29 LTD Classic RF gain issue

[999]

The meter on receive will stay on S9 even on my dummy load with the RF gain turned down. The RF gain will drop the incoming noise level, but the meter doesn't move. Adjusting VR2 will change the meter, but the RF gain does not have any effect.
My thought on this is that the RF gain potentiometer is working as it should so the issue will be on the board. My guess would be either D5 or C29 has crapped out.

Any thoughts? Thanks guys.

 

[999]

I'm looking for some help with this. I don't know if I'm on the right track or not.

 

[nomadradio]

The AGC circuit in this radio is a bit on the quirky side.

Fortunately it doesn't break that often.

Upshot from this is you won't find a lot of folks with first-hand experience troubleshooting that fault in this model. Just doesn't come up that often.

The S-meter isn't driven by the radio's AGC voltage, but from a separate diode D5. And that's all D5 does, is rectify a sample of the 455 kHz IF energy that also drives D61, the source of negative-polarity AGC and squelch-control voltage.

So that's what has me baffled. Where is the signal coming from to drive the S-meter up with no station transmitting on the channel?

Any chance this radio was connected to an antenna during a lightning storm? Does this radio transmit like it should?

For that matter, is it really receiving on the channel that's selected?

Troubleshooting typically begins with identifying those parts of the radio that aren't broken.

73

 

[999]

The AGC circuit in this radio is a bit on the quirky side.

Fortunately it doesn't break that often.

Upshot from this is you won't find a lot of folks with first-hand experience troubleshooting that fault in this model. Just doesn't come up that often.

The S-meter isn't driven by the radio's AGC voltage, but from a separate diode D5. And that's all D5 does, is rectify a sample of the 455 kHz IF energy that also drives D61, the source of negative-polarity AGC and squelch-control voltage.

So that's what has me baffled. Where is the signal coming from to drive the S-meter up with no station transmitting on the channel?

Any chance this radio was connected to an antenna during a lightning storm? Does this radio transmit like it should?

For that matter, is it really receiving on the channel that's selected?

Troubleshooting typically begins with identifying those parts of the radio that aren't broken.

73

It transmits and receives just fine and on frequency, it's just the S meter that is all out of whack. The trimmer will drop the level down, and the RF gain will drop the noise, but not the S meter.
At least I'm probably looking in the right place with D5. I'll have to pull it and see how it tests.

 

[Handy Andy]

In my experience, D5 (S-Meter) or D61 (AGC) - Pull D61 first, if the S-meter goes full scale or killed, You'll need to check the coil it goes to - L9 - it may have opened up or got crushed from a overzealous tune-up - you may need to replace it - even though it works just fine - the center tap that goes to the output wind that D4 shares - if that slug has been turned too far in to seat it, can damage the wires - I only hope the CT its pulled off the stem, a simple resoldering of the wire onto that post, then, it should be ok.

A simple continuity check from that pin to ground goes a long way here.

It's condition really shows up on strong signal - for those moments, AGC just doesn't work. Can't produce the power needed to back down the strong signal. Or would - by preset bias - thru VR1, that controls the 455kHz IF amp - kills the noise floor because the Pin diodes and part of the IF section R23 - gets this AGC correction signal so you'd have a quiet receive - and VR1 seems to control it.

​

Is VR1 and its' circuit ok?

This is a unique way of showing Cobras' colors when it comes to operation curve of the IF amp - it varies the RF signal - siphoning off some signal - into a cap - if they changed that cap - or altered VR1 in any way - the divider R26/R25 can put this section into overdrive.

It's that half scale - means to me, the divider of the AGC that 47k , R31 and the "Pull down sampler" R35 the 22K (could also be 18K) is shoving it's power into the S-meter - because it's not traveling all the way to ground thru L9's Center-tap lead. (But that's just me)

And...Yes, there's another quirk to this...

