1969 itt/kb ka2010

[Hunts smoothing bomb]

Evening all,

I've been working on one of these the last couple of evenings for a friend and I have the record deck and all bands working well.
I have had to change bad electrolytics in the PSU rails mainly to restore the audio to its former quality.

The unit concerned has the optional FM decoder installed and the issue I have is that even with a decent aerial (house roof via amplifier) connected I cannot receive FM stereo although the sound quality is very good, I also cannot get the FM beacon to illuminate.

I have done a search on the net and I cannot find much about how to test this area that I am having trouble with.

If the optional decoder module has not been factory fitted, the multiplex combined audio signal from the IF board is fed to the left and right inputs of the pre-amplifier. (this works, I've tried it)

With the decoder fitted, the multipex combined with audio instead feeds into a pin on the decoder module and the separate left and right audio outputs from this module then feed into the left and right audio inputs on the pre-amplifier that were originally connected to the multiplex (if you get my drift!)

Now firstly I would like to find out how to test to see whether the IF module is outputting the correct multiplex signal and pilot tone, is there a way?

Secondly, can I test the decoder by powering it up from a separate PSU of a suitable voltage and inject a 19khz pilot tone to the pin where the audio combined multiplex went to try and illuminate the stereo beacon?

I feel strongly that the issue is in one of these two modules.

Cheers

[Nuvistor]

Did it ever work? Just a thought if the decoder has been fitted at a later date have the de-emphasis components been removed from after the FM detector.

On silent passages on an FM stereo signal a scope should resolve the pilot tone from the FM detector, it will not be obvious when audio is present, all mixed together.

[DangerMan]

How is/was the decoder retrofitted? Is it a plug in modification?

Without the decoder fitted de-emphasis would have been provided in the signal path (50uS time constant for the UK).

When the decoder is added this must somehow be removed from the discriminator's audio output in order to preserve the high frequency multiplex signals to the decoder, and then must be added back to the output of that decoder to preserve the correct audio response.

Is there a possibility that the original de-emphasis was never removed when the decoder was added?
It may be as simple as a capacitor that was never removed when it should have been, but it could be a lot more involved.
Rule out the easy possibilities first !

[When I checked before submitting this post I see that Frank (Nuvistor) has covered this whilst I was typing.]

What type of decoder is it? (i.e. transistor, IC, and if IC what type, etc.)

Reason for asking is that electrolytic caps can cause trouble in discrete transistor ones, and MC1305/1307 type IC's have some inherent IC reliability problems.

One nasty possibility is that the tuner has inadequate bandwidth to recover stereo... even HJ Leak fell foul of this with their Troughline.
However, I think they actually indicated a stereo signal was present but failed to give a stereo output because the L-R signal centred on 38Khz (but with sidebands extending to 53kHz) was missing or badly degraded.

Pete

[AJSmith625]

Have you checked the stereo beacon bulb itself and that it is the correct voltage/current. I can remember some decoder designs that would not work if the actual bulb was OC or someone had fitted the wrong bulb.

Alan.

[DangerMan]

Originally Posted by AJSmith625

Have you checked the stereo beacon bulb itself and that it is the correct voltage/current. I can remember some decoder designs that would not work if the actual bulb was OC or someone had fitted the wrong bulb.

Alan.

Further to that, the bulb was often switched directly by the IC (obviously in the IC type of decoder) and it was usually a low current type... 1969 is a bit early for LEDs.
If someone has ever fitted an ordinary 300mA bulb it can be bye-bye IC.

Does the background noise increase when the unit is switched from mono to stereo? (If it can be)
That would indicate that it actually sensing and responding to a stereo signal.

Pete

['LIVEWIRE?']

If you have a frequency counter or oscilloscope, a 19khZ tone should be present at one of the pins of the decoder when it is set to stereo -which pin depends on the decoder fitted.

[Hunts smoothing bomb]

Originally Posted by Nuvistor

Did it ever work? Just a thought if the decoder has been fitted at a later date have the de-emphasis components been removed from after the FM detector.

On silent passages on an FM stereo signal a scope should resolve the pilot tone from the FM detector, it will not be obvious when audio is present, all mixed together.

I have been told by the owner that it did work and the stereo beacon bulb used to illuminate.

Cheers

[Hunts smoothing bomb]

Originally Posted by AJSmith625

Have you checked the stereo beacon bulb itself and that it is the correct voltage/current. I can remember some decoder designs that would not work if the actual bulb was OC or someone had fitted the wrong bulb.

Alan.

