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142 gtl alignment help
- Thread starter kaos513
- Start date
I have had this same problem more than once.
But not terribly often.
There is a 'cheat' for this. If you're sure that the driver and D46 are both perfectly okay, increasing the resistance value of R152 will extend the adjustment range of the bias-set trimpot VR8. R152 shows 220 ohms in the schematic. Pull R152 and substitute a resistor double this value, more or less to 470 ohms. Should get your bias adjustment closer. If it sets okay this way, that was easy. If it still won't turn down enough, doubling the resistance of R152 again to 1k would be the next thing to try.
73
But not terribly often.
There is a 'cheat' for this. If you're sure that the driver and D46 are both perfectly okay, increasing the resistance value of R152 will extend the adjustment range of the bias-set trimpot VR8. R152 shows 220 ohms in the schematic. Pull R152 and substitute a resistor double this value, more or less to 470 ohms. Should get your bias adjustment closer. If it sets okay this way, that was easy. If it still won't turn down enough, doubling the resistance of R152 again to 1k would be the next thing to try.
73
Time to genltly unsolder and lift the base leg of the driver transistor from the foil pad. If you still show idle current in SSB with the mike gain on zero your driver transistor is bad.
There is one other thing that comes to mind. If the sideband modulator's carrier balance trimpot is bad or set wrong, it will feed a weak carrier into the driver transistor.
That will drive up the meter reading even with the bias pot set to minimum.
73
There is one other thing that comes to mind. If the sideband modulator's carrier balance trimpot is bad or set wrong, it will feed a weak carrier into the driver transistor.
That will drive up the meter reading even with the bias pot set to minimum.
73
Any time the part tests okay, but behaves wrong in the radio the next step is to find out why.
The tester's opinion is all very nice, but the radio's opinion of a part counts for more.
Simplest explanation is that a fault in the radio is making a good driver transistor misbehave.
That's the point of isolating the base leg of the driver transistor. If it still shows current, you can't blame the radio's circuit when it is unhooked. The transistor alone can cause the bogus current reading when that leg is waving in the air.
If this stops the problem, and you get a bias-current reading of zero, this means the driver transistor is not the cause of the fault. The radio's circuit attached to the driver transistor's base terminal is the cause. Next step is to find why the radio is feeding bias current into that base leg when it should not be.
The two "way out there" things that could do this would be C152 or C154. They're both a0.0047uf, or 4.7nf, or 4700pf, depending on the markings. The three-digit "472" is more likely. Each of these blocks DC voltage from the base leg of the final. If either one becomes leaky, that would do it.
But those two parts are in the "once every few years" category for someone who fixes a dozen or more radios a week. Ceramic-disc caps can go bad on their own. They just don't do it very often.
Simply desoldering one pin of each, isolating the lead in the center of the pcb hole would make the bogus bias-test reading go away if either of those caps is the cause.
73
The tester's opinion is all very nice, but the radio's opinion of a part counts for more.
Simplest explanation is that a fault in the radio is making a good driver transistor misbehave.
That's the point of isolating the base leg of the driver transistor. If it still shows current, you can't blame the radio's circuit when it is unhooked. The transistor alone can cause the bogus current reading when that leg is waving in the air.
If this stops the problem, and you get a bias-current reading of zero, this means the driver transistor is not the cause of the fault. The radio's circuit attached to the driver transistor's base terminal is the cause. Next step is to find why the radio is feeding bias current into that base leg when it should not be.
The two "way out there" things that could do this would be C152 or C154. They're both a0.0047uf, or 4.7nf, or 4700pf, depending on the markings. The three-digit "472" is more likely. Each of these blocks DC voltage from the base leg of the final. If either one becomes leaky, that would do it.
But those two parts are in the "once every few years" category for someone who fixes a dozen or more radios a week. Ceramic-disc caps can go bad on their own. They just don't do it very often.
Simply desoldering one pin of each, isolating the lead in the center of the pcb hole would make the bogus bias-test reading go away if either of those caps is the cause.
73
ok i lifted one leg of the driver now the meter stays on the Madc scale but when i key up no output
Greetings!
