Philips 50IC36 No FM

[davidgem1406]

I have a Philips 50IC36 I am doing for a friend.
The AM side is working but none of the FM bands, there is noise but nothing of stations is to be had.

So far I have replaced all the electrolytic capacitors, my friend likes that done by default. I have lost track of how many I have changed for him.

Today I set about looking for the fault in the FM circuitry. All voltages were checked on the 3 front end transistors. They were not too far out from the given voltages.

This model uses the TBA570 IC. Checking the voltages on this I found only one that was out far enough to suspect. Pin 5 on FM should be 0.5V but is sitting at 2.5V a long way out. On AM the same point should read 0.3V but actually reads 0.6V, but the AM is working.

I have concluded that the IC has an internal fault affecting the FM side of the chip. A replacement was ordered.

I wondered if anybody may have other ideas as to what the fault may be. Avoiding having to replace the IC would be good, as removing it will not be too easy, the PCB tracks are very fine No doubt we all know what that is like on old boards.

Best regards
Dave.

[julie_m]

If the old chip is already knackered, there is nothing to be lost by cutting each of the legs and desoldering them one by one from underneath, pulling them up through the PCB so as not to risk lifting the copper tracks.

[davidgem1406]

HI Julie, that's what I would do for sure, but it's pretty closely packed in there.
I was looking for confirmation that the chip was indeed done for, just to be sure before I replaced it. I'm hoping there is enough room to put a DIL socket in there.

Dave.

[Nuvistor]

Have you a circuit? I cannot find it in the service data on this web site. Pin 5 on the I.C data seems to be AM audio out.
From the data sheet pin 7 is the 10.7Mhz IF output to a ratio det or similar.

[julie_m]

I purposely didn't suggest a socket because some TBA_ ICs come in an unusual package with two staggered rows of pins in a zig-zag arrangement; and also because, in the case of an IC handling IF signals, it would have been in violation of the three rules of high frequency circuit design (1. Keep all leads as short as possible. 2. Never let any lead be even a fraction of a millimetre longer than necessary. 3. If in doubt, see rules 1 and 2).

If your set has a chip in a straight DIL package, and you really want to use a socket, you might be better off with the cheaper stamped-pin type than a high-quality turned-pin type, just because there is less spare metal to cause stray capacitance and inductance. But I'd rather solder the IC straight to the board, than risk making the set unstable .....

[trsomian]

Are you certain the IC is faulty. I had a similar symptom with a radio based on a KIA6040P, with the voltage on a pin out of spec. It turned out to be a leaky ceramic decoupling capacitor. Changed the capacitor and it worked.
Be warned that the leaky capacitor may not be connected to the pin with the wrong voltage on it.

[davidgem1406]

Hello Frank,
Yes, circuit attached: 3 pages (1 pdf document).

Hi Julie,
The package I have coming is straight DIL, not the offset version.
Do you really think that at FM (10 Mhz) the frequency would be affected by the addition of the socket?
The sockets I have ordered to go with the chip, of course, are the turned pin version. just like it eigh,

Hi trsomian,
No, I'm not certain on that. It is just the only item that had a wide enough voltage variation to be suspect. That is what tempted me to make the posting. (An afterthought, bear in mind that the voltage on pin 5 is correct for AM, but up by 2V on FM. That is what makes me think the chip has an internal fault to the FM section.)

Regards to all
Dave

[crackle]

The TBA570 datasheet gives a max value of 4v for pin 5.
http://www.bg-electronics.de/datenbl...se/TBA570Q.pdf

Mike

[Nuvistor]

Pin 5 appears to be used as a signal strength indicator voltage, the IC could be faulty but before replacing have you equipment for checking the 10.7 MHz signal, it enters the IC on pin 15 and exits after amplification on pin 7 to be fed to the ratio det.

Instability in the tuner or IF could cause a high voltage on pin 5.

[davidgem1406]

Hello Frank,
Yes, I have a scope and frequency counter, I think you are meaning a scope though.
If I put a signal, from a generator, into the front end that will give me a clean signal to look for, but depending on where the problem is maybe not.

Thanks for the info.
Dave

[Nuvistor]

It depends on your scope specifications but yes if the scope is good enough give it a try, there should be no 10.7Mhz signal when no station is tuned in.

I don’t know what the fault is but if you have the test equipment it’s better to do some tests before changing major parts. I appreciate that is what you have to do though at times.

I was trying to work out what TS428, an oscillator was for then it “twigged” it’s a stabilised 12.5v supply for the tuning voltage,

[davidgem1406]

Hi, Frank,

I have 2 scopes a Telequipment D83 50 Mhz and a Tektronix 465B 100 Mhz.
I can't tune any stations, all I have is some noise that is not affected by tuning. It does go up or down when checking voltages around the 3 front end transistors.

Dave

[Nuvistor]

Are you using a suitable scope probe? I don’t know your scopes but a correct probe will load the circuit very lightly.
I must admit I would expect to see something however small but then I don’t know the radio either, just going off the circuit.

[davidgem1406]

I have good probes 100 Mhz and switchable to 1x or 10x. I can also go direct if need be.
As yet I have not made any more tests so far so can't say what the results might be.

I may get a chance tomorrow.

Dave

[tri-comp]

The FM Tuner/IF is made with Lockfit transistors and they shouldn't be trusted to operate normally any more.
If the Local-Oscillator/Mixer (TS422b) stopped working you'd get symptoms like what you're experiencing.
Another potential source of problems is the DC-converter (TS428) used for providing high-voltage for the FM tuner Vari-Caps.
Here the electrolytic cap's C602 & C606 should be checked and most likely replaced.
In any case check +4 and +5.

[davidgem1406]

Hello Tri-comp,
The 3 front end transistors, I voltage checked these and they were not far enough from that stated to say they were faulty. The only other way is to replace them.
TS428 I have not checked but will do so. All the electrolytic's have been replaced, nothing to do with finding the fault. My friend likes that done by default.
Thanks
Dave.

[crackle]

I have just had a similar fault on a radio, no FM.
Have you tried connecting an AM signal generator to the aerial and sweeping through 70 to 100MHz to see if you can pick up any stations..
If you can that should prove the FM oscillator is faulty, could be a duff solder connection on a coil, the coil its self, capacitor or the transistor.

Mike

[davidgem1406]

Hello Mike,
No, I have not tried that but it's another thing I can try.
You say to sweep through 70 to 100MHz to see if I can pick up any stations. But you then say "If I can that should prove the FM oscillator is faulty".
I would have expected you to say "If I can't". Is that a typo, or am I missing something here?

Regards Dave

[Nuvistor]

You are using the Signal Generator to replace the local oscillator , it will beat with the incoming signal to give the Intermediate Frequency.

The SG needs to be 10.7Mhz from the wanted station.

[davidgem1406]

Thanks Frank,

So it does mean if it beats then the local oscillator is not working.
And if it doesn't beat then the local oscillator is working and should tune to something anyhow, and produce the IF 10.7MHz.

Dave.