Seeing that this is exactly the same power supply board as in the Ham Jumbo, 007COX, there could well be other components on that board that are causing the fault - what about the Zener? I've not checked the specs on 2SD525 compared to the original 2SD704 type, but could it be a fake? There's a lot of fake components out there, especially if you bought the cheapest available and didn't use a recognised supplier. A mate of mine recently needed a particular CB radio transistor and I told him where I've got them from in the past and they've always been good, albeit a bit more expensive. He had three goes at buying this component, one from a German supplier, one from a Chinese supplier and one from a supposedly trusted UK supplier. All of them, without exception, were fake and useless. He brought them to me to test and they were all fake rubbish. So finally he bought from where I'd suggested in the first place, with absolutely perfect results and no more problems, other than getting refunds for the fakes from the previous suppliers. The German and Chinese suppliers gave instant refunds, however, the UK supplier was a lot slower and much more difficult to deal with, finally refunding once the parts were returned, sadly with no apology whatsoever...I won't name the supplier, as a lot of their parts are good and their prices reasonable, although I suspect that some will have already guessed who they are.dj56 wrote: 09 Aug 2026, 23:41I did replace the Regulator 2sd525 and had the same results replaced C1 C2 and same results
It's not the mains transformer. The incoming mains will vary during the day and cause that sort of variation on the transformer output. There could be a grounding issue somewhere, ie, a hot or bad connection that only shows up under load. For the purest, a rig is ruined once the original power supply has been removed and replaced, even if the modern replacement is actually better than the original. Value can be wiped of a radio with this kind of modification, but if you're not intending to ever sell the radio on to someone else, then fit a replacement power supply and be done with it.
I remember having a fault similar to this on a Jumbo years ago and I went through all the same tests as you are doing. The difference was that the radio was 'dipping' and 'warbling' on SSB TX. I verified that the mains transformer output was dipping by a couple of volts under load, just the same as yours is doing, that's how I can rule out the transformer being at fault on your radio. I also suspected C1 and C2 and bridged replacements across the originals (the rectifier can cope with the extra inrush surge) and this made no difference. I'd also then run the radio from an external 13.8 volt power supply with no difference to the fault, unlike what you have found, though. I had checked the 9.4 volt line at the Q44 AVR, as mentioned by someone in a previous post and all seemed good there. In the end, after a lot of head scratching, I isolated the Q44 regulator and tacked a 9 volt PP3 battery in its place and the fault disappeared. Replacing Q44 effected a complete cure, even though the original seemed to be doing its job and still tested good. Probably if I'd monitored the 9.4 volt line a bit closer I might have detected the small, but significant variation in the stabilised line voltage, however, although it's worth checking this component, I don't think that this has anything to do with the fault you are experiencing with your radio at the moment, as the fault on yours goes away when you run the whole radio from an external 13.8 volt supply.
So your fault is likely to be still on that 007COX board or a bad ground connection in relation to it.
