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Old 4th April 2013, 12:37 PM   #1
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Default Type of outputtransistors in a Classé Audio DR-3

Does anybody what type of TO3 outputtranssistors are used in a Class´s Audio DR-3 or maybe DR-2/DR-3B/DR-3 VHC.

Bias and DC offset are drifting in one channel so I can´t operate the unit properly.
Do have the service manual and I know bias must be 0.8 Volts measured over a one of the big 10 Watt resistors between input PCB and big blue PSU lytics and DC ±4mv max.
Suggestion handwritten on the servicemanual is to substitute both the inputtransistors MPSA53 TO92´s located at Q3 and Q4 to stabilize the fluctuating DC by MSP8599´s.

Any suggestions?

Did listen to this amp with reduced bias (130 Watts intake fom AC instead of the 500 Watts specified and it sounded so wonderful that I would love to keep it!

Thans in advance,

Robert
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File Type: pdf Classe DR-3 Service.pdf (31.3 KB, 88 views)
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Old 4th April 2013, 09:07 PM   #2
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Default Addendum

I just switched all the outputtransistors from one channel to the other but the problem remains in the same channel.
Whenever I connect a dummyload to the problem channel the bias immediately doubles and the DC Offset goes from ±15 mV to -50mV or even higher.
The other channel remains stable.

I can see that in the problem channel all outputtransistors haven been replaced by new pairs of 11015/16 highcurrent Darlingtons and the original 5 MPS8599 by MPSA53´s. So it seems they already tried to solve this problem and did not succeed or the problem was only temporarly solved.

Any help would be great?
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Old 4th April 2013, 09:33 PM   #3
Spiny is offline Spiny  United Kingdom
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do you have a scope, that sounds like the amp is oscillating when a load is attached. A scope would help to check.
Check the zorbel cap and resistor, C9 ,C5-7 and D3/4.
check the rail resistorsR33/34.
If it is oscillating a Miller? cap (100pf) across BC of Q10 may help
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Old 4th April 2013, 10:07 PM   #4
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Quote:
Originally Posted by Spiny View Post
do you have a scope, that sounds like the amp is oscillating when a load is attached. A scope would help to check.
Check the zorbel cap and resistor, C9 ,C5-7 and D3/4.
check the rail resistorsR33/34.
If it is oscillating a Miller? cap (100pf) across BC of Q10 may help
Thanks!
I´m not a technician and do not have a scope but all the caps you mentioned are those red and green WIMA´s.
The Zobel cap is C14 and the resistor is R31 inside the 2.5mH inductor probably?
The only way to check that resistor is to desolder the inductor I think.

With D3/4 you mean the Diodes CR-3 and CR-4.

On the attached image you can see that these have been replaced also in the problem channel.
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File Type: jpg Classe DR3 left and right frontends detail.jpg (512.2 KB, 127 views)
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Old 4th April 2013, 11:10 PM   #5
Telstar is offline Telstar  Italy
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Why don't you write to Classe' customer service? They have always been very helpful with me. The DR3 is a highly regarded amp and needs a proper repair to show his virtues
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Old 4th April 2013, 11:45 PM   #6
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when I repaired mine, I contacted Classe and they sold me all original parts and sent me a schematic and the whole nine yards! they are very nice!
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Old 5th April 2013, 11:49 AM   #7
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Originally Posted by Zero Cool View Post
when I repaired mine, I contacted Classe and they sold me all original parts and sent me a schematic and the whole nine yards! they are very nice!
Glad to here that!
If I only knew what to order
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Old 5th April 2013, 01:27 PM   #8
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Default Miller Cap and Scope

Quote:
Originally Posted by Spiny View Post
do you have a scope, that sounds like the amp is oscillating when a load is attached. A scope would help to check.
Check the zorbel cap and resistor, C9 ,C5-7 and D3/4.
check the rail resistorsR33/34.
If it is oscillating a Miller? cap (100pf) across BC of Q10 may help
I'm bidding on a Tektronix 2225 right now.

Miller cap like this:
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File Type: jpg Classe Audio DR-13 Schematic with Miller cap.jpg (170.7 KB, 88 views)

Last edited by RobertS61; 5th April 2013 at 01:37 PM. Reason: Wrong drawing sorry...
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Old 5th April 2013, 09:08 PM   #9
Spiny is offline Spiny  United Kingdom
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Thats the miller cap I was thinking off. It can tame some problems.

I would not be without mine, a very old Hameg HM812 probably 30 years old or more but still going strong. Maybe not as fancy as a DSO but shows oscillations. Good luck with the bidding.
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Old 5th April 2013, 09:15 PM   #10
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They are Darlington Output Transistors.
What an awfully unstable design!
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