First of all, I apologize I’m writing with some delay.
I also appreciate your suggestions about language –you’re right- English is my third language, and unfortunately I never learned it at school, so you can find many spelling and grammar errors here.
Panasonic FC-line capacitors are pretty good choice, especially regarding the price point. As I wrote already, all Sansui amps are direct-coupling, so the capacitors are not in the signal path. I used FC’s in hand-selection manner (the suppliers usually won’t send less than 5 pcs of each values), simply selecting them to be as equal as possible for both channels using simple capacitometer.
If you prefer to go with some better (an unfortunately pricier) caps, then I would suggest Nichicon Muse series, either FG (Fine Gold) or highest grade, KZ. The point is, use those with smallest ESR and lowest loss. I found KZ are most popular for this purpose in Japan.
You don’t need a technician for changing the bias and DC pots on power amp PCB. My suggestion why change pots too is simple: you must dismantling power PCB to resolder the caps and this procedure includes detachment of the PCB from heat sink. Once you’re there it is good idea to do that, since you will never go again to dismantle just for the pots.
As for the measurements and adjustments of bias/DC it’s easy (I’ve done this alone, and I’m also not a tech). Here how I’m doing this:
1. Listen the amp for some 15-30 min to achieve the operating temperature. Alternatively, you may just switch it on and just leave for some 1 hour or so.
2. Disconnect the speakers, or simply switch speaker selector to “B”, and accordingly measure DC between HOT/COLD (=pos/neg speaker terminals), and HOT/GND (pos. speaker terminal to any point of the metal case) noting the voltages. Both values should be less than 3mV DC.
3. Measure the DC voltage across the emitter resistors from underneath, which are located just before output transistors.
These resistors are usually 0,22 or 0,47 Ohms, big and rectangular, white colored, and usually two in parallels for each output transistors (8 of them per channel). You’ll obtain some voltage in mV, and dividing with the resistance, you'ill obtain the bias current.
Hint: one bias pot controls one pair of transistor, so you can measure only twice per channel (for example transistor 1 and 3 or 2 and 4). Ignore the polarity of the voltage, use absolute values only.
4. Mount the pots in “neutral” or “half-way” position, using ohmmeter.
Use some good tin containig silver.
Assuming everything is done correctly (good soldering, no shorts etc), after switching on the amp for the first time, you may have only one problem: the protector may flash indicating the amp is in protector mode. This is due to its activation if the DC is beyond his limit, and this is normal since the new pots for DC are not in the ideal position, and you will not have reading on the speaker terminals, since the speaker relay is open.
In this case just connect the voltmeter between the two various emitter resistors of both transistor pairs of each channel and roughly adjust the DC to some 50mV.
But, as you state, you may ask some tech just for adjustments.
I will describe my experience just to be complete…
5. First adjust the bias. Idle current should be around 60-80mA, but you can go higher if you want, since the heat sinks are same for 707 and flagship 907 model, but 907 has double the output transistors of your 707 (8 per channel), so the main limitation factor for higher biasing (cooling) is not an objective here.
If you want an excellent article with lot of hints and tips for biasing, please visit:
http://www.tnt-audio.com/clinica/bias_e.html
When you’ll complete the procedure, carefully adjust the DC on the speaker terminals as described. With 25 turn-sealed pots, it can be adjusted as low as 0,1mV, but it is not necessary to try to adjust less than 3-5mV, even up to 10mV is acceptable.
6. Finally, the amp may sound harsh on the beginning, but it’s normal since all the caps are new, and need some time to run-in.
Best regards from Croatia,
