Pioneer sx-780 vu meters dance but no sound

[ivalvara21]

I recently purchased a Pioneer sx-780 on ebay and it was working great for about 3 weeks. I had it hooked up to an Audio-Technica AT-LP60BK turn table and a pair of RSL Studio Monitor 3600s.

It was sounding great and then I started noticing that my left speaker was gradually dimming (wasn't as loud as the right). Eventually, the left speaker stopped producing sound and I was only able to listen from the right. Then, the right speaker stopped producing sound as well.

I surfed forums and people suggested to unplug the amp from the wall for a while and then plug it back in and try again, but that had no affect.

I've tried connecting my speakers on both the A and B connections, but still no luck.

The amp's lights come on fine when I turn it on and I do here a 'click' a few seconds after it turns on and when music is supposed to be playing I see the VU meters dance for both left and right speakers. Just no sound is being produced. My RSL Studio Monitors are in near mint condition so I doubt they're blown out.

I've popped the hood and used a duster to clean off all the cobwebs and dust since this thing is from the late 70s.

I've also checked the voltage of pins 0 and 1 on both Darlington packs (as I've seen suggested on multiple threads) and they both read 1.30V as expected so I know there is no issue there.

I'm a complete novice when it comes to vintage amps and this is my first one, so I'm a bit lost as to where to go from here. Any suggestions on how to proceed troubleshooting are highly appreciated.

 

[markthefixer]

Headphones? Do they work?

You have 2 sets of contacts between the amp circuit out and the speaker terminals.
One set is the protect relay you hear click.
The other set is the A/B speaker switches.

The headphones and the meters are tapped in between the two.

I suspect your A / B switches need Deoxiting.

If the relay contacts were bad, the load of the speakers would severely restrict the meter's "dancing". Does it?

Deoxiting - not just "cleaning". Cleaning leaves clean corrosion. Deoxiting loosens the corrosion so the friction of the switch contacts scrubs an UN-corroded path.

 

[ivalvara21]

Thanks for the response! I was actually able to diagnose the issue. The fuses on each of my speakers were blown out. Replaced them and it sounds great again! I guess there was nothing wrong with my amp. I did, however, deoxit it so hopefully that improves performance as well.

Is there anything I can do to prevent the fuses from being blown out again? Does this mean the amp is too powerful for my set of speakers? The type of fuse they take are 1.5A 250V and the speakers are RSL Studio Monitor 3600.

 

[rcs16]

Turning it down is the best way to prevent this from happening again.
A 1.5A fast blow fuse is on the low side. If the speaker states 1.5A, then I do not want to say to increase it since the mfg has made that recommendation and anymore will potentially blow more than a fuse.
Lets just say that speakers with fuses in them are generally not of the highest quality. I am not intending on saying that your speakers are crap, but just in general terms, fuses are usually frowned upon since they do add some degree of distortion.The SX-780 is not a very powerful amp, but saying that it can mean if you blast it, it could be clipping the amp which is hard on speakers and probably why the fuses blew. Fuses do help in this regard to protect a speaker from overload, so it may have helped you to not to blow the cross over or a driver, but as you have discovered fuses can be a nuances, that is why many speakers do not use them.

 

[markthefixer]

Look at the DC offset voltage present at each channel's speaker terminals. No sound for this.