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Old 27th May 2024, 12:38 am   #1
HatOfTheCat
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Default Stereo indicator lamp always on - stuck NPN transistor ?

I've managed to get my B&O Beomaster 1100 receiver up and tuning & playing FM radio nicely, sounds great, but the green FM Stereo indicator lamp is just staying permanently on (and thereby also the stereo decoding ) Its on as soon as I switch the FM radio on and doesn't even flicker at all between stations no matter how little signal there is or if its no more than just static, even tuned to the empty "dead zone" off the left of the tuning range it only sometimes goes out at all. Only way to definitely make it go out is to unplug the aerial. I'm thinking this is because of a "stuck" NPN transistor ? Or could the TBA460 IF amplifier chip be at fault ?

The original user manual I have clearly states "When you tune in an FM stereo program the green indicator light will turn on, and reproduction will automatically be two-channel stereo" so it definitely shouldn't be on all the time. I've refurbished two similar 901 model receivers and they were very fussy about the quality of the signal required before going into stereo mode and switching the lamp on.

Going by the attached schematic: my understanding is that for correct operation of the Stereo lamp OIL1, if there is too weak a signal from the IF amplifier TBA460 at the base of transistor TR3 it won't be drawing any current and max voltage will be present at its collector so that this same voltage will be applied at emitter of TR9 so also cutting off that transistor and not allowing OIL1 to light and not switching on the stereo processing.

I assume it could be either of TR3 or TR6 that is "stuck" on ? Or should I be also looking at the TBA460 IF amplifier and those extra components between it and TR3 ?
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Last edited by HatOfTheCat; 27th May 2024 at 1:00 am.
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Old 27th May 2024, 8:52 am   #2
frsimen
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

The lighting of the Stereo lamp OIL1 is controlled by the TBA460's pin 6. A suitable positive voltage on pin 9 will turn the lamp on. I notice that pin 9 is decoupled by a 10uF capacitor, C19, so try changing that first of all. If you have already changed it, make sure it is the correct way around!

As TR3's collector voltage is sourced from pin 6 of the TBA460, it will always be low when the stereo lamp is lit.

If C19 isn't guilty, check the resistors around pin 9 of the TBA460 and consider trying a replacement TBA460.

Paula
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Old 27th May 2024, 12:36 pm   #3
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

When the bulb is not lit, the high-ish voltage on its bottom terminal in the schematic is routed via a few resistors to bias off the frequency doubler later in the decoder. This is what turns the 19kHz pilot tone into 38kHz which in turn operates the switching diodes routing the full multiplex signal alternately between left and right channel outputs.

If there is no stereo light on, there is no 38kHz, so the determination of whether to light the light and enable the decoder has to come from detecting the 19kHz pilot tone.

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Old 28th May 2024, 3:07 pm   #4
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

Not having much luck with this one I'm going to have another poke at this today but here is where I am with it.

Video of how it is currently operating https://youtu.be/mFkm3MMHgWM

frsimen I checked both C19 and also C18 (as that is the one connected to pin 9 of the TBA460), BOTH measured OK but I replaced both and no change. Unfortunately, as the dotted outline on schematic indicates, TR3 and everything feeding into its base is enclosed in a shield box which I can't quite see how to to safely open as yet.

Radio Wrangler yes, it is detection of a suitably strong 19Khz pilot signal that should be condition for the Stereo light being lit and decoder enabled but what is happening is that once that has happened the Stereo lamp (and likely the decoder) are remaining ON, even when NO such signal is present But, oddly, detaching the aerial IS enough to leave insufficient signal and the lamp immediately goes out. I'm starting to get suspicious of the TBA460. I've popped a short video clip on Youtube, above link, which shows exactly what's happening. Power up with tuner set to "dead zone" around 87Mhz and Stereo lamp remains out until tuned to a reasonable signal but it stays on even when then retuned across the dial to frequencies with only very poor signal or none at all and even when returned to the "dead zone". It can eventually decide to go out when left in the "dead zone" but not in any way reliably I've looked at a few Youtube videos of examples of the same receiver and all show the Stereo lamp reacting going on and off as the tuning is moved across the dial so its definitely faulty in some way. Thus my question being about something "stuck" rather than not working at all (which would be easier to fathom). Is it TR3, as I'm assuming, or TR6 that actually provides the route to ground for OIL1 the Stereo lamp as its simply attached on the other side, just via a R24 (270 Ohm, 3W), to the +33V rail.
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Old 28th May 2024, 5:04 pm   #5
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

Ignore my brain burp re. "route to ground", it is pin 6 of the TBA460 that switches the Stereo lamp on (thereby the decoding).

