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Author Topic: Kenwood TS-850S Amp Buffer  (Read 1093 times)
KO4NR
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Posts: 221




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« on: September 10, 2019, 07:22:59 AM »

I have been using this circuit to key my old Swan Mark 1 amplifier for 14 years.
https://www.qsl.net/k0bx/amp.html

Everything worked fine until a few weeks ago.  Was using the amp and smelled a burning PCB.  Shut everything down and found the amp keying circuit on the RF board had burned up.


Reluctant to use this circuit again.  No idea what went wrong.

Any ideas?

73,
Bill KO4NR
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KD0REQ
Member

Posts: 2399




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« Reply #1 on: September 10, 2019, 08:03:43 AM »

pin pattern suggests you roached a power transistor. likely the amp keying line.  be interesting to meter the amp standalone and see if the keying voltage is off spec, I suspect something popped there.

whether that board in the 850 is rebuildable, I leave to others to guess. looks pretty toasty, have to tack wires over a bunch of burned traces if it is.
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KO4NR
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Posts: 221




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« Reply #2 on: September 10, 2019, 09:37:20 AM »

I replaced the board with a used one.  Rig works good now.  Hesitant to use amp now.  The boards are very difficult to find.

Forgot to mention the Swan's T/R relay was replaced some time ago with a couple 12vdcof PCB relays.
https://www.azettler.com/pdfs/az755.pdf

73,
Bill
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W1QJ
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Posts: 2978




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« Reply #3 on: September 10, 2019, 11:40:01 AM »

Hi Bill, check your schematic on the Swan Mark I.  I haven't looked at it in a long time but Swan was noted for using the AC transmit pilot light ground and the relay ground (DC) at the same time on the antenna relay ground line.  The AC component isn't good for the  transistor.  You might want to check that.
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KO4NR
Member

Posts: 221




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« Reply #4 on: September 10, 2019, 12:07:19 PM »

Hi Bill, check your schematic on the Swan Mark I.  I haven't looked at it in a long time but Swan was noted for using the AC transmit pilot light ground and the relay ground (DC) at the same time on the antenna relay ground line.  The AC component isn't good for the  transistor.  You might want to check that.
Many thanks for the heads up!!
I replaced the orginial T/R relay with two 12vdc PCB relays several years ago and installed a +12vdc power supply.
https://www.azettler.com/pdfs/az755.pdf
I checked the DC voltage on the RCA connector on back of the amp and it was a little over +15vdc.
Swan Mark 1 Relay Circuit


73,
Bill
« Last Edit: September 10, 2019, 12:09:44 PM by KO4NR » Logged
AC2RY
Member

Posts: 755




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« Reply #5 on: September 10, 2019, 01:20:51 PM »

Hi Bill, check your schematic on the Swan Mark I.  I haven't looked at it in a long time but Swan was noted for using the AC transmit pilot light ground and the relay ground (DC) at the same time on the antenna relay ground line.  The AC component isn't good for the  transistor.  You might want to check that.
Many thanks for the heads up!!
I replaced the orginial T/R relay with two 12vdc PCB relays several years ago and installed a +12vdc power supply.
https://www.azettler.com/pdfs/az755.pdf
I checked the DC voltage on the RCA connector on back of the amp and it was a little over +15vdc.
Swan Mark 1 Relay Circuit



73,
Bill


Flyback diode seems to be missing. That could be a cause of transistor failure.
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KO4NR
Member

Posts: 221




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« Reply #6 on: September 10, 2019, 01:37:22 PM »

Hi Bill, check your schematic on the Swan Mark I.  I haven't looked at it in a long time but Swan was noted for using the AC transmit pilot light ground and the relay ground (DC) at the same time on the antenna relay ground line.  The AC component isn't good for the  transistor.  You might want to check that.
Many thanks for the heads up!!
I replaced the orginial T/R relay with two 12vdc PCB relays several years ago and installed a +12vdc power supply.
https://www.azettler.com/pdfs/az755.pdf
I checked the DC voltage on the RCA connector on back of the amp and it was a little over +15vdc.
Swan Mark 1 Relay Circuit



73,
Bill


Flyback diode seems to be missing. That could be a cause of transistor failure.

