Sansui 7070 popping, dumping audio temporarily

[Steady Eddie]

I've replaced fusible resistors on the driver board, replaced all the electrolytic caps on the driver board except for the 470uf caps (C05 and C06) and I've also replaced all the caps on the power supply board except for the 10 uf caps (also C05 and C06 on this board) (somehow I overlooked these caps when ordering from Mouser)

I also replaced two zener diodes on the PS board that looked dodgy and two transistors.

Even though I didn't finish all the caps, I wanted to test to at least make sure that the unit still played, that at least I did not harm.

Right now, the receiver sounds a LOT better, the caps I replaced on the driver, I used nichicon FG series, and before it was popping much worse and farting out so bad I thought my speakers were blown. Now it sounds rich and detailed. But big bass transients make it pop, and there is some sort of protection circuit that temporarily shuts down the audio.

Am I on the right path? I will order the additional caps I missed, but I want to make sure that it's not a capacitor or other component on another part of the receiver.

 

[spark1]

Since you appear to be rebuilding it, do the the protection board as well. Have you checked the bias and offset? If you need to make adjustments, I would replace the trim pots....or, at minimum, given them a good cleaning with Deoxit. Power off of course, mark current settings, dribble a few drops into the trimmer, work it back and forth 5-10 times (they are not made for a high amount of usage), and let them dry thoroughly. When doing this, I blow air into them after letting dry for a while, to be sure.

I'm assuming you've already cleaned all the pots/switches/selectors.

 

[Steady Eddie]

Since you appear to be rebuilding it, do the the protection board as well. Have you checked the bias and offset? If you need to make adjustments, I would replace the trim pots....or, at minimum, given them a good cleaning with Deoxit. Power off of course, mark current settings, dribble a few drops into the trimmer, work it back and forth 5-10 times (they are not made for a high amount of usage), and let them dry thoroughly. When doing this, I blow air into them after letting dry for a while, to be sure.

I'm assuming you've already cleaned all the pots/switches/selectors.

Which board is the protection board? The main one? I'm looking at the service manual.

 

[Steady Eddie]

So now I have an extra problem. I was testing it again. Playing it loud, and the speaker protection relay shut off the sound. When it came back on, my right channel is super weak. Only passing thin treble.

 

[mbz]

Suggest you slow down a little, try to limit the damage.

Since you appear to be rebuilding it, do the the protection board as well

Have you checked the bias and offset?

If you need to make adjustments, I would replace the trim pots

Excellent suggest above

the speaker protection relay shut off the sound. When it came back on, my right channel is super weak. Only passing thin treble.

Excessive dc voltage will trip the protection relay, the voltage may be caused by a number of things like component failure.
Will try and get some test points, suggest basic health check of power supply then check dc offset then bias.
Since you are still getting some audio means the relay is closed/dc fault cleared... maybe...volume dependent???

 

[mbz]

Not familiar with the amp, causing some difficulty. Measure dc voltages on driver board, highlighted pins.
These are probably wire wrap pins accessable with bottom cover removed(?)
Black meter probe connected to ground/chassis. Consider using minigrabbers or similiar to connect to
the pins rather than probing, one slip...

 

[Hyperion]

These are probably wire wrap pins

I am pleased to say that I don't think Sansui ever used 'wire wrapping'

They will either be Molex connectors or wiring direct to the board, judging by the posted board layout. (I have never seen the inside of a 7070).

 

[Steady Eddie]

I'll check that out later but I think I came across something.
Doing random transistor tests with my Fluke, TR02 and TR01....
TR02 getting OL from B-E and B-C.
TR01, on the other side, is getting a .65 or so volt drop, which is what I expect.
I think TR02 is dead.
That side of the board corresponds with the right channel, which is dead.
Transistor is supposed to be a 2SC1708.

 

[spark1]

https://audiokarma.org/forums/index.php?threads/f2436-tr01-04-transistor-substitute.218518/

 

[Steady Eddie]

https://audiokarma.org/forums/index.php?threads/f2436-tr01-04-transistor-substitute.218518/

Thanks! While I'm building up a Mouser order, I may also change out those trimmer pots.

So TR01-TR04 are known as the differential pair transistors... What's the deal with them? Could they be causing other problems like popping?
I'll change all 4 out because I just read somewhere that they are most likely to be troublesome.

