Comms belt-pack: no output

[MidgetWillis]

So I'm currently trying to repair the unused (and quite broken) comms system at a school that I help out with to replace their current solution of cheapy walkie talkies. One of the belt-packs (tecpro BP111) is giving me quite a headache. When connected the microphone works fine, outputting through other packs but the headphones don't. There is no sound output from them of the connected mic or other packs in the loop. I've opened the pack up and can't see any clear component damage or bad soldering.

 

My question is: has anyone had this problem before? Is there a common culprit? My assumption so far is that some component has failed but which one is it likely to be?

 

Any help would be greatly appreciated as we simply don't have the money to buy replacements.

[mackerr]

So I'm currently trying to repair the unused (and quite broken) comms system at a school that I help out with to replace their current solution of cheapy walkie talkies. One of the belt-packs (tecpro BP111) is giving me quite a headache. When connected the microphone works fine, outputting through other packs but the headphones don't. There is no sound output from them of the connected mic or other packs in the loop. I've opened the pack up and can't see any clear component damage or bad soldering.

 

My question is: has anyone had this problem before? Is there a common culprit? My assumption so far is that some component has failed but which one is it likely to be?

 

Any help would be greatly appreciated as we simply don't have the money to buy replacements.

 

Can we assume you have tried known working headsets with it? Headset failure is far more common than pack failure.

 

Mac

[MidgetWillis]

So I'm currently trying to repair the unused (and quite broken) comms system at a school that I help out with to replace their current solution of cheapy walkie talkies. One of the belt-packs (tecpro BP111) is giving me quite a headache. When connected the microphone works fine, outputting through other packs but the headphones don't. There is no sound output from them of the connected mic or other packs in the loop. I've opened the pack up and can't see any clear component damage or bad soldering.

 

My question is: has anyone had this problem before? Is there a common culprit? My assumption so far is that some component has failed but which one is it likely to be?

 

Any help would be greatly appreciated as we simply don't have the money to buy replacements.

 

Can we assume you have tried known working headsets with it? Headset failure is far more common than pack failure.

 

Mac

 

Yes, I've tried multiple working headsets to no avail

[Dave m]

Open a known good one and try a meter to see where any differences are?

[boatman]

If it helps the schematic for this beltpack can be found here.

[timsabre]

There is basically a mic preamp, then a line driver / receiver hybrid, then a small power amp stage to drive the headphones. You should be looking in this last stage. Often its the volume control has been physically damaged, I'd check resistance on that first.

[MidgetWillis]

I've done some prodding around with a multimeter and it seems to be one of the transistors acting up. I've ordered a replacement so will see if that sorts it.

[Don Allen]

The output stage is a Class B Complementary Symmetry amplifier. The link will explain how it works. I usually replace both transistors as a set, in case the other one is starting to fail, but the output resistor R33 may be open circuit.

 

When testing belt packs, use the presence or absence of sidetone to help troubleshooting. If you can hear yourself on a beltpack, it is a quick check that mic and output amps are working. Next step is to add another beltpack to test audio quality.

 

These notes might be of use to you when troubleshooting talkback.

 

I had a Production Intercom, not Tecpro BP-1 belt pack with a faulty volume control and had to buy the volume pots from Production Intercom because of the sideways rotary control.

 

If you come across Production Intercom BP-1 beltpacks where you get hum when touching any XLR connectors, the system is using an ac adaptor that does not have an earthed output dc plug. No hum when not touching any XLR connector. Quick check is to look to see if the mains plug has an earth pin and the IEC input of the ac adaptor has an earth pin. I got caught with this fault recently. The earthed 24V DC ac adaptors are much dearer, which is why clients try to avoid buying them. You can get laptop ac adaptors that are earthed that are thrown out these days.

[MidgetWillis]

I've changed TR8 and all is working perfectly. ? I'll keep an eye on it incase the other transistors on the way out but it seems all good for now.

 

Thanks for the help ?

[wintec]

I have just found a belt pack with the same fault. I'll order a pair of transistors tomorrow! Thanks for telling us how you got on and what your fault was.

 

Best Wishes

David