FM Stereo indicator lamp in Tuner circuit
Wondering if someone could take a look at my Roberts Model 50 receiver schematic and be able to tell me what voltage the stereo indicator lamp should be and explain how that part of the circuit works.
I believe it should only light the lamp when a FM "stereo" station is dialed in. There is no value listed for the lamp and the existing one was burned out and the socket badly corroded. The lamp socket sits behind a small red jewel lens on the front panel.
There are also "dial" lamps which ARE listed in the schematic (3.2V x 6) but those are powered from a separate 3.2VAC winding from the power transformer and working fine.
The indicator lamp also has a different P/N in the service manual than the dial lamps but of course the P/N doesn't give you any clue as to the voltage or current.
So I'm pretty sure the stereo indicator lamp is a different voltage and current.
There is a voltage listed in the closeup schematic where the "star" symbol is shown (8.4V) and it is on one side of the indicator lamp socket but when I put a meter across the socket (no lamp installed) it measures 24V all the time. No change in voltage as I move the dial around to a stereo signal.
The two transistors Q13/14 look like what switches the lamp on when "stereo" is detected, but again I'm not very good at figuring out circuits and starting to think there is something wrong in that portion of the circuit.
I've attached a full schematic and then a closeup of the stereo indicator section.
Hoping someone can look at those and help me out....Thanks for any help and expertise you can offer!
Q13 and Q14 together form a Schmitt trigger, but this uses the stereo lamp as part of the circuit so it won't switch with the lamp open circuit. The 8.4V rail seems to come from a resistor near Q29/Q30 in the PSU but the bottom of this part is cut off your diagram.
My guess is something like 8V 40mA might work. Alternatively, an LED in series with a resistor (470-1K?)). The resistor would get hot so use 2W?
As above but I would suggest an LED in series with a suitable resistor (many LED's need 1ma or less) and then a parallel resistor across the lamp socket to get the correct current draw so the circuit works OK. The Schmitt trigger just prevents the bulb from being in an indeterminate state or from flickering. Its either on or off... in theory :)
That MPX decoder is clever, I like it.
24V is a little worrying, as the circuit implies that 8.4V is what should be available there, so I would have a look at the power supply before replacing the lamp.
The circuit works as follows: stereo multiplex is applied to the buffer amplifier Q10, and the signal split, with one path across the top filtered by a parallel tuned circuit to remove the 19Khz pilot tone and a second via the tuned transformers and Q11,12 tuned to a sharp 19Khz.
This path inclued a frequency doubler D9,10 so the drive to Q12 is at 38Khz and at sufficient level to saturate the diodes following the final transformer, the centre tap of which has the MPX signal minus the 19Khz carrier presented to it.
The effect is that the switching diodes and transformer both demodulate the 38Khz DSB S component and perform the addition/subtraction to give L & R.
There is an ALC loop around the 19Khz chain (Reduces the effect of multipath), from the emitter of Q12 to the emitter of Q11, and it is this voltage (which rises when the 19Khz stereo pilot is present) that drives the schmitt trigger.
Thanks to all for your help and explanations...
I attached the bottom part of the schematic and a close up of both types of lamps.
The (frosted) left lamp is one of the dial lamps "3.2V 0.16A".
The (clear) right lamp is the indicator lamp "6V 0.05A" I think!.....
The "6" is hard to see and all these lamps were solder blobbed into their sockets, so by the time I got the solder wicked off and unscrewed them...it was hard to read the markings!
I thought because of the 24V reading across the ind. socket maybe it was a "26V" marking on that lamp!
But if "8V 40mA" makes more sense and also needs to be "in circuit" then the "6V 0.05A marking makes much more sense than "26V".
So that is good....I think 6-8V 40-50mA is what I need to find.
Like Dan mentioned I thought there was also something wrong with that 8.5V power supply location so I lifted and checked R502 (400ohm 2W) and R503 (2.2Kohm 1/2W). They both read correct resistance and C507 is a new replacement cap.
I measured 30V at the top of R502, but 24V between R502/503 (across C507) but that was without the lamp installed!??
So I'm still confused if there is something wrong with the power supply, or I just need to get a lamp plugged into that socket and the voltage will be correct?
I'll see if I can find an LED and 2W resistor to test it, but I would eventually like to get a replacement incandescent...
Thanks Dan for that explanation of how the stereo signal is stripped out and detected. Tuner schematics are pretty intense and confusing for me. But I like to learn something new everyday!
I've managed to get this 1963 Roberts receiver back up and running very well, and these lamps are the last phase of repair.
So far I haven't touched anything in the tuner at all. It does seem to work fine.
There are a few electrolytic caps on the tuner board that I could replace, but haven't. Do you think that would be a good idea?
Thanks for all your help and suggestions!
Remember your bulb has R212 and R227 in series with it so they will drop some voltage.
Are we going to try and work it out as a series chain or just do it empirically ? :D
Hmmm... R212 and R227 and 24 volts and a 6 volt bulb doesn't sound right. If the resistors were a factor of 10 bigger then maybe.
Something doesn't ring true here with the circuit and the bulb. There is a 24 volt supply shown on the diagram....
Could the bulb be non original and someone has swapped it for the wrong rating ?
I'm hoping that is what's going on with the lamp being in series there and not some problem with that part of the power supply. Everything else on this unit is working and has correct voltages.
I can check R212 and R227 to be sure they are not open.
If there's a way to know that's why the voltage is high or you can do the calculation that would be a big relief and save me burning out a bulb or LED testing.
I'm not very good at doing the math...
I see your reposting of the circuit now shows the missing bits... I'll come back to you in a few minutes after seeing how it calculates out :)
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