I've done some probing of the TBA460 pin 6 voltage. Sort of interesting but may be nothing ?

- at power on with tuning in the "dead zone" and Stereo lamp OFF is 32.33v, just as expected given only 270r resistor between it and 33v rail

- as soon as first tune to any reasonable signal is 0.89v (schematic gives 0.8v) and Stereo lamp goes on, whether via tuning dial or any of the four presets. This figure is rock solid.

- BUT if return tuning to the "dead zone", or at couple of areas of no discernable signal across the tuning range, it very slightly rises to 0.91v and this is slight difference recurs consistently as you tune back and forth between strong signals (or presets) and the "dead zone" and Stereo lamp thus is remaining on. Almost like pin 6 is trying to change but can only manage a small change

Does this point to the fault being the TBA460 ?
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Old 28th May 2024, 5:33 pm   #6
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

I wrote this before seeing your latest post, but it might help a little!

This circuit operates in a loop, so it isn’t easy to say which part is faulty with any certainty. The subcarrier level is sensed by TR3. In the absence of subcarrier at a sufficient level, TR3 will not be conducting and its collector will voltage will approach the voltage at Pin 6 of the TBA460. The transistor is not responsible for turning on the lamp directly. The maximum collector current drawn by TR3 will be limited by the 18k resistor. Ignoring the lamp resistance, that current will be below 2mA, which is not enough to light the lamp. The lamp will light when pin 6 of the TBA460 is pulled low. That happens when there is a sufficiently large signal on pin 9.

Back to TR3, its collector voltage is used to control TR6. When TR3’s collector voltage is high, TR6 is cut off and the output from the 38kHz frequency doubler is stopped. When TR3’s collector voltage is low, TR6 will amplify again and there will be an output from the 38kHz frequency doubler stage. Pin 9 of the TBA480 is used to sense the level of the 38kHz signal from the doubler. Pin 6 will go high, towards 33V when there is no 38kHz signal, and to go low, towards 0V, when a sufficient level of 38kHz is detected. C29 adds a time constant that will stop a momentary drop in subcarrier level switching the circuit back to mono.

To return to the no subcarrier situation, both the TBA480 and TR3 need to turn off.

Two possible causes of the problem come to mind, one is C29 is faulty or has been installed reversed. The other is that the tuner is being overloaded, resulting in a lot of subcarrier being present even when not tuned in correctly. If the problem is the latter, an attenuator at the aerial input will help. That the stereo lamp will indicate correctly when the aerial is disconnected suggests that TR3, TR6 and IC1 are probably not faulty.
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Old 28th May 2024, 5:55 pm   #7
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

Quick correction to my previous post, the voltage on the collector of TR3 when it is not conducting will be more like 3-4V, not as stated earlier.

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Old 28th May 2024, 6:02 pm   #8
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

One extra test:

When tuned in the "dead zone" with the Stereo lamp stuck on. Then unplugging the aerial brings pin 6 back up to 32v and lamp goes out and stays out when aerial is plugged back in until tune to a signal and the lamp goes back on again... and so on... stuck
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Old 28th May 2024, 6:15 pm   #9
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

frsimen thanks for that I'll have a look at C29, its the original as far as I know as I've not touched anything but the electrolytics. An attenuator being required would be quite funny as I had problems with a similar style B&O 901 receiver not engaging Stereo at all as it needed the signal strength from the primary wall outlet from my building's FM antenna (converted old brewery building with cast iron framing & walls two feet thick, so indoor aerials are useless).
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Old 28th May 2024, 7:03 pm   #10
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

