Help Needed with Kenwood TS-440S FM Squelch Circuit - UK Vintage Radio Repair and Restoration Discussion Forum

kingbeau · 25th Oct 2018, 12:46 am

Hi everyone and thanks for allowing me to join the forum.
As per the title I have a TS-440S with a problem with the FM squelch not working. The squelch on AM & SSB works fine.

The FM squelch is controlled in IC2 (on the IF board) and is a MC3357P. Looking at the circuit diagram, the voltage on pin 13 should be 6.2v with squelch control at minimum, and 0v with squelch control at maximum. In my case though, the voltage stays at 6.2v regardless of whether the squelch is max or minimum.
The only thing that varies with the squelch pot is the voltage on pin 12, which goes from 1.6mV at minimum to 1.2mV at maximum control.

I'm a bit out of my depth with this to be honest so I'm hoping someone on here can help? In the meantime I've ordered a new MC3357P just in case.
Thank you in advance for any help.

http://www.radiomanual.info/schemi/K...-440S_serv.pdf

https://image.ibb.co/fKdhAq/Capture.png

SiriusHardware · 25th Oct 2018, 11:37 am

For others looking to help out, the relevant page of the PDF manual is page 88 (of 130).

Pin 13 is a bit of a red herring as it is a digital OUTPUT which is meant to indicate to an external circuit whether the squelch is open or closed. This would typically go to a scan control circuit, or in this case either provides or removes the bias to Q16 to unmute / mute the audio, by the look of it. That pin will only change state when the squelch gate changes state.

I haven't gone so far as to trace the incoming signal from the squelch control yet, but the squelch control itself seems to have two gangs / sections, one for FM and one for the other modes.

SiriusHardware · 25th Oct 2018, 12:14 pm

Further:

The squelch controls are shown at the lower right of page 126 of 130, the connection from the FM squelch control (which is just a variable pulldown to 0V) goes to J7 / FSQ on the right hand side of the IF PCB (Page 89 of 130) and from there via wire... 18, I think, which in turn eventually goes to pin 12 of U2, the MC3357.

If you have almost no voltage there at any setting of the squelch control then try measuring the resistance from pin 12 to 0V, if it is very low even with the squelch control turned both fully clockwise and fully anticlockwise then consider C78? (Diagram is poor) as possibly short circuit or low resistance.

kingbeau · 25th Oct 2018, 12:46 pm

Thank you very much for the help, it really is much appreciated. Resistance between pin 12 & ground varies from 1ohm with squelch control at minimum, to 46k ohms with control at maximum.
I've checked C78 & 79 with an ESR tester and they seem within tolerance.
Thanks again

SiriusHardware · 25th Oct 2018, 1:30 pm

Your observed resistance of 1 ohm to 46K is consistent with the Squelch control (which is 50K) and its wiring all all the way to U2 being OK.

If you have another MC3357 and feel OK about changing it, that may be your possible next step.

kingbeau · 25th Oct 2018, 1:52 pm

Thanks, I have one on order. I’ll report back once it’s been changed.
From the limited information you have about the fault, would you say that’s the most likely cause?

SiriusHardware · 25th Oct 2018, 2:11 pm

...should have asked this earlier, but is FM reception (of actual signals, not just the presence of no-signal white noise) working OK?

The problem is that you just can't turn the squelch 'on'?

kingbeau · 25th Oct 2018, 3:01 pm

Yes the reception is fine, (I was listening to the locals on 11 metres) just no squelch action.

Being an impatient kind of guy I had a rummage around and found a MC3357P in a little CB radio, I've swapped them round and the CB is working fine with the Kenwood's IC but the Kenwood still has the same fault with the CB's IC. I had really hoped that would've solved things

SiriusHardware · 26th Oct 2018, 1:21 pm

OK, can you visually examine / check by any means possible R108(?) (15K), D42 and D43. If the diodes are 'glass' diodes carefully desolder them one at a time and check that neither of them are cracked in half, as it is not unusual for that to happen. They can appear to be perfect, but fall apart as soon as one lead is desoldered. While they are out of circuit, check them and see if they measure the same.

If you see nothing untoward there, go wider and check soldering on / measure C80 through to C84 inclusive and R109-R111. My reading of the circuit is that these components along with the 'active filter' stage in the IC (Pins 10-11) sample only the high frequency part - the 'hiss' - from the received audio and generate from that a DC voltage at the junction of D49 / R108 / C79, the voltage there being proportional to the amount of hiss / noise present on the audio coming out of the demodulator.

