Cossor 524 rustling noise on FM

[PaulR]

I restored this set a few years ago and it worked really well. I have recently resurrected it and find that there is a constant rustling noise on FM. If I tune into it with my signal generator the rustling is a lot louder as the signal increases. I have substituted all the valves with ones from another set and that makes no difference. I have also used switch cleaner on the sliding and switches to no effect. Tapping and flexing the various joints makes no difference.

Other than the rustling the set works very well and is very sensitive on FM. I do actually have another chassis I could restore and substitute in the case but I would like to get this one working properly.

I wondered whether the AGC is causing the problem but I am not sure. Can anyone help please?

Thanks

PS I haven't replaced any resistors as I tend to leave the original ones alone if the set works ok.

Paul

[frsimen]

You could try removing the valve from the VHF tuner and applying a signal to the grid of the first IF valve. If the rustling is still present, the problem is somewhere in the IF strip, if not, it's in the tuner.

The noise getting worse with strong signals could be a component breaking down as the signal voltages increase. If the problem remains with V6 (the valve in the tuner) removed, check the components around the ratio detector which is where the largest voltages will be found with a strong signal.

Paula

[PYE 405]

One possibility worth bearing in mind is HT tracking on the selector switch. If audio and HT switching occur on the same wafer section, this can sometimes happen. It is worth digging out the circuit and having a close look at the switch sections to see. The HT will vary slightly to the front end according to signal strength, but whether this is enough to alter the level of rustling other than a fault elsewhere, is open to question.
It might also be worth switching the set to "Gram" and turning up the volume to be certain the rustling is only on the FM position.

[PaulR]

Thank you, both, for the replies.

After some further investigation it appears that the rustling is on both FM and AM. I thought it was just FM because that is the clearest signal. I applied an FM signal to the grids of both V2 and V3 with the previous valves pulled and the noise was still there. I also put a 470 KHz signal into the AM aerial sockets and the noise was there as well. It appears to be superimposed on the signal as it is much stronger when there is a strong signal.

I can hear it faintly with no signal as the valves are pulled up to the EABC80 which, of course, removes the pre amp AF.

There isn't a switch as such for Gram. You remove a jumper to disconnect the radio section and then use the PU sockets. I think you are intended to replace the jumper is such a way as to short the output from the radio section. Anyway, the rustling disappears when the jumper is removed. I haven only touched the PU sockets rather than put a proper signal into them but there is no rusting along with the hum.

The HT is switched but on a sliding switch rather than the wafer switch.

[PJC58-Hythe]

My initial thought would be a noisy resistor, which freezer spray might find. Less likely is a capacitor breaking down, I don't know if you have changed any capacitors?

[PJL]

Check the anode voltage on the first audio amplifier EABC80, the anode load (R28 trader) is often the source of problems.

[PaulR]

Well, the anode load resistor on the EABC80 was well out of spec (over IM as opposed to 680K) but changing it hasn't helped unfortunately.

I also wondered about noisy resistors and went over it with a freeze spray but nothing made any difference. I have changed all the waxy capacitors but not the lower value disk ones. There are a couple of flat brown moulded ones which I left in place. They are 100pF. I went over my soldering again just in case.

The set works so well other than the noise that I am sure that something must be staring me in the face.

[Ambientnoise]

Had the rustling on the same model and after much hassle, it turned out to be one of the FM if windings. I think it must have had internal green spot corrosion. A replacement kindly gifted from the forum was the cure. I hope yours has a simpler cause.

[G6Tanuki]

Does it have - anywhere - those ceramic trimmers that are made by metallizing sprayed direct on the ceramic base and the rotating part?. They are sometimes orange.

Problem is that the metallization-to-the-connection-points breaks downand can cause strange rustling/fizzling effects.

[G6Tanuki]

This is the kind of ceramc trimmer I referred to...

[PJL]

If it definitely occurs on AM it is not the FM front-end as the HT for this is switched off in AM.

When you tested it on gram and it did not rustle did you put the link in the gram position (short the radio output)? It might be worth confirming no rustle for gram in both AM and FM positions and with/without the radio short link.

HT tracking in the waveband switch has already been mentioned as a possible cause. HT is on S1 and S4 and the only way to check this is to disconnect the HT from these and hard wire for FM.

[PaulR]

Many thanks for all the suggestions. I will try to investigate further today. It seems to be on both FM and AM but is more noticeable on FM. I will look more into this and HT tracking.

The noise comes and goes with signal strength. On FM it can be heard in between stations but becomes louder as the signal strength increases. It is more difficult to ascertain on AM due to the general noise.

[Silicon]

The symptom of the noise increasing with RF signal level reminds me of the characteristics of 'modulation hum'.
There are many potential causes.

When you have exhausted all the other good suggestions you might want to try checking all the RF and IF bypass and decoupling capacitors.

[PaulR]

Right - it seems as though I hadn't got the whole story.

If I apply a 10.7Mhz signal to the grid of V2 the rustling is apparent until the volume control is turned to maximum when it is much reduced. Here is a YouTube link

https://youtube.com/shorts/T72oHZQ0Xh4

If I apply an RF signal to the "live" end of the volume control there is no crackling

https://youtube.com/shorts/04CjeWVJsr0

[PaulR]

Sorry the last sentence should read AF not RF signal applied to the volume control

[cathoderay57]

In post #1 you queried whether the fault could be AGC related, which makes sense since the noise amplitude seems to increase with signal level. Try temporarily disconnecting the AGC and see what happens. Jerry

[PaulR]

Thanks for the suggestion, Jerry. Would that account for the way the noise is much reduced when the volume control is turned to full?

I should have said, by the way, that the radio section was still connected when I put the AF signal into the volume control.

[cathoderay57]

Hi Paul. The Trader Sheet #1221 is in a more standard layout than the Cossor one and is easier (at least for me) to understand so I'll refer to component numbers in the Trader. The volume control is the AGC load resistor R26 although exactly why advancing it to max removes the rustling I don't know. maybe worth checking the resistance of the volume control pot and whether there is any leakage path between the track and case, with its cables disconnected. I would still try disconnecting one end of R21 the AGC feed resistor. You said that you cleaned all the switches so presumably that included the radio/gram switch? Jerry

[PaulR]

Thanks Jerry. I disconnected R21 but it made no difference. Isn't that part of the AM circuit, though.

I did notice that there is 12V on the hot end of the volume control. Is that right?

[PJL]

Is the 12V there on both AM and FM? Sounds like a leaky waveband switch to me.