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2SC458 in older Uniden chassis - Watch out !

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pabxone
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2SC458 in older Uniden chassis - Watch out !

Post by pabxone » Saturday, 01 October 2016, 19:47 PM

I've just had a '77 President McKinley MB8719 on the bench with very low level TX audio issues:
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 :icon_rolleyes:

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 :shock: (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 ?


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Re: 2SC458 in older Uniden chassis - Watch out !

Post by MDYoungblood » Thursday, 06 October 2016, 18:03 PM

This was a problem I had with my Uniden/President Washington, I replaced the transistor and like you said the popping went away, that was several years ago so I can't say what transistor it was, most likely a 2SC458, changed it because it was getting hot. Just last year I recapped the radio and it's working like new. Thanks for the info.

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pabxone
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Re: 2SC458 in older Uniden chassis - Watch out !

Post by pabxone » Saturday, 25 March 2017, 7:02 AM

The 2SC458 is not always the fault !

I caught myself out today working on a 3rd Generation Uniden Madison MB8719 chassis.

It had a loud clicking sound on receive - constant clicking about 2 times a second , after having a look around and also reading the circuit diagram , TR43 (AF Amp transistor) was a 2SC458 so I replaced it with a 2SC945 and assumed all would be fixed.

Not so , same problem was still there.
After a lot of chasing around the circuit board and getting lead on a wild goose chase looking for another (Non existent) fault it ended up being a faulty IC6 (AF Power Amp IC) TA7222 , After that was replaced problem was gone.

To compound the problems , The history of the radio is unknown and what/when/how the fault occurred is anyone’s guess.
It had an external variable AM power control pot installed in place of RV10 (AM power trimpot), some odd ball bypass on the AM power regulator ( 2 diodes and a electrolytic soldered onto the bottom of the PC) The AM power regulator is normally a 2SC1419 but someone had substituted that out for another 2SCxxxx transitor (Not a normal substitute for that part either) A FET had been substituted for some other very old FET from the early 1970's.

It's had a Re-Cap of sorts but none of the Tantalum capacitors were replaced.

Chasing faults often require you to take the radio back to as close to original as possible and that all takes time - Normally LOTS of time.

Just a thought - If you are going to modify a radio with uncommon mods or use non standard substitute parts - Just take 5 minutes of your time to write down what you have done (And why) on a piece of paper , fold it up , place it inside a small plastic bag and black tape it inside the radio.

The next person who works on the radio will thank you for it and if that person is you 5 years after you did the original work it's more than likely you will have forgotten what you had done years ago !

If you are doing a re-cap , Replace all the Electrolytic's & all of the Tantalum’s as well, That includes ALL the capacitors that are really hard to get at, hidden away on front panel circuit boards and/or have metal RF shields covering the solder side of the board.

If you don't do all that you are wasting your time & effort doing a re-cap

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Re: 2SC458 in older Uniden chassis - Watch out !

Post by Scipio Kid » Sunday, 26 March 2017, 14:33 PM

Okay, let me re-cap (yes it's a bad pun but I couldn't resist[or] ... wow really bad today ... warped sense of humor, can't help it). These old Unidens need help but if I understood you, the problems went away some time in the late 70's early 80's? I can't find a date on my Washington but it's Philippine made and uses a 5-pin mic. (Someone told me that might help with dating it [but I wouldn't recommend it ... girls are a lot better] Sorry, warped sense of humor again ...) I don't know the history but it appears to have had some serious work done on it. It makes some weird noises every now and then but basically is an outstanding unit.

Now I (and I hope a lot of us here on CBRT) understood very little of what you just said (I'll never admit that face to face) but caught just enough to know I should probably have it looked at. Now, since you thoroughly confused me while making perfect sense, I'd be more than comfortable having you work on it for me. Unfortunately, you're a few thousand miles farther than I typically travel to look for a good technician, of any sort. If you'd only posted this a moth ago. My boy was in Australia for two weeks last month, on his way to Hawaii (he tells me he's finishing college but all his pictures are of beaches and surf boards ... it's a rough life), could have sent the unit with him.

