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Old 26th May 2006, 05:34 AM   #1
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Default Trace Elliot V-type Bass Combo

Hello

Recently I bought from secondhand an Trace Elliot V-type Bass Combo (model 5001, 300W, build 1996). The combo is sounding verry good but it has one problem. It has verry loud 50Hz hum. It is disapear if I set the gain or volume at minimum. I have opened it and I saw that the power modul (bipolarbear) has been repaired - the output transistors were changed (now are BD249C and BD250C). From the service manual they must be 2SA and 2SD typ for 300W. Then i measured the DC offset and it was 0,5V. I tried to set it down but the minimum I was able to set it was 150mV.

Strange thing is that the power amp itself don't hum and the preamp is also quiet when is connected thrue DI.

Can somebody give me any advice how to repair this annoying hum?

Cheers
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Old 26th May 2006, 07:48 AM   #2
sreten is offline sreten  United Kingdom
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Hi,

Your hum is a problem with the preamp or the preamp wiring.

Start off by checking the ground connections at the input.
Also check the input sockets, you may need new ones.
(If the plating has worn through and your getting bad contacts)

/sreten.
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Old 26th May 2006, 09:07 AM   #3
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Quote:
Your hum is a problem with the preamp or the preamp wiring.
I agree but why when the amp is connected through the DI there is no hum. Also all the connections are made with snap in connectors and they look like they are all OK. The preamp is made around the two ecc83 tubes. The signal ground in the preamp is connected to the case (ground) through some 100nF decoupling caps. It is possible that this caps (if they are bad) cause the hum? Also in power amp the signal ground is decoupled from case ground with 100nF + 10k ohm resistor. I don't know but this 10k ohm resistor is not a bit high? I usualy use 10 to 100 ohm resistor for decoupling.

Quote:
Also check the input sockets, you may need new ones.
This will be done in no time!

Cheers
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Old 21st June 2010, 06:43 PM   #4
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Sorry to restart this old thread,

I'm trying to fix a Trace Elliot V-type 5001 power amp (Bass head)... for a friend, but I need a schematic... can anyone help.

The problem is in the solid state output stage, as signal is present at its input.

When power is connected to the power stage, the mains fuse blows - I suspect that the output needs new transistors - but it looks like somone have tried to fix it before.

Does anyone know why there are 2 heatsinks with 3 power devises on one and 2 power devises on the other??

Input is much appreciated

\\\Jens

Last edited by JensRasmussen; 21st June 2010 at 07:13 PM. Reason: Updated error info
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Old 22nd June 2010, 05:54 AM   #5
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The following is just speculation but I mention it anyway:

The reason for having two heatsinks might be because they don't insulate the transistors from the heatsinks and insulate the whole heatsinks instead.

Just sepculation but you will be on the safe side if you care about that.

Regards

Charles
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Old 22nd June 2010, 06:02 AM   #6
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Hi Charles,

I just got the thing working last night, and you are right...the two heatsinks are not isolated from the transistors collector, and thus each sits with either the positive or negative rail voltage.

Anyway I replaced the output transistors and the thing was alive.

The third transistor on the one heatsink seems to be part of the fan control circuit???

\\\Jens
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Old 7th July 2010, 12:10 PM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by JensRasmussen View Post
Hi Charles,

The third transistor on the one heatsink seems to be part of the fan control circuit???

\\\Jens
Hi,

Typically it would be there for thermal feedback for the biasing circuit.
It may be as you say the that the bias circuit also controls the fan speed.

/Sreten.
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