X-Force 600HD Smoked

[Shadetree Mechanic]

Before going any further. The DEI that were in it, did all 4 blow? If not, was it a transistor on each 2 pill section or were both transistors bad on one side? Trying to understand if maybe there was a problem before you changed devices.

Also, when you checked voltage, think you said it was 0.388V. Was this at the base of each transistor. ( what I’m getting at, does 1 or 2 show different voltage?)

The DEI that blew was the left most one, looking from the front. It did not take out any of the resistors around it, just the one on the output combiner. I just figured they were not very well matched to begin with. The supply that I run is a PowerMax 100a and before the transistor blew, the fan on the supply had kicked into high and I kept talking. Not long key times, but frequently and maybe a 50% duty cycle. Although nothing in the amp was really hot when I immediately pulled the cover and checked.

The voltage measurement was taken from the top of the bias diode on the side that blew. I will measure again at the base of each and report back.

 

[Shadetree Mechanic]

Why is that brass tuning cap in the amp to begin with? Someone may have thought it looked heavy duty, but it's in the way and unnecessary. All of that stray inductance from the long lead is counterproductive.

Rip out the trombone and save it for a 6 pill or larger amp that needs it. Run the wire straight from the output to the relay (where you put that coil) and solder in a good size cap to ground like every Dave copy box has been for the last 15-20 years. Long lead lengths are bad so keep everything short.

Do the voltage checks Crusher mentioned. If some pills are pulling more current than others the thing will be out of balance and keep failing until all are matched.

You have the right idea on the feedback resistors. Just take another 100 ohm 2 watt and parallel it with the one that's already there. More negative feedback will make it more stable.

Stick with the nominal cap values and figure out why it isn't stable before you jump too far down the rabbit hole.

The trombone was my attempt to tame the output SWR of 1.7 and I think the input SWR was hitting about 3 on SSB but not AM. I haven't taken very good notes and I go too long between times that I work on it. So yes the trombone will be coming out along with the coil. The original output cap was rated at 150pf and measured 130pf. As Crusher noted, it should be more like 170pf so I will be getting one on order along with the input transformer caps he recommended. Looking at the input trimmer, I can relocate it to the input combiner. I put it near the side so I could adjust it with the cover on, but once it is set it shouldn't need any further adjustments.

I will definitely be doing the additional feedback resistors and it makes scene to get back to square one with this thing.

 

[543_Dallas]

The trombone was my attempt to tame the output SWR of 1.7 and I think the input SWR was hitting about 3 on SSB but not AM. I haven't taken very good notes and I go too long between times that I work on it. So yes the trombone will be coming out along with the coil. The original output cap was rated at 150pf and measured 130pf. As Crusher noted, it should be more like 170pf so I will be getting one on order along with the input transformer caps he recommended. Looking at the input trimmer, I can relocate it to the input combiner. I put it near the side so I could adjust it with the cover on, but once it is set it shouldn't need any further adjustments.

I will definitely be doing the additional feedback resistors and it makes scene to get back to square one with this thing.

Just a note on the output SWR thing because I was also misled by someone I thought was smart. The swr meter can not see the circuit behind it. All it knows is how much power is going to the antenna and how much is coming back. If you are seeing high swr with the amp on it is due to harmonics that may be caused by oscillation. Low swr is not an indication that the output of the amp is tuned correctly.

If you were using a tube amp and cranked the plate tuning one way or the other the swr would not change unless the amp started oscillating. The amp should be stable enough that this doesn't happen. You may be able to tame it by playing with the tuning but the first time the wind blows the wrong direction it will all go to hell.

 

[Shadetree Mechanic]

Also, when you checked voltage, think you said it was 0.388V. Was this at the base of each transistor. ( what I’m getting at, does 1 or 2 show different voltage?)

Ok I just went and measured again. The two on the left where the transistor blew each read 0.384 v and the two on the right each read 0.380v.

 

[Shadetree Mechanic]

Just a note on the output SWR thing because I was also misled by someone I thought was smart. The swr meter can not see the circuit behind it. All it knows is how much power is going to the antenna and how much is coming back. If you are seeing high swr with the amp on it is due to harmonics that may be caused by oscillation. Low swr is not an indication that the output of the amp is tuned correctly.

If you were using a tube amp and cranked the plate tuning one way or the other the swr would not change unless the amp started oscillating. The amp should be stable enough that this doesn't happen. You may be able to tame it by playing with the tuning but the first time the wind blows the wrong direction it will all go to hell.

