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Sansui Au-888 driving me crazy! Ideas?

blhagstrom said:
You should be running a 60w bulb for small units like that.

Are the outputs still removed?
Click to expand...

Shoot, my bad on the 100w. In a youtube video I was watching the guy was using a 150w for receivers but I didn't question him. Did this cause the damage?

Outputs are still removed. I have tested every resistor on the driver board, all caps, diodes, and transistors. But the issue seems to be on the board. Both driver card connectors for the left and right channel have the same voltage readings, and when I swap the cards the problematic card has way higher voltage. I am using a variac at 50v due to the DBT smoking a resistor so the voltages are not reflected on the schematic. Trim pot TR803 is getting 7 volts on all legs and surrounding parts while the same part on the right channel card is reading nearly 0 volts.
 
Chriswashere said:
DBT smoking a resistor
Click to expand...
The DBT didn't cause the resistor to blow.. that went from some other issue. but it is a clue. You may have to review your work, go as far as solder trace continuity, cold solder joints? check vr's for accurate sweep. set to off. can't set bias w/out outputs but can compare i.e. base emitter v's etc. A transistor may check ok out of circuit but act diff. in circuit under power.
hope it helps.
 
From what I’ve pieced together.

You have matching voltages at the driver board connectors.

The outputs are removed

On dim bulb, a resister smoked.

One driver board is OK?

You’ve done work on the smoked resister driver board.

Something is really wrong here.
Either there is a mistake in your work or it’s some whacked like a broken part that tests OK or a bad trace.

I’m out of general hints and tips.
Never worked on an 888 and never had a smoking driver board with the outputs removed.
 
Binkman said:
The DBT didn't cause the resistor to blow.. that went from some other issue. but it is a clue. You may have to review your work, go as far as solder trace continuity, cold solder joints? check vr's for accurate sweep. set to off. can't set bias w/out outputs but can compare i.e. base emitter v's etc. A transistor may check ok out of circuit but act diff. in circuit under power.
hope it helps.
Click to expand...

Sorry for my miswording. I understand that the DBT would not smoke a resistor, but that an issue I've missed would allow the resistor to get smoked.

"A transistor may check ok out of circuit but act different in circuit under power"

THIS. I realized that I have the replacement transistors in stock for the cdc8002 and cdc9002, so why keep testing transistors when there's new ones right in the parts bin? I replaced all these transistors on the driver board and now have the left channel going again. Voltages check the same on each channel but the variac is only at 50ish Vac (meter is a bit junky).

The weird thing is I have never had transistors do this. It has always been a matter of finding a lack of conductance or a short.
 
blhagstrom said:
From what I’ve pieced together.

You have matching voltages at the driver board connectors.

The outputs are removed

On dim bulb, a resister smoked.

One driver board is OK?

You’ve done work on the smoked resister driver board.

Something is really wrong here.
Either there is a mistake in your work or it’s some whacked like a broken part that tests OK or a bad trace.

I’m out of general hints and tips.
Never worked on an 888 and never had a smoking driver board with the outputs removed.
Click to expand...



I had matching voltages on the board connectors but I decided to replace all of the cdc8002 and cdc9002 replacement transistors which seems to have brought the left channel back to life. The right channel was always good but I can see some repair work at some point. Whatever the issue was the last guy fixed it.

The issue is I have never had transistors that acted different when they had power. For years it has been a matter of finding dead shorts of lack of conductance on one leg. This one was one crazy experience.

I have it running at 50vAC on the variac so I will update things if there is an issue at higher voltages. Voltages will be checked as the Variac is turned up, but I am not sure if I should put the power transistors in just yet. It does sound pretty good with the junk shop speakers by the way.
 
slimecity said:
You generally want to replace these flying saucers with modern equivalents.

What did you replace them with, the same transistors?
Click to expand...

They were replaced with BC639 for the cdc8002 and BC640 for the cdc9002. They were recommended on another thread that was restoring one of these.
 
As a general update the crackle is back. I put the problematic board into the right channel and started to adjust the bias. Suddenly the crackling "thunder/wind" sound started and the bias SPIKED. I slapped the kill switch for the power supply and then lowered the voltage. At 50vAC the bias was jumping around between 50ma and up to nearly 100. But I cut the power again after seeing this for a second and hearing the speaker.

