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Thunderbolt
| | Posted on Monday, September 30, 2002 - 12:28 pm: | 
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I was talking on it when it went. I found that the 2 resisters that connects from the rear pills to the ground trace on the board where burnt out. The colors were red,black,brown,gold stripes. The amp stayed keyed up when this happened. I replaced them with one from an old 667v,same colors of coarse and it seemed to transmit fine but they smoked out again very quik. I had it on my base setup that time. SWR's were fine and input wattage was 2 going in. What now? |
307
| | Posted on Monday, September 30, 2002 - 2:30 pm: |
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If the resistors you are talking about are really "smoked", it is not the resistors fault. The transitors (pills) are bad. Where ever you see a "smoked" resistor at the base of a pill , means that the typical .7 vDC MAX at the base has come up to 13.8 vDC because of a short from the output to the input of the transistor.
307 |
2600
| | Posted on Tuesday, October 01, 2002 - 1:55 am: |
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I suggest strongly that you check the swamping resistor across the output combiner. It's right alongside the toroid (doughnut) coil about 2 inches directly to the rear of the relay. A 200 or 220 ohm 2 watt is typical. This one goes when one or the other pair of '2879 transistors has laid down. Also look for a smoked quarter-watt resistor, usually 6.8 ohm (blue-gray-black-gold, near the right (your right) edge of the front circuit board, a couple of inches back from the meter. Often the color bands will be burned black. If this one smokes,it could lead to overheating the new transistors.
I just hope you're not using this thing with a magnet mount antenna. Around here, this is a favorite method for popping mobile amplifiers.
73 |
Thunderbolt
| | Posted on Tuesday, October 01, 2002 - 1:23 am: |
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If the pills where bad would I still get full wattage out of the amp? When I replaced the resisters, I did get full wattage out of it but it only took a minute for the resisters to burn through again. |
2600
| | Posted on Thursday, October 10, 2002 - 10:53 am: |
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I checked back on this thread to see if you had looked at the little bias resistor I mentioned.
Am I right in assuming that the 10-ohm base resistors are okay UNTIL you key it? They don't start to get hot on standby, do they?
The quick way to see if the bias circuit is causing this trouble would be to find the 25-ohm (sometimes 30) 5-watt resistor on the front circuit board behind and to the left of the meter. Unsolder and pull (gently) one lead out of the board. Some Texas Star amps will also have another one of these 5-watt resistors on the rear circuit board. If so, pull one leg of it loose, or unsolder the wire that feeds back to it. This will turn off the bias current to the 2879s. If the base resistors STILL smoke, that's trouble. That PROBABLY means that one or more of the four 2SC2879 has failed. It only takes one to do this. If the base resistors stay cool when you key it this way, there's a quarter-watt resistor smoked on the front board, usually 6.8 ohm quarter-watt. Different production runs will use a slightly different value, as low as 3.9, or as much as 10 ohms. If it's too burned to read the color bands, 6.8 ohms is usually a safe bet to replace it.
A quarter-watt resistor is a LOT cheaper than four RF transistors. Anyway, that's still my best educated guess, from what you described.
Let us all know what you find.
