Pioneer SA-8500 Amplifier
A new amplifier for me to play with. There wasn't much needed in terms of maintenance here, but I did make adjustments to the DC offset in order bring it back to factory specifications which was a first for me.
I don't have much of a story to tell about this amplifier, other than I got it basically in mint condition with the original box and styrofoam packaging for a really good deal. It didn't really need any work other than a bit of a cleaning, but I did make a few adjustments to it which I'll explain later.
There are actually two versions of this amp, the Mk I and the Mk II. This is the Mk I, which has ever so slightly lower specs. As far as I can tell, the only difference between the two is a tiny bit lower THD rating (0.1% vs 0.05% on the Mk II) and slightly lower SNR rating (90dB vs 95dB on the Mk II). Not really much of a big deal; the Pioneer SA series is very highly regarded. Both versions are a respectable 60 watts per channel.
It also has a cool feature that I've never seen before: dual tone control knobs for bass and treble. There's two knobs for the low frequencies (50 & 100 Hz) and two knobs for the high frequencies (10 kHz & 20 kHz). Pret-tay neat.
As for the adjustments I mentioned earlier, I recently got a digital multimeter so I can actually test and troubleshoot various components without just poking at things in the dark. One of the most common things to check on these old amps is the DC offset. You hook up the leads of the voltmeter to the speaker terminals on the back, and measure how many millivolts are getting pumped out while it's idle. Typically if you are anywhere between 0 and 15 mV, you have a healthy amp. Up to 50 mV is decent, 85 mV and you know something is wrong, but anywhere over 100 mV and you really shouldn't be using this amp without a fire extinguisher nearby (or you don't mind if your speakers get destroyed).
The left channel of this amp was hovering around 1 mV (whew), but the right channel was over 150 mV. Not good. Luckily, there's a relatively easy procedure you can follow to correct this on most vintage amplifiers. If you check out the service manual (most are on Hifi Engine), there will likely be instructions for adjusting the idle current—sometimes called bias current—and also for adjusting the voltage (this sounds vague, but only because the terms used across various service manuals vary quite a bit). You can make these adjustments by hooking the voltmeter up to specific sections of the amplifier board, while also carefully twisting a specific variable resistor with a screwdriver. These variable resistors are extremely sensitive, so it requires patience and a steady hand.
While doing this, I actually discovered an error in the official service manual for the SA-8500. My suspicions were confirmed after I found others on Audiokarma.org who made the same discovery while making the same adjustments. The manual says to adjust variable resistors VR1 & VR2 for the idle current, and to adjust VR3 & VR4 for the voltage setting. This is actually the complete opposite of what you should do. After stressing out that I might have a more serious problem to troubleshoot, I found out that I should use actually VR3 & VR4 for the idle current instead, and VR1 and VR2 for the voltage adjustment. I have no idea why there would be such a glaring error, but apparently this happens more than you'd think. Luckily there are people out there smarter than me who can read circuit diagrams to figure this out.
So after carefully adjusting both values and getting them back into factory specifications as per the service manual, I checked the DC offset at the speaker terminals again. Both channels were reading roughly 0.5 mV. Heck yes.
I feel it's necessary to give a disclaimer that you should be absolutely certain you know what you are doing before poking around inside an amplifier while it's running. And especially so if you are going to be twisting things and making adjustments. You could destroy the amplifier or you could die (maybe) if you aren't careful. Triple check the service manual and ask questions before you do anything you are unsure of. The people at Audiokarma.org are extremely helpful, just check out the DIY section of the forums.