Yamaha AX-300 Natural Sound Integrated Amplifier

[Naughty Nigel]

Does anyone have a copy of the service manual for a Yamaha AX-300 amplifier?

I need to know the correct settings for output transistor bias.

I have a manual for the A-300 which looks very similar on paper, but is an older model. The A-300 workshop manual recommends that the bias is set to give 20 mV (± 5 mV) across resistors R161 and R162 (0.33 Ω), equating to a bias current of just over 60 milliamps.

The AX-300 seems to use a very similar output circuit to the A-300, so I have used the A300 settings in the absence of firm information. However, if anyone knows better I would be pleased to hear.

Nigel.

[brian2957]

Expensive . But if you're desperate :rolleyes: http://www.ebay.co.uk/itm/Yamaha-AX-300-Stereo-Integrated-Amplifier-Service-Manual-/221310149093?pt=LH_DefaultDomain_2&hash=item33871ce1e5
Ask the seller if he will give you the info. out of the manual . You never know he may be a helpful bloke Nigel.

[Naughty Nigel]

Thanks Brian.

Oddly enough I saw that, and apart from the cost I thought that by the time I get it I will be too late!

Maybe I will make an offer and ask at the same time. If I do end up buying it I will put it on the forum.

The amplifier is my son's, and is not a bad sounding unit, especially now he seems to have 'acquired' my Quad 11L speakers. (I put in an optimistic bid on a pair of 12L's not expecting to win, and to my surprise had to collect them last week.)

Nigel.

[Naughty Nigel]

I took up your suggestion Brian and asked the seller about the bias setting, and I'm glad I did.

Apparently, the correct setting for the AX-300 is 10 mV ± 3 mV - which might explain why the amplifier was running quite warm at its previous settings of 18 and 27 mV!

The A-300 manual that I have shows that the older amplifier had much larger [finned] heatsinks than the AX-300, so they probably ran in Class A until the output went above two or three watts per channel.

The bias preset pots on the AX-300 are extremely sensitive, and I found it very difficult to set them both to any particular value; which could explain why the bias currents had drifted so far since they were set in the factory twenty-five years ago.

I think I might try to replace them with fixed value resistors if I can work out their correct values, or perhaps a multi-turn preset pot.

Anyhow, I put in a 'best offer' for the manual which was accepted, so at least I will have the circuit diagram to work from.

Thanks again.

Nigel.

[brian2957]

Hah ! There you go Nigel , there are some helpful people out there .