A secondary culprit is the NFB R151 and C52 - These two were present on some of these radios for a time. If you have the 1306 or had one it there, this circuit kept it from producing harmonics - or at least knoced them down to keep the Driver from getting really hot from having to transmit all that bandwidth.
R151? 330 ohm - Orange-Orange-Brown and was rated about 1/6th watt.
C52 was a 0.0047 (472) Disc cap - nothing special but it had/has a semiconductor medium in it - meaning it's specific capacitance via substrate insulator - not by plate design.
Remember ever getting RF burned when you keyed up a radio? How it felt? I do, they get "hot" but yet the board remained cool to the touch - these caps were notorious for getting goofy on harmonics - like a dead short even when you only were checking SSB mA draw - the section could go into oscillation just by itself and these caps - once they've gotten enough hits and heat from RF cooking their chips inside - they'd go nearly a dead short on voltage pokes - not from bias, but from voltage breakdown across that sand grain - yet show 100% insulated checked via voltmeter.
So break that chain of R151 and C52 if you haven't already and see if the Bias goes away, (at least verify the traces are clean- not-shorts) then it was a quick and simple fix but I would not leave them out if you can keep them in there - just beef up both the R151 wattage (470 is ok here) and C52 up to 100V at 0.0047~0.0022uF (472 - 222) and you should be bulletproof on that section for quite a while - not including jumpers and such...
They did these on HR2510's back in the day on the 2166 Driver - so nothing unique but when someone drops another part in. It's usually best to check that ALL routes to the base are verified.
Regards!
:+> Andy <+:
A secondary culprit is the NFB R151 and C52 - These two were present on some of these radios for a time. If you have the 1306 or had one it there, this circuit kept it from producing harmonics - or at least knoced them down to keep the Driver from getting really hot from having to transmit all that bandwidth.
R151? 330 ohm - Orange-Orange-Brown and was rated about 1/6th watt.
C52 was a 0.0047 (472) Disc cap - nothing special but it had/has a semiconductor medium in it - meaning it's specific capacitance via substrate insulator - not by plate design.
Remember ever getting RF burned when you keyed up a radio? How it felt? I do, they get "hot" but yet the board remained cool to the touch - these caps were notorious for getting goofy on harmonics - like a dead short even when you only were checking SSB mA draw - the section could go into oscillation just by itself and these caps - once they've gotten enough hits and heat from RF cooking their chips inside - they'd go nearly a dead short on voltage pokes - not from bias, but from voltage breakdown across that sand grain - yet show 100% insulated checked via voltmeter.
So break that chain of R151 and C52 if you haven't already and see if the Bias goes away, (at least verify the traces are clean- not-shorts) then it was a quick and simple fix but I would not leave them out if you can keep them in there - just beef up both the R151 wattage (470 is ok here) and C52 up to 100V at 0.0047~0.0022uF (472 - 222) and you should be bulletproof on that section for quite a while - not including jumpers and such...
They did these on HR2510's back in the day on the 2166 Driver - so nothing unique but when someone drops another part in. It's usually best to check that ALL routes to the base are verified.
Regards!
:+> Andy <+:
r151 maybe the issue the 142gtl that i can read the bias has a resistor value of 330 ohms as the one that is giving me the issues r151 has a resistor value of 150 ohms.
Greetings!Ok, I'll try another approach to this and see what may be going on...
Ok, firstly - as I reflect on the NFB filter that R151 and C52 - I'm also reminded of the times that the rear panel "Mounting Lugs" the 142 and nearly every Base radio, CB radio at that time used - was not unlike a riveted "nub" of aluminum to a soldering lug.
Ok, Where am I going with this? To find the excessive current - unless the diode itself is a blob of conductive sand due to the dead short things like the NFB filter trying to tamp/clamp down and the cap wound up shorted, can make the diode overheat and still conduct as if it was a diode - just no polarity. Like blown shorted and it's now a pellet...
Or another scenario would be that the Transistor has failed internally and now the Emitter is open because the Collectors' Tab has shorted to the back panel due to a cracked Mica / Ceramic insulator - so the Collector is taking up the current as if it was a reversed biased diode - to ground.
But if that was the case - the middle foil pad would have blown like a fuse.
If you don't see these colors on your TV set...hmmm...
