Vintage Pioneer SA 720 amp question - please be kind, im old lol

[JoolsA]

Hey all, hope everyone well?

just bought a beautiful vintage Pioneer SA720 amplifier (same model as the one we had back in the day) and have an issue with the left hand channel speaker A&B output. Sounds awful, crackly and not defined. Right hand Chanel is fine on speaker A&B outputs.

I’ve swapped speakers to ensure it’s not a speaker fault. When headphone socket is used L&R channels are fine with great sound so I’m assuming that’s it’s likely to be R109 that’s faulty, as the signal to the socket is good? Just trying to check my logic that when headphones are used, R109 is omitted from the signal path but when headphones are removed, it is in circuit?
To recap, headphones sound is fine on L&R outputs. Headphones removed and through either speaker A or Speaker B selector, left hand Chanel is lousy, right hand Chanel,is good.

Caps all look in good nick, no signs of bulging or leaking and everything else seems in good order. Resoldered components ensure no dry joints, and since signal is on speaker A&B + LHS only, it has to be that or one of those resistors?

thank you for your time and apologies if really basic question lol.
Cheers
Jools

 

[Jacky_BEL]

Any signs of degradation on the switch assembly , like oxidation or corrosion?
Cracks in solder joints in the signal path to speaker connectors?

 

[JoolsA]

Hiya, thank you for the reply. All in remarkably good condition for age. Not taken switch apart as it’s a multi stage thing but used some electronic switch cleaner and all seems good. Signal path is literally from the switch board via wire to the speaker connectors. Resoldered the joints as was just wire wound to the peg from factory. So other signs of over heating or degradation that I can see by eye.
Signal seems great to the headphones with L&R Chanel’s sounding good. It’s just this LH Chanel when it goes via speaker A or B and it’s the same poor quality on A&B. Bit odd.

 

[Jacky_BEL]

It could still be a fault at the amp stage.
Driving headphones needs less power than driving speakers.
I doubt the R109 is the culprit , it is fairly high resistance , 56kohms.
It this resistor connected to a VU-meter? Does the VU meter work normally?

 

[JoolsA]

Hmmm, it has an electronic peak power meter (not traditional VU) and that’s operating correctly. By that I mean when L&R in balance, meters are reading the same. And when balance is either L or R, they change to show channel accordingly. I see what you mean about an issue at the amp stage mind.

 

[Shrek]

I tend to use silicone oil on switches and connectors

 

[JoolsA]

I tend to use silicone oil on switches and connectors

Do you have a recommended brand?

 

[Shrek]

For contacts I use 3 in 1 which has a solvent; for treating leather I use a type without solvent.

 

[Wirko]

For contacts I use 3 in 1 which has a solvent; for treating leather I use a type without solvent.

I'd recommend against using a lubricant (including WD-40, which people tend to use for just about everything) when you need a cleaner.

I use the Kontakt 60 spray (by CRC, formerly Kontakt Chemie, not sure if it's a familiar name in the US). Here are the data sheet and the safety sheet, and while it does contain some "white mineral oil (petroleum)", I can confirm that it evaporates completely.

But I find the "nothing" chemical even more useful. I keep moving the switches from one end position to the other until the sound improves. About 100x is usually enough. The same procedure often has good effects on potentiometers too but it requires caution and careful listening; it's helpful if they're dirty but does more damage if they're worn out.

 

[JoolsA]

I'd recommend against using a lubricant (including WD-40, which people tend to use for just about everything) when you need a cleaner.

I use the Kontakt 60 spray (by CRC, formerly Kontakt Chemie, not sure if it's a familiar name in the US). Here are the data sheet and the safety sheet, and while it does contain some "white mineral oil (petroleum)", I can confirm that it evaporates completely.

But I find the "nothing" chemical even more useful. I keep moving the switches from one end position to the other until the sound improves. About 100x is usually enough. The same procedure often has good effects on potentiometers too but it requires caution and careful listening; it's helpful if they're dirty but does more damage if they're worn out.

