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Author Topic: Kenwood TM-733A Duobander with no output on 2 Meters (VHF)  (Read 6126 times)
WA4ZYN
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Posts: 39




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« on: March 05, 2013, 12:40:33 PM »

I was recently given a relatively non-working Kenwood TM-733A duobander. I have been checking it out, and I find that UHF works fine on both transmit and receive. On VHF, receive is fine, but there is NO output on transmit. I am wondering if anyone else has had experience with this rig and this problem? I have seen many on-line comments mentioning that "VHF goes south after a few years", but no one has mentioned ideas on repair.

Thank you for your time and consideration.

Francis WA4ZYN
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KA4POL
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Posts: 1961




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« Reply #1 on: March 05, 2013, 09:46:02 PM »

First suspect is the power module. There have been cases of internal cracks and similar in such modules.
A low TX current could point toward a power module problem.
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KA4POL
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Posts: 1961




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« Reply #2 on: March 05, 2013, 10:09:33 PM »

There is a description of a repair under http://www.dk7in.de/TM733.html
The pictures are real good information, the description is in German though.
He removed the plastic cover and located cracks. Using some wire he soldered the connection back into service. I guess a magnifying glass will be real helpful.
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WA4ZYN
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« Reply #3 on: March 06, 2013, 02:42:42 PM »

Yes, I agree. . .I magnified it even more than that by using an HD webcam and my laptop computer.  I can't find any cracks!  I've also been all over it with ohm-meter millimeter-by-millimeter, and can't find any discontinuity.  I had hoped it would be such a "simple" and cheap repair. 

Thanks for your suggestion!  I guess I'll have to save up and buy a replacement (about 60 bucks).

Francis WA4ZYN
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W9GB
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Posts: 2613




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« Reply #4 on: March 06, 2013, 03:51:19 PM »

Francis -

Check Q232, the APC circuit transistor, used for VHF operation.
Resolution outlined in Kenwood Service Bulletin no. 1056 (28 March 1994)

Problems with the Toshiba S-AV17 RF Power Module
http://www.repeater-builder.com/kenwood/s-av17/s-av17-repair.html

Do not wait too long, as Toshiba has supposedly discontinued the part.
RF Parts is suggesting the Icom SC part as suitable replacement.
http://www.repeater-builder.com/kenwood/s-av17/s-av17.pdf

===
Check the Kenwood Service Bulletin, found here,
Scroll down to Service Bulletin no. 1056 (28 March 1994)
http://www.qsl.net/v/ve7lgp//kenwood...d%20TM-732.htm

23-04-2000 TM-732A/E APC circuit improvement
Author: Kenwood Communication, inc.
Service Bulletin no. 1056 (28 March 1994)

Symptom:
Failure of APC circuit transistor Q232 can occasionally be attributed to the
gain dispersion charcteristic of the VHF Power Module.

Some of these devices allow normal transmit power output even with reduced "DB" terminal voltages.
If the units is then operated for extended transmit key down periods Q232 can overheat and fail.

Corrective Action:
Measure the transmit power output and the "DB" terminal voltage at a dial frequency of 145.02 MHz.

Add the 1 dB circuit if the transmitter power is 60 watt or greater and the "DB" terminal voltage is 10 Vdc or less.

After adding the attenuator, re-adjust the transmitter output power for 50 watt and check the "DB" voltage.
The "DB" voltage should now fall between 10 and 11 Vdc.
If it does not adjust the values for R98, R100, and R99 until this voltage range is obtained.

Parts required:

Qty Description New Part No. Circuit Description
* 820 ohm chip resistor RK73FB2A821J R98, R100
* 470 ohm chip resistor RK73FB2A471J R98, R100
* 270 ohm chip resistor RK73FB2A271J R98, R100
* 5.6 ohm chip resistor RK73FB2A5R6J R99
* 10 ohm chip resistor RK73FB2A100J R99
* 18 ohm chip resistor RK73FB2A108J R99

* Note: Quantities vary depending upon level of attenuation required to meet requirement of step 3.

===
w9gb
« Last Edit: March 06, 2013, 04:15:56 PM by W9GB » Logged
WA4ZYN
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Posts: 39




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« Reply #5 on: March 06, 2013, 04:10:56 PM »

Thanks.  Mine has absolutely zero output, so I think the power module is toast.  I hate that!

Thank you for your input.

Francis
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W9GB
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« Reply #6 on: March 06, 2013, 04:18:38 PM »

Do NOT replace the VHF module, S-AV17 until you check the Q232 transistor.

