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eHam Forums => Amplifiers => Topic started by: KO4NR on September 10, 2019, 07:22:59 AM



Title: Kenwood TS-850S Amp Buffer
Post by: KO4NR 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.
(https://i.imgur.com/l0TW1oLm.jpg)

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

Any ideas?

73,
Bill KO4NR


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KD0REQ 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.


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR 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


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W1QJ 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.


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR 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
(https://i.imgur.com/YnoYOSZm.jpg)

73,
Bill


Title: RE: Kenwood TS-850S Amp Buffer
Post by: AC2RY 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
(https://i.imgur.com/YnoYOSZm.jpg)


73,
Bill


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


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR 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
(https://i.imgur.com/YnoYOSZm.jpg)


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


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9IQ 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


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR on September 10, 2019, 02:09:46 PM
Considering this circuit now:
https://kv5r.com/ham-radio/2018-projects/amp-interface/
73,
Bill


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9IQ 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


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR 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


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9IQ 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


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9IQ 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


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR 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


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9IQ 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


Title: RE: Kenwood TS-850S Amp Buffer
Post by: VE3TMT on September 10, 2019, 03:23:11 PM
With my 850, I use a simple external relay that feeds of the 12V and SEND lines from the radio to key the relay. The contacts in turn close the keying line on the FL2100B. Been using this simple approach for years, never had a single issue. Don't operate QSK so not a concern.

Max
VE3TMT


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR on September 10, 2019, 03:26:51 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
Sure can't hurt.  What value RFC should I use?
Bill


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR on September 10, 2019, 03:31:18 PM
With my 850, I use a simple external relay that feeds of the 12V and SEND lines from the radio to key the relay. The contacts in turn close the keying line on the FL2100B. Been using this simple approach for years, never had a single issue. Don't operate QSK so not a concern.

Max
VE3TMT
Sounds like a simple and reliable approach.  Thanks for the feedback.
73,
Bill


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9IQ on September 10, 2019, 03:56:58 PM
I would shoot for at least 100 uH in order to provide sufficient impedance on the lower bands.

- Glenn W9IQ


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR on September 10, 2019, 04:12:11 PM
I would shoot for at least 100 uH in order to provide sufficient impedance on the lower bands.

- Glenn W9IQ
Ok Glenn and Thank you!!
73,
Bill


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KM1H on September 11, 2019, 05:54:37 PM
RCA connectors and jacks are notorious for developing poor grounds over time, especially in a  environment such as high humidity NH in the summer. My pair of Drake C Lines and other gear that used them suffered until I regularly cleaned them. This was before DeOxit.

Carl


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR on September 11, 2019, 07:19:32 PM
RCA connectors and jacks are notorious for developing poor grounds over time, especially in a  environment such as high humidity NH in the summer. My pair of Drake C Lines and other gear that used them suffered until I regularly cleaned them. This was before DeOxit.

Carl
Thanks for the heads up Carl!!
73, Bill KO4NR


Title: RE: Kenwood TS-850S Amp Buffer
Post by: N3AJB on September 12, 2019, 07:12:18 AM
Below is a link to a video from Mr Carlson's Labs in which he discusses a buffer circuit for new transceivers and older amps which might use 120 VDC antenna relay keying voltages. 

Of course the circuit will work with any amplifier.  The transistor is the MPSA 92 which can handle up to 300V and costs $.22.  I had to buy 50 on Ebay for around $5.00.

It's a very simple circuit and easy to fabricate. 

If you advance to the 10 minute mark you'll find his discussion of the circuit.  However, I found the entire video very interesting.   JON


https://www.youtube.com/watch?v=FHxOI_iSmcM


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9AC on September 13, 2019, 05:04:01 AM
Of course the circuit will work with any amplifier.  The transistor is the MPSA 92 which can handle up to 300V and costs $.22.  I had to buy 50 on Ebay for around $5.00.

FWIW, I'll add a few comments...

Addition of the flyback diode across the relay coil is always a good idea, especially on relays with large solenoids since they can produce a large amount of current when the magnetic field collapses.  The diode will also accelerate the relay's release time since the collapsing field no longer holds the relay as current is diverted into the flyback diode.

It's hard to believe the level of damage done to the PC board was due to this alone.  I'll deviate from Carlson's advice on this one -- not because his advice is bad but if the work is being done anyway, choose a better switching device.  I would not use a MPSA92 here.  It's also a PNP type and not NPN which is what would normally be used - so just be careful of polarity.  

