Diagnosing loud popping from speakers connected to an SAE C3A amplifier

[fattyacids]

Hello electrical experts,

I just bought an old SAE "Two" 3CA amplifier and matching T3U tuner. As of yesterday, I had used the amp for a few hours and aside from a few oxidized switches, everything was working pretty well. My second day after plugging it in I left the amplifier on and connected to the speakers with all input sources off and the volume knob at zero, when I was awoken in the middle of the night by very loud popping noises from my speakers. I immediately turned the amp off and went back to sleep. Now whenever I turn the amplifier on, even with the volume at zero, I hear intermittent very loud pops through the speakers (can't tell if it is both channels and I don't want to turn it on again while connected to my speakers because I fear it will ruin them).

The fact that the popping occurs with the volume at zero tells me that the issue is with the amplifier, not the speakers. Is that logic sound?

I plan to open up the amplifier tonight to clean all of the switches and replace the meter lights and see if I can figure out what is going wrong. I would guess that the issue is a bad capacitor, but I could also imagine that it is related to my mains power in the house. The amplifier has a two prong power cord, and it occurred to me that it might be smart to replace it with a three prong cord. Any thoughts? One other possibly relevant detail is that I have the tuner and a turntable plugged into the switched outlets on the back of the amp and the turntable ground is connected to the ground terminal on the back of the amp.

To summarize my rambling, here are my questions:
1. What could be causing the popping and how should I go about tracking down the problem?
2. Should I replace the two prong cord with a three prong cord, and if so where should I attach the ground?

I have some knowledge of electronics and I have access to a scope, soldering setup, reflow oven, etc. A copy of the service manual (with a schematic) can be found here: SAE C3A Manual - Stereo Integrated Amplifier - HiFi Engine

Thanks!

 

[fattyacids]

Where did my post go?

 

[Mooly]

As a new member all your posts need to be approved before they go live on the board.

Don't worry, it doesn't last for long 🙂

 

[Mooly]

Patience 🙂 I've just deleted your duplicate post.

 

[fattyacids]

Thanks Mooly! I read the email about being under moderation - I was just confused because the post was up for a couple of minutes and then disappeared for a while.

 

[Mooly]

It disappeared because you edited it... and you could have put anything in its place 😉

 

[fattyacids]

Ahh, I see. I think it would make more sense to leave the original post up until the edit is approved so that there isn't a blank thread, but that may be an issue with vBulletin. I'll refrain from editing anything until I'm no longer under moderation so that doesn't happen again.

 

[fattyacids]

No one has any ideas?

 

[jitter]

To summarize my rambling, here are my questions:
1. What could be causing the popping and how should I go about tracking down the problem?
2. Should I replace the two prong cord with a three prong cord, and if so where should I attach the ground?

1. Sounds like DC is intermittently appearing on and disappearing from the speaker outputs. Had the same thing with a Marantz. If it happens very rapidly, the speaker protection relay won't kick in.

Two things to do first: measure all voltage rails in the amp and compare to the values in the service manual. If these are OK, closely inspect all solder connections for cracked or dry joints. Resolder any that look in the least bit dodgy.

Then I'd probably run the amp without speakers and with a multimeter connected set to measure DC-voltage and max-hold (if necessary repeated by a while on min-hold). If the output jumps to DC, then the max-hold function will hold a positive voltage, the min-hold obviously the negative voltage.

2. No. If the amp was designed with double insulation (look for the double square symbol on the rear of the amp), then it was not meant to be connected to an earthed outlet, hence the two-prong plug and cord.

 

[east electronics]

1. Sounds like DC is intermittently appearing on and disappearing from the speaker outputs. Had the same thing with a Marantz. If it happens very rapidly, the speaker protection relay won't kick in.

Two things to do first: measure all voltage rails in the amp and compare to the values in the service manual. If these are OK, closely inspect all solder connections for cracked or dry joints. Resolder any that look in the least bit dodgy.

Then I'd probably run the amp without speakers and with a multimeter connected set to measure DC-voltage and max-hold (if necessary repeated by a while on min-hold). If the output jumps to DC, then the max-hold function will hold a positive voltage, the min-hold obviously the negative voltage.

2. No. If the amp was designed with double insulation (look for the double square symbol on the rear of the amp), then it was not meant to be connected to an earthed outlet, hence the two-prong plug and cord.

Truly now jitter are you serious ???
Suggestions like that will work ONLY if the amplifier has been properly serviced and then still present this type of failure ....

For a 30+ years old amplifier changing all electrolytic capacitors is a one way ticket, All means all and no questions asked .
Expected to have soldering issues to be done
Expected to have pots and switches oxidized that will also apply to offset and bias trimmers existing inside which i don't think that can be cleaned with any of the sprays i know
Expected resistors inside to drift but that is a guess so while in service mode one will have to verify that also ...

After this is done you need to replace both notorious 2SA798 from inside either with the same type or starting to think of a modification that will fit and perform the same to cure the very well known problem to All amplifiers of that era using the specific transistor

Replace only the 798 to cure the problem and we will meet here much sooner than you expect ...

Regards
Sakis

 

[jitter]

Yes, I am, but this was not meant as a complete fix, just the first steps to be taken. What you propose, I would call "the works", something I'd only be prepared to do if the amp's repair was viable.

 

[east electronics]

No Jitter ... this is not the first steps these are the steps you make after you are done

 

[east electronics]

2Sa798 new will cost from 8-14 euro each , a handful of capacitors no more than 20 so yes i see ii as viable ....Also worth fixing ..sweet amp

 

[fattyacids]

Thanks jitter and sakis. Using a pair of junk speakers I've been able to isolate the issue to the right channel. I'm going inspect all the solder joints in that path on the power amp board and if they all look okay I'm going to use freeze spray to cool the pre-driver, driver, and output transistors individually so I can tell if one of them is the culprit. If I don't have any success with that technique then I will try the shotgun approach and replace all the caps and transistors.

Some of the transistors seems to be "two-in-one" types with 5 legs. How do I replace those with modern components?

 

[fattyacids]

Ahh, it appears those are the 2Sa798 transistors that you were talking about. Where would I order those from?

 

[jitter]

No Jitter ... this is not the first steps these are the steps you make after you are done

Perhaps we are in disagreement...

But the steps fattyacids has taken so far is what I would have done (more or less). First find the cause. Then fix it and test if it holds and if it does, think about replacing other parts that might fail soon if the amp were put into regular use again.

 

[east electronics]

agree but "might fail " is a rule that doesn't apply in 35+year old amplifier There is the point of our disagreement ....