Cobra 148 warbling

[163TM847]

The 1969 is a 20w the 2312 is a 25w but you would think the 1969 should work fine but for reasons i am not really sure of the 1969 doesn't work properly in most 148/360 [PB010], I do use 1969s and then in most cases i have to lower the value of C208 to get rid of the SSB warble, Pretty much the same as i explained about the 3900 issue.

[Mojo666]

Hello chaps, not yet had time to follow it up any further yet as I've had the fun police on my back all day.
I honestly can't say if it does it with the top lid off but I will give it a go Nick. Everything in there looks original so I don't think anything has been replaced the green paint is still on the nuts.
I should have an update tomorrow night.
Cheers

[Mojo666]

Well, I finally managed to have a look at the 148 last weekend and think I have sorted it. After spending a couple of hours reading through past posts and threads I decided to have a look inside and see if there was any distorted/damaged capacitors and noticed that one of them was Dome shaped on the top the (big one). I replaced it and omg no warbling. I left the radio switched on for 24 hours and still warble free.
' Touch wood' it will be all good.
Thanks for your help fellas, always appreciated.

[cb4ever104]

Glad you got it sorted .

S.

[Mojo666]

It's at it again. It's been away three times now and still doing it, I'm at my wits end.
Has anyone got a copy of the cobra project cd or point me in the right direction.
It's a mk2 pb010AA
Checked chassis screws
New caps
Full alignment
It starts with the bad audio after say 5mins of transmission.
Heeeeeelp.
Pls

[cb4ever104]

Is the bias ok on the driver and final ?

[andrew013]

you have tried new audio chip ???? and have you tried the finger test to see if anything gets way too hot and or alters the audio on tx , if fault comes on after 5min it looks like its after something has warmed up have you tried those ice cold spray tins they are good for locating over hot componentsas well maplins sell them I think
and im guessing its 5 min of ssb talking

[Mojo666]

Is the bias ok on the driver and final ?

I have been told all is ok however the fault is still here.
I have ordered some new bits and pieces and will get the meter out again on Sunday.
I have copied some info off the net and will report back.
It hasn't had a new audio chip. Maybe that's what I will try after the final and bias check.
Cheers guys

[cb4ever104]

Is the bias ok on the driver and final ?

I have been told all is ok however the fault is still here.
I have ordered some new bits and pieces and will get the meter out again on Sunday.
I have copied some info off the net and will report back.
It hasn't had a new audio chip. Maybe that's what I will try after the final and bias check.
Cheers guys

You should check it yourself . Just measure the transistor base voltage once the problem starts on SSB . It should be around 0.6-0.7V .

Sammy

[cjay]

OK, so after five minutes when the radio is hot does the warble start up again immediately you start to TX or does it take another five minutes?

Have you checked the regulator when it's warbling?

Does it do it into a dummy load (listen on another radio)?

Have you checked the PSU you're using by swapping it out?

[Mojo666]

OK, so after five minutes when the radio is hot does the warble start up again immediately you start to TX or does it take another five minutes?

Have you checked the regulator when it's warbling?

Does it do it into a dummy load (listen on another radio)?

Have you checked the PSU you're using by swapping it out?

Ok, the radio was left switched on all day while I was at work. I came on and it was working great on tx for approx five mins then I started getting the reports of poor audio. Another local operator told me to turn the mic gain down. This helped a little but then proceeded to get worse.
I gave the radio to the same guy to make sure it wasn't my power supply and the same outcome. I have listened myself on another radio too and it is distorted.
It has been rigged up to a doctors test equipment and I have been assured it was fine so I'm thinking it can only be after a good warm up when the symptoms are showing.
Cheers

[andrew013]

well it looks like something on the tx circuit if radio is on all day and warms up as normal and is ok still for 5 min it is something that warms up or breaks down under load in tx conditions this is where that freezer spray comes in handy
also im guessing that you have meticulously looked at the circuit board for dry joints as my mate would say , no matter what fault I have had his first words are the above lol

[cjay]

So something thermal is going on, could be that you're going to need some new finals and possibly thermal diodes.

It'd be interesting to see what the bias current is doing when it's hot enough to be warbling however you also suggested it was making an engine like noise, that could be motorboating, which would indicate it's in the audio section of the radio..

[andymuza]

So it has been away and no problems found which says to me something you have connected is the common denominator.

What current is your PSU capable of?
It could be that your PSU is starting to break down under load. I would start by trying a different PSU.

[Mojo666]

Another quick update,
Checked the bias with the test board out and got 50mA on on both.
Now here is a game changer I managed to touch the eprom and its red hot think this is the problem.
Any thoughts ??