Any time there's propensity for back current damage, I would use a Darlington at a minimum (e.g., TIP150 series).  A Darlington transistor is in effect two transistors in a single three-lead package.  A Darlington offers much better collector-to-base isolation than a BJT transistor like the MPSA92.  Several Darlington switching transistors contain an internal flyback diode.  The next level of protection is the addition of a Zener diode from the Darlington's base to circuit ground.

Stepping it up further, we can use one or even a balanced pair of IRF820 MOSFETS (See http://wb9kzy.com/keyallhv.htm (http://wb9kzy.com/keyallhv.htm) ).  These are good for switching 400V AC or DC and a pair make the circuit polarity independent.  The IRFBG30 will switch up to 1KV.

Finally, use a (4N35 or PVI5080N) optocoupler on the MOSFET gates.  With just three components, you have a buffer that can safely switch a couple amps of current on any key line, old or new, and have the ultimate protection of photo isolation.  

I've designed my own buffers on PC boards and these can be seen on my QRZ page.  They are shown under the "KEY LINE INTERFACE" heading.

One alternative I've been experimenting with uses an Ixys CPC1978 solid-state relay in place of IRF820s (See  http://www.ixysic.com/home/pdfs.nsf/www/CPC1978.pdf/$file/CPC1978.pdf (http://www.ixysic.com/home/pdfs.nsf/www/CPC1978.pdf/$file/CPC1978.pdf) ).  However, the on/off switching time is a few ms but in fact, they switch much faster than the spec and still plenty fast for nearly all keying and sequencing circuits.

Paul, W9AC


 


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9IQ on September 13, 2019, 05:21:22 AM
That is all valid, Paul but in what way is that superior to simply using the dry T/R relay contacts provided by the TS-850?

- Glenn W9IQ


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9AC on September 13, 2019, 05:59:34 AM
That is all valid, Paul but in what way is that superior to simply using the dry T/R relay contacts provided by the TS-850?
- Glenn W9IQ

Well, whatever caused the significant trauma to the circuit board would likely damage the TS-850s dry relay contacts.  I cannot imagine what caused that level of damage from the amp, but whatever it was, the amp should be fully isolated from the rig at least until the root cause is investigated.

Paul, W9AC


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KA5IPF on September 13, 2019, 06:52:24 AM
The burned spot on your board is because of Q43. Q43 supplies 12VDC to pin 7 on the remote connector and is 10ma MAX current draw. It is not part of the amp keying circuit. You amp should be connected to Pin 2 (common) and Pin 4 (switched). Neither is connected to ground. When activated the 2 pins close using a relay with contacts rated at 2A to key the amp.

Most likely. You were using Pin 7 to key an external relay to key the amp and that relay draws ~100ma, 10 times what Pin 7 is rated for and it finally burned up. Hook up the amp correctly.


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9AC on September 13, 2019, 08:07:27 AM
The burned spot on your board is because of Q43. Q43 supplies 12VDC to pin 7 on the remote connector and is 10ma MAX current draw. It is not part of the amp keying circuit. You amp should be connected to Pin 2 (common) and Pin 4 (switched). Neither is connected to ground. When activated the 2 pins close using a relay with contacts rated at 2A to key the amp.

Most likely. You were using Pin 7 to key an external relay to key the amp and that relay draws ~100ma, 10 times what Pin 7 is rated for and it finally burned up. Hook up the amp correctly.

His connections and schematic are correct -- and he indicated it was working correctly for 14 years.  Pin 7 sources current limited +12V as you describe but does not directly connect to the amp's relay.  Pin 7 provides +12V any time the TS-850's TX is engaged and its connected to his buffer circuit.  His schematic shows pin 7 connected to a pair of 1K/1K resistors to establish voltage-divider bias to the MPSA42 switching transistor.     

Your suggestion of using pin 2 and 4 is also correct when using the TS-850's internal amp relay.  For me and a lot of ops, it's way too loud -- and slow.  His buffer circuit can key as fast as any amp can.  My only issue with his circuit is that there's an easy opportunity to replace the MPSA42 with a better switching device for protection back into the TS-850 that does not increase complexity nor expense. 
 
Paul, W9AC


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KA5IPF on September 13, 2019, 10:26:25 AM
I guess I missed his keying circuit he added. I see the original amp circuit but not his addition. That transistor had been overheated numerous times ergo the large dark area. It finally gave up.

Have fun, I can only tell you how the circuit was designed to be used and normal usage.


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR on September 13, 2019, 06:41:38 PM
Excellent feedback and discussion!!  I appreciate it!!

