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Author Topic: Swan 270b no rx or tx hum changes w volume  (Read 3245 times)
KG7HVR
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Posts: 59




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« on: September 08, 2017, 03:49:06 PM »

I picked up a swan 270b for $75 it didn't come with the cord so I thought soldered acord on to the back of the jones plug-and jumpered the two different spots ( pins 7-8 and pins5-6) to test it out. it has a hum that increases with the volume it doesn't transmit or receive although I did adjust the bias and was able to see some activity. so if you pull the audio tube out the hum goes away so I'm not sure that it's a capacitor issue. the capacitors have been replaced by someone else. also the meter pegs all the way full scale when in receive mode. I have the schematic and I'm studying it to try and figure out what's wrong. I'm wondering if maybe there's a problem with the oscillator? I don't really know where to start on this so any advice from all you Swan experts out there would be great!!
I'm going to go through the troubleshooting chart and test for the proper voltage in all the places and see if I come up with anything weird I have an oscilloscope and I don't know where to really test to look for the hum or issues
 All tubes glow
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KG7HVR
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« Reply #1 on: September 08, 2017, 09:34:57 PM »

the swan 270b manual on bama has alot of mistakes or its for the 270 non b. I tested all the voltages at tubes. most were normal however i saw lower than rated high voltages like 187v instead of 237 onv11 pin 5. 193v insted of 250v on pin6 of 12be6 vfo amp. same with the driver . but certain tubes  are listed wrong so im not sure how well i can rely on the data given
bottom line lower voltages in places, higher in other. voltage on pins that should be zero. so maybe the cause is bad shorted tube?
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KAPT4560
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« Reply #2 on: September 09, 2017, 06:43:10 AM »

 I make it a point not to immediately trust other people's work, whether it was done correctly or not. I would go through the circuitry to double-check that it is wired correctly. Previous mods can be a cause of concern as well.
 Power supply and filter caps would be the first place to start. Use care around high voltages. I have seen NOS filter caps that were bad out of the box, caps installed backwards and shorted diodes, etc.
 Seeing a lot of mistakes in the manual is not good as you will depend on an accurate manual for repairs. You may have a later version with circuit changes or the unit was modified for other duties/hacks by a previous owner?
« Last Edit: September 09, 2017, 06:47:00 AM by KAPT4560 » Logged
KG7HVR
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Posts: 59




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« Reply #3 on: September 09, 2017, 08:56:32 PM »

I make it a point not to immediately trust other people's work, whether it was done correctly or not. I would go through the circuitry to double-check that it is wired correctly. Previous mods can be a cause of concern as well.
 Power supply and filter caps would be the first place to start. Use care around high voltages. I have seen NOS filter caps that were bad out of the box, caps installed backwards and shorted diodes, etc.
 Seeing a lot of mistakes in the manual is not good as you will depend on an accurate manual for repairs. You may have a later version with circuit changes or the unit was modified for other duties/hacks by a previous owner?
one tube listed they missed 2 pins. it think it was off by a pin on each listed.
serial #3408 not sure if that means much. bama only list the 260 under cygnet and the 270b. no reg 270 i found a regular 270 manual on cbtricks website so ill comparing differences. i also found a stray red wire that appears to be done on purpose.
think i read about this on a hellicrafters radio to pick up cw
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KAPT4560
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« Reply #4 on: September 10, 2017, 04:05:52 AM »

 Where does the red wire go to on the one side? Hallicrafters had a 'gimmick' wire for capacitive coupling to the IF for a 'sorta/kinda' BFO to receive CW, but that insulated wire wrapped around a lead going into the IF transformer.
 I have looked at a couple of 270B manual schematics and they have handwritten notes for microphone jack wire colors and a circled cap and choke off a plate. This leads me to believe that this is all copies of the same manual.
 There may be only one manual and your radio is different because of previous owner mods? It will require further investigation. The owner may have been trying to tailor the radio to his requirements?
There may have been published mods available in past issues of QST?
The mods.dk website has a 270B manual, but no published mods.
 In any case, you may want to put the radio back to what it was and then diagnose any problems from there. I have had to 'undo' hacks in order to get a radio back into original form.
 Then check tube socket voltages and make a list of them to compare with the voltages published in the manual.
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WG8Z
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Posts: 306




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« Reply #5 on: September 10, 2017, 11:17:01 AM »

Is the jumper plug in place on the Ext. VFO connector on the back?
If not you will need to jumper 2 pins. Forgot which 2.
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KG7HVR
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Posts: 59




