AM Modulation at best 15%
SSB TX power at best 2 watts on a Two tone test
Checked the board around the mike amp stages to see if someone had pulled a limiter transistor or some other mod had been done , but no it's all good there and the board doesn't appear to have ever been touched since new.
A quick and rough alignment of the TX stage was done and a new microphone was wired up but still the same issue.
The assumption was then "It's going to be a faulty electrolytic capacitor" , On the circuit diagram C121 looked like a good place to start as it was in series from the mike gain control to the first of 2 mike amp transistors , I replaced that capacitor and the radio came up with good modulation and good SSB TX power.
OK the first capacitor I've removed was faulty , the radio is almost 40 years old , I'll replace all the Electrolytic and Tantalums capacitors first before I do a full alignment.
A few hours later that was all done and I powered the radio up , everything seemed to be fine except the original low TX audio problem was back again.
OK, I've either fitted a capacitor in reverse, blobbed a solder pad or possibly fitted in a new capacitor that was faulty, Checked all that and all appeared to be OK.
Well it must be an alignment issue as I'd just roughly aligned it prior with all the old capacitors in place - it's bound to be way off the money now with all new caps in it - did another quick alignment and it was all pretty much as it was prior even with new capacitors.
I chased the audio around the mike amp stages with the scope, and was getting some very odd traces, The new mike I'd wired was a power mike so I tried looking around on the scope with the power mike on low and also flat out to see what I could see , it's was odd and appeared to be intermittent.
Back onto the power meter and then I saw it happen , initially it was low SSB power and then for no reason it jumped to full power on 2 tone and appeared to be happy to stay there for a short while and then collapsed back to 2 watts.
It must be a dry joint that I couldn't see on a visual inspection of the board , I tapped all around the board with a screwdriver handle and nothing seemed to change.
It's probably a thermal fault - But I don't have any freeze spray handy and it's probably a waste of time using a hair dryer to warm up components if I've got no freeze spray anyway.
The faults still there , it's getting more random in it's occurrence and it even looks to be noisy now on the scope traces.
To be honest I'd had enough of this and made a decision to go down a path that ALL technicians hate doing - Bush-range / Shot gun the fault down to find it.
It will be in the mike amp / limiter or audio amp stages TR25 -> TR32 or IC4, it's not going to be a Electro cap as they have all been replaced , it will be a Transistor , diode or the TA7222 IC most likely.
The transistors are all mainly 2SC945, 2SA628 or 2SC711, Diodes can all be subbed with 1N4148 - I've got all these parts handy - lets replace the lot till it's fixed
The 2nd Transistor I removed to replace ( TR29 the 1st Mike amp in the chain ) was immediately seen as the problem and replaced with a 2SC945 - The radio tested and the fault was gone permanently!
TR29 WAS a 2SC458 in this radio and not a 2SC945 as shown in the circuit diagram - This was the original transistor fitted from new in the factory and had not been replaced prior.
Where I'm going with all this is that if you work on early Uniden chassis you will (or should) be aware of the problems that occur with 2SC458 - They ALL go Noisy , Intermittent or outright just fail - it's very common problem that has occurred since the first use of them in the original Uniden 858 Chassis radio's.
In Cobra 138XLR , Early President Grant , TRC457 / TRC458 / TRC449 , Robyn SB510 and all others using the 858 SSB chassis most of them have a 2SC458 installed at TR35 (VCO amp) and TR21 / TR23 (AF Amp) (from memory but the diagrams I have here shows 2SC945 fitted in TR21/TR23 but ??)
If you see a 2SC458 fitted in anything you are working on - remove it and replace it with a 2SC945 as they all fail eventually.
2SC458 in audio circuits do create odd noise faults that crackle and pop and will drive you (and / or the customer) crazy so get rid of them when you see them.
TR36 (AF Amp) is a 2SC458 in most early MB8719 chassis - McKinley , 140GTL , SB505 , TRC450 , Stalker IX etc and are known to go noisy and should always be replaced with a 2SC945.
Uniden seemed to stop using 2SC458 in the early 1980's so later production gear shouldn't have these issues - but the changeover date to 2SC945 is unknown so best to check the board visually.
If you do a Google search on 2SC458 faults you will see a plethora of posts from folks who restore old high end 1970's audio gear suffering from 458 headaches !
I think this is an interesting case study for a number of reasons :-
If you were the owner of this radio and took it with this fault into a shop for repair you would get :-
1/ It's too old we don't fix that gear
2/ We gave it our best shot but can't fix it sorry.
3/ We can fix it but have no idea how long it will take or how much it's going to cost you (I'd hope that's what you would hear!)
4/ Yes that's an easy fix , we will replace TR29 then give it an alignment and tune for you 1 (I'll BET someone else has struck this fault before!)
5/ Here just buy a New 980 Bearcat and we will give you a $20 trade in for it cos we need a new door stopper for the shop.
6/ This one bothers me as I'll bet it has happened - Fit a power mike to it , crank the limiters wide off or pull the limiter transistors out and tell you to run the power mike wide open
(What I saw when testing this with a power mike on full there was close to 50% modulation and 5 watts SSB - with the ALC disabled it would likely show as almost respectable on a meter , but would look poor on a scope and sound pretty off on air)From my perspective as a Technician it is interesting for a number of reasons:-
. The end result was a fully functional radio
. The path to get to the end result was not "By the Book"
. Bushranging out faults even though frowned upon by myself (And most others) does often get results , sometimes it's the only way to get there But certainly not time or cost effective thou.
. Replacing ALL electrolytic capacitors in older radios very frequently gets results but in this case it "appears" to have been a waste of time and sent me down a different repair path that I need not have gone down. (Others may disagree - I even tend to disagree but that is what happened in this case - It's not to say Don't replace capacitors - just be aware it wont fix everything and often creates other problems due to reverse polarity, solder splatter and sometimes faulty components).
. 2SC458 is a known fault - it's been on my radar for decades , I'm always on the look out for them and replace them when I know they are there, But if the circuit diagram doesn't show them as fitted , unless you know there are there you are not watching out for them and then that when they "Get Ya"
. Uniden's designers have deliberately selected 2SC458 for a specific reason (Not cost based - most likely due to a nice audio gain curve and possibly a fast switch on time as a guess?) They were used in mike audio and receive audio circuits for a good reason - clearly they worked to design when they were new and they were not to know their lifespan was not perfect.
. 2SC945 is probably the handiest transistor to carry as a spare part!
. How many MB8719 chassis have been scrapped out due to intermittent problems with 2SC458 faults over the last 35+ years ?
