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Author Topic: Ham fest ft901dm popped fuse on tx after new caps  (Read 1373 times)
KG7HVR
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Posts: 69




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« on: December 04, 2017, 07:16:44 PM »

So I got this rig and looked through it real good. It tripped the breaker about the 3rd time I powered it on.
So I replaced all the 22mfd and 47mfd caps on the rectifier boards. I found one dead shrunk up 22mfd cap.
So all seemed well . I was transmitting on 40m into a dummy load and after about a min or 2 listening to it on another radio it blew the fuse.
Where should I start looking. The main filter caps are blue and look newer.. there's 4 axial caps that I haven't replaced or checked. The rest of the radio Looks ok minus these Chinese rf parts fake Taylor tubes. Tubes all give off a violet color on tx.
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KG7HVR
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« Reply #1 on: December 05, 2017, 08:09:54 AM »

Bias was way high. Like 150ma at idle . Turned it to 50ma per manual.
The radio does a odd deal when first turned on meter swings full scale and no audio. Then if you turn AGC off and on the needle drops to zero and you have audio. Needle never moves on signals. Sometimes it will move to 1/4 scale and stay there for no real reason.
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K5DH
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« Reply #2 on: December 05, 2017, 10:40:24 AM »

With the AGC engaged, does the receiver come to life after a short time?  Have you checked the position of the RF Gain control?  Make sure it's set fully clockwise at first.  If the RF Gain is set properly and the receiver still doesn't respond, it sounds like you may have a problem with the AGC circuit. 

With the AGC disengaged, the S-meter will not operate.  The meter reading is derived from the AGC voltage. 
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KG7HVR
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« Reply #3 on: December 05, 2017, 12:42:19 PM »

With the AGC engaged, does the receiver come to life after a short time?  Have you checked the position of the RF Gain control?  Make sure it's set fully clockwise at first.  If the RF Gain is set properly and the receiver still doesn't respond, it sounds like you may have a problem with the AGC circuit. 

With the AGC disengaged, the S-meter will not operate.  The meter reading is derived from the AGC voltage. 

I've checked all that it will never come alive until you turn the AGC off and on you can let it sit for an hour and it won't come on but once you turn it off and turn it back on the radio comes alive.
Also the radio made a high pitch noise turning the power switch off.  I flipped it on and off a few times and it kept doing it. It popped the breaker also doing this  flipping to off not on
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N8YX
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« Reply #4 on: December 05, 2017, 01:39:50 PM »

Did you replace the HV smoothing capacitors (located underneath the Counter module)?

Those are probably what's causing the blowing fuses issue.

Fix that problem and move on to the rest of the rig.
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KG7HVR
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Posts: 69




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« Reply #5 on: December 05, 2017, 06:22:46 PM »

Did you replace the HV smoothing capacitors (located underneath the Counter module)?

Those are probably what's causing the blowing fuses issue.

Fix that problem and move on to the rest of the rig.
yeah. They look new. Bright blue 120mfd at 500 wvdc weren't the originals 100mfd? It hasn't blown but one fuse and the bias was 150ma when it did that.
Where's the cheapest source for those caps? A brief look on ebay shows $50 . Seems kinda steep like the $100 power cord
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KG7HVR
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Posts: 69




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« Reply #6 on: December 06, 2017, 07:30:41 PM »

The agc doesn't work at all on this radio. When it's on rf gain doesn't work. Also when its off rf gain works
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KG7HVR
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« Reply #7 on: December 07, 2017, 03:44:53 PM »

Update.....
I was on  3.913 talking a while with the group down there and my AGC and meter started working. It worked for several hours. I went to bed. Got up and it still worked till i switched from 40m to 20m. Back to the usual bs
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AC5UP
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« Reply #8 on: December 07, 2017, 05:18:23 PM »

What would that suggest?
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KG7HVR
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Posts: 69




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« Reply #9 on: December 08, 2017, 01:25:54 AM »

What would that suggest?
that i havent a clue whats wrong. I pulled every board and cleaned them tonight. It was working for most of today. Switch to 40 meters from 75 andcit still worled then it quit. Audio becomes distorted and no signal strength on meter in any position on the agc. Sounds a bit louder with agc off.
With agc off the rf gain will work and show full meter reading when rolled back. Switch to agc slow or fast now no audio .
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KD0REQ
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Posts: 2021




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« Reply #10 on: December 08, 2017, 10:37:56 AM »

what have you got for a stabilizing capacitor in the AGC circuit? is there a DC filter on that stage that's electrolytic? those two spots and creepy solder joints are where I'd start.  also dirty switches that need a little DeOxit.  a leaky transistor could also do it, but sounds like somebody's been working on that radio before you got it, and with caps hosed to start with, I suspect them all in a bad stage. this is also why I'm suspicious about the state of the solder joints.
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KG7HVR
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Posts: 69




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« Reply #11 on: December 08, 2017, 12:18:27 PM »

what have you got for a stabilizing capacitor in the AGC circuit? is there a DC filter on that stage that's electrolytic? those two spots and creepy solder joints are where I'd start.  also dirty switches that need a little DeOxit.  a leaky transistor could also do it, but sounds like somebody's been working on that radio before you got it, and with caps hosed to start with, I suspect them all in a bad stage. this is also why I'm suspicious about the state of the solder joints.
there are 3 on the IF board that contains the agc circuit I have replaced two of them and I'm about to replace the third one.
 today the radio hasn't decided to work but I noticed that the problem seems to happen when I change bands.
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KG7HVR
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Posts: 69




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« Reply #12 on: December 08, 2017, 02:16:32 PM »

Now the stupid thing has no rx at all
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AC5UP
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Posts: 4420




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« Reply #13 on: December 08, 2017, 05:15:21 PM »

...but I noticed that the problem seems to happen when I change bands.

Which should make you wonder if the band switch needs to be cleaned.

If the covers are off, give every bandswitch wafer the hairy eyeball looking for bent contact fingers or broken contact slices.  Note the overall cleanliness of the switch as well.  Are the moving parts / contact points clean and shiny?

If the RX works on a band but quits when you change bands, try switching several times between those two bands to test if one band always works and the other never works.  If operation of the two bands is consistently the same there may be nothing wrong with the switch.  If one or both work sometimes, or not sometimes, you have an intermittent switch contact..............
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KG7HVR
Member

Posts: 69




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« Reply #14 on: December 09, 2017, 02:58:15 AM »

...but I noticed that the problem seems to happen when I change bands.

Which should make you wonder if the band switch needs to be cleaned.

If the covers are off, give every bandswitch wafer the hairy eyeball looking for bent contact fingers or broken contact slices.  Note the overall cleanliness of the switch as well.  Are the moving parts / contact points clean and shiny?

If the RX works on a band but quits when you change bands, try switching several times between those two bands to test if one band always works and the other never works.  If operation of the two bands is consistently the same there may be nothing wrong with the switch.  If one or both work sometimes, or not sometimes, you have an intermittent switch contact..............
I have barely any sound from the speaker. I have heard strong stations faintly. Tx is normal. I feel as though i may have had it on 40 meters while i keyed up another radio and amp near by it and blew out the rx. The sb220 and antenna tuner are a few feet away.
I completely took the entire radio apart. All i found was evidence of the main filter caps having been replaced. The radio is pretty clean.
I dont know what to do now. I should mention i can hear the marker generator but its very low volume.
I have a oscilloscope and basic test equipment. Any advice is appreciated.
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