Grundig HitBoy 310 Radio - FM dead

[richieyork]

Hi all

I've been handed a Hit Boy 310 - FM dead. MW and LW are fine.

I've cleaned the wavechange switch with contact cleaner but no result.

When switched to FM the only sound from the speaker is the hiss from the amp - nothing remotely RF sounding or any obvious IF activity.

So, before I start to tear it apart does anyone have a circuit diagram they'd kindly let me have

Does anyone recognise this fault with this set.

Is there an easy way to verify if the FM oscillator is working without resorting to test equipment I may not have?

I'd dearly like to get this going as it's elderly owner is attached to it (in spite of it's negligible worth).

Many thanks
Richard

[paulsherwin]

Check for plausible voltages at the transistor bases and collectors. You don't need service info to do this as you are looking for readings that are completely wrong, not some marginal variation.

Check all the FM IFTs for continuity. This is an early Malaysian Grundig, but I know the last of the Portuguese models sometimes had dodgy IFTs so this may be affected too.

If you have a signal injector, inject at the discriminator diodes and work backwards until the signal disappears.

If you don't have a signal injector, build one - you only need a couple of transistors and a few other bits.

[Nuvistor]

https://www.radiomuseum.org/r/grundig_hit_boy_310.html

http://www.free-service-manual-downl...-116515-0.html

[richieyork]

If you don't have a signal injector, build one - you only need a couple of transistors and a few other .

Paul, would a standard AF signal injector do for this test?
Cheers

[paulsherwin]

Yes it would, but a standard multivibrator injecter will shove a signal past anything because of the square wave waveform.

If you have a standard signal generator that will create a 10.7 MHz signal then you can use that, but it won't provide a frequency modulated signal, so you will need to monitor the received signal with a DMM, which is more difficult and counterintuitive unless you have experience.

Of course, professional test gear will generate the required signals, but I'm assuming you don't have access to that.

[richieyork]

Ok, I have a NE555 chip, can build a gen using that. I assume they output a square wave?

[Nuvistor]

Page 236 has a 2 transistor multivib for signal injection.
http://www.americanradiohistory.com/...PW-1963-07.pdf

[paulsherwin]

Or something like this: http://jestineyong.com/how-to-make-y...gnal-injector/

You can build something based on a 555 if you want to, but there's no advantage in doing it. You just need a squarish wave frequency generator running at about 1kHz.

[Nuvistor]

Paul, the transistors will be a bit cheaper in that one.

I built the one in PW, very useful, grief, 55 years ago.

[Boom]

Originally Posted by Nuvistor

http://www.americanradiohistory.com/...PW-1963-07.pdf

Note adverts. PX4 - 12/6d (62.5p) Leak Stereo 20 - £30

[paulsherwin]

You can build them with almost any transistors you have to hand, including ones scavenged from scrap equipment. You can build a highly professional instrument or just birdsnest the bits depending on your attitude. I built my most recent one inside an old TV remote control.

[richieyork]

Thanks to all for your suggestions. I've found a the circuit diagram for the Grundig on the Web so next will build the square wave gen and take it from there. Hopefully a positive report will be back on here in due course!
Kind regards
Richard