My Beolit 707 transistor radio has died. Mains power supply is a little low (6.9V vs 7.5V specified) but that's not the fault. DC before the coupling capacitor to the loudspeaker is about half rail voltage (as it should be). Loudspeaker cone is not offset, so output coupling capacitor is not leaky. Suspension seems fine (appararently, Beolit loudspeaker suspensions go stiff; can't imagine why). Turning the volume up gives hiss in the loudspeaker. So I'm figuring that the problem is somewhere in the tuner/IF strip. I'm keenly aware that PCBs of that era have a nasty habit of tracks lifting if you look at them in a funny way (even with temperature-controlled iron and careful use of good desolder braid). That means I need to find the fault first time, not a shotgun replacement of everything in sight that risks upsetting upsets the alignment.
I do have a circuit diagram, but it's quite compact inside, and the PCB is not easy to get at. But the biggest problem is that whilst I understand the principles of how it all works and even have an RF generator capable of FM, I have no idea of what sort of signal levels I should expect at each stage. And typical faults. The tuner and IF strip are silicon, so it's not germanium whiskers. Could anyone assist?
I do have a circuit diagram, but it's quite compact inside, and the PCB is not easy to get at. But the biggest problem is that whilst I understand the principles of how it all works and even have an RF generator capable of FM, I have no idea of what sort of signal levels I should expect at each stage. And typical faults. The tuner and IF strip are silicon, so it's not germanium whiskers. Could anyone assist?
PM me.
Do you have a schematic, or should I search?
I just fixed a clock radio for someone, 1970's Zenith. I did it for them because it reminded me on the products I started working on. Wayyyy back! lol!
Do you have a schematic, or should I search?
I just fixed a clock radio for someone, 1970's Zenith. I did it for them because it reminded me on the products I started working on. Wayyyy back! lol!
We call a schematic a "circuit diagram" over here, and I do have it. What I don't have is an appreciation of expected signal levels when poking around with oscilloscope.
Is there no reception on all wavebands?
I think this would all be better elsewhere on the forum... as it has yet to be fixed 🙂
Maybe so but its still 8% down and that is a stabilised and regulated supply (but an adjustable one) so that needs looking at.
I think this would all be better elsewhere on the forum... as it has yet to be fixed 🙂
Good point; I should have tried MW. I've just tried. Whether MW or VHF, signal level is way down and noise and distortion are up.
Agreed, power supply voltage is 8% down, but it could be the fault that is dragging it down.
Thanks for moving it and starting a new thread. Hopefully someone can offer some diagnostic advice.
Agreed, power supply voltage is 8% down, but it could be the fault that is dragging it down.
Thanks for moving it and starting a new thread. Hopefully someone can offer some diagnostic advice.
I don't see any classic limiter stages in the IF strip, so levels will depend on signal strength. One thing you can try is to feed in approx 10.7 MHz, then vary that until you find a peak at point "C". Keep the level low so you don't overload it. If you can frequency modulate it you should have audio.
Adjustments can drift out slowly and not by much. If the failure was more sudden or after sitting it would suggest a component failure. A transistor can go bad, reducing signal strength. Given they are germanium, this is very possible.
Your meter probe might upset circuit conditions and give a false reading. A scope probe is better, you can put a 100R damping resistor on the probe to isolate capacitance better.
Most of those I worked on were Japanese and I was familiar with which parts tended to fail. European components are a little different in brand and construction. So types of failure-prone capacitors or resistors are something a local service person may know better than me.
Edit:
The power supply is specified as a 9 VDC battery. I see the supply specified as 9V on the diagram.
Adjustments can drift out slowly and not by much. If the failure was more sudden or after sitting it would suggest a component failure. A transistor can go bad, reducing signal strength. Given they are germanium, this is very possible.
Your meter probe might upset circuit conditions and give a false reading. A scope probe is better, you can put a 100R damping resistor on the probe to isolate capacitance better.
Most of those I worked on were Japanese and I was familiar with which parts tended to fail. European components are a little different in brand and construction. So types of failure-prone capacitors or resistors are something a local service person may know better than me.
