Philips AH578 destroying resistors on driver boards, please help!

Check to make sure that the relays on the protection circuit board are not fused closed. That can happen when the output transistors are shorted and the protection circiut is activated. I had this problem and it took me a while to figure it out.
oops, I see you already looked at that!

After I found the fused closed contact on the relay K303 and checked the DC voltages on Q209 & Q208, I put back the power transistors on one channel and after a last check, I put the power on and .... no explosion, no smoke.
I adjusted quickly the bias current to read 20mV across R234. With a sinus signal applied on the input the output signal was correct.
I did the same with the other channel and now the amplifier is OK.

To summarize :
- 1- remove the covers and check the contacts on the 3 relays K301,K302 & K303
- 2- remove the two driver boards
- 3 -now, when you turn the power on, you must see K301 & K302 activated after 1 - 2 seconds but K303 must stay open. After you turn the power off, K303 will be activated for a short time but must return to it's open state.
- 4 - check the driver boards and remplace all the electrolytic capacitors, the R220 ajustable resistor and if necessary R232, R231, R230, the 0.47ohms resistors, the bad transistors ...
- 5 - plug back one of the driver board but don't connect it to the power transistors
- 6 - turn the power on and check the DC voltage around Q209 - Q208 - R232 - R220. The typical values are on the schema in the service manual. Check also that if you turn R220 the base voltage of Q209 & Q208 changes a little bit.
- 7 - if it's OK, turn the power off, and connect the power transistors to the board.
- 8 - connect a DC VTVM accross R234, turn the power on and adjust the quiescent current according the service manual. If you did'nt replaced R220, it may be a little bit tricky ...
- 9 - do the same with the second channel.

Concerning the transistors, here is the cross reference table I used with succes
(thank you very much Echowars :thmbsp
355N -> MJ15024G
357N2 -> 2SC2238
357P1 -> 2SA968
263N6 -> 2SC2632
148N3 -> 2N3904
158P3 -> ???
383P1 -> 2SA835
383N2 -> 2SC1124
213N1 -> ???
360P1 -> ???

CA3100S -> HA2525 (not tested !)
D206 -> 1N4148

As you can see on the last picture, the 22AH578 has a serious competitor on his top !

Hello !
I also working on 578. Left channel sometimes make siliy things: First only high freqency signals are on the speakers, then 5 minutes later, you can put the regulator to ZERO; but then the sound of the left channel is to hear on the right speakers, not so loud as the original left signal but you can haer it...????
a few minutes later only a 100Hz signal with less power is to hear, also when Poti R3 (50K) is on ZERO, Very strange, if there was an ELectrolytic Cap, I would say yes thats the Cap. There is an ELCO C201, but this could not be the matter.
When the unit is open, I wait with the Oscilloscope for the failure, but nothing happened. Some days everything is ok, then after 8 or 12 minutes the "spectacle" starts as described above.

Has anybody an idea on this error!

Thanx very much

Hi everyone. I realize that this thread is 9 years old and hope that I can get some insight... Recently, I inherited my fathers' Philips stereo system (AH578 Amp, 572 Pre-Amp, 673 Tuner, 222 Turntable, 477 Speakers, & N4504 Reel to Reel). Everything, when plugged in, turns on -- EXCEPT the Amp. Plug it in and try to turn it on... and Nothing. I would love to honor his memory by getting it up and running, but I'm not an electronics wizard. Should I hope that electronic repair shops are knowledgeable enough with these Philips systems in order to correctly work on - and, ultimately, fix them? I've read where only a mere 400 of these sets were ever produced ?? I find that hard to believe, and would need definitive proof of that number. Because of this, you can probably understand my hesitation to take it anywhere. I just hope it can be fixed.

Daniel Boyer said:

Hi everyone. I realize that this thread is 9 years old and hope that I can get some insight... Recently, I inherited my fathers' Philips stereo system (AH578 Amp, 572 Pre-Amp, 673 Tuner, 222 Turntable, 477 Speakers, & N4504 Reel to Reel). Everything, when plugged in, turns on -- EXCEPT the Amp. Plug it in and try to turn it on... and Nothing. I would love to honor his memory by getting it up and running, but I'm not an electronics wizard. Should I hope that electronic repair shops are knowledgeable enough with these Philips systems in order to correctly work on - and, ultimately, fix them? I've read where only a mere 400 of these sets were ever produced ?? I find that hard to believe, and would need definitive proof of that number. Because of this, you can probably understand my hesitation to take it anywhere. I just hope it can be fixed.

Click to expand...

Daniel - most electronic repair shops could fix this amp - you can ask for a cost to repair to see what it will set you back but to keep the amp in that matching set is something i would certainly want to do - the amp is rated at 220 w/channel and that's rather conservative - I know a great shop in the Toronto area that can fix this stuff - every pc of equipment you have listed here i have owned at some point in time have since gone from a N4504 to N4506 RTR to the N4520 - still have lost of Philips components as well this shop fixed my 30 year old CD960 and its like new again - based on you r equipment list im thinking you maty me in Canada or even Ontario ?? just a guess but if you want the name, address and # of the tech i can message it to you -

wazy