Lafayette LA215 voltage problem !

TJ · 10th January 2011, 03:44 AM

hi , this receiver drive me nut !! recently i did replace some capacitor due it age . as u can see from the circuit diagram the DC voltage to the opt transformer should be 255v . but now it is 345+v ... . as i know 130vac voltage double ~ 365v . the dc voltage at the tuner section i test it is ok same as the cuircut diagram . the weird part is the pre power amp dc voltage it show 355v dc on my multimeter . my el84 turn red . anyone can explain how they reduce the dc voltage significantly and which component i should replace ? thk

singa · 10th January 2011, 06:54 AM

Hi, This is a complex circuit so it would help if you give details of the
changes like component reference number in schematic and the values and
voltages of the caps.Off hand it appears the B+ of the power amp have
increased leading to higher bias at the affected tubes meaning they have
been over biased with too much current making them appear redder/hotter so it is not good to operate this amp before rectifing or correcting problem.This
may kill/damage the tubes.
Ps you should not modify the circuit as it worked originally.
Regards.

Tom Bavis · 10th January 2011, 02:37 PM

Is -21V present for preamp tube heaters? This is subtracted from B+ - also this the source for fixed bias on output tubes.

TJ · 11th January 2011, 12:17 AM

hi , thk for the reply . singa , i didnt mod the receiver . just change some cap only . tom , yes the pre amp heater voltage is -21 and at the G point is 6.3v . the vdc still remain 330+vdc . way to high for my el84 .

DF96 · 11th January 2011, 11:29 AM

Check R303, should be 220ohms. Have you accidentally wired it out of the circuit when you replaced the caps?

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10th January 2011, 06:54 AM	#2
singa diyAudio Member Join Date: Apr 2004 Location: near	Hi, This is a complex circuit so it would help if you give details of the changes like component reference number in schematic and the values and voltages of the caps.Off hand it appears the B+ of the power amp have increased leading to higher bias at the affected tubes meaning they have been over biased with too much current making them appear redder/hotter so it is not good to operate this amp before rectifing or correcting problem.This may kill/damage the tubes. Ps you should not modify the circuit as it worked originally. Regards.

10th January 2011, 02:37 PM	#3
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	Is -21V present for preamp tube heaters? This is subtracted from B+ - also this the source for fixed bias on output tubes.

11th January 2011, 12:17 AM	#4
TJ diyAudio Member Join Date: Sep 2001 Location: UK	hi , thk for the reply . singa , i didnt mod the receiver . just change some cap only . tom , yes the pre amp heater voltage is -21 and at the G point is 6.3v . the vdc still remain 330+vdc . way to high for my el84 .

11th January 2011, 11:29 AM	#5
DF96 diyAudio Member Join Date: May 2007	Check R303, should be 220ohms. Have you accidentally wired it out of the circuit when you replaced the caps?