I repair old cameras and WD-40 is a nightmare. Can’t tell you how many lenses and bodies I’ve seen over the years with WD dripping out of them. Tiny amount dropped via a syringe - fab, sprayed in, not so much.

 

[Shrek]

WD-40 is a no go in my book, even in moderation.

 

[Steevo]

For electrical connections use Corrosion X HD, it works wonders.

 

[Wirko]

Here are some ideas for debugging.

Listen to the left channel at very low volume, so soft that you need to hold your ear next to the speaker to hear clearly. Does it sound distorted in a similar way compared to when it's loud? Or is the distortion very much dependent on volume?

Can you make an adapter cable with a stereo headphone jack at one end and bare wires at the other end, so you can connect speakers to headphone output? That's how you can bypass the
switches. You'll hear the sound at low volume but loud enough to hear if it's OK. That's only partly useful though because you can make the output stages work at quite high output voltages, but not high currents.

Connect both speakers AND headphones, then silence the speakers in some way (blanketscarpetspillows or move them to another room?). Is the sound in the headphones OK on both channels?

If you have a voltmeter, measure the DC voltage at the speaker terminals of each channel, with volume set to minimum. It should be less than 100 millivolts, positive or negative, and ideally zero. Anything more than that points to a defective power stage.

If you don't have a voltmeter, just try switching the speakers on and off with the A-B-OFF switch, again at minimum volume. If you hear loud pops, it's DC voltage.

The R109 and R110 remain in the circuit at all times, with speakers, headphones, both or nothing. They carry the output signal to the VU-meter electronics - I checked, schematics are easy to find - but they can't cause anything like you're describing.

Hope this helps a little.

I repair old cameras and WD-40 is a nightmare. Can’t tell you how many lenses and bodies I’ve seen over the years with WD dripping out of them. Tiny amount dropped via a syringe - fab, sprayed in, not so much.

I have no experience with that but why don't you use something thicker? Some non-corrosive oil at least as thick as engine oil? WD-40 seems to remain thin forever so even if you apply it in tiny amounts it can get where you don't want it to be.
Also, the thing dripping out of lenses could be something else, not WD-40. Maybe even original oil or grease that has separated into thick and thin components over decades.

 

[Shrek]

For electrical connections use Corrosion X HD, it works wonders.

The trouble with carbon-based concoctions is that with time they attack plastics.

 

[Courier 6]

here are some usefull links for you -> https://www.hifiengine.com/ https://www.tapeheads.net/

 

[Grog6]

De-oxit and deoxit gold. Those do not damage pots, like everything mentioned above.

 

[Steevo]

The trouble with carbon-based concoctions is that with time they attack plastics.

Corrosion X doesn’t, it’s replaced silicone based products in all industrial applications, amp, deustch, delphi, peppers fuches, etc… all the major wide temperature sensor and connections for corrosion prevention solution.

How do I know, I have worked in that industry for 20+ years and seen the absolute train wreck that silicone based products can cause when applied to lite, heavy, incorrectly, or correctly even during simple temperature changes causing condensation inside a cavity almost filled with dielectric silicone oil or grease.

 

[Shrek]

I am all ears... seriously... but what is 'deustch, delphi, peppers fuches'

I still use silicone grease on my car bulbs and silicone oil on my electrical connectors; the one place I avoid silicone oil/grease is high current switches, where the spark can cause the silicon to oxidize and so make glass.

Now brake fluid (DOT 3 and 4) is designed to absorb water, but now there is DOT 5 which is silicone based.

 

[JoolsA]

Fantastic

Here are some ideas for debugging.

Listen to the left channel at very low volume, so soft that you need to hold your ear next to the speaker to hear clearly. Does it sound distorted in a similar way compared to when it's loud? Or is the distortion very much dependent on volume?

