V-fet Repair of a WEGA V4810 solid state heaven - Page 2 - diyAudio
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Old 29th August 2009, 11:04 AM   #11
Helmuth is offline Helmuth  Netherlands
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When step 1 is succesfull. so the 20V ,97V, +/-90V and +/-45V are OK we can go to the next step.


Step 2

We can test the end-stage with out V-fets attached to it. The driver stage of the V-fet amp is capable to drive the headphone output.

By this test we can see the if the pre-end-stage is working properly with out blowing the precious V-fets.

I attach the input connector of the preamp on the end-stage board and the power-supply connector of the left-channel.
Now you can check only the left stage voltages. I switch the preamp to phono 1 and use my finger to introduce hum to the left cinch connector of phono 1.
When this works properly you can also connect the power-supply of the right channel to the end stage. End test the voltages and listen if it amplifies normal. I did use my finger to make input noise, you can also connect a cd-player to the amp as source and listen music through the headphone output.

I made a custom made tool to clean the connector leads. It is a old monkey-wrench ( I do not know this is the right name) it is harden steel. It is cut in a triangle shape to scrape the surface of the leads of the connector.

I also cleaned TO3 foot for the V-fet it was polluted with coolant pasta. It would be a catastrophe when the gate would not make contact with the fitting. I also measured the contact resistance after fitting a V-Fet to it, before switching on the new fets.

The tool.

Click the image to open in full size.
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Old 7th September 2009, 07:53 PM   #12
Helmuth is offline Helmuth  Netherlands
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After finishing step 2 we can move to the last step and test with the V-fet.

Step 3

To prevent the fets from a possible dead, Piet told me he used resistors in series with the main voltage.

I thought a while, power resistors are expensive. Then I had a idea to use lamp bulbs as power resistor. I bought two Philips 500W halogen lamps for 8 Euro.

The behaviour of a light bulb is like a PTC the resistance can increase 10 times after glowing up. In this case the lamp had a resistance of 7.5Ohm in cold state. And when I shorted the 45+45 Volt with the two lamps in series the current was about 900mA.
Perfect to protect the V-fet.

I used a small piece off MDF and a angled aluminium profile and 12 wood screws to fit the lamps. A nice advantage was that the profile worked like a spring when I screw the lamp between it.
Click the image to open in full size.


To start I only fit 1 2sj18 and 1 2sk60 on the left channel to start with.
Then I bais the circuit to 25mV, test the voltage and listen to it.

*note when de potentiometer is in the left corner it is at his minimum value, the 25mV is measured over one 0.33Ohm resistor at the output.


Then I fit the second one on the left channel. Then I bais the left channel to 50mV.

If this works well I repeat this procedure for the right channel.

After fitting all 8 V-fets it was possible to listen to music on the connected speakers, YESS it worked. My occupation is electronics and i have repaired many switching power supplies in my career till now. But it was a bit a gamble to buy this amp and try to buy Nos V-fets in china find replacement transistors. It could turnout in a disaster and loose the investment.

You know what happens when your wife finds out.


Regards, Helmuth
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Old 10th September 2009, 02:47 PM   #13
Helmuth is offline Helmuth  Netherlands
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Here a picture of the leads to disconnect so you can put a lamp in series with it.
One lamp in series with the brown wire, and one in series with the violet wire.
(There is also a third violet wire, connected to speaker protection do not disconnect that one other wise the speaker protection switches on.)

The halogen lamp trick works with every end stage repair.

Click the image to open in full size.

Helmuth wishes you success with repairing.
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Old 12th September 2009, 03:35 PM   #14
Helmuth is offline Helmuth  Netherlands
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After fixing the endstage there was a bop noise and a little hum on the left channel.
After switching the prestage left en right on the back. I could find out that it was the prestage right channel who caused the problem.

I hoped after replacing al capacitors the amp would sound right. This was not the case so there was something broken.
I began checking the voltages of the preamp seemed ok, after switching on with the last stage of the preamp the bop still was not gone. So my eye fell on the mute FET , (Q206, Q256) Q206 acted as a resistor. I replaced them both with the 2n3819 it is only a switch, and saved the intact 2sk23a as a spare for later.

http://www.datasheetcatalog.org/data...shay/70238.pdf


Click the image to open in full size.


new list

2SA705 by BC560B the european low noise type PNP.
2sc634a by BC546B
2sa677 by BC556B

2sa639s by 2SB648A a nice toshiba alternative also obsolete
2sc926a by 2SD668A a nice toshiba alternative also obsolete

2sk23a by 2n3819 a good replacement for the mute fets not tested in the amplifier part.

Now the amp is working and sounding fine and warm.

Regards Helmuth
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Old 12th September 2009, 03:47 PM   #15
Helmuth is offline Helmuth  Netherlands
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The Tuner had a Little Service.

The light bulbs where broken and I made hardwood side panelsfor the wega set.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

The light bulbs are available at every car parsts shop because they also fit Japanese cars.

Regards Helmuth
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Old 13th September 2009, 05:04 PM   #16
Helmuth is offline Helmuth  Netherlands
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The V-fet is fully conducting current when there is no negative gate voltage. So in this design it is important to check that path to keep the amp healthy.

There is a lot said about the double diodes.

There are more threats to the V-fet than the doublediode's,

1)The bias potentiometer when the middle contact of the potentiometer fails to make contact=disaster.
I advise a 1K 10 turn potentiometer to replace the 2k5 one turn. You will have a smooth adjusting pot.
And you have to let the amp to heat up at normal ambient temperature adjust to the right BIAS.

2)The connectors are a weak point the +76V when they fail to make contact= disaster.

Like one previous poster you can skip the connectors and make a sturdy wire wrap connection.

3)Also the gate has to make contact with the fitting when ONE fails disaster. So clean the contact and make sure with a resistance measurement that it makes contact.
Click the image to open in full size.

Regards, Helmuth
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Old 13th September 2009, 06:03 PM   #17
Helmuth is offline Helmuth  Netherlands
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I cleaned the contacts, some remove the connector and make a sturdy wire wrap.

This is important to be sure of good contact of the +76V
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Old 20th September 2009, 04:12 PM   #18
Helmuth is offline Helmuth  Netherlands
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The set is playing for a while now. Low frequency like a tube amp and a high response without harshness. Nice sound.

I made some pictures with my home made Bankirai side panels.

Click the image to open in full size.

Click the image to open in full size.

Regards Helmuth
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Old 24th September 2009, 01:48 PM   #19
Bakmeel is offline Bakmeel  Netherlands
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Hey Hemuth, that looks like a job well done! I hope your amps are sounding well now... It seems they do ;-)

I especially like the good craftsmanship on the wooden side panels. How did you get the high gloss and keeping the dust away while the paint dries? I always get small specs of dust when trying to do a high-gloss paint job.
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Old 24th September 2009, 02:16 PM   #20
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wow very nice
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