This looks to be what I would call a 'noise' squelch rather than a 'level' squelch - it works out how weak the signal is by measuring the amount of white noise coming in with the signal, rather than by measuring the strength of the incoming signal.

The squelch control then adds further bias to the DC voltage before that is passed to the control input on the IC (pin 12), allowing the user to vary the squelch threshold.

kingbeau · 26th Oct 2018, 2:32 pm

SiriusHardware, you sir are a gentleman and a scholar. Thank you very much for your help.
I did exactly as you suggested, R108 measured correct at 15k, D43 was ok but D42 was O/C. According to the service manual D42 is a 1N60. I had a rummage around and found I had a handful of OA91 germanium diodes, popped one in to try it and hey-presto the FM squelch is now working! I'll have to try and find out if the OA91 is a suitable long-term substitute but it's definitely fixed it for the time being.
I'm extremely grateful for your help with this, thank you again!

SiriusHardware · 26th Oct 2018, 2:44 pm

You're welcome, enjoy the radio.

Radio Wrangler · 26th Oct 2018, 4:19 pm

Thermal stress or a bit of board flexing. Cracked glass bodied diodes are a fairly common fault in many things. A bit too much stress in manufacture and they'll fail years later.

David

Jon_G4MDC · 26th Oct 2018, 7:20 pm

Nice fix and not where my money was (capacitor after rectifier).
Good work.

Biggles · 26th Oct 2018, 11:22 pm

The OA91's will be fine to use as a substitute. I am sure that some circuits using the MC3357 family of chips specify OA91 type germanium diodes. The proof is that the set is working correctly. Strangely, similar faults have had quite a bit of discussion on this forum recently. Quite often it is glass encapsulated diodes that prove to be the problem.
Alan.

25th Oct 2018, 12:46 am	#1
kingbeau Tetrode Join Date: Oct 2018 Location: Dundee, Scotland, UK. Posts: 58	Help Needed with Kenwood TS-440S FM Squelch Circuit Hi everyone and thanks for allowing me to join the forum. As per the title I have a TS-440S with a problem with the FM squelch not working. The squelch on AM & SSB works fine. The FM squelch is controlled in IC2 (on the IF board) and is a MC3357P. Looking at the circuit diagram, the voltage on pin 13 should be 6.2v with squelch control at minimum, and 0v with squelch control at maximum. In my case though, the voltage stays at 6.2v regardless of whether the squelch is max or minimum. The only thing that varies with the squelch pot is the voltage on pin 12, which goes from 1.6mV at minimum to 1.2mV at maximum control. I'm a bit out of my depth with this to be honest so I'm hoping someone on here can help? In the meantime I've ordered a new MC3357P just in case. Thank you in advance for any help. http://www.radiomanual.info/schemi/K...-440S_serv.pdf https://image.ibb.co/fKdhAq/Capture.png

25th Oct 2018, 11:37 am	#2
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,556	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit For others looking to help out, the relevant page of the PDF manual is page 88 (of 130). Pin 13 is a bit of a red herring as it is a digital OUTPUT which is meant to indicate to an external circuit whether the squelch is open or closed. This would typically go to a scan control circuit, or in this case either provides or removes the bias to Q16 to unmute / mute the audio, by the look of it. That pin will only change state when the squelch gate changes state. I haven't gone so far as to trace the incoming signal from the squelch control yet, but the squelch control itself seems to have two gangs / sections, one for FM and one for the other modes.

25th Oct 2018, 12:14 pm	#3
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,556	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit Further: The squelch controls are shown at the lower right of page 126 of 130, the connection from the FM squelch control (which is just a variable pulldown to 0V) goes to J7 / FSQ on the right hand side of the IF PCB (Page 89 of 130) and from there via wire... 18, I think, which in turn eventually goes to pin 12 of U2, the MC3357. If you have almost no voltage there at any setting of the squelch control then try measuring the resistance from pin 12 to 0V, if it is very low even with the squelch control turned both fully clockwise and fully anticlockwise then consider C78? (Diagram is poor) as possibly short circuit or low resistance.