BINGO ! I think we have a winner

I tried the receiver plugged into the FM antenna wall socket in my kitchen which has a pretty lousy FM signal as its clear across the building, so at least a 25+ metre cable run between that and main FM socket in my living room which I've been using (my Tivoli DAB radio works just fine off kitchen socket on DAB band but rather less well in FM mode). Fed from this socket it reliably knocks the Stereo lamp off when tuned anywhere in the "dead zone" range around 87Mhz and even finds a few patches across the tuning range where the lamp will actually flicker or even goes off... which it was absolutely not doing fed from the main socket. I'll order a variable attenuator for it so I can get it set to a good compromise level.

Saved me looking at C29 as that is in the shielded box that I'm really not sure how to actually open
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Old 28th May 2024, 8:24 pm   #11
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

It still seems odd behaviour, that an excessive input signal is recognised as a stereo transmission (subcarrier present) and lights the LED!
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Old 28th May 2024, 8:49 pm   #12
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

This reminds me of the Cleartone CH800 handhelds where sometimes the mute was really hard, only opening at about 20-40 uV. It was tamed by adding a 1nF capacitor on the discriminator output pin on the MC3357 IF/Disc chip. Then it would open at 0.2uV and above depending on the mute knob settiing.
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Old 28th May 2024, 9:02 pm   #13
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

If there are a number of strong signals, the intermodulation products that are produced by the receiver front end will produce all manner of odd noises. It's quite possible that the sub carrier will appear at frequencies well removed from the source signal. The attenuator will tame the reduce the level of the intermodulation products and remove a lot of the inter station noises.

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Old 31st May 2024, 1:13 pm   #14
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

FIX CONFIRMED

Variable aerial attenuator (£5) arrived in post this morning and after 30 seconds of twiddling the adjuster to get a result, I can confirm that adding this to the aerial input has definitely fixed the problem Stereo lamp now only comes on when there's a decent signal and goes out otherwise. It also does not affect performance/sound when tuned into any good signal station and all my presets are coming through nicely. To be honest too strong a signal was the last thing I expected to be the problem, but I live in a large converted industrial building with a shared rooftop FM aerial which has boosters in the loft space for the long cable runs around the structure. Thanks to frisem for suggesting it as a possible cause.

Supplementary question (yup, there's always one). I'd probably want to replace the variable attenuator (0-20dB) with a slimmer inline fixed one for a neater installation. How do I go about measuring & working out what I've actually set the variable attenuator to
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Old 31st May 2024, 9:02 pm   #15
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

Hi HatOfTheCat

It's quite possible that your variable attenuator is DC coupled, i.e. has no chokes or coupling capacitors inside. If this is the case you can disconnect it and measure the resistance to ground at each end and the resistance from input to output using an ordinary multimeter. These three values allow the attenuation in dB to be calculated in a 75 ohm system. If your maths is rusty, post the three values here and I can help !
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Old 31st May 2024, 10:40 pm   #16
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

Mr 1936 - thanks, I'll take you up on the offer of calculation as its been over 40 years since I needed to do that one

Resistance to GND at INPUT 99.5 Ohms

Resistance to GND at OUTPUT 97.1 Ohms

Resistance INPUT to OUTPUT 67.6 Ohms

Its one of these https://www.sacelectronics.co.uk/var...1000mhz_ae5199
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Old 1st Jun 2024, 1:45 pm   #17
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

Hi

Ignoring a tiny bit of asymmetry, those values are equivalent to a "tee" network with 34 ohm series resistors and a 65 ohm shunt resistor.

The attenuation will be 8.5 dB in a 75 ohm system, so a 10 dB or a cascaded 3 and 6 dB fixed attenuator would be a good substitute.
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Old 1st Jun 2024, 6:33 pm   #18
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Default Re: Stereo indicator lamp always on - stuck NPN transistor ?

Thanks Mr 1936, actually found that 9dB fixed attenuators are available so I'll try one of those.
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