The only transistors not replaced are the A640. They will be the next step then, though they likely need to be ordered unless I have an equivalent.
 
I have advocated the use of resistors soldered over blown supply fuses leaving output transistors in, in more than one of such Sansui topics. (it also applies to other amps as long as supply fuses are accessible)
This eases troubleshooting A LOT, although many people do not see the benefit, it works for me. It protects the output transistors so measurements are not compromised by them lacking.
It is even possible to set both offset and bias having those resistors in.
One can use something like 390 Ohms 10W resistors for this.
fuseres.jpg888resamp.jpg

Have you replaced the input pair? Are those the 2SA640? Protection circuit lines taken loose?
If not, do that first, if having no "equivalents" putting BC640 in there, as I understand you have, MAY do fine during testing at the moment, although they will have to be replaced by proper ones later.

Also check and distrust the bias setting transistor, if somebody was there before it may even be wrong way around.
 
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Binkman said:
Don't quite understand your bias setting procedure. At 50vac? using the variac?
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I am not setting the bias at 50v. I had an issue with the bias and amp popping at normal voltage so it cut it. I check the bias at a lower voltage to see if it's still high meaning a blown component. I was taught weird..
 
As a general update I replaced the a640 transistors and all 2sc458. Now I have crazy distortion without the power transistors. What a pain.
 
I see a lot of questions and people not liking what I do. So I guess I have some explaining.

Why am I running the amp without the outputs? I was taught to do this. If you are getting good audio without the outputs, it should be safe to put them in. All amps I have worked on have worked without the outputs but they did not have the power. But the audio still passes through.

Why a Variac? I was taught to do this. If there's no smoke and the unit is working I was taught to keep cranking it up. This I learned in a hifi shop. When using a DBT things in my experience just instantly blow and I don't see the point of that. Maybe that's just my luck, but with a variac I can catch a problem at very low voltage, fix it, then test again.
 
Chriswashere said:
Maybe that's just my luck, but with a variac I can catch a problem at very low voltage, fix it, then test again.
Click to expand...
The problem with a variac is that if a semiconductor decides to let go, you cannot possibly turn the voltage down fast enough to prevent that happening. With a DBT - providing you select the right bulb wattage - it will automatically and instantaneously limit the current while giving you an almost instant visual indication (brighter bulb) that something is wrong - now do you get it? :)

Don't get me wrong, a DBT isn't a talisman that will prevent you ever blowing up a semiconductor in a piece of faulty gear, but it will sure as hell limit the ensuing damage.

As I said, with tube / valve gear the position is reversed, a variac is exactly right for this kind of gear, and a DBT is of very limited use. ;)
 
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Chriswashere said:
I see a lot of questions and people not liking what I do. So I guess I have some explaining.

Why am I running the amp without the outputs? I was taught to do this. If you are getting good audio without the outputs, it should be safe to put them in. All amps I have worked on have worked without the outputs but they did not have the power. But the audio still passes through.

Why a Variac? I was taught to do this. If there's no smoke and the unit is working I was taught to keep cranking it up. This I learned in a hifi shop. When using a DBT things in my experience just instantly blow and I don't see the point of that. Maybe that's just my luck, but with a variac I can catch a problem at very low voltage, fix it, then test again.
Click to expand...

Issues are

1. You can not run an amp with a signal and speakers lacking output transistors without major risks for other components being compromised.
2. You can not set any bias current amount without normal voltage over the voltage amp, as there is no current source in the voltage amp, just a resistor.
3. You can not set bias without the output transistors mounted, as the bias current flows through these transistors.

Your approach to tackle the problem is very unclear

Therefore, you may just get questions instead of answers.
 
Weird. I was taught by a couple shops to run gear without the outputs before putting them in. Could this have been specific units? That had to have come from somewhere so it’s a surprise that isn’t good for the unit.

I am not setting the bias without the outputs. I know it’s pointless. I’m also not setting the bias at reduced voltage.

The sansui is now working with the bias set and so far the “crackle” issue hasn’t returned and the bias seems to be holding steady. The trim pots do look rough so they’ll be replaced.
 
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