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 21 
 on: April 19, 2017, 03:32:54 pm 
Started by Mikeyod - Last post by Mikeyod
Got the parts today soldered in the BC140 machine slowed down. Put in BUV23 transistor machine slowed down.

O.K. things I've done on .372 card
All op amps (4) Changed for new 1) 1455 3)LM301AN
Electrolytic Cap 22u 40v (1)
D9 Diode bridge
BC140 transistor
Checked with magnifier for obviously burned/bad components.

Outside Circuit board

New Phase shift 8uf capacitor
New BUV23 transistor (in place of RCA411s)
Had capstan motor rebuilt .

What about rectifier card 1.080.355 wouldn't a problem on this card show up at the test points
on the stabilizer card 1.080.370 ? Those voltages are all good.

Going to change Q5 Q6 on .372 capstan control card.



 22 
 on: April 19, 2017, 12:17:18 pm 
Started by zerofly - Last post by klaus
Without intention to brag, I've seen thousands, but never seen one like this.
Plus: "attractive price? No, I would not buy it.

 23 
 on: April 19, 2017, 10:12:32 am 
Started by WolfbrotherB - Last post by Mikeyod
I would call KRK first 1-800-444-2766 looks like there part of Gibson.
Or yes open it up !

 24 
 on: April 19, 2017, 06:52:28 am 
Started by zerofly - Last post by zerofly
Hello Klaus, good people,

I have stumbled upon these tubes at a fair price, but, they look a little weird in aspect compared to what i'm used to.
They are clear, not black and the logo is horizontal.
i have attached some pictures.

Can these be fake? real?
Please!

many thanks.

 25 
 on: April 18, 2017, 11:39:47 pm 
Started by RuudNL - Last post by David Satz
The frequency response of a U 87 amplifier is tailored for the expected uses of the microphone, and according to Neumann's specifications (in copi0081.pdf from the Neumann "Infopool"), in order to sound right the U 87A amplifier (via its measurement input) should measure about -3 dB at 40 Hz and about -4 dB at 16 kHz, both relative to the gain at 1 kHz.

I'm sorry that I don't have full information handy about the exact way in which this measurement should be carried out, but if I recall correctly, the source impedance of the test signals can be critical, and a substitute for the capsule's capacitance must be in place. I suggest making extra sure that you're measuring the amplifier's response in the correct way before you base any circuit modifications on your results.

--best regards

 26 
 on: April 18, 2017, 05:58:48 pm 
Started by Mikeyod - Last post by Mikeyod
After yesterday I tried not to get excited. Good thing today my A80 slowed down so often I could not get anything done. By Thursday I should have both of the transistors in question, in hand.
  I noted that the BC140 on the card .372 capstan control was very hot. Normally it is warm/hot to the touch. I had to take my finger off of it today.
      I am unable to test the bridge under load. I did the normal diode test which I expected it to pass and it did.

 27 
 on: April 18, 2017, 05:00:56 pm 
Started by WolfbrotherB - Last post by WolfbrotherB
I've got a pair of old passive KRK monitors, I believe they are the old rokit series with 6" drivers. But one of the drivers has broken and I'm struggling to find out what driver I need to get to replace it. Is my best bet going to be to just take the monitor apart to have a look at the driver itself?

Cheers

 28 
 on: April 17, 2017, 09:25:35 pm 
Started by Mikeyod - Last post by radardoug
I've got the board on the way so I can test the diode bridge. Did you have anyway to test the diode bridge? Best way is a constant current supply, put it across a diode, push the current up to a good value, and see if the voltage across it increases.

 29 
 on: April 17, 2017, 06:21:23 pm 
Started by Mikeyod - Last post by Mikeyod
Got the bridges today soldered one in around 10:30 a.m.. Worked all day with no slow downs.
But, since that happened when I put in the new contactor, I'll believe it after about a year without any slowdowns  : )     I have the BC140 and BUV23 transistors on the way. Eventually we'll get it.

 30 
 on: April 16, 2017, 11:46:39 pm 
Started by RuudNL - Last post by klaus
The "thin" sound of your U87ai is more than likely the result of your specific K870 capsule (tight membrane tension), not the DC-filter coupling capacitor which is also serves as input point for calibration tones.

Of course, you increase the measured low end at the mic's output when you increase the value of C104 (corresponding to C5 in U87) through which the calibration tone is fed. But it affects only calibration audio going through this cap. The mic's audio signal from the capsule is not processed through this cap.

In mics with no calibration input, Neumann and others usually use an even smaller DC filter cap (typically 0.01mfd or even lower) for this function, which adds no low end attenuation of the mic's audio.

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