General Lee dual 520 bias question...

[Hawkeye351]

Wow, I been testing the output through my antenna the whole time, my SWR is lowest on 26.885 mhz (1.1) and it's a 1.8 on channel 40 (haven't taken the time to readjust it).

I decided to check it on my dummy load instead. Unholy crap, this radio is doing 60w PEP. Decided to try the output on the antenna again but on 26.885mhz where the SWR is lowest (1.1), wow, it's 62w PEP.

So far so good. Waiting on locals to get on the air to get reports and hammer on it a while.

 

[Hawkeye351]

Well, I figured I would hammer on it a few minutes before everyone else gets on the air just to monitor the output.

3w key with modulation - 40w, after 2 minutes the power dropped back again, same amount of drop as before.

MaxMod cool to warm (barely warm). C145 not hot. Chassis behind audio IC and Passthrough regulator is barely warm.

Gonna try some zener s across the two finals, then readjust bias. Will let you all know more in a bit.

 

[Hawkeye351]

Installed a zener on each final (I placed one on the driver previously). Reset bias. Reset key levels. Still drops after a minute or so, but doesn't drop back as much as before and climbs back up quicker than before. Nothing is getting anywhere near hot, Mosfets, Passthrough, AF, Audio IC, etc...all are cool to warm.

Talked to a few of my locals (even though it's still dropping back but not as much) for about 15 minutes on and off the key, they all said it sounds great, strong steady signal, no choppiness or issue on their ends like before.

I even spoke to the owner while he was on his mobile on his way home from work, he said he didn't hear any noises or strange issues from it this time.

Oh well, gonna run it overnight to see what happens.

Any suggestions as to why the output still drops (but not as much as before), even though it's new Mosfets, new MaxMod, new AF regulator, zeners across all the Mosfets, bias staying steady now?

 

[Hawkeye351]

So far, it's still dropping back on power (carrier and swing) after about 10 minutes of on and off talking (10 to 30 seconds at a time) and the heatsink behind the MaxMod/audio IC/8v regulator gets warm to hot, not scorching hot, the Mosfets get barely warm.

The locals said it sounds great and looks great on their meters. No fluctuations of the signal or any audio issues whatsoever on their ends the entire time, although I was seeing it on my watt meter.

Max is 60w PEP, after a few minutes it's drops to about 40w PEP, then steadily climbs back up to level out at 53w pep, until the next time I key up and does it again, but my locals can't see it or hear it on their end at all. Nothing is getting scorching hot at all.

This is with:
Zener diodes on all Mosfets
3.75v driver bias (VR 1/2 way)
3.50v each final bias (VR 1/2 way)
3w carrier (Lo power)

Gonna check a few other things.

 

[brandon7861]

Just wanted to throw this out there for max mod enthusiasts.
timestamp 12:58

 

[Hawkeye351]

It's what he wanted in it. I prefer 2SB817 myself, then a TIP36C. Those two work just as good in my opinion.

 

[brandon7861]

And since these radios are not 100% efficient, maybe 62w is a little ambitious for that regulator/heat sync combination. I wonder what would happen if you had a fresh regulator and backed it down a little like nomad suggested. Assume 70% efficiency, that's almost 90w. 60°C is 140°F, so if it feels hot to you, good chance you are pushing that derate curve.

 

[Hawkeye351]

Not sure what else to do. I've tried resetting the bias to 3.75v driver 3.50v each final and it still does it.

Driver 3.75v, finals 3.50v each
Driver 3.50v, finals 3.50v each
Driver 3.20v, finals 3.50v each
Driver 3.20v, finals 3.20v each

No matter what bias setting I adjust to, the radio still drops back on power after a bit of talking and then gradually climbs back up to settle.

Nothing getting scorching hot, only warm to slightly hot.

I've never ran into this issue with a 2SB817 or MaxMod mixed with old school BJT driver/final(s), or a 2SB817 mixed with Mosfets.

Gonna put it on the spectrum analyzer and see what the output looks like. If it looks good, then I'm letting it go. Everyone says they don't see or hear any strangeness whenever it does it.

 

[Hawkeye351]

Hmmm, sneaky people, lol...
Just found a diode (1N4001) tucked under some wires and parts in the Passthrough area. This diode tied into the R238 circuit but closer to the Passthrough regulator, hid pretty good.

Removed that diode, tested it out, so far so good.

I've never mixed any swing mods with a MaxMod, I can never get them to work together.

 

[Hawkeye351]

Again sneaky...
Noticed I couldn't adjust the modulation VR's, so I suspected a limiter still clipped somewhere. Checked around and saw nothing clipped until I pushed on R249, then I saw the snip. Very neat snip, couldn't even tell it was snipped.

Resoldered R249 back together, barely any audio. Adjusted both modulation VR's both ways, hardly any swing. Barely swings 6w PEP from a 3w key, barely swings 9w from a 7w key. Clip R249 and modulation is back up but can't be adjusted.

Any ideas as to why soldering R249 back together would drop almost all audio, regardless of where modulation VR's are adjusted?

 

[Hawkeye351]

D92 I'm thinking is bad. I'll give an update when I get home and try it.

 

[brandon7861]

I struggle with this part of the circuit, and I am most likely going to make an ass out of myself, but I am having a serious WTF moment looking at the DX33HP2 schematic. IMO, there is a missing variable resistor.

If the RF power switch is in the high position, what is after D92? I don't see anything. So even if D92 were good, it goes nowhere. This allows R249 to pull the base voltage up turning on TR53. With TR53 turning on, TR34 turns on, and that turns on TR32 muting the audio.



IMO, there should be a voltage divider at the base of TR53 at all times, not just in low and medium power. When the audio causes the passthrough transistor to drop output voltage, a fraction of that voltage (31%) is applied to the emitter of TR53. TR53 should be able to be set to start turning on when that passthrough voltage dips below the set base voltage (accounting for the be drop), but with no pot in line with the switch set to high power, TR53 would be on all the time.

I just don't understand how I can be right about this. Surely, if I am right, someone would have caught this by now. So, like I say, I am probably making myself look stupid for saying this, but I just cannot rationalize the schematic as is in high power. Sometimes you have to have the courage to question what you think is wrong. Maybe someone can explain it to me.

 

[Hawkeye351]

Turns out VR12 and VR14 is blown out. Checked them in circuit, nothing on ohm meter. Yanked them out and checked them, nothing on ohm meter. Checked others in the radio, all others show close to their stamped values. Hi and Lo carrier VR's are of newer types, as is all three VR's for the driver/final bias, so someone has replaced some of the VR's before, just not these, they clipped R249 instead.

 

[brandon7861]

I sure hope the VR's fix it. If it does, I'll have to have someone explain to me how it works in the high setting lol.

 

[Cable Guy]

I sure hope the VR's fix it. If it does, I'll have to have someone explain to me how it works in the high setting lol.

I think the switch may be wrong on the schematic. I will see if I have one here and verify. AMC HI is vr14 and according to the schematic, it just floats always. Errors happen.