Sansui eight trouble

[blhagstrom]

Hi guys,

I've been working on a Sansui Model Eight for a while, on and off.
It has "static" across all inputs. It sounds like AM garbage.
It's clearly there at zero volume but hides in the sound as you incease the volume.
It does appear in low volume.

I used the pre-out, main-in to determine that the issue is in the pre-amp.

The unit has been recapped. Main caps, power supply, pre-amp. Maybe more. That was done before I got it. I checked the caps and they are all the proper replacements.

I went through the threads and saw that the transistors on the pre-amp get "old". I shotgun replaced all of those.

I noticed that the tone sub board wasn't recapped. Hard to get to and uses bi-polar caps. I replaced all of those.

I've checked all the grounds about a dozen times.

Another thread leads me to believe it may be in the power supply board.

There is a "flying sauser" transistor. CDC80023. From the little word out here, it sounds like the "flying saucers" are typical culprits.

The power transistor (power regulator) is a 2sd223. It crosses to an NTE224 but mouser lists them at about $8.

Since the unit works and I don't have a scope, its tough to pinpoint. Replacing the transistors wouldn't be bad IF the cost were better then $8.

Anyone know replacements for the CDC8003? A cheaper alternative to the NTE224?


OR, am I down a garden path here chasing this issue.

 

[DCinDC]

Any transistor with the same case style, Sex (NPN, PNP) same of higher VCE and IC rating will work.

I don't have an Eight open right now to help you further, just letting you know that trying to find an old transistor is a waste. You just need something that works. Never buy NTE.

The worst scope on the planet would be adequate for finding the culprit. Spend the 50 bucks.

 

[blhagstrom]

Any transistor with the same case style, Sex (NPN, PNP) same of higher VCE and IC rating will work.

I don't have an Eight open right now to help you further, just letting you know that trying to find an old transistor is a waste. You just need something that works. Never buy NTE.

The worst scope on the planet would be adequate for finding the culprit. Spend the 50 bucks.

Thanks for the pointers. I try to avoid NTE. Which transistor are we talking about here? The CDC8003? I don't have any specs for that. Or, the 2SD223? I could work from the specs on that one, or at least the NTE sub specs.

I DO have an old scope. Heathkit tube type. Seems to be working, at least it lights up. I just don't have much clue how to use it. I "think" I should check the supply voltage on the pre-amp and look for the static. I don't have the leads and I "think" they are just wire probes. Any clues here for using the old scope?

 

[DCinDC]

Attach a wire, any wire to the vertical input. Touch it to the chassis of the Eight. That gives you the baseline noise that you will see. Then go fishing around.

For checking power supplies, put the vertical input to ac coupling and the horiz to .5-.25 sec/div sweep. You should see the 120hz ripple component easily that way. Shouldn't be more than about 100mv pk-pk ripple.

You may have a noisey diode or zener.

Check and mke sure that the trash is not on the AC circuit you are using by putting a different amp in place of the Eight.

 

[dr*audio]

Thanks for the pointers. I try to avoid NTE. Which transistor are we talking about here? The CDC8003? I don't have any specs for that. Or, the 2SD223? I could work from the specs on that one, or at least the NTE sub specs.

I DO have an old scope. Heathkit tube type. Seems to be working, at least it lights up. I just don't have much clue how to use it. I "think" I should check the supply voltage on the pre-amp and look for the static. I don't have the leads and I "think" they are just wire probes. Any clues here for using the old scope?

If you search AK there is a great tutorial on how to use an oscilloscope. There are also some on YouTube.

 

[blhagstrom]

hmmm

I don't know if this scope is that fancy, she's old.

I got ON/Intensity, Focus, V-center, H-center, V-gain, H-gain, V-input multiplier. These seem pretty straightforward.

Then there is the H selector with H-input, 60cy, 20, 120, 1400, 20KC, 150KC. Not sure where to set that.

Also Sync Select with ext, 60vy, int. Again, don't know about that.

I also have Phase and sync amplitude.

I don't know about those last controls.

I played with it a little. I don't know if its working enough.
I did just get a tube tester. I should check the tubes.
I wonder how good those old paper caps are....

 

[Tichu]

Can you try removing the jumpers on the back that connect preamp out and main in ?
Then listen and check if the noise is still there.

 

[blhagstrom]

Can you try removing the jumpers on the back that connect preamp out and main in ?
Then listen and check if the noise is still there.

There are no jumpers. The eight has an internal control on the speaker selector. I've verified that its in the pre-amp by running input to the main-in. That is nice and quite.

I've been round and round with this. I cleaned it (deoxit) and pulled the removable boards and checked the adjustments, etc.... I've inspected the boards a few times. Checked the re-cap work from before. I did find a broken trace 2 nights ago on a ceramic cap on the power board and thought I had found it. Nope.

 

[dr*audio]

You need to use a scope to troubleshoot this correctly. Go find the scope tutorial.

 

[kvflyer]

I don't know a Sansui Eight. But if it has carbon resistors, one or more may be noisy. I have had that issue with Allied Radio Knight-Kit amplifiers in the past, both tube and transistor.

 

[blhagstrom]

You need to use a scope to troubleshoot this correctly. Go find the scope tutorial.

:yes: But my scope may not be up to it....:sigh:
A buddy knows a tech that maaaaaybe I can access. Sort of a "private party" thing.

 

[dr*audio]

:yes: But my scope may not be up to it....:sigh:
A buddy knows a tech that maaaaaybe I can access. Sort of a "private party" thing.