 

[spark1]

Yes, replace all. Could well be source of some/all of your problems. Ideally, you would gain match each pair. Not sure, but you might also want to put a ring of shrink wrap around each pair for thermal continuity, if their positioning on the board allows it. Hopefully Hyperion is still watching this thread...he will know for sure.

 

[Steady Eddie]

Doing some research it’s suggested to change out TR 5-6 also. While I’m ordering and while my soldering iron is hot...

Any suggested replacement trimmers for this board? Specifically from mouser that will fit the spacing on the board. I’m thinking a multi turn trimmer.

 

[Skywatcher]

Thanks! While I'm building up a Mouser order, I may also change out those trimmer pots.

So TR01-TR04 are known as the differential pair transistors... What's the deal with them? Could they be causing other problems like popping?
I'll change all 4 out because I just read somewhere that they are most likely to be troublesome.

All 4 of the 2SC1400 should be gain matched as close as possible. They will effect DC offset. There are many threads here for the F-2624 driver board, which was also used in the 8080DB, 9090DB, and QRX-9001. Plus there are two variants. Early models used a transistor for BIAS tracking, and the later models used the STV-3H diodes for BIAS tracking and some of the components are a little different between the two versions. However, the complete boards themselves can be interchanged upwards meaning the 9090DB board can be used in the 7070, but the 7070 board can't be used in the 9090db unless you change a few transistors because of voltages.Otherwise they are the same basic designs.
It sounds like you will need to check both the DC offset voltage, and the BIAS current as it sounds like they are probably too high right now. The "protection" circuit is where you'll find the speaker relay. It sounds as though that circuit is working and doing it's job.

 

[Skywatcher]

Doing some research it’s suggested to change out TR 5-6 also. While I’m ordering and while my soldering iron is hot...

Any suggested replacement trimmers for this board? Specifically from mouser that will fit the spacing on the board. I’m thinking a multi turn trimmer.

Multi-turn pots are very nice, but not really necessary if you are careful and patient with single turn. You need to let us know which trimmers are on the board now. The 2 versions of the board used different trimmers.

 

[eiraved]

What speakers are you using? One pair or two?

 

[Steady Eddie]

What speakers are you using? One pair or two?

One pair. Bose 501 series II

 

[Steady Eddie]

According to this post https://audiokarma.org/forums/index.php?threads/restoring-the-f2624.709267/
the transistors don’t need to be matched. But let’s say I want to match them. My fluke 87v doesn’t have hfe testing (I used to have a radio shack meter that let you plug in transistors but it’s gone) is there a way to test gain on transistors with a regular meter?

 

[eiraved]

One pair. Bose 501 series II

The Bose 501 series are known to have issues with amps rated over 50 wpc. You may have overdriven and damaged the speakers by playing it loudly as mentioned, in turn damaging the receiver. You may want to check your speaker resistance at the terminals, they should measure around 4 ohms.

 

[Skywatcher]

The Bose 501 series are known to have issues with amps rated over 50 wpc. You may have overdriven and damaged the speakers by playing it loudly as mentioned, in turn damaging the receiver. You may want to check your speaker resistance at the terminals, they should measure around 4 ohms.

Possible, but I have a pair of original 501 (not series anything) I've re-foamed and they haven't caused any damage. Well, not yet anyways.

 

[Skywatcher]

According to this post https://audiokarma.org/forums/index.php?threads/restoring-the-f2624.709267/
the transistors don’t need to be matched. But let’s say I want to match them. My fluke 87v doesn’t have hfe testing (I used to have a radio shack meter that let you plug in transistors but it’s gone) is there a way to test gain on transistors with a regular meter?

He's right, they don't *need* to be matched. However closely matching them helps to steady the fluctuating DC offset that is common for these boards. Plus if you use KSC1845FTA all from the same batch, they're usually pretty close anyways and work well.
I have one of these I use quite a bit. https://www.amazon.com/gp/product/B00O0BMNX2/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1
And I have one of these for quick testing and identifying EBC, etc. Not super accurate, but close enough! And cheap too. https://www.amazon.com/Mega328-Digi...words=transistor+tester&qid=1569871168&sr=8-9
I made my own test leads using short pieces 22ga silicone coated wire, and minigrabber clips.