Trio KT-50L LW/MW/FM tuner - UK Vintage Radio Repair and Restoration Discussion Forum

SteveCG · 22nd Jul 2019, 1:04 pm

This tuner is from the 1981/1982 era - and is an analogue tuner, but with digital frequency readout. Apparently it is also known as a Kenwood KT-50L.

I have tried HiFiEngine and electrotanya for information on it (either user or service) but nothing doing.

The tuner essentially works - however there are two problems:

The first is that the signal strength led display only shows the first led brightly, the second one is dim and the others unlit. This is so even with a very strong local FM signal. However upon power switch on, the first 4 leds light up - with uneven illumination and then quickly fade to the situation mentioned earlier.

The second problem concerns the stereo/mono - mute/unmute combined switching. In the stereo setting both the stereo led and the 'locked' led light up brightly. If now mono mode is selected the stereo led goes out (as expected) but the locked led just dims. If now in this mono mode I tune off to a comparatively empty part of the spectrum then I just get silence. It is as if the muting function is still in operation. I had expected that in mono mode I'd be hearing hiss. Not having the user manual though I don't know what the tuner should be doing.

Can anybody help with information please?

Jonster · 22nd Jul 2019, 1:21 pm

I would be inclined to first check the connections between the main PCB and the front panel LEDs as it sounds like an earthing fault, the symptoms are similar to those when you get a poor earth on a vehicle light cluster.

paulsherwin · 22nd Jul 2019, 1:54 pm

The Kenwood KT-50 will be the model sold outside Europe without LW.

Many analogue tuners with frequency readouts sold in the early 80s were rush jobs based on older near identical designs. The manufacturers just bolted frequency displays onto earlier models as a stopgap. You may be able to find info on the ancestor model of the KT-50, though it sounds as if your fault is actually in the display subassembly.

It may help diagnosis if you identify the muting transistors and remove them temporarily. They will be quite close to the audio output. (Some people choose to remove them permanently because they think it improves the sound.)

SteveCG · 23rd Jul 2019, 12:30 pm

An update:

I've done a first attempt at circuit diagram of the display subassembly. I think that I have two problems here. The first is an open-circuit 2sc1694 npn transistor, the second is that I suspect at least two of the green 'rectangular shaped' green leds are not working properly (I'm measuring 4 volts across one that is only dimly glowing).

Anyway I want to go over the circuit again and double check.

I haven't done anything yet about the muting, but I've got a list of the ICs the tuner uses so my next step is to get the data on them.

Paul - Yesterday I tried looking for the KT-30 (which I assume is its predecessor) but could not find info on it either - mind you it might not be the predecessor).

paulsherwin · 23rd Jul 2019, 1:17 pm

As it happens, I actually have a KT-30L sitting in the loft, but I don't have service info for it and don't know how closely related it is to the KT-50L. It's possible they were adjacent models in the same range. It was quite common for manufacturers to offer essentially the same tuner at two different price points, one with a digital display and one without.

SteveCG · 24th Jul 2019, 11:41 am

Paul,

I've been trawling through the web and I reckon that the KT-30L is indeed the same tuner at the KT-50L, but as you say, without the digital display.

Anyway some chip info on the KT-50L, numbering as on the pcb:

IC1 HA12411 FM IF System
IC2 AN6551 Dual OP Amp
IC3 LA1130 AM Tuner
IC4 AN6551 Dual OP Amp
IC5 AN115 Stereo Decoder (apparently v. similar to MC1310)
IC6 AN6135 Hi-Fi Pop Noise Canceller

paulsherwin · 24th Jul 2019, 1:16 pm

The faults you've described don't sound like bad chips to me, though it's obviously a possibility.

Apart from the obvious connection and dry joint issues, lots of tuners from this era have bad electrolytics now.

SteveCG · 24th Jul 2019, 2:15 pm

My approach, now I have the IC info, will be to work back from the mute pin on the FM IF IC to the mute selection switch.

The signal strength LED issue I don't think is related and I'll look again at that later.

The electrolytics look fine - but that is no guarantee of not having dried out.

