Yamaha RX450 speaker protection cutting in

[moke007]

Hi Guy's

My brother in law has a Yamaha RX450 Amplifier that is playing up. He has had it for 20 odd years and it would be a real shame to chuck it and buy some crappy new amp. Anyway, I offered to have a look and this is what I found.

When I got it he said the speaker protection would cut in and out randomly. When I first tested it the protection relay just stayed off (no sound) and didn't click in at all.

I checked voltages as per the service manual and they were good. Checked the diodes around the circuit protection part and they were fine. None of the capacitors looked bad, no bulging tops etc...

Anyway, I fired it up and it has been working fine for a couple of hours now. Only problem, the LCD stopped working. Bugger!
EDIT LCD was just a blown back light END EDIT

Any suggestions on the speaker protection problem? Bad solder joints?

thanks in advance
Jim

[Reschs]

Definitely check for dry joints on the relay.
When in fault mode check the voltage across the relay coil so you know if the protection circuit is activated. Most activate the relay to connect the speakers, so no voltage on the coil equals no output.
Check the DC offset voltage at the speaker output of each channel before the relay.
It should not exceed +/- 100mv. If this is OK, do you have access to a CRO ? If so look for excessive noise on the output which will trigger the protection.
Without the circuit I am guessing.
Some circuits sense the DC offset via a series resistor from the speaker output and use a capacitor to remove the signal. This capacitor drops in capacitance over time and can cause this problem.

Stupid question, have you lubricated all the pots. (volume, bass, treble & balance).

[moke007]

Definitely check for dry joints on the relay.
When in fault mode check the voltage across the relay coil so you know if the protection circuit is activated. Most activate the relay to connect the speakers, so no voltage on the coil equals no output.
Check the DC offset voltage at the speaker output of each channel before the relay.
It should not exceed +/- 100mv. If this is OK, do you have access to a CRO ? If so look for excessive noise on the output which will trigger the protection.
Without the circuit I am guessing.
Some circuits sense the DC offset via a series resistor from the speaker output and use a capacitor to remove the signal. This capacitor drops in capacitance over time and can cause this problem.

Stupid question, have you lubricated all the pots. (volume, bass, treble & balance).

Thanks for all the tips. The thing just started working perfectly which was a bit of a pain. So I just gave the board a good clean. Re soldered a few joints that looked a little dodgy and have given it back to him to test.

I gave it a good test in my lounge room. Funny thing is this 20+ year old amp sounds heaps better than my Yamaha RXV450. I used to have an old Technics Amp and Ricter speakers. The speakers got damaged by the cats so I upgraded everything buying the Yamaha RXV450, Bose 301 fronts, 201 rears and centre speaker. I was always a little disappointed with the sound, it sounded a bit muddy compare to the old Technics amp and Ricter speakers. I always just assumed the Bose 301's were not as good as everyone makes out... I never thought it would be the amp.

I am now trying to convince the Minister for War and Finance that I need to spend $500 on the Silicon Chip 100W ch ultra low distortion kit :-)