Cobra 148 low volume and modulation

[Tolteca]

Hey guys,

This is the same 148 I was dealing with in the other thread but I decided to focus that thread specifically on the frequency mod. I've had this 148 for a while now and despite having aligned it and it being an overall good performer the one issue I never took care of was the low volume on receive and the low modulation on transmit. SSB modulation seems fine but when I compare the AM from this radio and another 148, my locals say the other one sounds much better. The receive volume is about half of what the other 148 is even when it's turned all the way up. This is not a receiver issue as it has been aligned and is quite sensitive. The problem seems to be in the actual audio stages.

My question is, does the radio use the same audio IC for modulating AM as it does for amplifying the receive audio? So far my best guess is that it could be a bad coupling cap somewhere along the audio chain of this audio IC. Anything else I should look for? Is the output transformer also shared for modulation and the speaker output?

 

[Handy Andy]

Yes it does - Tr47 and Tr53 - they use two Electrolytic caps.

Might want to check TR 53 by removing the cap that it uses to REMOVE audio so if and when Pin6 of the PLL is out of lock, it tries to send all the audio to ground thru it from that cap that goes to the audio line from the RX/Mic and PA stuff...does Pin 6 PLL ok does it still have that 22K in there too? If it's low - not out of lock - just low voltage - an electrolytic can pull down the line so that it will TX and RX but Tr47 and or TR53 may think something isn't right and try to "quench" the audio line.

In fact you may want to double check all the parts in and around the Audio amp - the thing is used for all the AM mod envelope as well as RX and PA - but all SSB stuff is still thru the AN612 - but RX is thru the Audio amp chip.

 

[Tolteca]

Found the issue, the problem was a resistor leaning onto TR47. Poked around a bit and it was fixed just like that. Thanks a lot for the help, did not know there was a muting function built into these radios.

The other issue now is what I call backward modulation. I aligned the radio and I get about 8 Watts output before turning down the AM carrier pot. Usually I get this backward modulation thing until I turn radios down to around 4-5 watts but this one does it a little even at 4 watts. People are telling me I sound much better now than I was before and of course the receiver is way louder than it was before and I know the radio is creating positive peaks. For some reason however on my RMS only meter the needle goes backward when I talk. I used the same procedure as I do in all my radios and I don't have that issue unless I turn the carrier way up. It's not so bad if I set it for 4 watts but I'd like to be able to set it for at least 5 watts carrier without having this problem. Any idea what that might be?

 

[Handy Andy]

Several, from the Audio amp chip towards the back...

But to keep it simple - requires you to do a side to side comparison to an known 148 that you know works.

Why because Caps, Coils and the quality of the parts used - all play a role...

So what I would look at first is determining if the "Audio line with Bias" is arriving to the Final and Driver properly.

And if the caps in that are fine, look at the pre-driver and alignment - anything back from the Final Driver towards the TX mixer and from the Driver and Final out to the back thru the network.

To save some time - when you have "backwards swing" it may mean as you already pointed out - excessive audio is getting injected in there and in doing so, "swamps out" the carrier and actually DROPS the power because the audio cant mix with carrier in the proper ratio.

 

[Handy Andy]

I use schematic values for starters.

Some parts may not even be in there so you know...its due to board revisions during the times the thing was made...


So there may be subtle differences in how they are biased as well as what parts are used in each production run and the "year" it was made...


There is also the issue of how the D50/D49 Bias for Class AB is done - there may be a goofy soldering "blob" grounding (or not grounding properly) to the spots on the board - one for RF ground (Chassis) and Foil Ground (Power supply and board ground) - be sure to watch for it...

 

[Tolteca]

OKay will be taking a look.

 

[Tolteca]

Okay so far the only weird thing I've found is that I cannot adjust the modulation limiter. I found a clipped diode and one covered with sony bond, I've replaced both of these diodes as well as the 10k adjustment pot with no solution. I'm not using a power mic or anything like that, the wave form begins to distort even with the dynamic at very low levels. I am not sure if this is a modulation issue or if it's because the final stage is collapsing everytime it is modulated. Either way the wave form looks like crap and no matter where I turn the modulation limiter adjustment pot, it doesn't get any better.

 

[Martian]

If AMC isn't working then the circuit is still disabled somehow.
There are not very many parts you can remove to disable it but everyone has their own "secret" way which achieves the same goal.

I had a 148 here that was a "tech special" that once repaired the AMC had no effect.
A scalpel had been used on the underside of the board and then colored well enough that it blended into the surrounding solder mask.

The idea being that the parts were still in place giving the illusion that the AMC was not defeated. But it was obvious.

My bet is that the circuit has been disabled in a clever way that you can't see - especially if all of the parts appear in place. I am betting they may be there but not intact.
This isn't a circuit prone to failure. The only failures of this circuit are usually caused by creatures of the human variety.