Ok, I come from a different background so what I'm about to tell you may or may not make sense right away, but this deals with "climbing" voltages that can affect the Driver of radios that use OPEN Circuit trimpots.
Below are some attachments to help explain what I'm referring to...
The above thumbnail deals with a problem I've seen from radios that have a Bias network that consists of the Trimmer Pot and the Diode...ONLY...
Review this as an attachment you can open in a separate tab as you see fit...
The problem lies in the amount of current the Diode is forward biased with - in some instances I have seen the 2166 blown shorted and the Diode fried due to a newbie overtrimming the drive to get a "Few more ghost watts" out of it. They put too much drive on the Base and the Diode - whichever one fried first didn't matter - they pretty much took each other out like Mutually Assured Destruction (MAD).
There was also the issue - and I'm currently on another thread about this - a drain current that if the Trimpot was left to it's own devices - it's current supply can go up and blow the diode easily enough - or when the user went Hard Over - Hard Over on the SSB drive - the Base or Diode would get dangerously close to their maximum current capacity (no buffer from RF - remember there is a NFB on these to keep output clean) but if the user produced a ton of harmonics while whistling into the mic - well that NFB was cycling back a lot of power back into the BIAS and Base circuit with nowhere to go but back in - current be (GOSH DARNED) or not...plenty of power can get fed back into both the base region, the bias diode and OOPS can't go anywhere else except UP - as in voltage exceeds ratings a POP - and the smoke appears...all because the BIAS TRIMPOT didn't have a shut to parallel to ground.
Look at any Galaxy radio, they have something there in the Driver and Final - Uniden and Cobra? Only in the Final. Can't offset a NFB kicking in to keep the Final from going south - it has nowhere to go - except cycle - circulate right there in the Base of the Driver and it's Bias diode...
So my fix was to place a 180 ohm resistor ACROSS the Bias diode and let the Trimpot current alone - as in ORIGINAL setting - the Driver lasted a LOT longer that way.
..
The above was from a SEPARATE discussion about the Foil versus Chassis grounds but applies here due to the nature that one of the many mounting lugs being dissimilar metal to the aluminum chassis back panel - may have oxidized and the RF ground it provided is now gone...
You may have a ground loop problem if the chassis ground and foil grounds are not all the same...
Or that the Diode and or the Base of that transistor - are blown and need replacement - but not without providing some form of protection from the runaway RF voltages the NFB and/with/or the excessive SSB drive that can damage the Driver stage...by excessive rectified current that will destroy both if left unprotected.
Hence the 180 ohm resistor - to keep RF at bay and save your day..
(Cliché's or not - it has to be said...)
I also describe this mod here - done to an older TRC-453 - again Open trimpot bias design - just a variable SERIES resistor - nothing to drain off the residual charges or keep excessive charges from affecting bias...
http://www.cbtricks.com/handyandy/PC-122/Red Dot Mod Change.htm
So as times evolved from those days, I came across a lot of older Cobra, Uniden and Radio Shack radios that had 2166 blown in them - all had the open trimpot design and all had blown bias diodes in driver section...replaced the parts but installed a 180 ohm 1/4 watt on the foil side across the diodes' bias line to ground - to shunt power that would have wound up turning on the transistor and caused a turn on thermal runaway event...
So I can't undo the damage done, but you can provide a means to shunt voltages to ground past the bias diode far easier and help keep your replacement parts lasting longer....
Since your radio has this "artifact" or "defect" in design - I wanted you to know...
Hope these hints help!
Regards!
:+> Andy <+:
Greetings!
Ok, here's some heavy reading for times when you're in quiet contemplation...
(Does anyone do that anymore?)
Anyway, I found another effect that led to the 180 ohm dissipation resistor and it dealt with SSB and low-level audio effects and PIN diodes...
Wow - three things ... could be a long story...
Well, er no. Just a noticeable effect that RF entering into a wire filled tapped with a DC voltage and the other end of the wire entering into a substrate that allows power thru but will always leave something present on the wire as a residual charge.
In every direction out of there (Referring to the junction of the Bias diode, Trimpot and the 1306 - from the Pre-driver output coupling found at L30) - the RF signal has to overcome the initial charge as if it was a static poke or even a squelch like activity or cutoff and push into the base region to start the conduction cycle.
