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TRC-451 predriver problem

codecxbox

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Jul 28, 2021
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The Emperors are on hold until I get new drivers and finals..
Im working with a TRC451 which was given to me and it was really messed up..so far I have fixed the RX section pretty much accurate possible with no osc, just a sdr and a TinySa as a signal generator..
this radio has a missing choke, that goes to the base of the driver, I installed a very old choke, I dont know if that would work..
Ok..this is the problem..
Upon the 451 Service Manual, the predriver T40 should have certain voltages but I get other..looks like it not conducting..but I do get a 27.015 Ch 5 signal as selected at TR41 which I think its a mixer buffer..Im seeing that signal on the SDR..Im including some pics..
I want this radio to work..Its a good 40 SSB and best of it , its small..fits in my car glove compartment..I really dont care about big radios, this one I would love to use as my daily driver..
 

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codecxbox

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Jul 28, 2021
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update!
the silkscreen placement sign on the board is wrong..its flipped..so switched the TR and now I get meanful voltage although not the SM values..getting 1.3 base, 8.3 collector and 0.62 emmiter, TX AM mode ch 5...test point for driver and final currents are not showing anything but I do feel the predriver and the driver getting warm..final is unattached from the collector pad..
btw the installed predriver is a C1973 but SM says C2086D..will the 1973 work as a replacement?
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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btw the installed predriver is a C1973 but SM says C2086D..will the 1973 work as a replacement?

Yes!

Found many PC-122 radios with Silkscreens showing wrong direction - unless the radio specifically stated in the schematic is used - say a Zener instead of a typical 1N4148

upload_2021-10-14_21-44-3.png
C170 can change value amongst the years this chassis was made, similar to the 640e in design but "crippled" by Tandy to lower your expectations compared to the others using this chassis during that time frame.

C170 helps reduce the peaking output "drive" to the 1730 Pre-Pre Driver - so a higher values of capacitance lessens the drive and heating - which is a trade off.

By cooler, will certainly make it last longer - use discretion...


Don't mean it can't be tweaked...

  • so yes, this radio is certainly a keeper - wish I still had mine but a story for later...
 
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codecxbox

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Jul 28, 2021
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Thks Handy! As I was looking at your post and preplanning the op, I found several ceramic caps on the radio that wasnt supposed to be there, most were too high..I changed them according to the manual parts list except 1, its supposed to be a 470pf but I only had a 390pf on hand..
Now, Im seeing a stronger and very clean signal at the drivers collector..its not getting warm though and current draw at the DC cable is 0.460 TX AM..but I do see a voltage drop at the collector from 14.35 to 11 volts..the final is not installed yet..does the driver rely on the final for current load?
 
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Handy Andy

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In light of the question...

Both the Driver and Final DRAW from the same source - so it will "drop" power - voltage and current - as the parts draw power thru their respective Collectors - both are sharing the same sourcing - so yes, to each their own.

As they draw the power - it is why you need to set the mA draw correctly for both - else one becomes unequally yoked to the other and can take nearly all the power and produce a lot of heat in the process.

Glad you saw the "caps" reference and was able to "tune" the radio's TX stage to stage "Admittance" so it can give you the results you want.
 
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codecxbox

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Well, after couple of days servicing car audio, back to what I strive to learn!
Ok, did Handy's 170 cap...and I got..drum roll...tada!
1 watt TX AM deadkey!
Now you may say pff but for me thats a huge achievement! this radio was dead, DEAD!
after tuning the TX related transformers and inductors per service manual spec, thats what I get..I going to suppose that I have the wrong TX transistors installed which I think they are probably bad..they do test ok with a dmm but Im seeing that RF transistors are not like audio transistors..its seems they go weak even if they test ok with the Dmm test..
the driver is a c1306 and the final is a c2166..and that 2166 is not supposed to be a final and I dont trust it either..
Collectors of both are at 10.25 V and 1.2A at the DC connector TX AM..so its not pulling enough current even with bias adjusting..comparing a good trc 449, collectors should be at least 5.6 volts...
 
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nomadradio

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The driver is right, but the 2SC2166 is only used as a final in AM-only radios.
Or as a sideband driver where the 1306 is found in this radio.

It won't deliver the peak power to show full sideband wattage.

The 2SC1969 the radio is meant to use was discontinued about 15 years ago.

Counterfeits abound. Some of them work, some just don't. The original manufacturer was Mitsubishi. They made that part from about 1978 until 2006 or 2007. The process had become highly refined, and the result was a tough component that would tolerate some abuse without breaking down too fast. A genuine original Mitsubishi part won't be cheap. Not extinct, just more and more rare each year.

And a chinese company called "Eleflow" has made and sold a legitimate modern-day version. Lately we have found the "HG" brand our best current alternative.

And those get counterfeited, too.

73
 

codecxbox

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thks Nomad..theres a fellow around me that has a room full of vintage radios and sells for cheap..Im considering to make an offer for similar sbb radios and pull out that 1969 if I find it within.
what 23 channel radio carry the 1969?
 
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nomadradio

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The 1969 didn't show up until 40 channels were well under way. If there is a 23-channel model that used a 1969 I have never seen it.

The transistor Uniden used before the 1969 was the 2SC1307. It should work in that radio, and that's what you'll find in most 23-channel SSB CB models. Unless it's so old the final is in a metal package.

73
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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Unfortunately the 2166 and 1306 are "considered" Driver transistor so you can get wattage, just not a lot of it - to get the "oomph" you'll need to either use a 1969 or the 2312 - both are "clones" now so you kinda take chances on the supplier...
 
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Handy Andy

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Apr 23, 2018
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Andy, I think you meant 2312

Updated...

Hey 8:14, the first cup hasn't woke me up yet...

Thank you!

giphy.gif

Don't stare at this too long, it will make you want to go pee...
Reminds me of days I had to go pick up the "kids" ...
upload_2021-10-21_16-7-1.png
I miss those days...​
 

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Handy Andy

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Back then, they were the "kids" - they've gone onto better places now ...

As I get older I miss that routine because I had time to wake up - let alone wake up the "kids" for day-care - mom-care and grooming ....

Look thru my past posts - thousands of them, every now and then I let you folks know I'm human...
:)
 

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