Can you make an adapter cable with a stereo headphone jack at one end and bare wires at the other end, so you can connect speakers to headphone output? That's how you can bypass the
switches. You'll hear the sound at low volume but loud enough to hear if it's OK. That's only partly useful though because you can make the output stages work at quite high output voltages, but not high currents.

Connect both speakers AND headphones, then silence the speakers in some way (blanketscarpetspillows or move them to another room?). Is the sound in the headphones OK on both channels?

If you have a voltmeter, measure the DC voltage at the speaker terminals of each channel, with volume set to minimum. It should be less than 100 millivolts, positive or negative, and ideally zero. Anything more than that points to a defective power stage.

If you don't have a voltmeter, just try switching the speakers on and off with the A-B-OFF switch, again at minimum volume. If you hear loud pops, it's DC voltage.

The R109 and R110 remain in the circuit at all times, with speakers, headphones, both or nothing. They carry the output signal to the VU-meter electronics - I checked, schematics are easy to find - but they can't cause anything like you're describing.

Hope this helps a little.


I have no experience with that but why don't you use something thicker? Some non-corrosive oil at least as thick as engine oil? WD-40 seems to remain thin forever so even if you apply it in tiny amounts it can get where you don't want it to be.
Also, the thing dripping out of lenses could be something else, not WD-40. Maybe even original oil or grease that has separated into thick and thin components over decades.

Fantastic faultfinding guide Wirco. I’ll work my way through that. Will be a couple of weeks as away with work.

I use watchmakers lubricants and occasionally something developed by NASA for Nikon during the Apollo space programme called Astro-oil. Horrendously expensive. Old helicoid grease does separate over time and causes apertures to stick but the equipment I’ve received in the past you can smell in the box before opening. Can’t underestimate the ability of the bodger to astound and make you wonder why….

thanks for the tips, will get the multimeter out.

 

[OneMoar]

no no and no save the WD40 and cooking oil and cold pressed avocados

DeoxIT D5 or G5 is what you want for contacts, general cleaning and lubrication
DeoxIT FN5 For Faders,Sliders and Pots where you need something safe for bakelite and other vintage plastics

anything else and you could very well end with melted pots or plastic goo everyware

 

[Frick]

WD-40 is a no go in my book, even in moderation.

It does have its uses, but not around electronics. Around heavy machinery and rusted iron and steel it be really handy though.

 

[OneMoar]

water
displacement
formula
40
no where in the name does it indicate its a lubricant,nor a solvent or penetrant

 

[Grog6]

Yup; dw was developed to get water out of electronics, back when they were using tubes. It was very popular with the military. It's even in a time tunnel episode, iirc.

 

[Steevo]

I am all ears... seriously... but what is 'deustch, delphi, peppers fuches'

I still use silicone grease on my car bulbs and silicone oil on my electrical connectors; the one place I avoid silicone oil/grease is high current switches, where the spark can cause the silicon to oxidize and so make glass.

Now brake fluid (DOT 3 and 4) is designed to absorb water, but now there is DOT 5 which is silicone based.

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[Wirko]

This discussion about lubricants is interesting to me too but let's not get carried away too far ... it's not necessary for the OP to make his equipment resistant to acid rain, freezing water, pressure washing, electrolysis, literal bullshit and dog pee ... it's an audio amplifier for the living room! If it has an issue, it's a transistor or two that broke down, or a cold joint somewhere on the PCB (those are my best guesses).

One more thing about switches. Some types of switches are also self-cleaning, which means that metal parts slide and rub against each other as they make or break a contact. The layer of dirt and oxides gets rubbed off as the switch is used, but if it hasn't been moved for years, the angry cleaning procedure that I mentioned above will clean it. Rotary switches, pushbutton switches, and slide switches probably are like that. Rocker switches and toggle switches probably aren't - metal parts just touch one another without rubbing. But it all depends on the construction of course.