Cobra 21 GTL tiny bit forward swing

[Puttputtinpup]

I bought this nice little 21gtl for $10 this weekend. It didn't appear to have been worked on and was anxious to try my hand at a few mods. 1st I checked the PA. Had audio. Then hooked dummy load and meter. Dead keyed 5 watts and virtually no forward swing. So I powered it down. VR5 was all the way clockwise. It turned it counterclockwise. That didn't affect the forward swing at all. I clipped D9 limiter. Same thing. Needle swings to maybe 6. Found a couple of ground solders at the back of the radio from board to radio frame. Fixed those and still no change.

I hooked up a known good radio and transmitted through it using dummy load. The 21GTL near it receiving. Audio from the external speaker can be heard. No audio can be heard with ext speaker unplugged. Speaker wires are soldered on. Vice versa is true. Transmit with the 21GTL into dummy load and known good radio picks it up.
What would cause the weak forward swing? Like I have 29ltds and uniden pc66 that dead key 2 - 4 then swing to +15

 

[Handy Andy]

Ok, slow down...IF you go too fast something might go poof...

Board number?

Pc-417? I may have an answer...

Age of the radio can play a role here including a "stuck" speaker cone - seized up, frozen, from lack of use...

Does it thunk?

IF Pa works, that's half the battle but you need to figure out of the Jack on the rear panel has dirt in it or you have a bad speaker or the spyder holding the speaker may have been damaged and the cone is sitting on the bottom of the throw in the magnet...(the "thunk" is important - for the cone of the speaker will move as a low-frequency event so if the speaker core moved and now it's jammed - well, you have an answer...)

The Age of these things is what I'm cautioning you on, for those bootstrap and other polarized caps in the radio can blow because of dissipation problems if you're not letting it "charge up" if it hasn't been used in a while...

 

[Puttputtinpup]

Ok, slow down...IF you go too fast something might go poof...

Board number?

Pc-417? I may have an answer...

Age of the radio can play a role here including a "stuck" speaker cone - seized up, frozen, from lack of use...

Does it thunk?

IF Pa works, that's half the battle but you need to figure out of the Jack on the rear panel has dirt in it or you have a bad speaker or the spyder holding the speaker may have been damaged and the cone is sitting on the bottom of the throw in the magnet...(the "thunk" is important - for the cone of the speaker will move as a low-frequency event so if the speaker core moved and now it's jammed - well, you have an answer...)

The Age of these things is what I'm cautioning you on, for those bootstrap and other polarized caps in the radio can blow because of dissipation problems if you're not letting it "charge up" if it hasn't been used in a while...

Yes. The board is a PC417ac (of the ac makes a difference). Re. The internal speaker...should i unsolder it and wire up a known working speaker? I have some tuner cleaner. Would spraying some on the ext speaker jack help? I did shoot some into the Dynamike pot. No change.

 

[Handy Andy]

First, examine the speaker you have ON the radio - that one just may need to be reheated in the action of adding some extra solder just to free up the cone from getting caught in the spyder - the heat can help free the cone from being jammed - you can also use an Ohmmeter and unsolder those wires to it, remember the color code for it is polarized (again back to first post) and check for ohmic reading.

IF that is ok, then you'll need to see if the EXTERNAL speaker jack that routes IN SERIES with the speaker wires to that speaker - meaning - it routes thru speaker and thru the EXT SP jack, if the jack is dirty or bad, or shorted, there goes the ballgame...

 

[Puttputtinpup]

First, examine the speaker you have ON the radio - that one just may need to be reheated in the action of adding some extra solder just to free up the cone from getting caught in the spyder - the heat can help free the cone from being jammed - you can also use an Ohmmeter and unsolder those wires to it, remember the color code for it is polarized (again back to first post) and check for ohmic reading.

IF that is ok, then you'll need to see if the EXTERNAL speaker jack that routes IN SERIES with the speaker wires to that speaker - meaning - it routes thru speaker and thru the EXT SP jack, if the jack is dirty or bad, or shorted, there goes the ballgame...

so, fixing the speaker will get me forward swing past 6?

 

[Handy Andy]

Look back on that schematic...

Look to your right side of the page by the External Speaker jack and CB/PA stuff...

It can be just a simple wiring fault...you just have to track it down...

We'll fix the "swing thing" in a moment or post or two... may mean other things like the Caps used to send audio power into TF-129 - your Reactance Modulator or Mod Transformer - is going out or something in that section or the power that is sent by it to the Driver and Final are affected...

 

[Handy Andy]

so, fixing the speaker will get me forward swing past 6?

Each and every thing along this chain of parts can sap power from your audio and signal - if we don't fix them as we go, we'll never get anywhere...

 

[Puttputtinpup]

Each and every thing along this chain of parts can sap power from your audio and signal - if we don't fix them as we go, we'll never get anywhere...

I'm familiar with the speaker jack sw and how it disconnects/connects the internal spkr. I could remove the speaker for now just in case it's interfering with the audio circuit. and clean up the sw plug a bit also, and press down with a plastic tool on the part that reconnects the internal spkr.

The other items you mentioned on the board are outta my league. I've changed caps, done a couple of swing mods for fun, found broken solders, etc. I have a meter than checks caps. Point me to the component #s to check and I'll do my best to rule them out.

