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Author Topic: TS-50 gets hot when power applied  (Read 3911 times)
K9SMC
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Posts: 33




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« on: July 28, 2012, 01:44:08 PM »

I was testing my Kenwood TS-50 into a MFJ can dummy load through an MFJ-828 wattmeter in cw when the radio started to show very high power output, then no output and the heatsink gets very hot.  Shut the radio down until it cooled off, when I turn it on it gets hot immediately - suspect rf transistors shorted.  What are the part numbers and what is the easiest way to disassemble the radio to replace them?

Thanks, Steve K9SMC
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KA4POL
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Posts: 1910




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« Reply #1 on: July 28, 2012, 10:07:51 PM »

I think you are right with your diagnosis. The symptoms seem to indicate the PA started oscillating and at least one of the final transistors shorted itself. They are 2SC2879. I can't help you about the disassembly. Get the service manual from the web. Most likely you'll also find a Yahoo group. Usually there is good support there.
What I am asking myself is, how did the dummy load influence the whole failure. Could there be a problem?
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TANAKASAN
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Posts: 933




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« Reply #2 on: July 29, 2012, 01:05:21 AM »

I have a copy of the service manual, it's a 40 Mb PDF. If you post your email address I will send you a copy.

Tanakasan
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K9SMC
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Posts: 33




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« Reply #3 on: July 29, 2012, 07:17:33 AM »

Tanakasan, my email is scapp@comporium.net.  KA4POL, thanks for the part number, at least I can get them ordered and have them on hand.  I have enough projects, didn't need this one.

Steve K9SMC
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K9SMC
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« Reply #4 on: July 29, 2012, 10:12:06 AM »

I have taken some ohm meter measurements and found nothing shorted, not what I was expecting.  What can I look for as the cause besides pulling the finals out and replacing them?  Is there a possibility the finals are fine and something is driving the amp into oscillation causing it to get hot?

Steve
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KA5IPF
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« Reply #5 on: July 29, 2012, 10:39:55 AM »

Most likely. One of the finals has a B-C short. When the power cord is plugged in that applies voltage to the base of the other final and turns it on causing a large current draw and lots of heat. That will not show up as a short when looking at the power cord.

Clif
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K9SMC
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Posts: 33




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« Reply #6 on: July 30, 2012, 05:05:03 AM »

Checked the finals in circuit and no short.  No problem when power is applied, only when radio is turned on - it get hot immediately.  Possible bias issue, can find no info about that.  Does anyone know who works on Kenwood TS-50 radios??  Would tackle myself but no time and have sold most of my equipment - only have a meter and soldering iron left.

Steve K9SMC
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W6EM
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Posts: 710




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« Reply #7 on: July 30, 2012, 05:58:05 PM »

Steve:  The 2SC2879 datasheet is available from RF Parts dot com.  Based on its test parameters and circuit, you shouldn't be drawing more than 100 mA or so collector current each with no signal under ssb operation.

Also, the Toshiba test circuit uses a single diode junction as a voltage regulator.  Assuming its a silicon device, it should saturate at about 0.6v and hold the voltage to that value.  There is a 10/12 voltage divider between the diode and the base.  So, the bias on each base should be about 0.5vdc with no signal.  If that diode were to have opened or a bad solder joint floating it, that might explain too much bias and too much no signal collector current.

Lee
W6EM
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KA4POL
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Posts: 1910




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« Reply #8 on: July 30, 2012, 09:53:21 PM »

You said you did check the finals. Did you measure collector to emitter for short as well?
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TANAKASAN
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Posts: 933




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« Reply #9 on: July 31, 2012, 05:06:17 AM »

Sorry Steve, I have made a number of attempts to send the file but I just get a bounce message from your ISP saying that the file is too large. The file can be downloaded from:

http://96.9.26.247/schemi/TS50S_serv.pdf

The schematic diagram can be found at:

http://96.9.26.247/schemi/TS50S_sch.pdf

Tanakasan
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KE3WD
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Posts: 5694




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« Reply #10 on: July 31, 2012, 01:57:35 PM »

Try checking the Base-Emitter voltage drop by placing one probe each of your DMM set to DC Volts on those two pins. 

Power up the rig and ascertain that the heating is still happening, if that B-E drop is anything other than about 0.6 to 0.7VDC, you have likely got a bad device. 

The good news is that these particular devices will not break the bank. 

The bad news is that if the damage was really caused by oscillation, you could take out the replacements if that oscillation condition still exists, so be cautious and aware at turn-on.  I'd place a 12V bulb or even a 120V filament bulb in series with the + power lead for the first turn-on with the new devices, see if it was drawing heavy current or not before removing the bulb. 


73


73
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W5FYI
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Posts: 1044




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« Reply #11 on: July 31, 2012, 03:11:31 PM »

As an aside, if you bring the rig to Oklahoma within the next month or so, you can get it hot without applying power. Today's record-setting temperature in Oklahoma City was 107°F!
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KE4DRN
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« Reply #12 on: July 31, 2012, 03:17:51 PM »

hi,

Many manuals including the TS-50 service and operating

http://www.radiomanual.info/KENWOOD.html

73 james
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K9SMC
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Posts: 33




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« Reply #13 on: August 01, 2012, 04:49:46 PM »

I tried in-circuit measurements and came up with the following: E-B was 7.5 ohms, C-B and E-B were around 10-22Mohm - did not sound right to me, memory of transistor readings is from late 70's.  Will replace the transistors but will check drivers and bias before I do - don't want to waste $60 for nothing.
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KA5IPF
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« Reply #14 on: August 01, 2012, 08:39:14 PM »

Plug in the power cord and turn on the supply. Measure the C and B voltage on the finals. The E is ground. Should be 14v on C and -0- on B. If OK then put DMM plus lead on B of drivers and turn on power to the radio. There should be -0- volts. If ANY voltage then one of the drivers is leaky and turning on the other one. Lift both bases and do it again. One of the lifted leads will be high. Replace both if that is the case.

E-B ohms reading is the bias resistors of 15 ohms each.

Clif
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