Hi Paul, yes they are pretty basic, and I'm an electrician by trade, but don't have enough amp knowledge/schematic experience to trust myself, nor do I have a cap discharger and all the other tech tools the amp men have at their disposal, plus the 135's have the extra twist of the ultra-linear output transformer, which isn't much of a difference I guess, especially not to the guys who work on em for a living.
What it did initially was simply stop volume-wise. It remained powered up, tubes getting juice, standby still engaged, it just stopped converting bass signal to dB's. So I took it to this guy's place who I'd been recommended to, with a cab and a P Bass, set it up in his lounge and it worked perfectly. He had a fiddle with it, we couldn't get it to fault. So I took it home, continued using it at home for a few days, then it quit again. Back to his place, this time DOA. So left it with him, and it dragged on for two years, him trying to get it to fault, then eventually he just started changing out parts as he couldn't get it to fault. In the end, it sounded amazing with fresh caps and a new set of tubes, but like I mentioned, a few months after he'd finished the job, it quit in exactly the same fashion.