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WA4ZYN
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Posts: 39




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« Reply #7 on: March 06, 2013, 06:38:03 PM »

Thanks!  Will check Q232 on your advice. ..
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WA4ZYN
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Posts: 39




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« Reply #8 on: March 06, 2013, 06:40:24 PM »

Can you tell me where exactly Q232 is located?  I've got my magnifying glass out. . .my schematic is tiny.
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WA4ZYN
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Posts: 39




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« Reply #9 on: March 06, 2013, 06:43:49 PM »

Q232 seems to be on the UHF board. . .do you perhaps mean Q28 on the VHF board?  What would be my best way to check it?

Thanks again.
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KA4POL
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Posts: 1961




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« Reply #10 on: March 06, 2013, 09:44:37 PM »

You are right about Q232, it does not have any influence on the VHF part. If you have a chance to check the drive IC10, however. It would be interesting to see if it does have RF output going to the power module before checking other parts like APC or High/Med power circuitry.
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WA4ZYN
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Posts: 39




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« Reply #11 on: March 07, 2013, 07:16:08 AM »

Thanks for helping clarify.  A very knowledgeable ham sent me that info on Q232 in a previous post--it was the text from a Kenwood Service Note.  I just can't see (in my non-engineer brain) how that transistor would affect something on the VHF board. . .I'm hoping he will weigh in and clarify that for me, as Kenwood says it could be the culprit.  As you may know, the schematic is very complex, and covers many pages in the manual.  It is hard for me to keep track of what connects to what over several pages, while studying it through a magnifying glass!

 
I have checked at pin 1 of the power module, and there IS 1.2 volts present there, which leads me to believe that IC-10 (driver) is producing output.  Am I correct about that?  I'm not an engineer; I have no training in electronics, other than being a ham for 43 years.  I grew up with tube equipment, and its components are big enough to find, check, and replace.  I'm just "nervy" enough to take the covers off and try to fix a piece of this more modern equipment rather than just junking it because of some glitch or other. I've been studying the power module under high magnification.  I may have discovered a crack in a tracing last night, and if I get time today, will try to solder that spot.  My hands are just about too shaky to do the work, which is very frustrating to me. 

Thanks again for your valuable time and input.

73,

Francis  WA4ZYN
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KA4POL
Member

Posts: 1961




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« Reply #12 on: March 07, 2013, 10:31:33 AM »

If you got a second radio like an HT you can get close to the preamp and see if it transmits. I can not tell you what value you should find. A diode in series and a capacitor to ground make a good means of measuring RF presence. Let's hope you located something in the module.
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KA5IPF
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« Reply #13 on: March 07, 2013, 11:10:30 AM »

Thanks for helping clarify.  A very knowledgeable ham sent me that info on Q232 in a previous post--it was the text from a Kenwood Service Note.  I just can't see (in my non-engineer brain) how that transistor would affect something on the VHF board. . .I'm hoping he will weigh in and clarify that for me, as Kenwood says it could be the culprit.  As you may know, the schematic is very complex, and covers many pages in the manual.  It is hard for me to keep track of what connects to what over several pages, while studying it through a magnifying glass!

  
I have checked at pin 1 of the power module, and there IS 1.2 volts present there, which leads me to believe that IC-10 (driver) is producing output.  Am I correct about that?  I'm not an engineer; I have no training in electronics, other than being a ham for 43 years.  I grew up with tube equipment, and its components are big enough to find, check, and replace.  I'm just "nervy" enough to take the covers off and try to fix a piece of this more modern equipment rather than just junking it because of some glitch or other. I've been studying the power module under high magnification.  I may have discovered a crack in a tracing last night, and if I get time today, will try to solder that spot.  My hands are just about too shaky to do the work, which is very frustrating to me.  

Thanks again for your valuable time and input.

73,

Francis  WA4ZYN

If you look at the schematic you will see that Q232 provides bias for both the 2m and UHF modules.

First test; What is the current draw in 2m tx? If less than 2A then look at the module itself.

From left to right on the module should be ~1vRF, 12VDC, and 13.8VDC. The 4th pin is the output. If you don't have ~12VDC on Pin 2 look at Q232. If you have the voltages and the current draw is low and no power out it's the module. Simple as that.

Clif
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WA4ZYN
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Posts: 39




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« Reply #14 on: March 07, 2013, 07:33:20 PM »

2 A current?  Are you talking 2 amps at the RF power module?  Or are you talking 2 Amps of A/C line current?  Sorry if I missed a point somewhere.

I didn't have an RF probe put together yet, but I did get 1.2V A/C indication on my digital multimeter on pin 1 of the RF power module on transmit.  Pin 2 shows 12V on TRANSMIT, 0V on receive.  Pin 3 shows the 13.8V all the time.  Of course, absolutely ZERO output on pin 4.

I just tried your suggestion.  Using my HT set to same frequency, I held its antenna near the rig, and pegged it's signal meter when pressing PTT on the TM-733A.  In fact, it pegs the receive bars on the HT wherever I am in the room.  I think that shows me that the exciter is working fine. 

Couldn't find any trace repairable in that S-AV17. . .

73,
Francis
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