General Lee dual 520 bias question...

[Hawkeye351]

Got a General Lee on the desk for an issue that reminds me of something heating up.

The radio has a mosfet (520) driver and two mosfet (520) finals. Radio is not stressed out in any way, runs cool, receives great, transmit great, just an odd issue.

The radio does 53w Pep, everyone says it sounds great, but after talking for about 5 minutes the power will drop down from 53w to 43w and within 1 minute it starts gradually climbing back up to settle at 53w for another 5 minutes then repeats itself. Alignment went great, caps are great (practically brand new radio), no bad solder joints, all wiring is tight and making firm connections, bias settings went great, output adjustments went great, no sudden changes in any adjustments, all went smoothly. Regulator not running that hot, audio IC not running that hot, only the heatsink the driver/finals are mounted to get warmer than anything else. I noticed a cracked insulator on the last final (same as last radio I did) so I figured they must have ran too hot and are now weak so I would replace them all.

Well, I got to thinking about something, bias diodes!!!
I can't seem to locate a schematic for a dual final mosfet General Lee to check and see if there should be bias diodes in this radio, there are none at the moment and the radio doesn't show a marking for them (typical D89, D112, D88). But this issue does resemble output without those diodes.

Question is, do the dual final (520 mosfet style) General Lee radios have bias diodes, although parts side of radio doesn't show parts locations for them?

 

[Cable Guy]

I was thinking bias diodes as well, strange that there aren't any, I would add them and check again. It wouldn't be the first time a needed component was sacrificed for cheaper cost. I bet if it wasn't just an am/FM only rig, it would have them to keep ssb linearity due to junction heating, but a trucker radio, sold to the masses for it's AM power output, might neglect the diodes. Just a hunch.

 

[Eldorado828]

Got a General Lee on the desk for an issue that reminds me of something heating up.

The radio has a mosfet (520) driver and two mosfet (520) finals. Radio is not stressed out in any way, runs cool, receives great, transmit great, just an odd issue.

The radio does 53w Pep, everyone says it sounds great, but after talking for about 5 minutes the power will drop down from 53w to 43w and within 1 minute it starts gradually climbing back up to settle at 53w for another 5 minutes then repeats itself. Alignment went great, caps are great (practically brand new radio), no bad solder joints, all wiring is tight and making firm connections, bias settings went great, output adjustments went great, no sudden changes in any adjustments, all went smoothly. Regulator not running that hot, audio IC not running that hot, only the heatsink the driver/finals are mounted to get warmer than anything else. I noticed a cracked insulator on the last final (same as last radio I did) so I figured they must have ran too hot and are now weak so I would replace them all.

Well, I got to thinking about something, bias diodes!!!
I can't seem to locate a schematic for a dual final mosfet General Lee to check and see if there should be bias diodes in this radio, there are none at the moment and the radio doesn't show a marking for them (typical D89, D112, D88). But this issue does resemble output without those diodes.

Question is, do the dual final (520 mosfet style) General Lee radios have bias diodes, although parts side of radio doesn't show parts locations for them?

Yes there are supposed to have them but for some reason certain production runs would leave them off. I went through your situation 4 or 5 years back with the galaxy 33hp2 and a general lee. They're supposed to be on the soldering side of the board.

The correct zeners are 1N752A 5.2V, ask me how I know.

 

[Hawkeye351]

Guess I'll throw some on. I just checked this radio out over the air after replacing Mosfets and a few caps. Everyone says it's loud and strong, but when the watt meter jumps down to 43w and starts gradually creeping back up they said my audio gets weak and creeps back up. I checked around for anything getting hot, only thing getting hot is C145 and the Audio IC. I did monitor the driver and finals gate voltages before and immediately after it does this, the driver bias jacks up to around 3.92v from my setting of 3.80v, both the finals remain steady at 3.80v each. This tells me the driver may be the only one that needs that diode across it.

 

[nomadradio]

We set the bias trimpots by first tacking a tiny wire jumper across the 10.695 carrier crystal. Lets you key it with no drive to the driver and finals. Like a sideband radio. A meter on the 10 Amp scale goes in line with the positive power wire. The trimpots are first set for zero Volts. Key the mike and the indicated power draw is now your "zero" bias reference. We found the IRF520 was safer around 20 mA. Turn up the driver trimpot to increase the reference current reading by 20 mA. Then turn up one then the other final the same increment.

Take the wire jumper off the carrier crystal and you should be good to go.

73

 

[Hawkeye351]

Haven't taken care of this querk yet, but I did speak to a few of my locals over the phone and asked them to describe what they saw on their meters and what they heard the whole time I was conversing.

They all claimed the same identical thing. My signal (carrier) was very strong and the audio was very loud but crystal clear with no distortion or any issues. Then after a bit of talking, my carrier on their meters would drop dramatically (one guy said I would drop from a 9lb carrier down to a 1lb carrier) and I would obviously be extremely choppy, almost to the point of sounding off frequency.

On my end, the carrier would drop from a 10w key, down to a 7w key and gradually climb back up to a 10w key, the PEP swing would drop from 53w down to 43w and gradually climb back up to 53w.

