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Old 2nd April 2008, 06:46 PM   #21
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> OK I've uploaded the pics to Photobucket:

Very useful photos.

> A couple of things: I was right the first time, the bypass cap is
>10uf 50v, not 50uf.

This KN928 seems like a lower performance version of the KN734. Nothing wrong with that, but they probably saved a few nickels going with a smaller cap. I wouldn't worry about it for now. Maybe after it's working again, some of these items can get revisited.

>If I replace the 47k resistor, what power rating should I get?
>Is quality an issue here? (i.e. metal film vs carbon vs unobtainium
>$50 resistors from the back of the Absolute Sound?)

Assuming we're talking about R55, you'd be fine with a 1/2 watt. There's no real power moving through there. Even a 1/4 watt might do. In what way did the old one fail?

>Looking at the KN-734, I see what looks like 2 12ax7s, but that's
>actually one isn't it? The diagram shows each one as a triode,
>but this tube is a dual triode as I understand. Am I reading this
>right?

Yup. Two triodes inside one bottle. The schematics routinely show only half the tube if that is all that is being used for that piece of the circuit.

>I'm gonna owe you a case of beer (assuming you like beer that is)

Love the stuff.

I'm still trying to figure out where the problem lies. The only voltage that looks odd to me is the ~20 volt reading you see on the grid (pin 2) of 12AX7 #1 and #2. Can you recheck these? Measure the voltage with respect to the cathode (pin 3), not ground. The grid should be sitting about one half to one volt lower than the cathode. Make sure the amp's inputs are shorted, or the volume knob is turned all the way to the minimum setting while you're checking this. You don't want a signal running through the tube while you're trying to check idle voltages.
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Old 2nd April 2008, 07:10 PM   #22
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Check this and see if it is close.

Click the image to open in full size.

I can't see R48 in your photos, but it shouldn't be hard for you to find. C26 is the light brown ceramic disc cap sticking straight up in your photo. I'm assuming it's a .047 since I can't read its numbers.

I've been deliberately ignoring the two 12AX7 tubes in the far corner (#3 and #4, as you call them). One half of them is almost certainly used for the phono preamp. The other half is probably used to make up for losses in the bass and treble controls. If the stage that provides the gain for the tone controls is whacked, that might be responsible for your low volume. But first, I'm still curious as to why you see 20 volts where is should be closer to 60 (on the PI).
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Old 2nd April 2008, 09:18 PM   #23
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R48 is most likely a 47k part, same as R49.
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Old 2nd April 2008, 09:52 PM   #24
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R48 is most likely a 47k part, same as R49?

Try to figure out what voltage is on either side of R49. You already know the voltage on one side is 240V (at pin 1 of 'X7 #1/#2). I'd like to know how many volts are on the other side, or how many volts are dropped across it. Max plate current for the 'X7 is 1.2mA. There could be as much as 55 volts dropped across R49, but something tells me that tube isn't conducting as much as it should be.
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Old 2nd April 2008, 10:28 PM   #25
kirk57 is offline kirk57  United States
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OK I replaced R55 with a 1/4 watt resistor for now (I'll put a 1/2 watt in there when I get one). The old one was physically broken, the lead had come off the body. No apparent change in volume
with this change.

I measured the voltages on 12ax7-1, this time wrt pin 3
(previously I had done this wrt ground)
I think these are the readings you'd expect?

pin1 177v
pin2 -1.15

12ax7-2
pin1 182
pin2 -1.12

As far as C26...the ceramic disk cap between pin 2 and 6 has these markings:
RMC - This is the manufacturer I think?
.01 .01uf
Z5U is this tolerance?

57 volts are dropped across R49
I am not seeing R48..is it possible it's not there?

I tried using headphones to listen instead of these tiny monitors,
I can defininitely hum 60 hz hum through the phones (I don't think the speakers go that low!)

This means I need to replace the filter caps?
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Old 2nd April 2008, 10:40 PM   #26
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Here is another Knight 724 circuit:
http://wb9k.tripod.com/


Sorry for hijacking your thread, but please can you give me the correct values for the RC-circuits behind 6c4/EZ81?

I have a Knight 735 on my workbench...somebody has make the attempt to better the original circuit with much higher Cs....but as far as I can see without knowledge....and maybee some false connections.


So...for you, it could be a great learning excercise to take a look at the original circuit and post the original values...this will help others.

