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 on: April 19, 2017, 06:36:34 pm 
Started by J. Mike Perkins - Last post by uwe ret
These U87us do look legitimate and in surprisingly decent cosmetic condition. The very early series (not really 'prototype') did not have the internal battery option, thus no meter either.

 on: April 19, 2017, 06:34:56 pm 
Started by J. Mike Perkins - Last post by klaus
Aside of the capsules which are not shown, the amps look authentic.

 on: April 19, 2017, 05:38:08 pm 
Started by J. Mike Perkins - Last post by J. Mike Perkins
A seller on ebay is offering what they claim to be Neumann U87 prototypes #032 and #035.  Interesting photos show a different circuit layout than anything I have ever seen on a U87.  Comments anyone?  Price is high, but these look like they would be a great addition to someone's Neumann mic museum. 


 on: April 19, 2017, 05:02:18 pm 
Started by Mikeyod - Last post by Mikeyod
That didn't do it. I don't have a vari-speed hooked up to the machine. But, I wonder on the 1.080.372 card schematic it shows where the vari-speed hooks in. Could any of those components cause a pseudo vari-speed, if it were to begin to fail. Like C24,C11, R58, R59. Maybe R28 in series with the pot R27 ?

 on: April 19, 2017, 04:25:15 pm 
Started by Mikeyod - Last post by Mikeyod
changed Q5 & Q6  no change and wave looked the same on the scope
Will now do Q7,8,9.

 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.

 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.

 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 !

 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?

many thanks.

 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

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