Sir Weng,
My Brother knows a bit about electronics. He didn't actually repair the Sony 5650. What he did was follow the Service Manual instructions (and reading on the net regarding the vfets0 on how to adjust the bias and dc offset setting of the amplifier which has strayed from the factory settings from the time the unit was produced. Then he replaced the diodes which AK gurus claim are the cause of permanent damage to the transistors. It is said that if the VFETs are gone, the amplifier is just a Huge Paperweight. He still has my 4650 which was already adjusted but still has problematic pre-amp section.
Actually, he didnt follow the advice of the AK gurus exactly who recommend a bias setting above 0. He set it to full 0 to totally avoid the heating problems.
By the way I have a soft copy of both 5650 and 4650 service manuals if you are interested. Your technicians will need these to work on the vfets. My brod couldnt do anything about the adjustment till he obtained a copy of the SM's.
Also, my brod mentioned that although the 5650 is more powerful than the 4650, he connected the power amp section to another pre-amp to bypass the problematic preamp of the vfet to test the unit.
He claims that the 4650 sounded as good as the 5650. It is thus a rare unit that deserves restoration.
Now as to whether he will accept non-brother projects, i still have to check with him.
I am based in Quezon City.
Foo
Guys,
I too started my real audio set-up back in 1989 with a surplus amp I got from Raon. It was a NEC Django (forgot the model). It was really %$@#$ heavy due to the huge transformer. It has an orange VU meter going sideways indicating 180watts.
After all these years, I had been very interested with all these equipment parts upgrades and modification. Here are some tips so that you could keep your newly acquired surplus amps in tip-top condition and improve its performance.
1) Electrolytic capacitors are the one that will normally fail over time. The liquid inside these capacitors tend to dry up or in some cases leak out. As these amps are already more than 10 to 15 years old, it is the best to replace all these electrolytic caps. To be safe, just use the same capacitance and voltage rating (you could increase the value of the voltage rating but don't go lower in vale). For the power supply caps, you could increase the capacitance to twice the value and by doing so will improve the bass performance of the amp.
2) Get a service manual so that you could set the correct bias setting. Some amplifier runs hotter because they are more into class A, meaning although the amplifier is considered Class AB, biasing it higher will make the first few watts more into Class A. A higher bias means a smoother sound, with the bass becoming more controlled and rhythmic.
3) Bias the dc offset to zero or as close to zero if the amp doeasn't comes with a circuit to automatically check the dc offset.
4) Check for any rusty connectors or cables (corroded/rusty). Replace them with better types you could get.
5) Clean all the contacts and connectors with a contact cleaner like a CAIG contact cleaner or if you are on the budget, get the Philips (red can) contact cleaner.
Here are some tweaks/parts upgrade you could do to further increase the performance of your amp.
1) Replace the powersupply diodes to ultrafast/soft recovery type. Get the HFA series from International Rectifiers. Source is Farnell.
2) Replace the Zobel Network circuit. This circuit is a combination of a capacitor and resistor found before the speaker terminal/connector with a good polypropylene cap like Auricap or Mundorf and resistor like Kiwame.
3) Replace the internal cable leading to the speaker terminal and to the output RCA jacks with better cables.
4) Replace the speaker binding post and RCS connectors to better quality type.
Happy tweaking.
