These are just some short notes I took while inspecting an aged Sony TA-F220 amp some days ago. I have seen several of these over the last years, pretty decent amp with a nice sound. They all have some regular aging flaws in common, though.
First, download the service manual. It is freely available from eserviceinfo.com or elektrotanya.com. Also, I will not go into details about the disassembly & reassembly of the amp, this should be pretty obvious. The manual contains explosion drawings and some more hints can be found printed onto the solder side of the preamp PCB.
As always, sufficient knowledge is implied. Remember to keep your fingers in the right places and the device unplugged while working. There are many open line voltage contacts near the fuse that can be easily touched by accident.
Errors & solutions
Amp turns off completely when reaching higher levels, sporadic, DC relay clicks off.
This is a fault that can be found by sight. Looking into the case (Fig. 2), a big, circular, wet looking spot can be seen in the front left corner of the main PCB.
The spot is caused by a bottom seal failure of the C703 220µF electrolytic cap in the center of the picture, just below the relay. Logical analysis of the schematic also helps to identify this part, I found it this way even before I noticed the dirty PCB.
After ruling out the overload protection as the cause by simply shorting the corresponding pin (#1) of the protector IC to GND (see datasheet of µPC1237HA for details), I tested the DC protector the same way. When shorting the pin (#2), no shutdown occured. On the contrary, I could not measure DC at the outputs (before relay) while in shutdown, but some DC-ish signal at the protector input. The reason is the following:
A capacitor is a frequency-dependent impedance Zc(f) (low f -> high Zc, high f -> low Zc). In this case, it is connected in parallel to the input of the protector IC and works as a lowpass filter together with the series resistances in the sensing lines (several K-ohms). For low frequencies, the voltage drop across Zc is larger than for high frequencies – that’s just basic RC filter theory.
If the cap becomes defective – which is usually a creeping process for electrolytics – its low impedance region will begin at higher and higher frequencies the more it degrades. This increases the filter edge frequency, letting more and more low-freq. components through to the protector IC input where only true DC components should arrive. In consequence, low frequencies of sufficient level in the audio signal will trip the DC protector. The error will grow more and more pronounced with continuing deterioration of the cap. Replace it by an equal or higher rated one (voltage/temperature/ESR depending on usage scenario, not capacity!). In the case of the F220, the original part seems to be a bit underrated in terms of voltage (only 6.3 V).
Oh, and clean the board if the cap leaked. A Q-tip soaked with a mixture of water and alcohol is a good choice for cleaning electrolytic fluids. The mess needs to be removed as the electrolytic sometimes shows corrosive behaviour. Hell, you don’t want anything fluid in the circuit, conducting or not. You can see the effect in the next picture below (Fig. 3), look at the resistor in the center.
Amp loses one channel from time to time, mechanical shock or turning it off/on brings it back.
Dirty relay contacts. The contacts are usually silver-plated to enhance their contacting ability. Unfortunately, this has the big downside of being a good medium to oxidize. The dust cap of the relay can be taken off, but I don’t recommend cleaning the contacts as they are too fragile in the original part. The durable solution is to replace the relay. 24V coil voltage is required, and Sony has made this easy by providing through-holes for two different relay footprints. You can barely make out the different marking in the top left of the following picture.
Crackling, noises, etc.
The usual problems with old amps: Consumer products almost always develop bad solder joints. Typical weak spots in this amp are: Input RCA sockets (Fig. 4a), output binding posts (Fig. 4c), the connector between the preamp board on the right and the front panel (Fig. 4b, solder joints are on the front panel). Also check the underside of the main PCB by sight, but usually this is ok. Check any parts that appear heavy and might move a lot when the case is handled, heatsinks, voltage regulators, large resistors, connectors. If in doubt, reflow the solder with some flux.
The other part are the switches. These are also silver plated, and they develop hard oxides. Cleaning sprays like Contact 60/61 have never helped me here as the oxides are too baked in, so I always unsolder the two channel select switches and the speaker selector and open them up. This can be done by carefully uncrimping the metal tabs keeping them together and taking them apart while memorizing (!) where the four contact blades go. After cleaning the contact strips with a special silverware cleaning fluid (NEVER USE ABRASIVES HERE!), I apply a thin layer of grease to keep oxides out. Then the contacts are closed and re-crimped again. I admit that this is more for the mechanically gifted, but it can be done. Unfortunately, I forgot to take pictures. Yikes, not going to redo that.
The volume and tone control pots are mostly not to blame for this. The quality is decent even if they are not dust-proof. If a mild crackle is present, turn the knob over the crackling section very fast 20-30 times and it will eventually smooth out. Potentiometers can generally also be disassembled and cleaned, but you risk destroying a part for which you might not get spares. Also, cleaning has rarely kept the problem out for long.
Another hint towards the end: When loosely placing the main PCB into the case for testing, always insert the center screw that makes contact to the central ground metal ring contact. The amp will not turn on if this is disconnected ;-)
In this case, that was all. On another occasion, I worked the sister model of the F220, the F120, which was completely dead. If so, check the two fusible resistors R801/R802 first. They are pretty well hidden and the F220 does not have them. Sometimes they will simply blow, probably because of insufficient airflow. Just take care that you replace them by suitable spares! Fusible resistors are special types that will specifically open on fail, and will not produce a flame. Many producers still make them, but they are mostly only available at Farnell, Digikey, etc.