There's only one guy in Utah I'm aware of, who knows much about and still works on CB equipment. How can I tell if he's capable of doing what you explained? I know another guy who's great with fixing TVs and computers and he says he's sure he can work on CB's too but I'm worried that'd be like letting a guy work on my wife's Jaguar since he knows how to change the oil in a Kenworth. I don't want to spend a hundred bucks making things worse.

I talked to a guy in Illinois who, like (both of) you, seemed to really know his way around CB radios but I was leery of shipping my radio across the country based on his ability to impress over the phone. So is there any good way to tell if a tech is any good? Any key questions to ask? Or does anyone have any recommendations? Flat rate shipping isn't too bad so a good reputation and recommendation would ease my concerns about sending it out there somewhere. In fact I have several units that I figure would be worth putting a few bucks into if I knew they'd be really good units after the investment.

Also, what kind of money are we talking for doing some of the things you were talking about and what kind of longevity would one expect out of a radio after having it fixed? I'm sure I'm not the only one wondering.

Thanks,

Scipio
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Re: 2SC458 in older Uniden chassis - Watch out !

Post by pabxone » Tuesday, 28 March 2017, 2:22 AM

Wow, Lots of good questions there.

President/Uniden Washington - They switched to the 5 pin microphone configuration in 1979 whilst the radio was still badged as a President Washington , Yours is a Uniden Washington (From Memory the President name was dropped around 1980 ? ) Being a Philippines built radio would date it somewhere from the mid 1980's thru to the end of production in the very late 1990's.

There are quite a few "Techs" here on CBRT so I'm sure someone local to you will chime in and say hello.
It's always best to find someone local who you can physically drop off and pick up the radio from in person - failing that posting it off to someone who comes well recommended is fine too , it just adds a little more cost to the work.

A good Technician will never make a good Salesman and a good Salesman will never make a good Technician.

So "beware" of people who claim the radio is "Too old" , "Not repairable" , "Parts are too hard to get" "It won't be reliable for very long" BUT are in a real hurry to sell you a shiny new radio.

The MB8719 Washington has been around since 1979 (38 years) there is no reason with careful use & basic maintenance that they shouldn't still be working fine in another 38 years.

There are very few parts that cannot be bought new from various sources , most other parts can be scavenged off older "junked" radio's that are floating around out there.

Personal recommendations are normally worth their weight in gold - No one will ever recommend someone who has done bad work for them in the past.


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Scipio Kid
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Re: 2SC458 in older Uniden chassis - Watch out !

Post by Scipio Kid » Tuesday, 28 March 2017, 12:01 PM

Thanks Paxbone,
A lot of wisdom in those words. It's clear you've been around a while.

Also, thanks for the info on my radio. It's definitely a Washington without the President as you surmised.

I'm still looking for a recommendation on a good tech in Northern Utah. The fellow there at Sapp Brother's caters to the truckers who come through but so far as I've seen, he really knows his stuff. Maybe I'll have him do some work for me and then give my recommendation on him.

Also, (you may have answered this numerous times already but I haven't seen all your posts) do you skip into the US often? I don't get out too far here in Herriman (20 miles south west of Salt Lake) but I have a base out in the desert that skips all over. I'm just not there very often. I've made contacts in Texas, Florida, Georgia, California, all over Mexico and one fellow who claimed to be in England but I think he was drinkin' a lot and pullin' my leg a little. I've picked up contacts as far away as the Philippines but they apparently can't here me. It'd be nice to see how far I can really get out. I'm doing everything barefoot (for now) but It'd still be fun to listen in even if I'm not putting out enough power to join in. Let me know where and when to find you and I'll try to give you a shout (or at least a good listen) down under.

By the way, my son lives in Hawaii and recently spent 2 weeks traveling around Australia, his first time there. He was in Victoria for a friend's wedding and then flew all over the country to see the sights. (If you recently had a near fatal accident with a tall, blonde American kid driving on the wrong side of the road, well that was him ... sorry). Now he wants to give up the wonders of Hawaii for the splendors of Australia!

Ahhh, to be young, foolish and unfettered!
Happy Trails




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