Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Drake "hot transistor"




 
The AM Forum
August 03, 2020, 10:42:05 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Drake "hot transistor"  (Read 482 times)
0 Members and 1 Guest are viewing this topic.
k7mdo
Contributing
Member
*
Online Online

Posts: 369


« on: July 25, 2020, 10:59:57 PM »

Restoring a recently acquired Drake 2C.  When I got it it would just "hummmm" looking at the power supply it had a significant sawtooth to the low voltage (transistor) supply so I re did the supply board with new diodes and capacitors....  now it plays, and seemingly well.  

Except one thing that bothers me.  The audio output transistor that is heatsinked to the chassis gets pretty warm.... I looked it up in my old 1969 transistor manual and it is listed as "normal" operating temperature 25 degrees Celsius.  To my touch it is quite a bit hotter than that.

Anyone out there with a 2C that has tried the "finger" test on that rig?

Transistor: 40310. TO66.

It plays so well and the audio is very good.  In fact the set is going to replace my 2B that I have used for years....  

73, Tom
Logged
Pete, WA2CWA
Moderator
Member

Offline Offline

Posts: 7763


CQ CQ CONTEST


WWW
« Reply #1 on: July 26, 2020, 01:38:31 AM »

Should have roughly 12-12.5 volts on the transistor case. Check the .82 ohm emitter resistor. Check the collector current.
Maybe your finger(s) are extra sensitive to heat generating components. I always have a hand held Laser IR Infrared Thermometer on the workbench to check the heat of components. I never trust my fingers. I think I got mine at Harbor Freight for roughly $22.
Logged

Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
k7mdo
Contributing
Member
*
Online Online

Posts: 369


« Reply #2 on: July 26, 2020, 04:39:20 PM »

I dug into the underside this morning... the .82 ohm resister is .84 ohms.... should be OK... the case of the transistor measures about 11 VDC....  what I did find was the C-64 capacitor actually measured 17 mfd.... not the specified 10... so, nothing else to do I replaced it as I had one on hand...  

Radio still plays fine and for now I have installed a heatsink that I made in the shop to help dissipate a little....  

I'm starting to think the temp is not going to bother but have not actually measured it...  

I thought my Drake 2B was a little tight in the underside when I re-capped it.... this 2C is worse but having been a watch repairman in the 1970's made me a little less daunted by the tight spaces...  They didn't put it together with the expectation that someday it would need some parts replaced.

I even see a few black beauties (surprised me) that will need a hard look at some time.

Anyway, it plays so well and the wider bandwidth is a big difference on AM.  Great SSB too.

Take care, I will move on.

Tom, 73
Logged
Pete, WA2CWA
Moderator
Member

Offline Offline

Posts: 7763


CQ CQ CONTEST


WWW
« Reply #3 on: July 26, 2020, 06:30:42 PM »

I dug into the underside this morning... the 82 ohm resister is 84 ohms.... should be OK... the case of the transistor mrasures about 11 VDC.... 
Tom, 73

It's .82 ohms and hopefully yours is really .84 ohms and not 84 ohms  Cheesy
Logged

Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
k7mdo
Contributing
Member
*
Online Online

Posts: 369


« Reply #4 on: July 27, 2020, 01:18:22 AM »

Yes, digital typo!

Thanks
Logged
W1RKW
Contributing
Member
*
Offline Offline

Posts: 4217



« Reply #5 on: July 27, 2020, 04:25:24 PM »

this is just a guess but it is probably biased for class A. Power dissipation is rated at least 40watts and max collector current is 3A.  If Ic is within 3A the heat is probably normal.
Logged

Bob
W1RKW
Home of GORT. A buddy of mine named the 813 rig GORT.
His fear was when I turned it on for the first time life on earth would come to a stand still.
k7mdo
Contributing
Member
*
Online Online

Posts: 369


« Reply #6 on: July 27, 2020, 04:37:25 PM »

Thanks for the note....  I am about ready to believe that the temp is not a problem either....  Tom
Logged
Tom WA3KLR
Contributing
Member
*
Offline Offline

Posts: 1994



« Reply #7 on: July 27, 2020, 07:06:19 PM »

Just out of curiosity, what is the voltage across the 0.84 Ohm emitter resistor?
Logged

73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
Tom WA3KLR
Contributing
Member
*
Offline Offline

Posts: 1994



« Reply #8 on: July 31, 2020, 11:47:34 AM »

Hi Tom,

I emailed a friend of mine, Mark, who is a big Drake owner and restorer.  He has a R4C for 40+ years, has the same audio circuit.  Yes, he says he will burn his finger if he touches the output transistor, but it has been working fine for all those years.  I looked at the R4C manual and the voltage across the emitter resistor is 0.45 V which would be 0.55 A. The collector voltage is 14.5 V, 14.5 - 0.45 = 14.05 V collector to emitter, 14.05 v X 0.55 A. = 7.7 Watts dissipation; a decent amount of power.

Looking up the 40310 in my 1967 RCA SC-13 Transistor Manual, the maximum power dissipation is 29 Watts. Maximum operating junction temperature is 200 C (quite surprising to me as usually 150 C is the max for silicon junctions I thought). The thermal resistance junction to case is 6 degrees C/Watt.  There is reference to a derating curve earlier in the book.  7.7 Watts/29 Watts = 27 %, allows ambient temperature of about 150 C., by the manual.

Looking at the situation my way, 7.7 Watts X 6 degrees C/Watt = 46 degrees C case to junction temperature difference, very close to eye-balling the graph (200 C - 150 C). However using 125 C as a maximum junction temperature for good reliability, 125 C 46 C = 79 C case temperature.

If you can measure the case temperature and it is 79C or less you are in great shape.  Per RCA manual, 200 C 46 C = 154 C case maximum, pushing to the limit.

Looking at the case with an IR thermometer, I assume the case is a shiny metal surface.  The emissivity is lower that the IR meter is designed for and usually gives an erratic reading.  Put a piece of black or dark colored masking tape on the case to get a good reading.
Logged

73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
k7mdo
Contributing
Member
*
Online Online

Posts: 369


« Reply #9 on: August 01, 2020, 12:11:35 AM »

I have come to the conclusion of is normal...  Other folks have mentioned the same result.

The good news is that those transistors are still readily available! 

I made my own heatsink and it dissipates a lot of heat but maybe for no reason.

That 2c is quite  nice radio!

Tnx for your comments, Tom

Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.062 seconds with 18 queries.