I can’t go much in detail since I’m not familiar with BTECH kit, but I’d be inclined to test/assess the DC regulator.
If you can borrow a high current variable and supply the right range of voltages (13.8V input is the nominal, fused direct connection to the battery would see around 11-15V whilst the radio is getting a suitable input voltage I’d guess. If it’s demonstrating the same sort of characteristics across the board, it’s pretty certainly regulation fault territory - I say regulation because it could be an ancillary component that’s part of setting the regulation range could be iffy.
But the stuttering points towards momentary and inconsistent regulated outputs and shutdowns or output cutout, so it’s probably sounding like a switch mode PSU if you slowed it’s switching to an audible range rate.
if you determine what outputs it normally generates (there’s bound to be a 5v/3v, possibly 1.8v depending on the internal logic at play, as well as a few higher voltages to the PA stages (higher current)), and directly supply either the correct regulate voltage that should appear directly as a bypass (it may output one supply with further changes later in circuitry) you should be able to conclude if it’s regulator or a more logic related matter (there’s some quite highly integrated complex logic mimicking some expensive DSP type logic chips within the ceramic or polymer lump.
If a basic regulator test or reg bypass fails to give clues or change, you’re looking at logic and that’s a hellish road to tread to diagnose without docs and service info.
In fact, if it’s looking seriously like logic investigation is in order, there’s a slim hope that solder reworking could correct a poor connection or dry joint cause at the logic end, since there’s almost certainly some logic loop back to regulation.
But when it comes to replacing obscure, nearly unobtainable logic to replace a component, it’s going to rapidly become a costly note viable venture beyond (if successful in fixing) the challenge.
It’s probably the reason, the fault, it got sidelined in the first place…
If it were a Baofeng, I’d not waste time beyond a rudimentary test and maybe reworking some parts as they literally aren’t viable to fix (you can still use a Duff one as a Fishguard donor up to a point). But that maybe worth trying to save as it maybe a pricey set to get new.
Well, it’s food for thought - maybe someone who’s got BTECH servicing experience can shed real light on the fault.
I had a similar fault on an aged IC-251 multimode that would randomly draw max current on power up for 5 mins like it had jumped into Full power TX, but no output. It turned out to be a supply instability to the logic, so it tried to put out an empty carrier in bypass of all front panel settings (it had real rotaries not encoders on most dials), but no output because the PLL was in an unlocked state ‘miscode’ state. It took some digging and a really good factory service manual to resolve (thankfully I was working at an LMR service center and had no shortage of solid test gear at the time).
The big difference was whilst there were some complex (relative to the 80s radio world) logic, a lot of it’s guts were fairly off the shelf TTL and non-TTL and common CMOS stuff.
Still occasionally pulls the odd random intermittent stunts, but given it’s age and sentiment (it was a fully itemised Mutek front-end modded, and aligned to LMR grade tolerances example setup by a real old school engineer and Ham who literally saw things through to the bitter end or absolutely awesome success) and the association with it’s only other owner which means it survives until it dies from old-age (being 40-something and many NFD and other contest veteran, it deserves to be a bit temperamental and mischievous).
Anyway, good luck but don’t get drawn into a wasted weeks of eventually pointless attempts to fix if it’s an internal logic flaw.
