[Amps] Ten Tec 425 Repair

[Jim Brown]

Good advice. And if that's bad, other stuff on the bias and key logic 
boards may be bad. It's worth pulling those boards out and checking all 
the semiconductors with an analog VOM (like a 260).

Yes, I think the input VSWR is a possible clue. But also, those padder 
caps are switched in parallel as the bandswitch goes lower in frequency.

Another possibility is a weak tube. The amp runs and tunes fine with 
only one tube in place, so I suggest trying it with one tube at a time.

The first time I had to chase problems on the keying and bias boards, I 
took the time to draw a nearly complete schematic (everything but the 
tuning components in the RF section). It's on my website as several pdf 
files.  k9yc.com/publish.htm

73, Jim K9YC

73, Jim K9YC

On Wed,8/31/2016 11:56 AM, Joe Giacobello via Amps wrote:
> Ken, I can't comment re L4, but high grid current is often associated 
> with a failure of R17 (4.7 ohm , 5% resistor) on the bias board. Why 
> this would only manifest on 75/80M is a mystery, but since it only 
> occurs after the amp warms up, it's possible that it's changing value 
> with temperature.  The board is accessible from the removable plate 
> under the RF compartment.  After you remove the four plugs and screws 
> holding it in place, it's easy to work on.
>
> 73, Joe
> K2XX
>
> On 8/31/2016 10:46 AM, Ken Claerbout wrote:
>> A friend asked me to look at his 425. First thing I noticed was the 
>> plate choke was burned. I think it came from trying to operate on 12 
>> meters. I replaced that choke with the Ameritron model, which I 
>> understand is not resonant on 12 meters. The amp also had a burned 
>> padder cap on the 160A position. That's replaced. The amp is working 
>> now on every band except for 75 & 80 meters. It tunes fine to about 
>> 500 - 600 watts. Advancing the power further, the grid current goes 
>> from zero to pegging the meter, with no in-between.
>>
>> I've replaced the padder caps for those segments with no change. The 
>> band-switch and contacts have been checked. As far as I can see they 
>> look fine. I thought I was on to something, when I found the 
>> connection from the tap on L4 had a cold solder joint on the 
>> band-switch. L4 is used on 160, 80 , and 75 meters. I Re-soldered 
>> that but same thing. I'm beginning to think that the toroid L4 is 
>> wound on is fried although, I don't see any obvious signs. I haven't 
>> taken it apart and removed the tape. After several tests on 80 & 160, 
>> I took the cover off and felt the core. It was very warm, almost to 
>> the point of being hot. I'm mindful that L4 also comes into play on 
>> 160. But something appears to be heating up, or arching, once power 
>> is advanced much past 500 watts output. Also when I first turn on the 
>> amp, it's cold, I can get close to legal limit on 80 & 75. But after 
>> several attempts the grid current problem pops up. Seems to indicate 
>> the toroid is heating up?
>>
>> I should also mention the input SWR on 80 is quite high, 3.5:1 but 
>> much better, 1.6:1 on 75. I suspect that's a separate issue and will 
>> look into that once I get the output side squared away.
>>
>> So the question, especially for those who have experience with this 
>> amp, I don't, have you seen the core in L4 go bad? Is there something 
>> I'm overlooking? I did try a spare set of tubes, nothing changes.
>>
>> 73
>> Ken K4ZW
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