Wow, yes I know what you are saying about the tube amp SWR. It appears that I was already half way down the rabbit hole. Haha.

 

[2NC995]

Shade, I know you’ve got a couple unanswered thoughts out there, so I apologize for the hijack...

I ask for my own curiosity; have you by chance identified the other components in this bias circuit?

I see the diode providing the voltage drop...got it. The 103 cap, which I assume is for RF bypass. And the electrolytic I can only guess provides a little stability to the DC voltage?

But what are the green and yellow components paralleled to the diode?

I’m also surprised the diodes weren’t mounted over a transistor for thermal tracking. I thought that was the norm?

Do you have any thoughts on this setup after monkeying with it?

 

[Shadetree Mechanic]

Shade, I know you’ve got a couple unanswered thoughts out there, so I apologize for the hijack...

I ask for my own curiosity; have you by chance identified the other components in this bias circuit?

I see the diode providing the voltage drop...got it. The 103 cap, which I assume is for RF bypass. And the electrolytic I can only guess provides a little stability to the DC voltage?

But what are the green and yellow components paralleled to the diode?

I’m also surprised the diodes weren’t mounted over a transistor for thermal tracking. I thought that was the norm?

Do you have any thoughts on this setup after monkeying with it?

View attachment 37613

The green and yellow components I believe are inductors. The only thing I can guess is that they provide a small clamping action to help suppress voltage spikes? Not sure. Maybe they just help swamp any RF off the line so the bias is pure DC.

 

[2NC995]

The green and yellow components I believe are inductors. The only thing I can guess is that they provide a small clamping action to help suppress voltage spikes? Not sure. Maybe they just help swamp any RF off the line so the bias is pure DC.

Thanks Shade. I was thinking an inductor would be a dead short across the diode at DC, and there might be a thermistor in there. I just don’t know

Edit and a side note...The bias voltage you’re seeing is less (nearly half) the typical voltage drop across a rectifier diode. Hmm. Still don’t know

 

[Crusher]

I’m not sure if those small yellow and green things are. I have one here, but going to have to get a magnifying glass to read. Really fine print. The diodes are 1N5420. I have some here from Arlyn.

Bypassing B+, yes. I try to use 2-3 per transistor.

Feedback resistors- yes, parallel 100ohm resistor across the 100ohm resistor there.

What radio are you using to drive amp?

 

[Shadetree Mechanic]

I’m not sure if those small yellow and green things are. I have one here, but going to have to get a magnifying glass to read. Really fine print. The diodes are 1N5420. I have some here from Arlyn.

Bypassing B+, yes. I try to use 2-3 per transistor.

Feedback resistors- yes, parallel 100ohm resistor across the 100ohm resistor there.

What radio are you using to drive amp?

I am using a Stryker 955.

 

[Crusher]

One of those little caps are a 104 / 0.1uF cap. The ones I have are yellowish orange. I bet the green is a 1000pf. Using different values to filter out any RF. They are not inductors

 

[Crusher]

Are all 4 Rfp transistors matched?

 

[Shadetree Mechanic]

Are all 4 Rfp transistors matched?

Yes, I bought them as a matched set.

 

[Crusher]

Ok, just asking on the matched set. I don’t know if rf parts tests or if from HG testing. I know of 1 guy that bought 100 to truly match 32 for his box. I’ve had regular HG going into oscillation by what I think was power supply issue. Had to upgrade power supply and problem went away. Issue in regulator circuit. So many possibilities it’s hard to say what issue is. I’ve tried the best I can.

 

[Shadetree Mechanic]

Ok, just asking on the matched set. I don’t know if rf parts tests or if from HG testing. I know of 1 guy that bought 100 to truly match 32 for his box. I’ve had regular HG going into oscillation by what I think was power supply issue. Had to upgrade power supply and problem went away. Issue in regulator circuit. So many possibilities it’s hard to say what issue is. I’ve tried the best I can.

Thanks for all your help, the transistors I believe are matched by RFParts and they have a number 62 in pencil on the top. I will change the input transformer capacitors and add the extra feed back resistors and +B bypass caps. I am having trouble finding the 170pf output capacitor or even any value of the correct type to add to the existing capacitor to bring it up to 170pf. The original cap was 150pf and measured 130pf. Could you point me to a source for the correct capacitor to use for this? Also, is the 6KV rating needed? I have seen capacitors with a 3KV but am not sure what's needed.

Thanks again for your help, I will get it back to square one with the updated parts and we will see what it does.