Broken audio generator

[sawdust]

I purchased an old Heathkit AG-9A audio generator yesterday for $2. Of course it didn't work, I traced the problem to an open filter choke. I had 420v on one side and nothing on the other. I unsoldered it and it was open. I tried to figure out a value for a replacement resistor, I came up with a 6k at 29 watts. The manual says I should have 420v on the recifier plate and 410 after the choke. The 6X4 recitfier has a max 70 ma draw. Could this be correct or did I miss something. The schematic is at the link below.

http://i255.photobucket.com/albums/hh135/dave30019/heathkit/heathkitag9a.jpg

Thanks, Dave

[sluckey]

If you have an unknown resistor that drops 10V when .07A flows thru it, the value of that resistor must be 10/.07=143Ω. The power dissipated would be 10vx.07a=.7W, so use a 2 watter.

I doubt that there is really 70ma being drawn from that recto. I'd start with a 250Ω/5W and see where the numbers land. Then adjust up/down until you're happy. Should only take a few minutes if you have several value resistors on hand. I bet it will work with a 250.

I usually don't trust hand written numbers on a schematic unless I recognize the handwriting (and trust that person). Those numbers sound reasonable though. Probably one of the smart guys can analyze the circuit and tell you exactly what value to use.

And you can probably find the entire manual online. Maybe it will have a voltage chart or maybe even a DC resistance for the choke. HK was very thorough with their manuals.

[sawdust]

Thanks for the reply. The voltage chart from the manual I have is where i got the voltages; 420 and 410. The 70ma was from the tube data specs. And yes i didn't think it was drawing 70ma. So I should just look at the 10v drop and figure from that value; well, use your values that you came up with. Those hand written values are mine that i copied from the manual. I did find the manual online but it did not have any usefull info on the choke. I do have 2 chokes in my junk box, one is 56 ohms and the other is 125 ohms. But they are much larger than the original and won't fit in the chassis. Which is one reason why I thought I could go with a resistor instead. Thanks very much i will try the 250/5w this afternoon.

Dave

[sluckey]

Those hand written values are mine that i copied from the manual.

Ahhh! Yes, I'd trust those numbers. More and more I find that the notes I write on schematics mean less and less a year later! I still recognize the handwriting but often the meaning is gone!    ;D

If the sig gen works when you put that resistor in, you can just measure for a 10v drop across it. If it's not close enough for you, just increase/decrease until you're happy.

[PRR]

> 6X4 recitfier has a max 70 ma draw

Yeah, well, I have a thick board which could take a 1,000 pound load, and I keep the dogs' bowls on it.

The actual current can be inferred from the voltages and resistances. There are only 3 paths out of the B+ and back: 6CL6 plate, 6AU6 plate, 6AU6 screen.

All 6CL6 current flows in the cathode resistance, 5,047 ohms, and drops nominal 210V. 200V at 5K is 40mA.

6AU6 plate current flows in 47K and drops 410V-200V= 210V. Under 5mA. 6AU6 screen is just over 2mA. I won't sweat the decimal because I see the 6CL6 is the big pig here. 40+5+2 and rounding-up we have 50mA.

> I came up with a 6k at 29 watts.

That's the TOTAL. 400V and 50mA is, indeed, 8K and 20 Watts. Using your numbers you get your answer.

BUT we want a low-loss filter, not the total load. Throwing darts, try 10% loss. To drop a 8K load 10% requires about 0.8K or 800 ohms. This will drop 35V-40V DC, a lot more than the choke, and filter the hum less than the choke (why Heath budgeted for choke cost). But it will work. It will need a trim, and it will be 10% down on original rated output level, but should be good for general bench work. As long as you are bodging it, feel free to try any similar value like 1K or 470. 250 will work, though I bet there is generous hum on the output even IF the filter caps are still good.

Oh: if your resistor smokes, you found out why the choke smoked: the second filter cap or something down-stream is shorted. Which is no big deal. Replace the C-L-C filter with 40uFd-100R-100uFd-100R-100uFd filtering: caps are cheaper than chokes today, and this will put you near-enough the original DC voltages with fairly low hum.