Sansui 7070 popping, dumping audio temporarily

[Skywatcher]

761 and 753 are a close match. The idea is to keep the DC offset as balanced as you can, with what you have to work with. I always buy transistors in 100 increments to have a better "pool" to match from, and get the price break. The ZTX are quite useful too. As far as the difference between L/R, there still won't be all that much to worry about, and you might not even notice it..

 

[Steady Eddie]

761 and 753 are a close match. The idea is to keep the DC offset as balanced as you can, with what you have to work with. I always buy transistors in 100 increments to have a better "pool" to match from, and get the price break. The ZTX are quite useful too. As far as the difference between L/R, there still won't be all that much to worry about, and you might not even notice it..

Skywatcher,

Let me first thank you for your helpfulness and your patience. It is truly appreciated.

I've replaced all four transistors, and also TR5-6 for good measure. But that didn't fix my problem. It DOES sound better in the channel, clearer, more detailed... But I think that it's the output transistor, and I need help verifying and replacing it, if it is indeed the case. I do believe that replacing the differential transistors is considered part of the driver board rebuild, so I don't consider this a wasted effort.

There are two plugs on the board by the driver board, they feed the output transistors. I unplugged them and swapped them. I figure, if the problem is on my driver board, then my problem will switch from the right channel to the left channel.
It didn't. My problem remained on the right channel.

 

[Skywatcher]

Skywatcher,

Let me first thank you for your helpfulness and your patience. It is truly appreciated.

I've replaced all four transistors, and also TR5-6 for good measure. But that didn't fix my problem. It DOES sound better in the channel, clearer, more detailed... But I think that it's the output transistor, and I need help verifying and replacing it, if it is indeed the case. I do believe that replacing the differential transistors is considered part of the driver board rebuild, so I don't consider this a wasted effort.

There are two plugs on the board by the driver board, they feed the output transistors. I unplugged them and swapped them. I figure, if the problem is on my driver board, then my problem will switch from the right channel to the left channel.
It didn't. My problem remained on the right channel.

Swap speakers from L/R to verify.

 

[Steady Eddie]

Swap speakers from L/R to verify.

It's not the speakers, I checked those.
I swapped the output transistors, I didn't notice that there were two types.... I put them back correctly in the right spots, but I may have messed something up, the output sounds distorted and weak and the right channel is still out. I need to figure out how to test the transistors.
TR11, one of the driver transistors, was getting very hot. I need to walk away from this for a while and read up more. I may desolder the driver transistors to test, but that's a bitch with the heat sink.
Also, I had the DC offset dialed in, but the bias won't. It keeps moving on me. I am pretty sure this is key. So I replaced the red dot diodes with 2x1n4148's, and tried to bias it. One channel is only going up to 24mA, the other channel is way out of range, like 140 mA.

 

[eiraved]

It's not the speakers, I checked those

Skywatcher is correct, check the speakers again. Have you checked the speaker resistance yet?

 

[Steady Eddie]

Skywatcher is correct, check the speakers again. Have you checked the speaker resistance yet?

4.2, 4.3 ohms.

 

[Skywatcher]

4.2, 4.3 ohms.

But did you swap L/R to see if the distortion follows the speakers?

 

[Skywatcher]

4.2, 4.3 ohms.

Was that before, or after the protection kicked in when you had it up "loud". If before, check 'em again. Also note that just because you read DCR at 4.3 Ohms, that does not mean a speaker system (with crossovers) is good. A voice coil can still be damaged, a resistor in the crossover could be damaged, and crossover caps can be blown. We just have to eliminate any possibility to prevent chasing a red herring. Check speaker fuses as well.

 

[Steady Eddie]

Was that before, or after the protection kicked in when you had it up "loud". If before, check 'em again. Also note that just because you read DCR at 4.3 Ohms, that does not mean a speaker system (with crossovers) is good. A voice coil can still be damaged, a resistor in the crossover could be damaged, and crossover caps can be blown. We just have to eliminate any possibility to prevent chasing a red herring. Check speaker fuses as well.