Regards, Carsten

PS: the 47k from the opt is located in a feedbackloop (for better performance)...1/2W is enough.
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Old 3rd April 2008, 09:41 AM   #27
kirk57 is offline kirk57  United States
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Carsten-

I checked and the RC values on my KN928 are exactly the same
as those shown on the stock power supply on the 724 page you provided a link to.

At this point I'm confused. It seems like the voltages and DC resistance of the filter caps are OK, but I'm hearing hum through
my headphones that might indicate the filter caps are bad.

Should I replace both cans? Looks like close to $70 in parts
from Antique Electronic supply (www.tubesandmore.com)
to do both.

Kirk
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Old 3rd April 2008, 10:30 AM   #28
korey is offline korey  United States
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Kirk57, of all the things you tried, i would most definitly replace the filter capacitor, but also check that 6CA4 tube. When you tried it on that Knight 680 tuner, that tuner does not draw the current that a power amplifier draws, so if you can, get another 6CA4 tube.
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Old 3rd April 2008, 02:36 PM   #29
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Replacing the filter cap probably isn't a bad idea, simply due to its age. But, seeing reasonable looking voltages in the power supply, output stage, phase inverter, and AF amp suggest to me there is still a problem somewhere else.

Swapping the 6CA4 again surely can't hurt anything. I certainly agree that a tuner probably doesn't put the same kind of load on the rectifier as a quad of hungry EL84. But again, the voltages seem to check out. If the 6CA4 were dying, wouldn't we see a lot of voltage drop? 320 volts on the plate sure seems right to me.

Kirk mentioned he has an old single channel oscilloscope. I'm wondering if it would be worthwhile to try using it to diagnose where the problem lies. I'm thinking the signal path through this amp goes something like this:

line in > select switch > 1/2 'X7 > tone > volume > AF > PI > EL84

The line in from a CD player, etc. goes through an input selector switch. Then it passes through a first stage audio frequency amplifier (gain stage) which is built out of half a 12AX7. Then it goes through the tone controls (bass & treble), then through the volume control. After that it goes into the next AF amp, the phase inverter, and finally the EL84 outputs.

We've already looked at the AF/PI/output stages and I didn't notice anything glaring. If you feed a signal into the input, then carefully choose places to stick your 'scope probe, you should be able to see the signal as it goes through the amp. It might be smart to check the point you intend to probe to see if there is any significant DC offset, or large AC swings before you stick your 'scope probe on it. No sense frying the scope, or yourself.

The input should be easy enough to scope out. I figure you'll have a waveform somewhere on the order of +/- a several tenths of a volt, depending on your input source. I'd think you'd have the same signal immediately after the input selector switch.

Finding the first gain stage (immediately before the tone controls) might be a little tricky. We haven't really looked for it yet - there might not be one there. Looking at the photo of the 'X7s in the corner (#3 and #4) it seems like one half of each triode is cap coupled straight into the other half. This suggests to me that the whole tube is used up for the phono pre-amp, and there are no triodes left over for a pre-tone control gain stage.

If there is one, then it will probably have a ceramic disc cap coming off either pin 1 or 6, which will then lead into the tone control circuitry. You don't really want to stick the scope probe directly on pin 1 or 6, as there is probably 120 volts DC there. On the other side of that cap it should be nearly 0 volts DC, but with an AC signal on it. That's where you want to put the scope. This signal should be greater in amplitude than the original input signal.

After the tone controls, the signal should be smaller in magnitude than it was going in. After the volume control, the signal might be anywhere from equal (at maximum clockwise rotation of the volume knob) to nearly zero (at minimum clockwise rotation).

You should be able to safely probe at pin 7 of 12AX7 #1 and #2. This signal is the input to this tube, and should be +/- 1.0 volt. In fact, this is probably the first place you should starting probing. If you can't get a signal approaching +/- at least a tenth of a volt or so, I'm going to wonder why. You can also check the signal at pin 2 (or pin 1 on the oddball) of the EL84 tubes. This signal is going to be several times greater than it was going into the AF/PI stage.

Be careful probing. I'm going to stare at the photo of #3 and #4 tubes and see if I can figure out how they are connected. I've updated my drawing of the AF/PI stage slightly - can you spot any mistakes? It's hard for me to see where all the pieces go, since some parts go underneath others.

Click the image to open in full size.
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Old 3rd April 2008, 03:16 PM   #30
kirk57 is offline kirk57  United States
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Ty-

OK I'll get the scope out.

I hope this doesn't lead to a "need help to repair vintage B&K scope" thread....

Kirk
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