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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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22nd Mar 2023, 9:41 am | #1 |
Triode
Join Date: Mar 2019
Location: Lytham St Annes, Lancashire, UK.
Posts: 11
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Kenwood TS-700G
I pulled my old TS-700G from under the bench (it had been there maybe 10 years) with the intent of selling it on. When I powered it on I noticed the TX power output was down - CW/USB/LSB is at 7Watts instead of 10W and FM is at 5 Watts instead of 10 Watts.
I've been through the radio it appears to be working fine (a couple of blown dial lamps) - the oscillator chain has all frequencies where they should be, carrier suppression is >40dB, repeater shifts are fine and so on. Low power is usually a classic PSU issue, the 21 volt line fails - but I've recapped the PSU and it is fine. You can't get 7W out with a blown PA stage. The fact that the power output is different on FM and CW suggests maybe there are two issues? Not sure. A frustrating side issue is that my copy of the TS-700G/A Service Manual is incorrect - you can't put a scope probe on a FET if there isn't one on the board. This TS-700G is a later one, with the TS700A HET board. Any TS700G guru's got thoughts on where to look? I just want to get this thing off the bench and away. |
22nd Mar 2023, 11:39 am | #2 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,013
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Re: Kenwood TS-700G
Lack of drive? There is a varicap tuned filter on the HET board. Is the tune Voltage VCV correct for the band?
It comes from Relay 2-5 and the Repeater / Band Switching. Any of these could have dirty contacts and mis-tune it. |
23rd Mar 2023, 9:58 am | #3 |
Triode
Join Date: Mar 2019
Location: Lytham St Annes, Lancashire, UK.
Posts: 11
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Re: Kenwood TS-700G
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23rd Mar 2023, 10:04 am | #4 | |
Octode
Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
Posts: 1,648
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Re: Kenwood TS-700G
Quote:
The first thing I would ask is....."Can your power measuring device be trusted?" 7W is only a couple of dB down on 10W. 5W is only 3dB down on 10W. Can you check the accuracy of whatever power meter you are using? Richard |
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23rd Mar 2023, 11:54 am | #5 | |
Triode
Join Date: Mar 2019
Location: Lytham St Annes, Lancashire, UK.
Posts: 11
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Re: Kenwood TS-700G
Quote:
What I've just found is that T3 on the GEN board doesn't peak, it hits the 'max inductance stop'. I suspect the cap inside the can has failed. I've had that fault before on that type of transformer on another TS700G quite some years ago now. I need to fix that first and see if that resolves things. 73 Steve |
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23rd Mar 2023, 1:08 pm | #6 |
Octode
Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
Posts: 1,648
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Re: Kenwood TS-700G
Steve,
yes I would expect the error on a Thruline to be a fraction of a dB, so that's fine. And the reflected power is minimal so no problem with your dummy load. Sounds like you may be on to something with the non-tuning inductor. Richard |
23rd Mar 2023, 1:13 pm | #7 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,013
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Re: Kenwood TS-700G
Thrulines are not that great actually, read at or below midscale.
I still think that following the dodgy inductor is more likely to resolve it all the same. |
23rd Mar 2023, 2:18 pm | #8 | |
Octode
Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
Posts: 1,648
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Re: Kenwood TS-700G
Quote:
Your comment got me wondering what the accuracy actually is quoted as? So I googled "bird thruline accuracy" and got the advice that it is +-5% of full scale reading. So we are looking for a 10W reading with 25W full scale. Actual reading is 7W or 5W depending on mode. 5% of 25W is 1.25W. So the actual power corresponding to the 7W reading could be anywhere from 5.75W to 8.25W. And for the 5W reading, its 3.75W to 6.25W. I think that's accurate enough to indicate that there is a real problem with the equipment, and that the low power readings are genuinely low. Richard |
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23rd Mar 2023, 2:35 pm | #9 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,013
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Re: Kenwood TS-700G
Yes that's the point. 5% at full scale not across the scale.
There would be no need for the 10W 5W 2.5W 1W inserts otherwise... |
24th Mar 2023, 1:12 pm | #10 | ||
Triode
Join Date: Mar 2019
Location: Lytham St Annes, Lancashire, UK.
Posts: 11
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Re: Kenwood TS-700G
Quote:
I had hoped to get that inductor dealt with this morning but got distracted with other stuff... 73 G0AIN Steve |
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29th Mar 2023, 9:10 pm | #11 |
Triode
Join Date: Mar 2019
Location: Lytham St Annes, Lancashire, UK.
Posts: 11
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Re: Kenwood TS-700G
So, I swapped the faulty transformer and now both TX chains provide exactly the same amount of drive / power out. The output voltage from the mixer unit to the PA stages matches the service manual. The varicap control voltage on the VCV pin varies in line with what the potential divider is supposed to deliver.
My initial assumption that it wasn't the final is looking badly flawed now, as it is all that is left to test. I've pulled the PA unit and now need to test the transistors. I have spares of both driver and final so hopefully this is at an end. Many thanks for your help. 73 Steve G0AIN |
3rd May 2023, 10:48 am | #12 |
Triode
Join Date: Mar 2019
Location: Lytham St Annes, Lancashire, UK.
Posts: 11
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Re: Kenwood TS-700G
Well I tested the PA transistors and they were seemingly OK. I swapped the entire PA unit for one in the junk box, it produced 15W on FM and CW as per manual, ALC adjusted to wind that back to 12 Watts exactly as per manual.
Is it possible for a PA transistor to be damaged but not blown? I tested the transistors in and out of circuit and they seemed fine. Thanks for the help! |
3rd May 2023, 11:11 am | #13 |
Dekatron
Join Date: Aug 2007
Location: W.Butterwick, near Doncaster UK.
Posts: 8,923
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Re: Kenwood TS-700G
Normally transistors in my experience transistors are 100% or duff completely. No low emission like a valve.
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3rd May 2023, 11:31 am | #14 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,013
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Re: Kenwood TS-700G
It is possible. RF transistors are often built with several devices on the same die linked up by bond wires to the connecting tabs. You can blow some junctions but not others.
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3rd May 2023, 2:31 pm | #15 |
Nonode
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
Posts: 2,851
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Re: Kenwood TS-700G
A simple diode probe is handy for such fault tracing. Even without a circuit of the pa, you get a feel of what gain (if any) there is or where it stops.
Rob
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4th May 2023, 1:21 am | #16 | |
Pentode
Join Date: Sep 2019
Location: Perth, Western Australia, Australia.
Posts: 199
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Re: Kenwood TS-700G
Quote:
You could get an output stage that tested OK for gain at the low power level of the RF sweeper, but under full drive "couldn't hack it". |
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