I listened to my carrier on my nearby monitor radio, just carrier, no modulation. After about 5 seconds on the key, I start hearing a slight crackling/frying sound coming in with the carrier when it drops to 43w and starts gradually climbing back up to 53w. I tried to modulate when this happened and noticed the audio got so loud that it was either distorted or over modulated (yeasu 950 receive doesn't like seeing low carriers with high swing). I noticed C145 (between Passthrough regulator and audio IC, typically 330uf or 470uf) getting hot on top (it's a brand new cap, original was 330-16, I put in a 470-25), the audio IC and the heat sink directly behind the Passthrough regulator (Palomar MaxMod - new) gets scorching hot.

No swing mods
All limiters intact
No regulator mods
New 520 Mosfets
New Passthrough regulator
New AF regulator
New audio grade Nichicon 2200uf 25v caps at C194, C195.
New Nichicon 470uf 25v cap at C145.

Receive is never affected when this happens.

I have none of those specific diodes (1N752), can I use a regular zener or a 1N4001 instead? Could this cause the Passthrough regulator to get scorching hot?

 

[Cable Guy]

I would try a regular 5.2v zener and if it fixes it let it ride. A 1n4001 would work but provide no overvolt protection. Maybe that driver is experiencing thermal runaway? Did you try reducing bias to it? Seems it could also be asking too much from the regulator if it is runaway, causing cap heating.

 

[Hawkeye351]

I'll try the zener first, then reduce the bias if need be.

 

[nomadradio]

heat sink directly behind the Passthrough regulator (Palomar MaxMod - new) gets scorching hot.

A glance at the power supply current draw should reveal a hint. The MOSFETs are just drawing too much DC current. I'll guess that if you were to check the bias current with no carrier, it's a half an Amp or more per transistor. About twenty times what's safe. If it's drawing (say) 6 Amps to produce a 10-Watt carrier, this means the final and driver are consuming over 70 Watts to deliver 10 Watts of RF. The remaining 60 Watts is heating up the modulator, driver and final transistors. The Max-Mod is tough, but melting temp is still melting temp no matter the Watt rating.

73

 

[Hawkeye351]

Well, tried using a zener across the driver (that's the only one the gate voltage climbs up with when it does this issue), no go, still same result.

Left the zener on the driver and adjusted current draw like Nomad stated above (not his last post, but his post before that one) to 20ma for the driver, same result, actually 20w less output. Checked gate voltage after setting ma for driver, it was 2.49v, so I brought the gate voltage up to 3.20v (turn on voltage), same result with 20w less output. Brought the gate voltage up to 3.50v, wattage was back up but the power drop continues to do this.

Only time the Max mod didn't get hot was with 20ma (2.49v) going to the driver only, but it was very very low on output this way.

Maybe the MaxMod is flat due to the old flattened Mosfets I replaced. Mosfets don't get hot at all, barely warm. Only the top of C145 gets very hot within 5 seconds of holding a key and the heatsink directly behind the MaxMod gets scorching hot after about 20 seconds of holding the key.

The owner told me it was a new MaxMod, but he never told me how long he ran it with these flat Mosfets in it after he put the MaxMod in.

 

[Hawkeye351]

Current draw:
Idle, receiving with volume all the way down - .25 amps.

Transmit, carrier - 1.99 amps. Lo power, 3w carrier.

Transmit, modulation - 3.45 amps

Transmit, carrier - 3.65 amps. Hi power 10w carrier.

Transmit, modulation - 4.88 amps

When the power drops and gradually starts climbing again, there is no change at all in the current draw on carrier or modulation.

 

[Hawkeye351]

I thought this was a new MaxMod he had put in here before he got it to me.

When I got it, the MaxMod wasn't tightened down firmly, there was a gap big enough to slip just the tip of my ceramic tuning screwdriver in, just the very tip, and the regulator would actually move side to side with slight force.
When I tried to tighten it up firmly, I noticed someone had drilled the hole in the chassis slightly larger than the nut/screwhead and was slipping through. I replaced the hardware and added washers which allowed me to tighten it down firmly, but I left the MaxMod that was in it, in it.

Still haven't gotten back to the desk yet, but wanted to explain more in detail for others and future reference.

 

[Cable Guy]

It may have been cooked, I would replace it. I wonder what it's output looked like when first key and as the power declined. If it wasn't getting good contact, can't it over heat, reducing output? Especially if you say modulation suffered during it's fits.

 

[Hawkeye351]

Decided to monitor the MaxMod voltages before and during this issue.

Passthrough voltages:
Receive
Base - 13.23v
Collector - 6.57v
Emitter - 13.71v

Transmit (Hi power - 10w carrier)
Base - 12.12v
Collector - 5.87v
Emitter - 12.86

Collector increases to over 6v+ when the Passthrough starts getting scorching hot (within 20 seconds). Base and emitter remain unchanged when getting hot.

Enough playing, replacing it.

 

[Hawkeye351]

Got the new MaxMod installed along with a new AF regulator (just to be on the safe side).

So far so good.
Gonna hammer on it later this evening with the locals. I'll give an update later.