Your symptoms aren't all that related to RF Gain - C29 - if that has been modded, then yes - it can affect the receive - which you have - but even if C29 is open - it's a smoothing cap - you'd hear more by playing with that value as well as with R128 - check both parts...R128 can open up

A goofed up re-work on the Output Network - you're getting signal bias from the internal IF thru the SWR bridge back there - like an RF carrier - all channels - look for the Chassis and Foil Board ground - bonding tabs, to the back panel - poor joints and cracked cold soldering shows up because the radio can't dump IF before it goes to the antenna. It's milliwatts - but that's all you need.

So, per @nomadradio - suggestion - try that avenue first, else to me this may indicate a goofed-up tune up or a mod to get more power out now generates a birdie you can't get rid of.

 

[999]

OK, so I hooked the radio up to check things out and the RF gain is working again, nice and smooth. Needle moves down as the noise level drops off..no issues.

I'm stumped.

HA...thanks for the help. I'll keep all of this info tucked away just incase it rears it's ugly head again.

 

[TM86]

OK, so I hooked the radio up to check things out and the RF gain is working again, nice and smooth. Needle moves down as the noise level drops off..no issues.

I'm stumped.

HA...thanks for the help. I'll keep all of this info tucked away just incase it rears it's ugly head again.

Makes me suspect an iffy solder joint somewhere when the problem comes and goes like that. But if it's working there's no point digging around and maybe breaking something else.

 

[Handy Andy]

Makes me suspect an iffy solder joint somewhere when the problem comes and goes like that. But if it's working there's no point digging around and maybe breaking something else.

And you'd be correct.

Found several return radios 29's 25's 78's even 68's all having some kind of AUX/Piggyback amp in there - removed - and they don't resolder all the parts back into places.to make the radios' own inside stuff - stay inside itself.

• - the open ended sections act like mini-antennas and throw the radio out of whack. The returned radio usually winds out of tune simply because of some of the soldering rework done to remove the amp - just wasn't complete.

This approach may help those that get a used eBay special that has "one previous owner" Untested - yet claims Excellent Condition. Hmm. Will send up a Red Flag on my end - but one patient with the patient - will find where the rework was done and will be able to finish the refurb and return the radio back to nearly brand new stock.

It's the holes left behind from them removing the bolts the amp used to keep itself on the back panel that kills me...

 

[999]

I've owned this radio for close to 20 years and this was the first time that it had done this. I have never had it mobile as I prefer sideband radios, but I toss it on the bench every now and again just to make sure it's all up to snuff. The only mod done to it are a variable power. That's it.
That's why I suspected that something died/went out of spec to cause this issue. I'll have to go over the solder side and see if anything jumps out at me as needing touched up.

 

[999]

So I did a quick video on a "cold start" to show the issue.

 

[999]

OK, I left the radio on for 5 or 10 minutes and came back to it and the RF gain was functioning again. I decided to pull diodes D5 and D6 and check them...all good.
I also pulled c29 and c30. They are both 103 caps (10nf).
C29 tested at 12.1nf and C30 tested at 14.2nf. This was after the radio sat for about an hour and I turned it on just to check functionality, and it was still working. When these caps fail do the raise in value, or just generally short like an electrolytic can?

I will have to let it sit again overnight and see what it does in the morning. Maybe the reflowed solder fixed the issue, maybe not.

 

[Handy Andy]

How does the noise sound on the antenna - change when the NB/ANL is turned off, or just to ANL.

This may mean the TR6 is damaged.

Asking this to help understand - for the way you lose the linearity of the RF gain control - it's all scrunched up near the top of it's rotation - meaning the "noise floor" and the RF Gains' power used to adjust RF gain - is very close to each other - as if one sets the mid point for the other.

Quick question - did you check the PIN diodes for shorted or blown open condition...

TR6 is more on than off when the RF gain knob is turned down, sending power thru itself to make it appear more as a open line as far as the PIN diode section is concerned - even though it is nearly acting more like a dead short- Power present (RF) at the antenna input section to the PIN diodes see the Diode as a reverse Biasing no power flow thru the section - it appears as "nothing" until the power present in the received signal forces Pin diodes so they only conduct in spiking or extreme input level condition, it's also pulling the power from D5 / Last IF can - if that power balance is right, you see carrier power when someone is keyed up and you have a signal to hear - when it's "Dead" on the channel - there is still an IF carrier - it's a balancing condition so you're seeing IF carrier power all the time - tells me TR6 or the Pin diode section can be bad.