Swapped out the Boses for some Infinity SM80s. Still distorted. TR11 still getting hot. I don't run it hard or let it run long.

 

[Skywatcher]

Swapped out the Boses for some Infinity SM80s. Still distorted. TR11 still getting hot. I don't run it hard or let it run long.

TR11 running hot is a clue. I would not be running it until the issues are fixed or you'll cause more issues. Something is shorted, or open somewhere. Check the 0.33 Ohm 5 Watt emitter resistors for starters. Make sure none are open. They won't read 0.33, but as long as they're not open, they are fine. Sub 1 Ohm is all but impossible to accurately measure, even on some Fluke meters so being close is good enough. If your BIAS reading is misbehaving, you need to look at everything in that circuit.

 

[Steady Eddie]

TR11 running hot is a clue. I would not be running it until the issues are fixed or you'll cause more issues. Something is shorted, or open somewhere. Check the 0.33 Ohm 5 Watt emitter resistors for starters. Make sure none are open. They won't read 0.33, but as long as they're not open, they are fine. Sub 1 Ohm is all but impossible to accurately measure, even on some Fluke meters so being close is good enough. If your BIAS reading is misbehaving, you need to look at everything in that circuit.

Those big ceramic resistors read fine.
I tried reading the bias current again and I'm getting nothing.
I understand a little bit about reading schematics for simple amplifiers, guitar tube amps, etc. This is a bit out of my league.

 

[Skywatcher]

Those big ceramic resistors read fine.
I tried reading the bias current again and I'm getting nothing.
I understand a little bit about reading schematics for simple amplifiers, guitar tube amps, etc. This is a bit out of my league.

Perhaps there is an AK member close by who would be willing to take a look at it. Having no BIAS current can be several things. An open output transistor is one of them, but who really knows at this point. You can test the output transistors with that little tester to check for shorts, or open junctions. A cracked mica insulator under one of the output transistors can cause heaps of trouble too. The collector (the case of a TO-3 transistor) on this unit must be electrically insulated from the heat sink. That's what the mica does while still conducting heat. Telling us where you are from might help finding someone nearby.

 

[eiraved]

I tried reading the bias current again and I'm getting nothing.

If using a Fluke (and a few others), you might try checking the meter fuse first. They do blow sometimes, leading to erroneous current readings.

Also, I would continue to use the Infinity speakers for testing. You may as well take a meter to them and verify their impedance (8 ohms) before further testing just to rule them out as being a potential problem.

I wouldn't trust 4 ohm speakers with my Sansui 9090db. It didn't like anything with nominal impedance at 4 ohms or below. And yes, I'm aware the 7070 manual says 4 ohm speakers are ok. The 7070 is too similar the 8080 and 9090, and I believe it is also more sensitive to lower speaker loads like they are, regardless of what the 7070 manual states.

 

[Skywatcher]

If using a Fluke (and a few others), you might try checking the meter fuse first. They do blow sometimes, leading to erroneous current readings.

Also, I would continue to use the Infinity speakers for testing. You may as well take a meter to them and verify their impedance (8 ohms) before further testing just to rule them out as being a potential problem.

I wouldn't trust 4 ohm speakers with my Sansui 9090db. It didn't like anything with nominal impedance at 4 ohms or below. And yes, I'm aware the 7070 manual says 4 ohm speakers are ok. The 7070 is too similar the 8080 and 9090, and I believe it is also more sensitive to lower speaker loads like they are, regardless of what the 7070 manual states.

I forgot about the Fluke fuse. Well if that's it, there goes another $10. . I run 4 Ohm on my 9090db without any issues. I just watch the heat when turned up. But the 9090db speaker switch runs the speakers in series when using a combo of A+B or A+C. The 7070 does not do that, they are in parallel making a pair of 4 Ohm actually 2 Ohm.. That draws too much current through the output transistors. The TO-3 transistors are pretty robust and can take it (4 Ohm) as long as they don't overheat.

 

[eiraved]

I run 4 Ohm on my 9090db without any issues.