73 |
Thunderbolt
| | Posted on Monday, October 14, 2002 - 9:26 am: |
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I am still going over this amp. I did not find any visable burnt resistors in the front or any other place. The resistors by the rear finals are fine until I key up. They burn out quickly. I get full wattage and good swing until they go which is about 5-10 seconds.the resistor that you speak of that is behind and to the left of the meter I asumme is the closest one to the front and directly behind the variable,one leg solders to one side of the meter? which is fine. The only others are next to the ground point of the coax connection which are 2 side by side and they are fine too. I dont really know which one to lift the leg on. The other one is under the fuse connections? This is puzzling since there are no visiable signs of damage anywhere. I have a 250V sitting here that came in with the same problem. |
Thunderbolt
| | Posted on Tuesday, November 26, 2002 - 7:03 pm: |
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Ok,I am back at this amp again. I have replaced the rear 2879's and the resistors at the base of them. Now when I key the amp it does swing ok but the rear 2879's get very hot fast. I turned the dial a watt all the way down and keyed up with almost no output and they still get hot. I also noticed the amp draw from 12 volts will rise quickly as they warm up. I am afraid of burning these new ones out so I cut it of.I can not see anything else visibly burnt. What now? |
2600
| | Posted on Wednesday, November 27, 2002 - 12:52 am: |
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Well, if there are no "visibly burnt" parts, you need to find the "invisibly burnt" bias resistor on the rear circuit board. If the front two transistors aren't getting hot like the rear ones, the problem is on the rear circuit board. Find the 5-watt (usually 25 ohms) ceramic power resistor at the right (your right) end of the rear circuit board. The front end of this resistor has a wire leading to the front circuit board. The rear-facing lead is feeding into a circuit board trace that leads to the center-tap of the input transformer. Somewhere along that trace or at one end of it, will be a quarter-watt resistor to ground. My diagram says 6.8 ohms, but the factory often selects a different value to suit a batch of transistors. This little resistor to ground is bad, even if it looks good. Since it isn't burned, you should be able to read the color bands okay. The third band really is GOLD on this one.
73 |
Thunderbolt
| | Posted on Wednesday, November 27, 2002 - 10:46 am: |
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I am a little confused on what the value of this resistor that is there now and what the colors should be. The one that is there now looks to be(burgandy,grey,gold,gold) but that dont seem to add up to 6.8. On my chart a 6.8 should be (blue,grey,gold,gold) I dont know. I have many resistors here but not labeled so what colors should I look for to work in this spot. What colors could I get away with using until I can get the correct one. Thanks alot for your time 2600. |
2600
| | Posted on Thursday, November 28, 2002 - 1:08 am: |
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How about "blue,gray,gold,gold. I'll bet the burgundy was blue before it got cooked.
5.6 ohm would be green,blue,gold,gold(or silver)
5.1 ohm is green,brown,gold,gold
4.7 ohm is yellow,violet,gold,gold or silver
I wouldn't go much below 4.7. Higher than the original, and getting the front two transistors to balance the rear ones may suffer. Using a physically larger wattage resistor is okay. The resistance value is what really matters. It doesn't throw a lot of heat when everything is working right. One or both of the old transistors in the rear board developed a short inside that fed power back out the base lead. This little resistor takes the same surge as the 10-ohm resistors that smoked. It just didn't show as much visible damage.
Oh, and you're welcome. Replacing "almost" enough parts in an amplifier can be pretty frustrating.
73 |
Thunderbolt
| | Posted on Tuesday, December 03, 2002 - 1:08 pm: |
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ok,back again. I replaced that resistor with a (burgandy or maybe brown,grey,gold,gold)and it worked. Keyed it down and evrything stays cool or normal. The wattage doesnt seem to be as it was but it is good enough. only swings around 400 or so now. Thanks for all the help. |
2600
| | Posted on Friday, December 06, 2002 - 2:11 am: |
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Hey congratulations T-Bolt, you stuck with it and got it tamed. The resistor you used may be holding down the bias a little lower than factory setting. They use the value of this resistor as a "trimmer" function. The factory selects the right resistor value for the batch of 2SC2879 they are using that week. If you use it mostly for AM, it shouldn't be a problem. If SSB modulation is "gravelly", the resistor value you used may be too low. The drop in power is nowhere near enough to affect your transmit range. That difference is a lot like turning OFF the nitrous oxide valve in a racing motor. Sure, you saw a little more power with the nitrous turned on, but not for long enough to matter.
Since I don't know what kind of meter you are using, 400 watts swing on SOME meters is just about what a 667 should show. I seldom see more than 550W on a true PEP meter.
73 |
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