Edit:
The power supply is specified as a 9 VDC battery. I see the supply specified as 9V on the diagram.
Power supply is definitely 7.5V or five D cells, not 9V. I'm going by the diagram in the little envelope inside all Beolit radios. Apart from the AC187/AC188 germanium output stage, all the transistors have a B prefix, indicating silicon. I know earlier Beolits had more germanium, but not this 707.
What sort of level of 10.7MHz at test point C? I can do levels down to uV. In my diagram it appears to be the input to the FM discriminator. 50mV? I think I have found test point C on the PCB. I have set 10.7MHz carrier with 10kHz deviation and 400Hz modulating signal on generator.
I haven't tried the Aux in, but I'm fairly sure the amplifier/loudspeaker combination is fine - its quiescent DCs are as expected.
What sort of level of 10.7MHz at test point C? I can do levels down to uV. In my diagram it appears to be the input to the FM discriminator. 50mV? I think I have found test point C on the PCB. I have set 10.7MHz carrier with 10kHz deviation and 400Hz modulating signal on generator.
I haven't tried the Aux in, but I'm fairly sure the amplifier/loudspeaker combination is fine - its quiescent DCs are as expected.
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Never assume, either try it or look with a scope for audio on the top of the volume control.
Also check the supplies and DC voltages at key points in the RF stages. For example there is a 68 ohm series feed resistor. Check voltages both sides. Look for something common to both FM and AM sections... starting with supplies.
I must have a different version. Similar, but it indicates 9VDC. No matter, the diagram I have is close enough to work with.
I'll assume you checked DC voltages. That's troubleshooting 101 and you're well past that. As I mentioned, there does seem to be any limiting stages, so signal levels will be highly dependent on input signal strength. You indicated common low volume and distortion on FM and MW bands. Logic then points you to past the selector switch to the AF section. My next test would be the Aux input, or finger on volume control with it maxed. Functional test in that case.
For the FM detector, I would expect a few volts on TR5 collector. Might be lower for a portable, but higher than 50 mV.
I'll assume you checked DC voltages. That's troubleshooting 101 and you're well past that. As I mentioned, there does seem to be any limiting stages, so signal levels will be highly dependent on input signal strength. You indicated common low volume and distortion on FM and MW bands. Logic then points you to past the selector switch to the AF section. My next test would be the Aux input, or finger on volume control with it maxed. Functional test in that case.
For the FM detector, I would expect a few volts on TR5 collector. Might be lower for a portable, but higher than 50 mV.
As it happens, I haven't checked the DCs in the IF strip. Mainly because (being a 1980s PCB) silk screen labelling hadn't been invented yet. It took me a good five minutes to find TP4 on the circuit diagram and a further minute or two to mentally mirror the PCB diagram and find TP4 on the PCB. Good point about never assuming. It's a long while since I did faultfinding on something I didn't design myself. I can easily bodge up a phono to DIN lead and squirt some music into the amplifier. >50mV noted at TP4. I've another job to finish before seriously attacking the Beolit, but thanks for the advice everybody. I'll see what I find and report back.
it's a strange thing, but working with valves, I hesitate to put my finger onto live electronics, even though I know it's a perfectly good 50Hz source.😉
My last electric shock came from line scan (1200V peak) as I was groping round the back of a display tube whilst looking at the cross-hatch on the front; that was about thirty years ago (and I'd like to keep it that way). Although the B&O can run off batteries, it needs five expensive D cells, so I'm running it off mains. Its mains lead doesn't have an earth connector, so I may add an explicit earth to its chassis just in case whilst poking around. It's not a problem to knock up a DIN lead, so that will almost certainly happen. A closer look at the (tiny, and difficult to read) audio amplifier diagram shows that the four transistors are not DC coupled, so just because the quiescent at the output is correct does not mean the input stage is correct. But what really drives me nuts is that it is a "siliconised" circuit that started out as germanium, and that means all the voltages are the wrong way up and it's full of PNP silicon when they should be NPN.
I got nailed with an oscilloscope HT, more than once. Touch the intensity control and you're part of the circuit. Intensity controls were located conveniently behind the face on the top corner.