 

[Handy Andy]

Was there any work done to the AM Mod section?

There are two resistors that affect the AMC SENSE line to the AMC amp.

IF R131 has been fixed, that goes to the BASE of TR26...

But did you check for the existence of R166 a 3.3K or R165 a 1.5K resistor? These are the tap and voltage divider off of the AM Mod amp for the AMC amp TR26, then TR25 takes that and amplifies it for TR24 - TR25 is a PNP and is special to note that the ALC amp and AMC amp signals are applied here.



...[The following paragraph was edited by author]...
IF ALC amp is still on - try and see if adjusting it - affects the AMC - then you know you have more work to do including a steering diode issue - check D42 - by ALC amp. It applies the AM mode 8V signal to essentially turn off any amplification TR34 can have - both Emitter and Collectors are now equal no power can flow thru - but if D42 was removed or the line powering it was turned off or interrupted somehow - ALC will still work and attempt to affect the envelope and "pull down" modulation. This impacts the Limiter AMP TR25 - it gets both a modulation (TR26) and signal envelope component (TR34) at the same time. You cannot adjust one successfully because the other will simply take its' place and exert its' influence.

The biggest clue is if you BACK down the audio carrier and the audio seems to recover. In some instances this type of compression will generate a backward swing meaning there is not enough carrier power or RF signal (if SSB) being produced at the TX mixer to handle the ratio of Audio Envelope and RF carrier power and produce a strong enough signal so the base region of the pre-amp and stages after it, will drop power as they lose their ability to produce RF power at the frequency of interest.
In AM a similar backwards swing can develop from LACK of RF carrier - the pre-drivers' Collector cannot produce enough RF to overcome the Drivers' reluctance to conduct at it's Base because it's own Collector is swamped with an Audio drive signal with very little DC Bias to operate from - it becomes unstable - you lose power - your backwards swing.
So double check even the D42 and AN612 Shunt TR21 - by checking and making sure TR21 is working. R113 can verify this.
IF BOTH ALC AND AMC are on at the same time - both AM and SSB modes you would not be able to have any envelope power - about 4 watts and then audio distortion when you try to produce more power above that ​

What seems like its cranking up modulation - is due to the clipping even on the audio causing "compression" within the RF envelope at the same time (AMC and ALC on)

Well, do you hate me now???

 

[Robalo]

modulation pot (vr7) can not be adj. if tr26 been removed,chech value of r124,r126,r114and c18,c176 HANDY ANDY what is the work of c191 (1uf 50v)?
Andy you have to many likes one more from me

 

[sonoma]

Andy puts up a lot of good info here. glad he decided to join here. I have always liked his post. I have so many of his write ups saved to my computer. have been saving them for years.

 

[Handy Andy]

There are various interpretations, but I look at it as the smoothing cap.

The envelope itself goes into two stages - so some filtering or smoothing is needed to prevent the previous stage from feeding into the next stage thru an unchecked loop - that is the purpose of the cap. Just DAMPENS down the audio loop effects both stages have on each other.

In a way, a type of compression and conversion is occurring in this section - at the same time. Audio and bias are applied at the same time, but the stages are processing inversions. One has a "positive signal wave" while the other stage is processing a negative signal portion of the wave - both are applying power as wattage, but an offset is generated - the offset is the combined shift-up in audio frequency (at that moment in time) and subtraction -shift-down of audio frequency - at the same moment in time. This duty cycle shifts back and forth between the stages 27 million times a second back there....

So the cap - in a way is not just a smoothing cap, but a reserve of energy to put into and pull from at a given rate (hence if you change this value you affect bandwidth too).

 

[sonoma]

Andy a few years ago I had a 148 that some one did a diode mod to R228 at the transformer and it killed the TX audio , do not remember if it had any effect on the RX. after changing out the diode and the large cap right by it the radio was fine. could there be some thing in that area .
just a thought.

 

[Handy Andy]

There's some mods I had to take on the chin - that was one of them - too many people put the resistor lead to the wrong part of R228 - should have been the "cap side" and the diode used the banded end TOWARDS the Emitter leg output to the Finals and Driver - SIGH...

I put in a list of mods others including Bill Eitners' from CB Tricks. Just a bag of tricks that you may find in radios' needing repair - have to have them undone to recreate original problem and some instances - they were the original problem.

Didn't put in how to do it - just showed it - found too many I've had to undo and redo - so some mods I just won't post anymore because of improper installation and they blame us for it...

 

[sonoma]

I just happened to think about that mod and had a problem with it being bad one time. so many things can be changed on a radio and cause all kinds of problems if not done right. if the did work and the part fails it is so hard to tell some one to look for the fault areas.
you do such a good job with pics and helps a lot.