Interesting Cybernet modulation fault PLL02A / Hygain V 2705

[pabxone]

Just had an interesting fault go over the workbench.

Philips SSB/AM 201 (Cybernet PLL02A 40 AM/SSB 048AOX chassis)
You folks most likely would never have seen one as they were only sold here in Australia, but for all intents and purposes they are the same as a Hygain V PLL 40 Channel AM/SSB with a different front panel and control layout.

The radio worked fine with the only issue being AM modulation was VERY LOW - could only get 40% modulation at best with a 4 watt carrier.
With a 2 watt carrier - it could barely scrape out 100% modulation.

The radio is 39 years old and the fault showed all the signs of a shorted or resistive electrolytic capacitor so I replaced all the electrolytic and tantalum capacitors in the TX/RX audio chain.

Still no better , I pulled the 2 AMC modulation limiter transistors and the 2 associated DC switch transistors out of circuit but still the same.
I even went as far as subbing out the 3 chokes that feed the modulated audio to the driver and final transistors but still the same problem.

Cybernet radios are not know to be loud modulators but this one at 40% was seriously down and pretty useless as it was.

I had noticed early on that IC5 (TA7205P) Audio IC had been replaced at some stage , I pulled the IC out and noticed that it had been replaced with another TA7205P which indicated that it was very likely that the IC had been replaced a VERY long time ago since the TA7205P has been out of production since around 1980 ( It was superseded by the TA7205AP which is a far more reliable device )

I replaced it with a TA7205AP and the problem disappeared.

IC 5 is a very common failure , If a TA7205P is fitted it WILL fail at some stage (They started to fail at a high rate when the radios were 12 months old back around 1978/79) - when they fail the radio will have several faults.
1/ No receive audio - completely dead - you may get a click or pop out of the speaker when you turn it on and off - but that's it.
2/ No AM modulation - you will get a full normal carrier but no modulation at all
3/ No SSB transmission - NOTHING
4/ No PA - You may get a low level click on key down but that's it.

This fault was interesting as the IC was working to a point and certainly wouldn't have been a suspect for a fault like this in the past.

So it's definitely a wise thing to replace any TA7205P still in any Cybernet chassis with a TA7205AP when you see it as it's going to fail at some stage.

And if you are holding onto any NOS ( or used for that matter ) TA7205P's in your spare parts bin - Can I direct you towards a rubbish bin where you can throw them all into !

TA7205AP are cheap enough to buy these days ( $1.50 each if you buy in quantity )

[sonoma]

good info to have, thanks. there is a shop in your neck of the woods that is a husband, wife and her father that repairs radios and they are great with radios. their main work is commercial radio repairs and she can change every thing inside a radio and make it cover so many channels. they build and program the complete PLL circuits. she has been a member on cbtricks for a few years and her work is fabulous. cbtricks has been down for 1 and 1/2 months now so do not know if it willl come back. their shop is name LBcomms. not sure where she is at in Austailia but if you ever see that shop name they work wonders with radios.

[ Post made via Linux Laptop ]

[pabxone]

I've just had another Cybernet 02A doing the exact same thing - Very low level AM modulation but the receive audio is fine.

This one is a Hygain V Export multimode radio with a PTBM121D4X circuit board - Similar to the last radio but slightly different in layout.

This really had all the hallmarks of a shorted electrolytic capacitor so I replaced all the electrolytic's in the audio stage but still the same problem.

Must be another TA7205P fault I guessed so I pulled the original Audio IC out and replaced it - But still the same fault.
The audio IC I had pulled out was a TA7205AP so I doubted it was going to be the problem after I had removed it.

The problem ended up being C164 ( 4.7uf Tantalum capacitor ) short circuited.

I missed that Tantalum when I was replacing the electrolytics in the audio chain - It's tucked away at the end of the audio IC right up against the heatsink.

These things keep you on your toes !