• The above description is to see RF gain knob all the way down - but some people they see RF Gain goes UP - they think it fully receives, and they'd be correct, but the operation appears backwards for as you turn the control - you're changing the amount of power that goes thru TR6 - in parallel to the Pin diodes off of D3.
• AGC action affects the BASE of TR6 - which in turn affects how much the RF Gain knob has control of R9 and D3
• Did you ever locate R101? D20? R113? Locate L3 and it's mess, you'll see something...
  
  • I don't want to get into an argument with anyone over it - but...it is interesting...
  • It's part of the FEED to both RX/TX switch but also for voltage to the RF amp, thru R12/R13 and R15 - R18 and R26.

Why this comes up? Well, if you have an older Cobra 29 without ST, WX or non-LX models, you would see a normal compliment of parts located all around the TR21 RX/TX switch

On these newer models - you might want to re-check that.

The above is from a KEPC 1141 board.​

The issue lies in how you work the RF gain, one way it's an attenuator - another way, it is an amplifier for attenuation...

But both use what is derived off of D5 C29 and R128 - AGC goes up and down all the time varying with noise level and the incoming signals - all of which will reflect on the S-meter.

[There is so much more to add, this will have to wait . for I'll still have to stick with the adage of IF leakage but the route to give you this answer was a strange one!]

Here's why...

There are several errors in schematics
that parts don't match up to the radios' 100%
Traces too...​

So to wait for the next round...

If that is not the case, then again, IF signal leakage is going on somewhere and the radio's picking this up.

 

[Handy Andy]

What is interesting here is - if RF Gain control opened up, can do this too.

The RF gain control, is the resistor side of a power control. Meaning, the TR6 is controlled by the AGC line - to siphon off power from the power being supplied to the PIN diodes ALL THE TIME.- thru another Resistor, R9 a 3.9K

When D5 senses a signal it conducts more than the "usual". Because D5 is also seeing a carrier per-se from the IF noise and such brought in by Air noise and the noise floor of the entire strip added in.

TR6 sees this - but needs help in figuring out where middle of the road is. So R31 a 47K and R35 a 22K (or 18K per radio) provide a means to act like a pull up or pull down "lever" to the Base of TR6 to control just how much control is left for the Pin Diodes to use - along with how much offset the IF gain needs to back down by to maintain order R24 (1K) and R26 (27K).

 

[999]

How does the noise sound on the antenna - change when the NB/ANL is turned off, or just to ANL.

This may mean the TR6 is damaged.

Asking this to help understand - for the way you lose the linearity of the RF gain control - it's all scrunched up near the top of it's rotation - meaning the "noise floor" and the RF Gains' power used to adjust RF gain - is very close to each other - as if one sets the mid point for the other.

Quick question - did you check the PIN diodes for shorted or blown open condition...

TR6 is more on than off when the RF gain knob is turned down, sending power thru itself to make it appear more as a open line as far as the PIN diode section is concerned - even though it is nearly acting more like a dead short- Power resent at the antenna input section to the PIN diodes see the Diode as a reverse Biasing no power flow thru the section - it appears as "nothing" until the power present in the received signal forces Pin diodes so they only conduct in spiking or extreme input level condition, it's also pulling the power from D5 / Last IF can - if that power balance is right, you see carrier power when someone is keyed up and you have a signal to hear - when it's "Dead" on the channel - there is still an IF carrier - it's a balancing condition so you're seeing IF carrier power all the time - tells me TR6 or the Pin diode section can be bad.

If that is not the case, then again, IF signal leakage is going on somewhere and the radio's picking this up.

The diodes checked out as no current in one direction and they showed just over .300 with my DMM on continuity and no tone.
I am uploading a video that is a little over 11 minutes of the warm up and the needle dropping off over that time back to zero. It also shows much better linearity with the RF gain knob after the warmup.

 

[999]

Here is the video of the needle dropping. You can skip forward to see it drop instead of waiting through it, but this is in real time.