Philips FR-740 working but no audio output

[R_J]

Re: Philips FR-740 working but no audio output

Good to here it's working, If that's a fusable type resistor, it might not show that it is bad.

[seve91]

Re: Philips FR-740 working but no audio output

Fixed! -VPl was missing, found an high resistance on 3901 (should be 47ohm). Replaced and now seems to be working good.

Usually when a resistor is broken, i can see a black spot or something. This one was perfect like new, and i can measure an high resistance instead of the usual open circuit.

Thank you everyone very much for your help!

[seve91]

Re: Philips FR-740 working but no audio output

IC7412 got 2 faulty output. Replaced, now i have 4.9V on AMP LR LINE, but still no sound.
On the C of 7309 i have -21V instead of -34.5V, and I have -40V on C of 7304, and I have almost -40V on pin 14 CCC instead of -33.5V (C of 7310).

HST & DC Protection Circuit seems to be good.

Very low voltages on 7909 and 7902.
All those transistor have already been replaced.
Any advice?

[seve91]

Re: Philips FR-740 working but no audio output

Hi everyone, due to Lockdown here in Italy, I decided to disassembly the Philips FR740 again to find out the issue. Hope to fix it this time.

[seve91]

Re: Philips FR-740 working but no audio output

Maybe put it away for a while and come back to it later, sometimes a break is needed and when you look at it another time the answer may come easier. It is likely something simple that is being over looked.
I also looked at the service manual for the FR975, it is very similar and a lot clearer to read, although the voltages are a bit different (slightly higher for more power) they are very close

i don't know...maybe you're right.
For sure I don't have so much experience... I go on with attempts.
The amplifier is not mine, the owner is a friend.
If she wants to bin it, then i will take it back.

[R_J]

Re: Philips FR-740 working but no audio output

Maybe put it away for a while and come back to it later, sometimes a break is needed and when you look at it another time the answer may come easier. It is likely something simple that is being over looked.
I also looked at the service manual for the FR975, it is very similar and a lot clearer to read, although the voltages are a bit different (slightly higher for more power) they are very close

[seve91]

Re: Philips FR-740 working but no audio output

sorry friend, i've just reassembled it. I give up.
Too much time wasted, and can't find a solution still.
Thanks to you for all your kindness and patience, your tips were useful to improve my troubleshooting skills.

[R_J]

Re: Philips FR-740 working but no audio output

I don't know why either, what ever it is effects both amp ic's correct? as both are turned off.
What are the voltages on transistor 7261 & 7260 ?

[seve91]

Re: Philips FR-740 working but no audio output

It should'nt be hard to find a transitor to use for now, almost any pnp rated higher than 60v should work.
So with that transistor out of circuit do you have audio out of the speakers?
did the voltage on 7310 (C) drop a bit lower from before?
With that transistor out of circuit, any comand from the front panel should have no effect on turning the amp ic's on or off, so if there still is no audio (and you have it at the stk inputs) then that voltage on the CCC line must still be too high

No still no audio voltage on CCC too high, don't know why. Without transistor, C of 7310 is -37.7V.
Tried replacing cap 2350 without success

[seve91]

Re: Philips FR-740 working but no audio output

replaced 7309, 7304, 7310, 7902, 7909. same voltages remain.
Could be a bad capacitor somewhere? Why the voltage is too high?

[R_J]

Re: Philips FR-740 working but no audio output

It should'nt be hard to find a transitor to use for now, almost any pnp rated higher than 60v should work.
So with that transistor out of circuit do you have audio out of the speakers?
did the voltage on 7310 (C) drop a bit lower from before?
With that transistor out of circuit, any comand from the front panel should have no effect on turning the amp ic's on or off, so if there still is no audio (and you have it at the stk inputs) then that voltage on the CCC line must still be too high

[seve91]

Re: Philips FR-740 working but no audio output

i know that the voltage on C of 7309 is wrong, but i don't know how to fix it.
I'm trying to desolder and test.
Desoldered cap 2365, nothing changed. Cap tested with ESR meter, is good.
Resoldered the cap back and desoldered 7309.
So as you said, 7309 was E-C shorted, because now on C of 7309 i got -36.6V (without the 7309).
So i need to order a replacement for sure. But probably there is another fault in the main board or in the front board

[R_J]

Re: Philips FR-740 working but no audio output

7309 could be bad cap 2365 could be bad, something is killing that voltage. all the other voltages seem to be close except this one. and it is the "AMP ON/OFF CIRCUIT"

I checked the schematic for a FR975, which is similar (more power) and that same circuit shows a negative voltage on transistor 7309 that is more than the 1.44 volts you measure there. so I assume the schematic is correct

[seve91]

Re: Philips FR-740 working but no audio output

base 7902 = -40V

so what could be the next step? try replacing a transistor?