It should be across the relay coil, correct?
73,
Bill
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W9IQ
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Posts: 3521




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« Reply #7 on: September 10, 2019, 01:40:07 PM »

It would also be a good practice to place a reversed diode across your keying transistor. Belts and suspenders.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KO4NR
Member

Posts: 221




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« Reply #8 on: September 10, 2019, 02:09:46 PM »

Considering this circuit now:
https://kv5r.com/ham-radio/2018-projects/amp-interface/
73,
Bill
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W9IQ
Member

Posts: 3521




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« Reply #9 on: September 10, 2019, 02:23:52 PM »

Since you already have a low current, 12 volt keying system that would be quite redundant.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KO4NR
Member

Posts: 221




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« Reply #10 on: September 10, 2019, 02:32:54 PM »

Since you already have a low current, 12 volt keying system that would be quite redundant.

- Glenn W9IQ
You mean the one in the 850?

Don't want to fry another RF board.  Very hard to come by now.  I have read some use the 850 with an amp with no interface.  Just plug it into the Remote socket then the amp and operate.   Maybe I am being over cautious but the cost is very high when you loose the board.

73 and Thank you,
Bill KO4NR
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W9IQ
Member

Posts: 3521




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« Reply #11 on: September 10, 2019, 02:57:07 PM »

It should be across the relay coil, correct?

Yes, with the cathode oriented up in your schematic.

- Glenn W9IQ
« Last Edit: September 10, 2019, 03:07:53 PM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
Member

Posts: 3521




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« Reply #12 on: September 10, 2019, 03:03:10 PM »

Since you already have a low current, 12 volt keying system that would be quite redundant.

- Glenn W9IQ
You mean the one in the 850?

Don't want to fry another RF board.  Very hard to come by now.  I have read some use the 850 with an amp with no interface.  Just plug it into the Remote socket then the amp and operate.   Maybe I am being over cautious but the cost is very high when you loose the board.

73 and Thank you,
Bill KO4NR

Since you updated the keying circuit in the amp, you now have a low voltage, low current keying circuit whereas the referenced circuit is for a higher voltage DC circuit.

In addition to the flyback diodes, check that your ground connections through the keying line cable are solid on both ends. This should be a shielded cable interconnect.

Do you have any symptoms of RF in the shack?

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KO4NR
Member

Posts: 221




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« Reply #13 on: September 10, 2019, 03:14:53 PM »

Since you already have a low current, 12 volt keying system that would be quite redundant.

- Glenn W9IQ
You mean the one in the 850?

Don't want to fry another RF board.  Very hard to come by now.  I have read some use the 850 with an amp with no interface.  Just plug it into the Remote socket then the amp and operate.   Maybe I am being over cautious but the cost is very high when you loose the board.

73 and Thank you,
Bill KO4NR

Since you updated the keying circuit in the amp, you now have a low voltage, low current keying circuit whereas the referenced circuit is for a higher voltage DC circuit.

In addition to the flyback diodes, check that your ground connections through the keying line cable are solid on both ends. This should be a shielded cable interconnect.

Do you have any symptoms of RF in the shack?

- Glenn W9IQ
No RF symptoms in the shack.

Keying line is grounded.  Used RCA phono jacks.  I will double check the connections.

Thanks Glenn.
73,
Bill
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W9IQ
Member

Posts: 3521




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« Reply #14 on: September 10, 2019, 03:21:10 PM »

Your may also wish to place an RFC in the amp in series with the center connection of the keying circuit. Despite the two 0.01 bypass caps, you may be passing RF from the amp onto the keying line.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
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