Hi Alan, the lamp was open circuit, it was a 6v 0.1A type and so I have fitted a 6v 0.05A type, it's all I had but should work no problem as it is simply switched by an AC128

Cheers

[Hunts smoothing bomb]

Originally Posted by DangerMan

Further to that, the bulb was often switched directly by the IC (obviously in the IC type of decoder) and it was usually a low current type... 1969 is a bit early for LEDs.
If someone has ever fitted an ordinary 300mA bulb it can be bye-bye IC.

Does the background noise increase when the unit is switched from mono to stereo? (If it can be)
That would indicate that it actually sensing and responding to a stereo signal.

Pete

Hi Pete,

The decoder is too early for IC and uses discrete components (AC126's all throughout except the stereo beacon driver and switch)
The unit does not have a mono/stereo switch, it is in stereo mode all of the time.

Cheers

[Hunts smoothing bomb]

Originally Posted by DangerMan

How is/was the decoder retrofitted? Is it a plug in modification?

Without the decoder fitted de-emphasis would have been provided in the signal path (50uS time constant for the UK).

When the decoder is added this must somehow be removed from the discriminator's audio output in order to preserve the high frequency multiplex signals to the decoder, and then must be added back to the output of that decoder to preserve the correct audio response.

Is there a possibility that the original de-emphasis was never removed when the decoder was added?
It may be as simple as a capacitor that was never removed when it should have been, but it could be a lot more involved.
Rule out the easy possibilities first !

[When I checked before submitting this post I see that Frank (Nuvistor) has covered this whilst I was typing.]

What type of decoder is it? (i.e. transistor, IC, and if IC what type, etc.)

Reason for asking is that electrolytic caps can cause trouble in discrete transistor ones, and MC1305/1307 type IC's have some inherent IC reliability problems.

One nasty possibility is that the tuner has inadequate bandwidth to recover stereo... even HJ Leak fell foul of this with their Troughline.
However, I think they actually indicated a stereo signal was present but failed to give a stereo output because the L-R signal centred on 38Khz (but with sidebands extending to 53kHz) was missing or badly degraded.

Pete

Hi Pete,

It is a transistor decoder module using AC126's, I am wondering now after what you say whether those little blue Philips style electrolytic caps on the module are giving me trouble?

I think the de-emphasis is involved with the removable link of the 5 way adaptor plug as it has two very small value caps from the MX linking to the L & R inputs, this plug is removed and discarded when the 5 pin decoder is plugged in, hopefully a schematic attached of the RF/IF module and also the decoder.

Cheers

[Nuvistor]

The decoder circuit is straightforward, presume it’s in a small Mullard module that’s a pain to work on.

With there being a good mono signal from the IF strip and has the IF was designed for stereo I think the decoder will be at fault.

[Hunts smoothing bomb]

Originally Posted by Nuvistor

The decoder circuit is straightforward, presume it’s in a small Mullard module that’s a pain to work on.

With there being a good mono signal from the IF strip and has the IF was designed for stereo I think the decoder will be at fault.

Thanks Frank, I am leaning towards this theory also and you are quite right, the decoder module (after removing the screen) is well populated and very tightly packed with spidery print, I'm not too sure how easy it would be to change all of those caps.

With the decoder powered, can I inject 19khz into the MX input pin to see if the stereo beacon will illuminate or is there a little more to it than that?

Cheers

[Nuvistor]

You could inject a signal but why not trace it with a scope while you have the cover off. It uses AC126 transistors which could have tin whiskers and voltage checks may show the fault.
There is a 19khz doubler, 38Khz driver for the diode matrix, it’s unlikely the fault is in the two audio amp stages of the module. Presume the first tuned circuit is 19Khz and the second in the collector is 38Khz with the one driving the diode matrix also 38Khz.
A scope will also trace the L+R audio signal.
The circuit is straightforward but I couldn’t work in that tiny PCB, too small for me these days.

[Hunts smoothing bomb]

Thanks for the info Frank,

I will try and apply this useful information here maybe Saturday, it looks like the weather is going to be too grim for doing things outdoors!

Cheers

[Nuvistor]

The tuned circuit in the collector of the first transistor could 19Khz and the doubling is double by the diodes.

[vidjoman]

Just noticed that the circuit diagram for the decoder shows 'voltages measured in the LP1163 are measured with respect to IT'S pin 3'. The decoder is 'upside down' to the normal power supply.

[crackle]

You know the service information for this Unit Stereo is on my website.
http://www.kbmuseum.org.uk/itt_servi...KA2010_ITT.pdf

Mike