Hi Bill,
Teac A-H500i is on top of A-H500 (highly advise to go for the "i")
Usually, when you buy a SH amplifier, it totally depends on their condition. Here is just luck. But I always change capacitors from power source with something superior( Mundorf, Elna Silmic II, Panasonic FMs, Nichicon FG, KZ).
So for both Amplifiers I spend almost 200 $ in upgrade as I tend to stay on these Teac pieces. If I would be just experimenting sound I wouldn't have upgrade it, as it sounded splendid. Aging capacitors can do a lot of damage inside, but these amplifiers are having short-circuit protection inside, and I can say at to a certain point are safe to keep them.
I am not a repair engineer, I just studied Electronics and Telecommunication University, and I can say I know quite more than basics.
What to look when buying:
1. Test a piece of song where you can detect distortions, weird sounds.
2. Test it at very low level, and then gradually increase, see any signs of weird noises.
3. give some power and see how it sounds.
4. Let it run for 15-30 mins and see how hot it gets. it should be worm, not hot.
5. Put your year close to the unit. No buzzing sounds should be heard. just quiet
6. User a pair of headphones, remove any signal input, and run the amplifier from 0 to Max, there should be no noise, just some hiss at the maximum level (NO Signal INPUT just empty amplifier working on)
And that should be it.
NOTE: B & W 601 NICE Pieces btw, and Mission SUB as well. Teac 300 series is fine, but definitely would recommend move up to "i" reference series. For this money, there is nothing better, you will definitely be enjoying it, I am sure of this.
Regards,
Claudiu M.