FYI, I did find 2 stink bugs nestled inside on the solder side of the board when I took the top cover off. I took them outside to play and brushed the board with an old toothbrush.

I've gotta head off to work (3rd) in a bit so I'll dig into this thing some more.

 

[Puttputtinpup]

I do appreciate your input on this HandyAndy.

I did read on a similar thread on about a cobra 25 checking the audio chip 1st and last leg for 13.8v. That's about where those pesky stink bugs were huddled up at on the board.

I'll check those voltages and check receive on the 21gtl using an ext speaker as soon as i get a chance.

 

[Handy Andy]

Sounds good!

While you have the radio out - look for the trace side where those "stinkbugs" were at. The Edge pin row that the SIPP TDA722AP Audio Chip sits on, Pin 10 to Pin 1 - if there is any evidence of corrosion - I can say that with assurance the Pin 10 and 9 use a "bootstrap feedback" cap of 16V 47uF - if any "juice" from their mating/Sleepover happens - it can digest or affect parts like the Electrolytics' in there...

Pin 2 uses a 10uF 16V...another bootstrap one that gives the radio that characteristic "thunk" in the speaker as it powers up.

Now also pay attention to those voltage ratings of the caps - they're 16V and are pretty marginal - you may have to get some caps to replace those like the ones we're talking about and even more - for if they "Dry out" they may not recover. You can use 16V to get by, but 24~36WVDC do better in mobile setups - at least you wouldn't have to replace them in ten years or so or if they get too hot in a car interior on a sunny day...

Sometimes reheating the solder pads they're mounted to might bring it back but not for long. It usually means that they are done and no longer can hold a charge for the function they were used for.

You may have some work to do but if we can get your swing back - I'd just hate to have you swap out parts until you get the one that caused all of this. But it may come to that.

Next of the "cap to replace" list would be still in the Audio section - but it's that large size and value 470uF 16V cap that takes the audio to TF-129 - your mod transformer. But hold that for more thought on this...

I hesitate to make you do that cap swap because to get PA to even work, it uses that cap so that means it works for now...

The "pinch" effect you get or lack of swing can also mean a bad tuneup or other failed parts along the chain...

But for right now, let's get the Audio straightened out first, it may be our only whole problem - and you'll get your swing back...

Now before I let you go, when you say "6" do you mean 6 watts or just "6" on a meter scaled from 1 to 10?

• Because if you're getting "6" watts - that means they've overpeaked for carrier and you have to retune the strip - its' not tricky but you have to keep your ducks in a row for that to work.

The biggest culprit may be L10 - that big brown slug tank. Located back by your final - that is your output tank circuit and uses a slug, make sure you have a slug in there.

 

[Puttputtinpup]

Thanks. That's great information! The power adjustment at the rear still has wax on the top of it, so doubt it's been messed with. Lemme try to upload at photo of my meter.

 

[Hotdog505]

First, examine the speaker you have ON the radio - that one just may need to be reheated in the action of adding some extra solder just to free up the cone from getting caught in the spyder - the heat can help free the cone from being jammed - you can also use an Ohmmeter and unsolder those wires to it, remember the color code for it is polarized (again back to first post) and check for ohmic reading.

IF that is ok, then you'll need to see if the EXTERNAL speaker jack that routes IN SERIES with the speaker wires to that speaker - meaning - it routes thru speaker and thru the EXT SP jack, if the jack is dirty or bad, or shorted, there goes the ballgame...

Omg first try your power supply, your doing too much at once,check voltage at final out.

 

[Puttputtinpup]

Omg first try your power supply, your doing too much at once,check voltage at final out.

At the risk of sounding ignorant, check at the final at the rear of the radio, right? Which legs to chk? The process. Elaborate please. What readings will tell me they're good or bad?

 

[Puttputtinpup]

I just home from work and still had the cover off the radio. Before going upstairs, i took another peak at the caps that handy andy mentioned..well, I found one that I definitely will be changing out. It appears to be C66 on the board. 470uF 16v. Note the white crust on the top edge and the rippled side.

 

[Handy Andy]

There ya' go!

Although not a obvious in the smaller ones...you're looking for it and that's good...

No pressure - you got this...

BTW - they "modded" that TF part, note the 161 (164?) versus the schematics reference to the 129 - so there are changes in this design from even the original schematic that pertains to the board.

See Graphic...

Minor revisions, but you'll see what I mean as we work thru this...Heck can even have been modded by another tech and they swapped out parts to achieve a performance level on ANOTHER radio that can use the same parts - so they just swapped out the Transformer---be ready for things like that when it comes to buying used equipment.

There's a "brown Disc Cap" between those two caps (one with "chalk fuzz is the 470 - other is the POWER supply Choke Ripple filter part of the power smoothing filtering process) The Disc - I believe that is part of the "Tonal filter" this radio uses to match the higher frequency range the Audio chip throws at this section of the board - and so it is used as a Hi-Cut filter to lower or lessen the chances for squealing audio caused by mis-match from it's input (ability to take in power the audio chip sends) and it's ability to Transform it (convert from Audio signal to Audio + Bias) to drive the Final and Driver.

Welcome to Class D operation...this is where that process begins...and it is then applied thru the Driver and Finals' Collectors.