 (3k9) and replace with 100R in series with a 1N4148 diode with the cathode (stripe) end towards the edge-connector. Also remove C201 (15uF). Add a 9V Zener across C206, stripe to +.
On PC300 card, change C320 to 47uF (from 15uF), change R315 to 1k5 (from 1k) and change R314 to 1k (from 3k3). Add a 5v1 Zener across R312, stripe to the chip.
Both mods are drawn in on the diagrams reproduced on the circuits page.
AI2Q's SWR bridge mod.
Alex has re-wired the SWR bridge (which generates the ALC and PA protection signal) to place it on the output of the PA, before the c/o relay. This means that a slow relay or a dirty contact will not destroy the PA because the SWR protection will act. He did this mod to his 210X some years ago and hasn't had any problems since.
Alex describes the mod thus:
Re-route the RF output from the PC-500D board to the input on the PC1010 (low-pass filter) board. The output from the latter goes to the changeover relay. From the common terminal on the relay, the transmit RF is routed to the input of the filter board and thence directly to the antenna. It take more time to explain it than it does to perform the wiring change.
VFO stability improvement.
The Atlas low-voltage regulator circuit which feeds the VFO is not very stable and replacing it with a modern IC regulator is a good idea. The pass transistor (usually Q1 on the main chassis diagram), Zener and 100 Ohm resistor should be removed and replaced with something like an LM317 set for 9.1V (suggested by AD5SI). I managed to find a "low-dropout" 10V regulator chip LM2937ET-10 at Farnell which works well too.