[R_J]

Re: Philips FR-740 working but no audio output

That voltage on the collector of 7309 is way too low, the schematic says it should be -34.5 volts,
Whats on the base of 7902, it should be -35 volts
7309 could be leaky e-c killing that voltage
Like said about the other stk486-430, it seems to only take a 2 volt difference on the ccc line to turn it on or off.
You have -40v on ccc line, the schematic says it should be -33.5v

[seve91]

Re: Philips FR-740 working but no audio output

I wonder if 7309 is'nt bad, there should be -34.5 volts on the collector. I guess you could check the 2 100k's 3329 & 3936
-------from other post-------
ok, page 79:
A of 6305: 0V
K of 6305: 3.2V
C of 7309: 0.04V <------------the schematic indicates there should be -34.5 volts here
C of 7304: -41.5V
C of 7310: -41.5V
--------------------
I see from before
STK496-270
11 = 39,5V
14 = -33,5V
10 = -39,5V
3,4,19 = -40V
7,8,9,12,13,15,16,17,18,20 = 0V <------------pin 12 listed as 0 volts
7309: E= 3.4V, B= 3.05V, C= -1.4V
7304: E= -40.3V, B= -39.6V, C= -40.3V
7902: E= -40.3V, B= -39.6V, C= -40.3V <-------7902 (C) listed as -32V
check the voltage on resistor 3918 and then 3935

I checked a data sheet for a STK486-430 and the CCC pin only changes BY 2 volts between amp on and amp off (it uses -50v amp ON -52v amp OFF)

the resistors are all SMD and all are in good conditions.
Voltages:
3329: -39.6V (B of 7304) and -1.44V (C of 7309)
3936: -39.8V (B of 7909) and -38.15V (to 4910)
3918: -40.28V (to 3935) and -40.27V (C of 7902)
3935: -40.16 (C of 7909) and -40.18V (to 3918)

is it possible that one of these transistors is not working good?
I don't know if it can help, but every time i touch the tip of oscilloscope probe to C of 7310 (pin 14 CCC of STK496-270) i can hear a click coming from the speaker (i have connected a speaker, just to test, to the Front L output)

[R_J]

Re: Philips FR-740 working but no audio output

I wonder if 7309 is'nt bad, there should be -34.5 volts on the collector. I guess you could check the 2 100k's 3329 & 3936
-------from other post-------
ok, page 79:
A of 6305: 0V
K of 6305: 3.2V
C of 7309: 0.04V <------------the schematic indicates there should be -34.5 volts here
C of 7304: -41.5V
C of 7310: -41.5V
--------------------
I see from before
STK496-270
11 = 39,5V
14 = -33,5V
10 = -39,5V
3,4,19 = -40V
7,8,9,12,13,15,16,17,18,20 = 0V <------------pin 12 listed as 0 volts
7309: E= 3.4V, B= 3.05V, C= -1.4V
7304: E= -40.3V, B= -39.6V, C= -40.3V
7902: E= -40.3V, B= -39.6V, C= -40.3V <-------7902 (C) listed as -32V
check the voltage on resistor 3918 and then 3935

I checked a data sheet for a STK486-430 and the CCC pin only changes BY 2 volts between amp on and amp off (it uses -50v amp ON -52v amp OFF)

[seve91]

Re: Philips FR-740 working but no audio output

these are the voltages measured:

7309: E= 3.4V, B= 3.05V, C= -1.4V
7304: E= -40.3V, B= -39.6V, C= -40.3V
7902: E= -40.3V, B= -39.6V, C= -40.3V

[R_J]

Re: Philips FR-740 working but no audio output

did you check the voltages on 7309, there should be 3.1 volts on the emitter, if not that 6304 zener could be shorted, did you check any of those transistors like 7309, 7304, 7902

[seve91]

Re: Philips FR-740 working but no audio output

That does'nt seem like it works right, so TEST EEPROM = PASS
and EEPROM FORMAT = NEW
Well the reason the amp is not turned on is the missing +4.9V on diode 6306 (page79) amp lr on.
I guess you could lift the anode of diode 6305 and feed it +5 volts to see if the amp turns on and is ok. You could have a bad 7412 (hef4094bt)

Yes exactly, test = pass and format = new.
Tried to connect anode 6305 AMP IR ON to AMP S ON that was at 5.2V, but now they are both 0.4V

I tried also to desolder the anode from the mainboard to disconnect it from the circuit, and solder the anode directly to pin 2 of 1426 (5.2V).
Now i have 4.6V on that pin, but no audio. Could 7309 be in short?