What could be heard was a type of pinching of audio signal, or degraded quality of it as a distortion, the SSB low-level and volume - power - going into the Driver was getting pinched off - which started me thinking (dangerous - I know) about how to get that current to flow thru without damaging the audio information present on the line and the interesting aspect of this is from that moment of reflection - I see the resistor swamping used more often now - and in many ways has helped clear up the audio artifact that some radios' even when new - had their first few consonants and vowels "trimmed off" due to the bias on the line that transferred from the Pre-driver to the driver...took the words right out of their mouths...
Another circuit on a CB radio that has SSB (AM Regulator) also exhibits a similar trait but I'll save that for a later time - anyone wishing to prime themselves on it can review any PC-122 or even the TRC 465 - D42. Many people remove it but they shouldn't - just install a 100~330 ohm resistor IN SERIES with the diode - prevents a pinching effect from the diodes' own intrinsic effects...
So in a way - I'm reliving that moment - nothing serious. I hope these methods help you keep a radio working. I'm familiar with that chassis and had owned one. It's the kind of chassis that were used in the homes of many of the friends I used to know. They opened their doors into a life around radio - whether it's a Cobra 142 the 148GTL or a Washington or even an Excalibur - those people are long gone but at least the radio you have - still exists - and I don't want that to pass...
Good luck!
Regards!
:+> Andy <+:
Ok, here's some heavy reading for times when you're in quiet contemplation...
(Does anyone do that anymore?)
Anyway, I found another effect that led to the 180 ohm dissipation resistor and it dealt with SSB and low-level audio effects and PIN diodes...
Wow - three things ... could be a long story...
Well, er no. Just a noticeable effect that RF entering into a wire filled tapped with a DC voltage and the other end of the wire entering into a substrate that allows power thru but will always leave something present on the wire as a residual charge.
(And to think that people would argue the finer points of charge carriers - disregard them as cast aways like Stray Cats - er Caps leaving behind little Coulombs of Charge wandering the Alleys of Circuit Traces - only to be Barked at and Chased after by the Dogs Of Inductance...To save the town, that city, the whole planet - a Resistance movement was founded - called Swamping Resistance - to mop up such areas of insolence...and properly, purposely - perfunctorily - sent to their rightful place, bound to ground - potentially saving the human(e) race from such parasites known as parasitics...)
The effect is used in limiters e.g. - ANL - and the Drivers' Bias network setup like the Cobra 142 / 146 as well as the PC 122 or TRC 453 on up to the TRC 465 - all had source voltage bias with no dropping resistor just the base and doping regions of the diode.
In every direction out of there (Referring to the junction of the Bias diode, Trimpot and the 1306 - from the Pre-driver output coupling found at L30) - the RF signal has to overcome the initial charge as if it was a static poke or even a squelch like activity or cutoff and push into the base region to start the conduction cycle.
What could be heard was a type of pinching of audio signal, or degraded quality of it as a distortion, the SSB low-level and volume - power - going into the Driver was getting pinched off - which started me thinking (dangerous - I know) about how to get that current to flow thru without damaging the audio information present on the line and the interesting aspect of this is from that moment of reflection - I see the resistor swamping used more often now - and in many ways has helped clear up the audio artifact that some radios' even when new - had their first few consonants and vowels "trimmed off" due to the bias on the line that transferred from the Pre-driver to the driver...took the words right out of their mouths...
Another circuit on a CB radio that has SSB (AM Regulator) also exhibits a similar trait but I'll save that for a later time - anyone wishing to prime themselves on it can review any PC-122 or even the TRC 465 - D42. Many people remove it but they shouldn't - just install a 100~330 ohm resistor IN SERIES with the diode - prevents a pinching effect from the diodes' own intrinsic effects...
So in a way - I'm reliving that moment - nothing serious. I hope these methods help you keep a radio working. I'm familiar with that chassis and had owned one. It's the kind of chassis that were used in the homes of many of the friends I used to know. They opened their doors into a life around radio - whether it's a Cobra 142 the 148GTL or a Washington or even an Excalibur - those people are long gone but at least the radio you have - still exists - and I don't want that to pass...
Good luck!
Regards!
:+> Andy <+:
- No one is chatting at the moment.