I’ll go with the SSR.
http://www.crydom.com/en/products/catalog/mp-series-ac-pcb-mount.pdf
73,
Bill KO4NR


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9IQ on September 13, 2019, 06:55:01 PM
Bill,

For that SSR to work, you will need a minimum load current of 20 mA. The relay circuit you added to your linear will not be sufficient. You will need to put an additional load on the output (e.g. a resistor, LED or lamp).

Also make certain you order the DC version of the SSR (MPDC...).

- Glenn W9IQ



Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9IQ on September 13, 2019, 07:52:47 PM
I would also suggest that what fried your board is the the 850 12 volt on transmit pin sources 12 volts. You modified keying circuit in the amp also sources 13+ volts.

If you wish to use this type of interconnect, simply remove the diode in your linear switching circuit that is powering the relay and remove the electrolytic filter cap. This will make your amp a current sink for the 850 voltage source.

No matter which way you go, don't forget the flyback diode on the amp's switching relay.

- Glenn W9IQ


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR on September 13, 2019, 07:59:22 PM
Bill,

For that SSR to work, you will need a minimum load current of 20 mA. The relay circuit you added to your linear will not be sufficient. You will need to put an additional load on the output (e.g. a resistor, LED or lamp).

Also make certain you order the DC version of the SSR (MPDC...).

- Glenn W9IQ


I'm using two PCB relays in the amp for T/R.  I've had them in there for about 14 years now.
https://www.azettler.com/pdfs/az755.pdf
At 12vdc and a coil resistance of 275 ohms they should draw about 43ma each.

73,
Bill KO4NR


Title: RE: Kenwood TS-850S Amp Buffer
Post by: W9IQ on September 13, 2019, 08:01:50 PM
Yes, that should do it then.

- Glenn W9IQ


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR on September 13, 2019, 08:12:29 PM
Yes, that should do it then.

- Glenn W9IQ
Appreciate your well thought out feedback. All the information shared in this thread will be a big help to others in the same boat.
73,
Bill KO4NR


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR on September 13, 2019, 08:30:01 PM
The burned spot on your board is because of Q43. Q43 supplies 12VDC to pin 7 on the remote connector and is 10ma MAX current draw. It is not part of the amp keying circuit. You amp should be connected to Pin 2 (common) and Pin 4 (switched). Neither is connected to ground. When activated the 2 pins close using a relay with contacts rated at 2A to key the amp.

Most likely. You were using Pin 7 to key an external relay to key the amp and that relay draws ~100ma, 10 times what Pin 7 is rated for and it finally burned up. Hook up the amp correctly.

His connections and schematic are correct -- and he indicated it was working correctly for 14 years.  Pin 7 sources current limited +12V as you describe but does not directly connect to the amp's relay.  Pin 7 provides +12V any time the TS-850's TX is engaged and its connected to his buffer circuit.  His schematic shows pin 7 connected to a pair of 1K/1K resistors to establish voltage-divider bias to the MPSA42 switching transistor.    

Your suggestion of using pin 2 and 4 is also correct when using the TS-850's internal amp relay.  For me and a lot of ops, it's way too loud -- and slow.  His buffer circuit can key as fast as any amp can.  My only issue with his circuit is that there's an easy opportunity to replace the MPSA42 with a better switching device for protection back into the TS-850 that does not increase complexity nor expense.  
 
Paul, W9AC

I was using a NTE94 transistor in the circuit because I had a bunch of them.
https://www.nteinc.com/specs/10to99/pdf/nte94.pdf
I tested it with my meter and found that it was shorted(Base to Collector).  The failure occurred while I was tuning the amp.

I was so focused on finding a replacement board and checking out the amps keying circuit I overlooked the obvious.
73,
Bill


Title: RE: Kenwood TS-850S Amp Buffer
Post by: WE6C on September 20, 2019, 04:35:06 AM
I've been using this...

https://www.qsl.net/k0bx/amp.html

I did add a diode and used a transistor with higher ratings than what's shown in the link. With this I can turn off the amplifier relay in the 850. Been using it for 3 or 4 years now.


Title: RE: Kenwood TS-850S Amp Buffer
Post by: KO4NR on September 20, 2019, 05:59:17 AM
I've been using this...

https://www.qsl.net/k0bx/amp.html

I did add a diode and used a transistor with higher ratings than what's shown in the link. With this I can turn off the amplifier relay in the 850. Been using it for 3 or 4 years now.
I was using the same circuit.  Had been operating fine for 14 years then it failed.  I used a NTE-94 transistor which had higher ratings as well.

73,
Bill