Your scope should be fine for audio use. All you need to do is look for where the noise is coming from. Alternatively you can buy a can of freeze spray and start spraying the transistors in the output stage of the preamp. the one that makes the noise worse or better when you spray it is the culprit. I forgot to ask, is this noise in both channels or only one channel? If it's only in one channel it's not the power supply, since that is common to both. If it's in both channels, suspect the supply.

 

[blhagstrom]

Ok

I was trying the scope an got some sizzle and smoke, from the scope!

I went through the scope. Tubes are all good. She has Mullards in her! :yes:

The sizzle and smoke must have been some wax on the tranny from a cap or something.

I finally figured out the sweep. 20 cycles give me a reasonable wave. A little choppy, I suppose the caps are "funky".

I went through some of the o-scope info and tutorials. DCinDC's thread was cut short by unemployment.

I get the feeling that the probe wire is usually sheilded? ToneLizard talks about BNC to Banana adaptors. I know I have some banana adapters around somewhere. I'm wondering if a coaxial probe wire would be better. Ground the sheild. Thoughts?

The "static" is both channels, any input. It seems to fade as the receiver runs, I suppose as it warms up. It sounds like an AM tuner picking up a thunderstorm.

SOOOooooo, how do I "go fishing"?


Oh, I checked the pre-in/main-out switch by jumping the jacks in the back. I can see the switch and cleaned the hell out of it. The jump proved that the switch is OK.

 

[dr*audio]

I was trying the scope an got some sizzle and smoke, from the scope!

I went through the scope. Tubes are all good. She has Mullards in her! :yes:

The sizzle and smoke must have been some wax on the tranny from a cap or something.

I finally figured out the sweep. 20 cycles give me a reasonable wave. A little choppy, I suppose the caps are "funky".

I went through some of the o-scope info and tutorials. DCinDC's thread was cut short by unemployment.

I get the feeling that the probe wire is usually sheilded? ToneLizard talks about BNC to Banana adaptors. I know I have some banana adapters around somewhere. I'm wondering if a coaxial probe wire would be better. Ground the sheild. Thoughts?

The "static" is both channels, any input. It seems to fade as the receiver runs, I suppose as it warms up. It sounds like an AM tuner picking up a thunderstorm.

SOOOooooo, how do I "go fishing"?


Oh, I checked the pre-in/main-out switch by jumping the jacks in the back. I can see the switch and cleaned the hell out of it. The jump proved that the switch is OK.

You're making progress. The scope should have BNC connectors for the probes. If so, you should buy an inexpensive scope probe. MCM Electronics has them, use the link at the bottom of the page. If you can take a picture of the front panel of the scope it would be helpful.

 

[blhagstrom]

You're making progress. The scope should have BNC connectors for the probes. If so, you should buy an inexpensive scope probe. MCM Electronics has them, use the link at the bottom of the page. If you can take a picture of the front panel of the scope it would be helpful.

She's an old Heathkit. Looks like the one on ToneLizard's page.

I picked it up dirt cheap along with a few other old test rigs.
I have another Heathkit o-scope at my other place that my grandfather used to build their first TV. Its someplace to start to learn.

I have a bunch of adapters that may have come with it like this.

Hmmm, I have to look in that stash, maybe there is a probe set up too.

Edit: Wow, managed to find the stash and yes, there is a banana to BNC adapter AND I found a BNC probe.
SO, it has a wire off the side on the probe. I imagine you ground that to the unit under test chassis while probing?

 

[DCinDC]

yep, clip that to the chassis.

now follow the flow thru the preamp to find where the noise starts. You are going to have to twist the v/cm dial to get an appropriate waveform as you go thru. The idea is to have the signal look the same on the scope face at each step. At one of those steps you will see the trash along with the test signal. The previous stage of the preamp is the culprit of the noise.

As you go along, you will get familiar with what waveforms look right.

When this is all done, sell the Mullards and buy a $50 scope. There's plenty of them out there.

 

[dr*audio]

She's an old Heathkit. Looks like the one on ToneLizard's page.

I picked it up dirt cheap along with a few other old test rigs.
I have another Heathkit o-scope at my other place that my grandfather used to build their first TV. Its someplace to start to learn.

I have a bunch of adapters that may have come with it like this.

Hmmm, I have to look in that stash, maybe there is a probe set up too.

Edit: Wow, managed to find the stash and yes, there is a banana to BNC adapter AND I found a BNC probe.
SO, it has a wire off the side on the probe. I imagine you ground that to the unit under test chassis while probing?

Yes, the wire goes to ground. Looks like you have what you need.

 

[Tichu]

Using a scope is like cheating.

There are a couple of simple checks you can make without one:
-> same hiss on both channels ?
-> does the High filter has any impact on the hiss ?

 

[blhagstrom]

Using a scope is like cheating.

I sure hope so. Figuring it out otherwise has been quite a guessing game.

There are a couple of simple checks you can make without one:
-> same hiss on both channels ?
-> does the High filter has any impact on the hiss ?

It's not really a hiss, that term implies consistant noise. This is much more random. Static is a better term I think. It seems to be the same on both channels.

High and Low filter ON seems to dampen it

 

[Tichu]

I have seen (or heard) some cases of random crackling, and they were caused by defective transistors in the preamp section.
In the case of the Eight, that would mean TR7001 through TR7008.

But the fact that both channels are similarly affected is disturbing, this leads to a power supply issue, or an epidemic failure among these transistors (all 2SC693).

Of course, if you can see the noise on your scope by probing in this area as DC explained above, this will point you directly to the culprit.