22nd Jul 2019, 1:04 pm	#1
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Trio KT-50L LW/MW/FM tuner This tuner is from the 1981/1982 era - and is an analogue tuner, but with digital frequency readout. Apparently it is also known as a Kenwood KT-50L. I have tried HiFiEngine and electrotanya for information on it (either user or service) but nothing doing. The tuner essentially works - however there are two problems: The first is that the signal strength led display only shows the first led brightly, the second one is dim and the others unlit. This is so even with a very strong local FM signal. However upon power switch on, the first 4 leds light up - with uneven illumination and then quickly fade to the situation mentioned earlier. The second problem concerns the stereo/mono - mute/unmute combined switching. In the stereo setting both the stereo led and the 'locked' led light up brightly. If now mono mode is selected the stereo led goes out (as expected) but the locked led just dims. If now in this mono mode I tune off to a comparatively empty part of the spectrum then I just get silence. It is as if the muting function is still in operation. I had expected that in mono mode I'd be hearing hiss. Not having the user manual though I don't know what the tuner should be doing. Can anybody help with information please?

22nd Jul 2019, 1:21 pm	#2
Jonster Heptode Join Date: Apr 2008 Location: Eastbourne, East Sussex, UK. Posts: 671	Re: Trio KT-50L LW/MW/FM tuner I would be inclined to first check the connections between the main PCB and the front panel LEDs as it sounds like an earthing fault, the symptoms are similar to those when you get a poor earth on a vehicle light cluster.

22nd Jul 2019, 1:54 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: Trio KT-50L LW/MW/FM tuner The Kenwood KT-50 will be the model sold outside Europe without LW. Many analogue tuners with frequency readouts sold in the early 80s were rush jobs based on older near identical designs. The manufacturers just bolted frequency displays onto earlier models as a stopgap. You may be able to find info on the ancestor model of the KT-50, though it sounds as if your fault is actually in the display subassembly. It may help diagnosis if you identify the muting transistors and remove them temporarily. They will be quite close to the audio output. (Some people choose to remove them permanently because they think it improves the sound.)

23rd Jul 2019, 12:30 pm	#4
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: Trio KT-50L LW/MW/FM tuner An update: I've done a first attempt at circuit diagram of the display subassembly. I think that I have two problems here. The first is an open-circuit 2sc1694 npn transistor, the second is that I suspect at least two of the green 'rectangular shaped' green leds are not working properly (I'm measuring 4 volts across one that is only dimly glowing). Anyway I want to go over the circuit again and double check. I haven't done anything yet about the muting, but I've got a list of the ICs the tuner uses so my next step is to get the data on them. Paul - Yesterday I tried looking for the KT-30 (which I assume is its predecessor) but could not find info on it either - mind you it might not be the predecessor).

23rd Jul 2019, 1:17 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: Trio KT-50L LW/MW/FM tuner As it happens, I actually have a KT-30L sitting in the loft, but I don't have service info for it and don't know how closely related it is to the KT-50L. It's possible they were adjacent models in the same range. It was quite common for manufacturers to offer essentially the same tuner at two different price points, one with a digital display and one without.

24th Jul 2019, 11:41 am	#6
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: Trio KT-50L LW/MW/FM tuner Paul, I've been trawling through the web and I reckon that the KT-30L is indeed the same tuner at the KT-50L, but as you say, without the digital display. Anyway some chip info on the KT-50L, numbering as on the pcb: IC1 HA12411 FM IF System IC2 AN6551 Dual OP Amp IC3 LA1130 AM Tuner IC4 AN6551 Dual OP Amp IC5 AN115 Stereo Decoder (apparently v. similar to MC1310) IC6 AN6135 Hi-Fi Pop Noise Canceller

24th Jul 2019, 1:16 pm	#7
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: Trio KT-50L LW/MW/FM tuner The faults you've described don't sound like bad chips to me, though it's obviously a possibility. Apart from the obvious connection and dry joint issues, lots of tuners from this era have bad electrolytics now.

24th Jul 2019, 2:15 pm	#8
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: Trio KT-50L LW/MW/FM tuner My approach, now I have the IC info, will be to work back from the mute pin on the FM IF IC to the mute selection switch. The signal strength LED issue I don't think is related and I'll look again at that later. The electrolytics look fine - but that is no guarantee of not having dried out.