V-fet Repair of a WEGA V4810 solid state heaven
After some speaker posts.
I have a nice vintage solid state project!
To be more precise I wanted a nice vintage set because they look so good. What is more attractive as an vintage amp then one with a J-fet preamp and V-fet end stage.
A V-fet is on the solid-state area the best match to a Triode Tube. When you look to the amplifying characteristics. So the damping factor of a solid state and sound of a tube.
All V-fets are defect are defective except one 2sk60.:bawling: Picture of the endstage and defect V-fets.
This amplifier is really beautifull..congratulations.... i hope you fix it
Despite your amplifier has a very good look, the yellow motorcicle looks even better!
I downloaded a unreadable schematic at http://www.hifiengine.com/manuals/sony/ta-5650.shtml
after that I got a readable one from Jerry.
Low noise J-fet preamp in combination with bipolar:
Bipolar and very special V-Fet Endstage end power supply.
A white paper on mods on the V-fet amp I found on karma. It is from the belgium service cennter of Sony. Very important the lower bias-current to prevent a temperature avalanche effect.
I introduced the mods on my amp.
The end stage was unsuccessfully repaired, may be more then one attempt, that I could see.
The result all V-fets defective except 1 2sk60. You can do a diode test to see it is defective. So open without negative gate voltage the V-fet measures as a short. Most of the transistors where not original ones any more.
I tried to become them by brokers but they won't sell part below a quantity of 50 pieces or more.
So I searched for alternatives for my self. And use common European alternatives. It seemed to me that in America it is easier to find Japanese transistors.
I replaced the missing:
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
I also replaced al capacitors by new ones. Electrolithic capasitors dry out inside after a long period. This affect increases by high environment temperature. So the capasitors of the endstage will be faster worn then the ones on the preamp board.
I advise to replace them by 105 degrees types they are designed to have long lifetime at high temperature.
They have high voltage types in the design to prevent leakage current and sound and losses produced by this current. So today capacitors are smaller you can even choose types with even higher voltage rating.
I will replace some capacitors by Wima MKS types. They have lower absorbent losses then eletrolithic capacitors, almost as good as a MKP capacitor only much smaller. So they fit in the place of the old electrolithic types. And their have an almost unlimited live expectancy.
The proportionality constant relating each material's capacitance enhancement over that of a vacuum is known as its "dielectric constant."
In A.C. applications, where signal handling is involved, factors which affect the rates of both charging/discharging become key issues. Even though dielectrics with larger constants allow smaller size/capacitance devices, the properties of such dielectrics contribute deleteriously to audio signal processing.
The residual charge from dielectric relaxation is known as the "dielectric absorption." When an audio signal is passed through a capacitor the dielectric absorption prevents full charging and discharging of the capacitor at the frequency of the alternating current signal. When the signal reverses the charging on the plates the dielectric absorption presents a lagging current of the former polarity, a hysteresis effect results. This effect becomes more acute with increasing frequency.
Obviously now, not all dielectrics are equal. In audio applications it is desirable to seek the insulating material with the lowest practical dielectric absorption; hence, lowest dielectric constant, barring size and economics. Dielectric materials can be classified based on their relative polarity/polarizability properties, which the dielectric constants and dielectric absorptivities parallel.
What follows is a qualitative categorization of dielectric materials in decreasing polarity/polarizability based on chemical structure considerations (Dielectric constant data "K" given when available):
I. Metal oxide corrosion layers (electrolytic capacitors):
1) Tantalum oxide (K = 11)
2) Aluminum oxide (K = 7)
Both consist of polar metal oxide bonds possessing large permanent dipole moments, polarizability factors are negligible.
II. Ceramics and Glasses:
1) Ceramics - typically alumina or aluminosilicates (K = 4.5 - thousands)
2) Glasses - typically borosilicate (K = 4-8.5)
Similarly, the polar inorganic oxide bonds in these materials have large permanent dipole moments.
1) Mica (most common) - an alkali metal aluminosilicate, hydrate (K = 6.5 - 8.7)
Same as II.
A. Polymer films - functionally linked - ranked in order of decreasing functional linkage polarity (brackets "[ ]" indicate guess based on functional group polarity):
1) Polyesters (ex. Mylar) - ester (K = 3.2 - 4.3)
2) [Kapton - ether and imide]
3) Polyamides (ex. Nylon) - amide (K = 3.14 -3.75)
4) Polycarbonate - carbonate (K = 2.9)
5) [PEEK - ether and ketone]
6) [Poly(phenylene oxide) - PPO - ether]
7) [Poly(phenylene sulfide) - PPS- thioether]
The members of the above list can essentially be ranked based on polarity considerations alone, though polarizability considerations are significant for the latter members of the list.
B. Polymer films - carbon chain backbone - ranked in order of decreasing attached-group polarity/polarizability:
1) Poly(vinyl chloride) - PVC - chloro-substituted (K = 3.3 - 4.55)
2) Poly(chlorotrifluoroethylene) - chloro- and fluoro-substituted (K = 2.48 - 2.76)
3) Poly(p-phenyleneethylene) - Parylene - exception to list phenyl ring in backbone (K = 2.65)
4) Polystyrene - phenyl-substituted (K = 2.54 - 2.56)
5) Polyethylene - essentially unsubstituted carbon chain (K = 2.3 - 2.37)
6) Polypropylene - methyl-substituted (K = 2.1)
7) Poly(tetrafluoroethylene) (ex. Teflon) - perfluoro-substituted (K = 2.0 - 2.1)
So how lower the K factor the lower the absorbsion losses.
MKS is polyester = K3.4-4.3
I would be very surprised if you will find any V-FETs.:rolleyes:
Before I bought it I first checked if I could buy new V-fets of course.
I bought the NOS V_FET's in Hongkong quite a risky job but it turned out to be OK.
First step in testing and rebuilding the endstage.
Test of the power supply.
Remove all connectors of the endstage board.
Switch on the power. On the front plate in the right coner there is a powersupply board with one potentiometer to adjust the +20 Volt. The +97 volt shout also be automatic the right value.
Over R419 an R414 we must see about + and - 90Volt. Then also over the big capacitors the main powersuplly voltage - and + 45Volt.
The new EPCOS caps 10000uF 63V
New caps and transistors on the endstage.
After step1 testing the voltages the 20V and 97V where OK.
But the +/- 90V and +/-45V where not symmetrical distributed and started slowly to drift.
I could not under stand what was going on??
Then I discovered the problem why the previous owner couldn't fix the amplifier and blew it over and over again. The was a open in the circuit board of the rectifier.
Probably caused by short of defective V-fets and switching on the amp with new fuses. The open was from the junction of the to power capacitors to the ground junction of the two secondary.
Look here to see the open it is the bottom of the circuit board.
I hope I did other service Mann a favour here by posting this cause.
Regards and keep them running Helmuth
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