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Old 25th July 2012, 10:24 PM   #221
spurlte is offline spurlte  United States
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Quote:
Originally Posted by Kaplaars View Post
Sorry for kicking the topic, but I could not edit the post above.

Well guys, I blamed the clips, because when changing them the problems were over. But, believe it or not, but 4 minutes after I sended the post above, my oscilloscope went blank and loads of smoke came out of it. It was pretty impressive . Pfew, that stinks. Took the oscilloscope outside. It kept smoking for a wile.

I had an earlier discussion with David (Spurtle) about the so called 'rattle capacitor'. These oscilloscopes have one too. They are from Rifa. Altough quality is not that bad, after 30 years they go short circuit (X2 capacitor my a**). Here is a picture of it, I would not even imagine what would have been happened if I don't was in my room:

Click the image to open in full size.

(blue one is the replacement)

Click the image to open in full size.

Unbelievable how much smoke can come out of such a little capacitor. Replaced it, cleaned the print and decided to clean the potentiometers and switches with some Tuner 600 spray. This is very good spray btw, leaves absolutely no residue. Prayed that it would work again, because Kaplaars is a bit broke nowadays And IT DID!!!

The oscilloscope had some stability problems earlier with triggering and al sorts of strange distortions. Thought that was due to the clips, but it turned out it was actually caused by the oscilloscope itselve! So another note to myself, when testing equipment NEVER leave the room.
Amazing, in it self, the X-CAP lasted 30 years. Will you have to recalibrate the oscilloscope?
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Old 25th July 2012, 11:26 PM   #222
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Quote:
Originally Posted by spurlte View Post
Amazing, in it self, the X-CAP lasted 30 years. Will you have to recalibrate the oscilloscope?
Hi David,

To be honest, I don't know how I have to calibrate it. I am afraid I don't have adequate equipment to do that. I think that is not nessesairy because the X-cap does not have a direct influence on measurements. Only indirect for example when it is broken it can cause the scope to give 'strange' measurements, because it leaks current. I also noticed the last few days that when I switched a light on or something like that, it influenced the measured values a bit. With the new cap, it is like it was before; perfect

It seems measurements are now as it has to be, nice 10kHz square sine wave at 40 VAC RMS, no oscillaton to see. Very happy with that, Jim did an excellent job with creating this boards I have to be carefull, the amp amplifies even 50 kHz signals when I apply them to it. I know, have to measure very short because of possible oscillations that can occur when injecting signals of high frequency.
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Old 26th July 2012, 03:31 PM   #223
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Little update, now my scope is entirely fixed up and measurements are realy good (even 20 kHz ssquare wave at max power no distortion at all!!!), it is time to deal with temperature.

I felt realy stupid yesterday, turns out my DVM has an option to measure temperature with a probe (max 3 degrees deviation)! So removed one of the fans and insterted the probe into the outtake of cooling tunnel. Turns out temperature is about 68 degrees inside @ 444 mV bias, ambient temperature of 32 degrees (for Dutch standards it gets very hot in my room when the sun is shining) and 1,5 hour power on:

Click the image to open in full size.

It turns out my spreadsheet (http://michaelq.home.xs4all.nl/Proje...0derating.xlsx) does a very good prediction! Calculated that C/W-value is slightly lower, it is about 0,21 C/W.

But this way I could not measure accurately how hot the TO-3 devices became. Also compairing the intake and outtake temperatures was a bit more difficult. I got an offer I could not refuse, some one offered me an IR-thermometer for 10 euro's! So I bought it .

At 444 mV bias, one fan at 50% I measurethis temperature at the intake (device has 2 degrees deviation):
Click the image to open in full size.

And this temperature at the outtake:
Click the image to open in full size.

AVwerk and Klaas were absolutely right, difference between in and outtake in the current setup is 18 degrees. 75 degrees should not harm the TO-3's when I calculate values. It is well within it's SOA. What dou you think guys? Will this shorten lifespan of the TO-3 drasticly? It has to work for at least 50 years No kiddin, but I want that it will be an relieable amplifier.

As far as I am considered I can an do two things; separate the cooling tunnels. Going full bias would not be an problem then, or stick to this design and go for lower bias.

I shall do further measurements @ .625 mV bias. Temperatures should get a bit lower when adding fan two and when temperatures drop a bit in here.
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Old 26th July 2012, 07:15 PM   #224
lgreen is offline lgreen  United States
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Why don't you just use bigger, quieter fans on the in/out? Seems like that would be easier and cheaper.
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Old 26th July 2012, 09:57 PM   #225
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Hi lgreen,

I was thinking about that, but... I can't mount a bigger fan to the cooling tunnel. I could buy some fan convertors, but than the heatsink can't fit in the cabinet. When I mount a bigger fan some of the airflow will not go through the tunnel, but go via the sides instead, so I think air pressure will be lower. But I haven't did that experiment, will do, maybe it will be better. And..... bigger fans are quieter

I've biassed it right now to 500 mV. Ambient temperature (measured at the TO-3 sockets with my new toy) is 35.5 degrees Celsius. With fans at full speed and mounted the two of them I get 70.2 degrees Celsius (=158.36 F) at the outtake and 53.8 C (=128.84 F) at the intake. The second fan did improve temperature a bit more. With one fan running at 50% I got @ 440mV bias 71.2 degrees Celsius at the outtake, with both fans running at 50% speed I got 68.4 C at the outtake.
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Old 27th July 2012, 12:34 AM   #226
lgreen is offline lgreen  United States
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not sure if that is air temp or metal temp? anyway i bet you can monkey with it sufficiently to have 2 fans relatively silent with nice flow. if you are well below or at 100c at junction than dont sweat it. (hah!..."sweat") TO3 are good w heat so nice design choice. will heat your room a degree or 2 per hour. nothing is perfect so you may have to accept potential output stage failure at 15 years out vs. 25 years with optimal cooling.

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Old 27th July 2012, 12:53 AM   #227
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Has anyone on here tried adjusting bias on his KSA-50 or 100 using a distortion analyzer at 10 khz while looking at the distortion content outpput on a scope? This is the way many an amp used to be biased (within limits of course). Just curious... I have the equipment...but not the time to do it.

Mark
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Old 27th July 2012, 12:53 PM   #228
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Hi lgreen,

That is metal temp, straight on the top of the TO-3's. The heatsink itself gets ~3 degrees less warm, the air coming out of the tunnel cooler is about 42 degrees C. Will certainly try it. I have some big fans in my computer, but I need the computer and the computer needs the fans, so have to buy ones instead

Heeeeeyyyyyy that is good news, tested it yesterday at .625 mV. Got about 80 degrees C on top of the TO-3's at outtake. Intake was about 63 degrees. That would mean that even full biassed there is not a real problem. Nice to hear. Dean was right again, temperature is actually lower than I thought. I've red in 'designing audio power amplifiers' that you could put your finger on a 60 degrees heatsink for 30 seconds without feeling pain. Well I have a high pain tolerance, but at 60 degrees my fingers did realy hurt even for 2 seconds. The quote from mister Pass which Dean quoted is far more correct.

@ Mark, I haven't got a distortion analyser A good one is a bit too expensive for my budget. I was wondering if I could make cross over distortion visible with a scope and a signal generator. How would that look like? Tried it, but I did not see real changes when looking at the sinewave at the scope while lowering bias.
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Old 27th July 2012, 02:48 PM   #229
spurlte is offline spurlte  United States
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Default Image of PCB

nechi,
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Old 27th July 2012, 03:03 PM   #230
spurlte is offline spurlte  United States
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Default Zenith Clock Radio

Kaplaars, here is the Zenith Clock Radio
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