KLV-550

[liquidh8]

well, I replaced d18 (with a 1n4002)with another diode anyway, and I am now reading about .58 volts at L9. (fluke meter) I haven't gotten any local buddies yet to give me a report. But, I have been talking skip to stations on your side of the pond all morning! :laugh:

I am still going to replace the rest of the diodes and transistors for kicks.

So I want to see .6vdc on l9? or do i wanna see less or more for the bias to be right?

 

[bob85]

after adding the resistor my stock bias circuit started @6.2v at very low drive and droped to slightly under .6v with 10w drive fm, 10w ssb it fluctuates but stays just above .6v

 

[liquidh8]

ahh, well, I talked the same 4 people I talked to last night, and apparently it is fixed. The voltage I am reading at L9 is right under.58, I think it is staying biased correctly, if I turn the amp off and run barefoot, the unkey and turn it on and talk, I am told I sound the same. So all is good.

I am still gonna change the rest of the diodes in it it, I ordered them already. In the grand scheme f things, the total tally to rebuilt it was wayy less then a new or good used one on ebay.

I wanna thank you VERY MUCH! If I could buy you some beers I would! Maybe I'll catch you with all the skip we've been getting on 11m here. I am 231, Pennsylvania.

 

[bob85]

glad to hear you got it working 231 ,
i just noticed i put the . in the wrong place, bias started @.62v and dropped just under .6v at 10w drive.

 

[liquidh8]

well, I can say it has all new transistors, diodes, and resistors. The 2nd cooling fan work for a nit, now it doesn't when keyed up. The IC1 has ben replaced, and like I said, all the transistors. If I put both fans to the constant ground, they both run. When I key up and talk, there is still 1 volt sitting on the ground wire for the 2nd fan. Also, I don't see .6vdc on the top of L9, I see between .496 and .502, ughhh, this amp is angering my chi!

 

[liquidh8]

After hearing it myself, I realized it still didn't sound jus right. I ended up finding a bad resistor feeding TR9. So after replacing it, the voltage is up to .66-.68 on low, to .61 to .63 on high. Is this too high of a bias voltage? Now it sounds great, but I don't wanna kill the brand new pills ya know?

 

[bob85]

i would say its too high at low drive, mine is only .62v with no drive and the bias activated, it drops a little as you add drive,

there is a clamping diode in there to stop destructive levels of bias, the diode is shown the wrong way round on the schematics,

you can change r30 on the version 301 to a variable to set bias but be carefull you don't go too far,
preset the variable to a higher resistance than the fixed resistor you remove before you solder it in,

i find the best way to do it using minimal test gear is,

adjust the bias for about 200ma increase in current draw from the dc side of the psu when the bias is manually activated in standby,

or remove L8 and insert an ammeter, set idle current to 200ma in standby with the bias manually activated,

if you don't understand what i mean leave it alone

 

[liquidh8]

I kinda though so, but it is .60-.62 in high. It also rises as I add drive. I have been talking SSB mostly, so as I talk it starts at .60 in low, and climbs to .67-.68vdc.

I was going to put a variable in there R30 (3.3k) when I took it out to check it and replace it,and do it like that, but the variables I can get at the rat shack are only 1/8 watt, it calls for a 1/4 watt. I know when the bias was down at .56v it sounded like crap, Sucks it fluctuates like that as the power gets turned up. If I get it to .62, it may very well be .56 on high, and sounding crappy again

The clamping diode is d17 right?

I could remove L9, I don't know how to manually active the bias though I don't think set the bias? I just don't wanna fry the new pills with to much bias voltage.

Thanks for the replies, this has helped me out tremendously!

 

[bob85]

its very likley you are getting rf into the meter if you are measuring bias while transmitting,
try putting a .1uf cap across the meter terminals or a ferrite choke on the leads,

if its .62 at low drive that is ok and should fix the scratchy audio, the bias circuit is unregulated so it will drop voltage with increased drive,

you remove L8 the choke that feeds the finals to measure idle current not L9 the bias choke,

you can activate the bias manually by grounding the tr8 side of r29 while in standby, the same place i soldered the variable to fix the bias switching issue, i would ground it with a 10ohm resistor,

yes d17 is the clamping diode.

 

[liquidh8]

copy that. so maybe i should go to am and try it and reread the meter, and add the cap and go from there.

 

[liquidh8]

the 1uf cap worked well. I ended up going up a bit in resistance, not much. the orignal was about 3250 ohms, I went to like 3400 or something, brought me down to .63 in low and .60-.61 on high. now to get some audio reports.

thank you for all the help again, this thread will help others I am sure.

 

[bob85]

glad you got it sorted

 

[liquidh8]

I think I spoke too soon. The bias voltage continues to rise as I am talking. I can't seem to get it right. What is the voltage from the power supply supposed to be? I am getting 34volts. right to R28 says a "1,0 ohm" resistor, so I assume that is 1 ohm. that don't work too well, lol I blew a BD179 transistor. Ughh, This thing is starting to anger me. The reason I changed it is the original R28 was like 100K, even though the color code said 1 ohm, so it was bad.

 

[bob85]

supply volts depends on your mains voltage and your transmformer and when under load the value of reservoir capacitance in the power supply, we have 240-250vac mains and i have 30v dc @ r28 1ohm resistor,

sounds like you have too much voltage from the psu compared to me, i have no problem with the bias rising too much when using all stock components,

i did lots of experimenting/measuring with mine, feeding the bias circuit with regulated and unregulated voltage and noting the results,

the bias voltage rises from cold as the bd179 heats up because its not thermal tracked, mine does not rise much when in circuit under load,

d15 and d16 really should be in contact with the heatsink,
d16 can be placed on top of a pill,
if i use a diode for tracking i file a flat on it to increase contact area, its a fiddley job to do and you could damage the amp if you get it wrong,

you can use the regulated power supply that feeds the front terminals to regulate the bias supply,
regulating the voltage to the bias circuit does not in practice make much difference in the regulation of the bias voltage seen at L9 in that type of bias circuit.

 

[liquidh8]

Yeah, I have 34 volts there, 110-120VAC. I could run a high resistor to get it down to 30vDC after R28, I could also mount both of those diodes under the cb instead, not a big deal really to do that, I can file a small flat on them. I could even run D15 and 16 on top each pill. I also notice the power going down as the bias voltage goes up. I guess I should move the diodes, get the voltage down after R28, this way I can get the bias at about .60 on high, it'll probably be .64 on low. I can get the bias down by adjusting R30, but it it almost 10k, a far cry from 3.3k.

As I side note, I noticed after not keying for a while, when I first key up, bias voltage is .49-.50, after talking for a minute it climbs back up to .68.

Would I just lift R28, and feed the 13.5 volts to it? Then adjust R30 to get my .60-.62 voltage.

I am done messing with it tonight, lol. Kinda tired of looking at it. Sucks, cause almost everything is New, except for the Transformore. bridge rectifier, and the big 4700uf filter caps, all the transistors, diodes, the small relay, and half of the resistors have been replaced.