25th Oct 2018, 12:46 pm	#4
kingbeau Tetrode Join Date: Oct 2018 Location: Dundee, Scotland, UK. Posts: 58	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit Thank you very much for the help, it really is much appreciated. Resistance between pin 12 & ground varies from 1ohm with squelch control at minimum, to 46k ohms with control at maximum. I've checked C78 & 79 with an ESR tester and they seem within tolerance. Thanks again

25th Oct 2018, 1:30 pm	#5
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,556	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit Your observed resistance of 1 ohm to 46K is consistent with the Squelch control (which is 50K) and its wiring all all the way to U2 being OK. If you have another MC3357 and feel OK about changing it, that may be your possible next step.

25th Oct 2018, 1:52 pm	#6
kingbeau Tetrode Join Date: Oct 2018 Location: Dundee, Scotland, UK. Posts: 58	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit Thanks, I have one on order. I’ll report back once it’s been changed. From the limited information you have about the fault, would you say that’s the most likely cause?

25th Oct 2018, 2:11 pm	#7
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,556	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit ...should have asked this earlier, but is FM reception (of actual signals, not just the presence of no-signal white noise) working OK? The problem is that you just can't turn the squelch 'on'?

25th Oct 2018, 3:01 pm	#8
kingbeau Tetrode Join Date: Oct 2018 Location: Dundee, Scotland, UK. Posts: 58	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit Yes the reception is fine, (I was listening to the locals on 11 metres) just no squelch action. Being an impatient kind of guy I had a rummage around and found a MC3357P in a little CB radio, I've swapped them round and the CB is working fine with the Kenwood's IC but the Kenwood still has the same fault with the CB's IC. I had really hoped that would've solved things

26th Oct 2018, 1:21 pm	#9
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,556	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit OK, can you visually examine / check by any means possible R108(?) (15K), D42 and D43. If the diodes are 'glass' diodes carefully desolder them one at a time and check that neither of them are cracked in half, as it is not unusual for that to happen. They can appear to be perfect, but fall apart as soon as one lead is desoldered. While they are out of circuit, check them and see if they measure the same. If you see nothing untoward there, go wider and check soldering on / measure C80 through to C84 inclusive and R109-R111. My reading of the circuit is that these components along with the 'active filter' stage in the IC (Pins 10-11) sample only the high frequency part - the 'hiss' - from the received audio and generate from that a DC voltage at the junction of D49 / R108 / C79, the voltage there being proportional to the amount of hiss / noise present on the audio coming out of the demodulator. This looks to be what I would call a 'noise' squelch rather than a 'level' squelch - it works out how weak the signal is by measuring the amount of white noise coming in with the signal, rather than by measuring the strength of the incoming signal. The squelch control then adds further bias to the DC voltage before that is passed to the control input on the IC (pin 12), allowing the user to vary the squelch threshold.

26th Oct 2018, 2:32 pm	#10
kingbeau Tetrode Join Date: Oct 2018 Location: Dundee, Scotland, UK. Posts: 58	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit SiriusHardware, you sir are a gentleman and a scholar. Thank you very much for your help. I did exactly as you suggested, R108 measured correct at 15k, D43 was ok but D42 was O/C. According to the service manual D42 is a 1N60. I had a rummage around and found I had a handful of OA91 germanium diodes, popped one in to try it and hey-presto the FM squelch is now working! I'll have to try and find out if the OA91 is a suitable long-term substitute but it's definitely fixed it for the time being. I'm extremely grateful for your help with this, thank you again!

26th Oct 2018, 2:44 pm	#11
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,556	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit You're welcome, enjoy the radio.

26th Oct 2018, 4:19 pm	#12
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,868	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit Thermal stress or a bit of board flexing. Cracked glass bodied diodes are a fairly common fault in many things. A bit too much stress in manufacture and they'll fail years later. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

26th Oct 2018, 7:20 pm	#13
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit Nice fix and not where my money was (capacitor after rectifier). Good work.

26th Oct 2018, 11:22 pm	#14
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Help Needed with Kenwood TS-440S FM Squelch Circuit The OA91's will be fine to use as a substitute. I am sure that some circuits using the MC3357 family of chips specify OA91 type germanium diodes. The proof is that the set is working correctly. Strangely, similar faults have had quite a bit of discussion on this forum recently. Quite often it is glass encapsulated diodes that prove to be the problem. Alan.