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« Reply #6 on: September 11, 2017, 02:39:22 AM »

yes the vfo is jumpered. I'm not a expert on using a scope but I saw a signal from vfo on out to a tube and followed it a bit but then had to really adjust the scope to see the signal. honestly I have no idea if I'm am looking at a vfo signal . it didn't change a lot with the band switch. I tried to test the power supply board which is wired weird and looks aftermarket with the scope on a 10x probe my scope is a 100mhz Tektronix.
the caps are wrong sizes on the board. 150mfd and 100mfd. originals were 80 and 40mfd. I replaced the paper caps. no changes. the noise varies with volume from low to very loud so the amplifier audio part works.
I have to order the high voltage caps. I cant find any here in phoenix az
so anyways I think the wire looks like a gimmick wire. since photo bucket is not free I have no idea HOW TO POST PICS HERE.
I will keep messing with the radio in my spare time. hopefully once working its better than the swan 350 I once had. that thing drifted so bad it was barely usable but sounded good
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AJ8L
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Posts: 14




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« Reply #7 on: September 11, 2017, 05:20:35 AM »

You might look at the 4 rectifier diodes. I have had them fail on the 270 and 270b a cause a loud hum with no receive or transmit. I replaced all 4 of mine with 1N4007 diodes. Geoff, AJ8L.
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KG7HVR
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Posts: 59




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« Reply #8 on: September 11, 2017, 02:04:30 PM »

You might look at the 4 rectifier diodes. I have had them fail on the 270 and 270b a cause a loud hum with no receive or transmit. I replaced all 4 of mine with 1N4007 diodes. Geoff, AJ8L.
aren't there 8 in total?
whats the most economical diodes to buy at frys electronics. I found the originals and they were $5 each so it would have cost $45
if they are bad they should test bad normally? or can they leak?
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KD0REQ
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Posts: 2018




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« Reply #9 on: September 11, 2017, 02:16:34 PM »

1N4007s are nickle pricing. Look at DigiKey or Mouser. Get those PS caps while you're at it.
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KAPT4560
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Posts: 383




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« Reply #10 on: September 11, 2017, 02:18:59 PM »

 There are 8 diodes. It looks like 2 full wave bridges in series.
Early silicon diodes were known to be leaky after some years. You may want to test them before just replacing them. They should read open in one direction and a nominal voltage drop the other direction. All 8 should show fairly equal results to each other.
If they are leaking back current, that could cause a hum.
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KG7HVR
Member

Posts: 59




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« Reply #11 on: September 11, 2017, 11:03:24 PM »

There are 8 diodes. It looks like 2 full wave bridges in series.
Early silicon diodes were known to be leaky after some years. You may want to test them before just replacing them. They should read open in one direction and a nominal voltage drop the other direction. All 8 should show fairly equal results to each other.
If they are leaking back current, that could cause a hum.

I went to frys and got 8 of them. they were $2 each. i had them match vetco's 67 cent price. there's another one going to the 12v supply I didn't account for. non tested bad on my fluke 16 and all read similar numbers (0.555)

I traced the red wire to v12 the 100khz cal marker its labled "output coupling" so i guess its normal? it just acts as a antenna?
I haven't powered up the radio yet. I just finished the diodes on both bridges
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AJ8L
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Posts: 14




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« Reply #12 on: September 12, 2017, 03:30:33 AM »

It has been so long since I repaired mine I forgot there were 8 of them. They are a nice radio. I have 3 of them. Geoff, AJ8L.
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KG7HVR
Member

Posts: 59




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« Reply #13 on: September 12, 2017, 11:17:06 AM »

It has been so long since I repaired mine I forgot there were 8 of them. They are a nice radio. I have 3 of them. Geoff, AJ8L.
theres another one on the power supply board too. looks like a drop in aftermarket board.. guys call is writen on it too. pencil marks labeling stuff too.
I double checked everything. pulled every tube and found that it wants to rx with the agc tube removed. i was even hearing a distorted marker. but as soon as the tube is put back in the meter swings full scale and radio quiets.
i tuned in a a very distorted rtty signal from another radio transmitter. also hooked my mfj 259 to it and tuned the signal. very diatorted. still lots of hum but now with the agc tube pulled i can see its trying to work.
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KG7HVR
Member

Posts: 59




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« Reply #14 on: September 12, 2017, 12:15:49 PM »

ok ...I have a update. with the 6av6  v10 agc amp tube pulled i get crappy rx and the marker works badly till warmed up then it fades and no longer is heard. but it still can pick up a garbled signal from a near by radio in the room. put the tube back in and it gets quiet and no rx at all. it will also transmit and actually alot better than expected but it does sound drifty and odd. with tube in tx audio is reduced to almost nothing.
so now what?
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