Philips FA-443 – Low Sound, One Channel Only

 

Initial Faults

Volume pot is really itchy and scratchy; heat sink runs very hot; one channel missing (left); the volume LEDs flicker to higher values on pot increase but don’t stay lit for the pot position and return to unlit; other channel too quiet.

So for the experience, I cleaned up the old messy, white, SVI2129 and power transistor thermal grease and applied new copper coloured paste. There’s an insulating silica heat pad behind the regulator – the old paste was only on the heat sink side, not the transistor to pad side so not sure if this was factory work, so unsure if the SVI2129 is the original or not.

My logic behind the heat sink being hot, is that if the old stuff is too insulating, then the heat will rise in the SVI2129 and regulator making them too hot, which will eventually heat up the heat sink as the the rate of cooling is insufficient, so the default temperature becomes above normal.

After doing this, the heat sink did run cooler, but not definitive yet until full signal is restored.

The main board needs removal for access to the dirty/damaged volume pot..

…and what a pig to get to! Had to dismantle the whole case and so many screws to get it out! Pain to wick as well, nearly lost a track.

Now cleaned, it has made a big difference already – really easy to turn and the almost dead channel is a bit more audible and the good channel now shows continuous LEDs. The trimmers on the LED board were also measured and made equal at 6.2k Ohms almost exactly back where they were paint locked before.

After a lot of wasted time checking components that were fine, I found it was the pot again!

This volume pot caused me no end of grief after testing OK out the chassis –  it had me chasing down other components for the missing channel when it was the pot being intermittent bad contact even after replacing and cleaning! It took a second removal, clean, check and some solder heat damaged tracks to get this thing working properly and re-installed despite the bad tracks. Only then did all else work correctly.

The chase down was good experience though, going through the signal chain at convenient points though some components were hard to find with my eyesight.

The odd thing that caught me out too was the speaker AB system – when I had signals at the SVI2129 I got no speaker sound! The connections are separate grounds so you can have 2 or 4 speakers without the usual switches you find on amps so the single system connections aren’t next to each other like usual!:

There is no difference between speaker loaded and unloaded output, using 8 ohm dummy loads to test with a 500mV input signal (I can’t get a clear sig gen/scope signal at much less than this level with the cheap sig gen), showing 40Vpp at full volume just shy of clipping:

Power Output:

Gain at max volume (0.5V in) before clipping = 40Vpp/o.5Vpp = 80

P=IV = 40Vpp = 20Vp/root2 RMS = 14V RMS

14Vp x 14Vp / 8 ohm = 24.5 W per channel

About right for the higher input signal of 500mV over nominal 150mV line input 25W per channel at minimal harmonic distortion:

A solid little amp – well built as expected from Philips. I don’t like the horrid big heat sink sticking out the back though, and it can get quite hot – be aware what it’s resting against.

The SVIs are all the same in most amps so should all sound the same power for power output.

The minimus 26s make the difference – they have a really nice imaging quality and are pretty bassy for their size with the Loudness button on!

Lessons Learned

Beware soldering under board horizontally, as solder can run down larger component legs – like volume pots! – and cause similar problems to the ones you are trying to solve by cleaning!