The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

I nabbed a faulty Icom 735 much to my excitement and went about fixing the problems with it.

0. Dry joint on antenna wire on filter board - fixed by previous owner and checked ok
1. Cracked pcb track on recieve phono connector - fixed
2. PLL unlock until warmed up for a little while - very slight adjustments to PLL trimmer caps cured problem, timmer caps seem in good condition and there was no wax in the pll can [Have since aligned according to service manual]
3. Low TX power 25W max at 27Mhz, unable to test other bands at present. According to the previous owner, low power on other bands.

After dismantling the unit and removing the cover of the PA unit it was discovered that the power output had increased to around 40W. Then moving the rig as far away from the pa unit as the wires would allow returned the power output to 100W. At first I thought it was dry solder joints so i dismantled the PA and resoldered. This didn't cure the problem.

Further analysis of the problem indicates that the nearby metal of either the PA cover or the rig reduces the output power.

On the filter board is a circuit to reduce output power in case of high SWR. Something here must be the problem, I suspect L36 (section 6.5 of the service manual).

Has anyone come across this problem before?

I think i'll dismantle the rig (again!) and bring a coin near to the inductors and see which one exactly is causing the problem.

I can post an image of the filter board circuit diagram with the SWR circuit on if required.

Tried to find which inductor was sensitive but it didn't work. Rig TX's more power when the PA is uncovered and in a certain position. Didn't manage to get the full 100W this time, more like 60-70. At least the output transistors are ok Definately sure this isn't dry joints.

The problem with most ham radios that have been used by CB?s is that they run them at full chat, which over a period time causes caps and resistors to fail (This is why I do not repair one?s which are owned by CB?s) you will probably find bad caps in the filter?s that is causing RF feedback, the only way to find the bad caps is to change them and hope for the best.

What is the performance like on 80, 40, and 20 meters?
Also check relay circuitry, I have had a bad relay causing RF feedback in the PA stage.
Also check the Bias as hi Bias will cause RF feedback.

Rick.

Ok thanks for your advice Rick.

The performance is similar on all bands, however to be fully sure I need to get myself a dummy load capable of 100W. Or, an antenna matcher. I certainly don't want to risk further damage by raising the power on an unmatched longwire or silver rod.

Will check the bias and check the relays and their operation and post any findings.

Checked the driver and final bias and they were ok (110mA, 780mA). Adjusted them back to 100mA, 600mA.

As far as I know, this hasn't previosuly been owned by a CBer (apart from me of course). There are no obvious signs of overheating.

I have checked the operation of the relays by listening to them and removed the covers of any suspect ones and applied 12V to the coil. They all are ok.

I have made a list of electrolytics and mica capacitors to replace them, hopefully this will fix it, tho I've got a hunch this may not be the problem, as power output appears to be low on all bands and the electrolytics look in ok condition. The 4.7ohm non-wirewound resistor check ok too.

Will post back when I have changed them.

What still puzzles me is the varying output depending on pa unit position with the cover off. At first I thought nearby metal was changing something - maybe its RF altering something in the nearby circuitry (pll can area).

Excellent news!

Before ordering all the new caps, I had one final go at trying to find the problem.

I tested the PA unit using many silly but workable ways of affecting the filters like putting a screwdriver in the air cores that form part of the 22-30Mhz filter. Could not get any variation in behaviour so the filter was probably good. Did loads of other things trying to determine if it was the PA itself, like bypassing the filter completely. Could not prove it was the PA, so I assumed it was good.

I worked my way back through the circuit and came to L13 and L14 which form part of the adjustment for output power. I touched the metal nib of a very small screwdriver against the core and wham, up shot the power.

Visual inspection of the core showed a missing piece that had broken off either from adjustment by person unknown or from the factory, and had worked its way loose over time.

Anyway a few quick adjustments to L13 and L14 and a reliable 100W+ of power appeared!!!

When the PA was in a certain position the RF must have affected the inductor which would have been situated very nearby in the service position and caused the strange effects of varying power.



There are still a few things left to adjust no doubt, but they will have to wait until my test equipment is more suitable for the job.

Anyway very happy that its back in good working order! And that I didn't have to order ?20 of capacitors.

I have a question about the Noise Blanker on this rig.

How can I test its operation? It doesn't seem to do much at the moment, the only effect i can get it to do is make the sound distorted on a strong signal am station.