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Old 14th December 2009, 02:14 AM   #1
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Default Counterpoint SA-1000 buzzing problem

hey ya boys
since you've been talking counterpoint thought I'd throw out my needs-
a friends' sa-1000 has a nasty buzz coming out of the main outputs but not the tape outs-
I've replaced all the electrolytics, the pre-amp tube(vs the 12ax phono stage)
and the volume control and still have the buzz-humph
I am thinking that the two transistors and the buffer would be next but I don't have the part numbers or the voltages-
anyone help out with some advice or a parts list/schematic?
thanks for your time
rob
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Old 14th December 2009, 02:48 PM   #2
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Hi Rob;

They are INFINEON BSS101. I believe that there is some substitution information on the Altavista Audio website (a Zetex part I believe)...from their website...

Failed Output Stage
The itty-bitty TO-92 Siemens BSS101 MOSFETs used as source followers after the line stage tube fail on rare occasion. If you cannot find this exact part, then the Zetex ZVN4424A is a good replacement (TO-92, N-Channel, 240V). I recommend replacing the right and left channel MOSFETs at the same time to keep then two channels identical. DigiKey Corp. | Electronic Components Distributor | United States Home Page carries Zetex parts.

Good luck

Matt
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Old 14th December 2009, 06:49 PM   #3
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Matt
thanks so much for your input-much appreciated.
quick question-
am I focussing on the wrong thing (transistors and mosfets)if music passes through, its just overladen with a very healthy dose of buzzzzzzzzz/hummmmmmm-I've measured about 20 millivolt dc on booth channels
another respondy suggested one of grounds on the volume control could be compromised and as I've poke around the volume does seem to be an influence- sometimes when you touch it the humm will change-not go away just change- though I've jumped the phono post ground to the pot and other than shorting it out it didn't seem to fix anything
your input is appreciated
rob
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Old 22nd January 2010, 03:59 AM   #4
anatech is offline anatech  Canada
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Hi Rob,
My first question is ... have you checked the voltage regulator for proper operation?
Have you checked the heater power supply for regulator drop outs?

Swapping mosfets and capacitors are not the way you're going to solve these issues. You need to use your mind before you engage your soldering iron.

Let's think about touching the volume control. You do know the case and shaft are grounded, do you agree? If this is the case, what would cause the hum to appear if you touch the case? An innocent question. Does the volume control feel solid or sloppy? Or, is it that you are touching a location that normally will have signal on it?

It is possible that the power transformer has problems as well. To check, unplug the preamp and undo the bolts securing the power transformer to the chassis and set it on top of a compliant surface (like a folded paper towel). Turn the unit on and take some AC measurements from the transformer core to ground (on your scope would be fine as a reference). Note whether your hum got better or worse.

Another clue is the nature of the hum or buzz. You indicated that it was a buzz, and that makes me think of a power supply problem rather than induced hum. Hum would be 60 Hz and a buzz is more like 120 Hz with a lot of higher harmonic content (a rectified and poorly filtered power supply).

Can you supply pictures of the chassis in it's present state?

Also, I split your posts to their own thread in the tube forum. This is a totally different device as it's in the solid state forum right now.

-Chris
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Old 22nd January 2010, 01:06 PM   #5
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Default counterpoint fix

Anatec
thank you so much for your input and consideration.
Your thoughts on putting down the soldering iron and turning on the brain were "spot on". It is easier to thrash around than to think and observe.
Initially I replaced the electrolytics and the volume control and regrounded the balance pot with no abatement of the buzz. Humph
When I finally got hold of the schematic(thank you RM) and the voltage references for the test points I found the high voltage zenners were blown. I replaced those as well as the transitor. Put it all back together and now the low voltage for the timer ic was out. So I checked the voltage regulator and found it was grounding out on the case(internal socket was worn and allowed the screws to touch the chassis).Made rubber grommets and got my low voltages back. Buzz gone!!
Finally noticed that the output was low in one channel and the voltages across the two 12AX7 where fairly divergent(20-30% off reference and each other).Replaced those and the voltages returned to within spec.
I most note that Mr Elliot responded to at least a dozen questions and was very helpful in the process. I have read that he would repair but not help and this makes sense from a time and money point of view. His time is worth something and he is not a forum contributor.Yet he obviously showed an interest in his product performing properly and should be commended for his efforts. Thank you Michael.
That's my story and I'm sticking to it!
Stay warm and remember to keep one hand in your pocket when working on tube electronics
rob
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Old 23rd January 2010, 12:36 AM   #6
anatech is offline anatech  Canada
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Hi Rob,
Congratulations on a job almost well done! (one problem that I'll get to)

As far as Mike being unwilling to help someone, that's not true unless that person hits Mike with an attitude. Michael and I have exchanged email several times, and he has always been helpful. If you look into those reports, I think you will find that there was more to the story. He and I don't agree on everything to be sure, but that has no bearing on my impression of him as a person.

Quote:
Made rubber grommets and got my low voltages back. Buzz gone!!
Now for your problem. When you repaired the low voltage power supply, you said you used rubber. This uses an LM317 (data sheet attached -file too large), and the case is the regulated voltage output terminal. Look on National Semiconductor's site and search for LM317 or LM117. So when this was shorted, you were drawing excessive current. You should certainly just replace the filter capacitor (1,000 uF 25 VDC). As well, the socket for the LM317 needs to be replaced. It normally has a boss that fits into the hole in the chassis. Yours is either broken off, or squashed between the socket and chassis. Rubber will degrade over time and faster when very warm. For the cost of an LM317 and the quality of your preamp, just replace that as well. It's been running into thermal shutdown or current limit - take your pick. Either way, well beyond it's designed parameters. It may be or become noisy.

Now, as for the plate voltages being different by 20 ~ 30 %, that can be normal depending on the tubes you are using. With the original National brand - normal. They were all over the place. If you are using Sovtek, they should be close. If you are using Electroharmonix, I would expect them to be pretty close to each other as well. I would highly recommend the latter brand. What is very important to note is that the bias is adjusted for minimum distortion. That also means that "tube rolling" isn't a casual thing. Each tube must be set up individually, which is a drag. Did you set the bias up this way? If not, your unit is not meeting it's specifications for distortion. The bias setting makes a large difference on sound quality. No, you can not set this by ear either.

So, best of luck with your working preamp. Do go back an look after those last few things. Order the parts and do it soon. It shouldn't take you very much time to do this work. While I'm thinking of it, you should probably replace the bridge rectifier as well. I'm really glad you didn't lose your power transformer.

-Chris
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