I'm glad it's working well, but the Sansui manual gives no leeway for anything other than 8 ohm speakers. I assume yours has been restored, yes? That may explain why it's handling the low impedance, that and not playing it like a rock star! Mine was not restored, and I used an EV Regal 300 pair with it which sounded glorious...until the music ended and never came back, LOL!

The 7070 does not do that, they are in parallel making a pair of 4 Ohm actually 2 Ohm.. That draws too much current through the output transistors. The TO-3 transistors are pretty robust and can take it (4 Ohm) as long as they don't overheat.

The OP likes it loud, which may account for the issues he's been experiencing in repeat mode while using speakers with low impedance.

 

[Steady Eddie]

If using a Fluke (and a few others), you might try checking the meter fuse first. They do blow sometimes, leading to erroneous current readings.

Also, I would continue to use the Infinity speakers for testing. You may as well take a meter to them and verify their impedance (8 ohms) before further testing just to rule them out as being a potential problem.

I wouldn't trust 4 ohm speakers with my Sansui 9090db. It didn't like anything with nominal impedance at 4 ohms or below. And yes, I'm aware the 7070 manual says 4 ohm speakers are ok. The 7070 is too similar the 8080 and 9090, and I believe it is also more sensitive to lower speaker loads like they are, regardless of what the 7070 manual states.

It's the fuse. My Fluke 87V has amp and milliamp terminals, the amp one has a higher rated fuse, so I plugged into that one, and it's good. Will continue testing with Infinity speaks.

 

[Steady Eddie]

Perhaps there is an AK member close by who would be willing to take a look at it. Having no BIAS current can be several things. An open output transistor is one of them, but who really knows at this point. You can test the output transistors with that little tester to check for shorts, or open junctions. A cracked mica insulator under one of the output transistors can cause heaps of trouble too. The collector (the case of a TO-3 transistor) on this unit must be electrically insulated from the heat sink. That's what the mica does while still conducting heat. Telling us where you are from might help finding someone nearby.

I got most of the problem sussed out. I'm kind of embarrassed about it. I was moving the power transistors from the left channel to the right to see if the problem followed, and I thought that the collector on the TO-3 transistors went to the chassis, so I didn't screw them in. Well, my fluke showed me they were actually isolated, so when I put in the screws, the channels sound great now.
I've got the DC offset and bias on the money on one of the channels, on the other one, it's close but not as exact. I am offshore today and don't have the numbers in front of me. I will report more later.

Part of the channel problem may have been with the FM tuner, and I think that is actually due to poor reception with the built-in antenna. This is something I will explore more later, but for now I am using my iphone as a source with a cable going into the tape input.

One thing I did find was that when using the L/R balance pot, that when I turn to the left channel, it actually diminishes the volume. The left speaker gets fuller sounding the more it goes near the center. I never noticed this before. The pot doesn't sound scratchy, but should I give it a shot of DeOxit?

 

[eiraved]

when using the L/R balance pot, that when I turn to the left channel, it actually diminishes the volume. The left speaker gets fuller sounding the more it goes near the center. I never noticed this before. The pot doesn't sound scratchy, but should I give it a shot of DeOxit?

Yes, and I would work the balance control back and forth fully about 50 - 100 times. Hopefully this solves your issue.

I'm kind of embarrassed about it.

We've all been there, some of us more times then we'd like to admit...
We're here to learn, and thank God we have great help here at AK to limit our excursions LOL!

 

[Steady Eddie]

Yes, and I would work the balance control back and forth fully about 50 - 100 times. Hopefully this solves your issue.

We've all been there, some of us more times then we'd like to admit...
We're here to learn, and thank God we have great help here at AK to limit our excursions LOL!

I'm extremely thankful for your help, and also Skywatcher, for being patient and helpful through this.
I'm not a total noob, I've got some tech chops and I've worked on my guitar tube amps and pedals for years, but this is a bit more complex.
This site is just a great resource! Thank you guys!

 

[Steady Eddie]

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