Rangefinder low output

[Svenson007]

Hey guys. Out of all the complex builds I have finished in the last few days the “more basic circuit” is giving me trouble. I’ve built maybe 4 of these with great results, until now.

My other builds I wound simply populate the board with the stock components, and play with the 5k trimmer. It almost seemed there was a ton of leeway in this circuit, and I always had a hefty amount of output. Instead of swapping biasing resistors etc I would just switch out germaniums until I was happy with the sound (also miraculously came within the 7v range). It works for me. Also no matter what transistor I swapped in…. The volume was minimally different between them. A great boost level.

This time I am getting low output (next to no boost). So I have swapped out the 68k for many different values and got close to 7v on transistor. Still low volume. I was also using transistors I know sounded good in the past. Something isn’t right this time.

Wondering if anybody can look at my pic and help me understand what resistor does what in this circuit. Which other resistors are used in biasing / output level etc (to put it in a crude way, in fairly new to germanium)

I didn’t take a pic of the bottom of the board as I’ve reflowed / checked for bridges / added extra solder. Basically it’s darn near perfect. The 68k spot isn’t soldered in in the pic, as I was experimenting with different values.

So a basic run down of which resistor is doing what would be awesome. I checked that my values are correct so I dunno…

Thanks guys l. This is one I didn’t think I’d ever need help on!

 

[Chuck D. Bones]

5. Pic?

4. What value did you end up with for R3?

3. What part number did you use for Q1 & where did you get it?

2. Measure & post the voltages on Q1-E & Q1-C.

And the #1 question: did you perform a thorough visual inspection? (not just the solder joints)

 

[Svenson007]

5. Pic?

4. What value did you end up with for R3?

3. What part number did you use for Q1 & where did you get it?

2. Measure & post the voltages on Q1-E & Q1-C.

And the #1 question: did you perform a thorough visual inspection? (not just the solder joints)

Thanks Chuck. When I put the circuit down I believe I had a 39k in for R3. I was experimenting with many values. I did perform a thorough inspection. I’ll add more details (measurements etc) when I’m able to pick this up again. Probably later today. I know my post is lacking detail.

 

[Chuck D. Bones]

39K is too low, it will load down the pickups. Keep R3 at 68K. If you can't get to the desired bias point, then either use a less leaky transistor for Q1 or make the Bias trimmer 10K. Q1-E should be around 1V and the desired range for Q1-C is around 5V for maximum headroom. You can dial up or down from there for more harmonic content. Why are you shooting for 7V on Q1-C?

 

[Svenson007]

39K is too low, it will load down the pickups. Keep R3 at 68K. If you can't get to the desired bias point, then either use a less leaky transistor for Q1 or make the Bias trimmer 10K. Q1-E should be around 1V and the desired range for Q1-C is around 5V for maximum headroom. You can dial up or down from there for more harmonic content. Why are you shooting for 7V on Q1-C?

Thanks Chuck. I haven’t been able to get back to this build until today. I really should have tried harder on my own to get this working before asking for help. I was in a bit of a hurry to get it working well that day. Thanks for the advise… I was almost considering popping a 20k trimmer in there, as I wasn’t sure 10k could even get me there. But I’ll try that first.

About the 7v. That was just some general advise I read in another thread. Seems very high to me.

Time to experiment. Cheers

 

[Chuck D. Bones]

Asking for help is always OK.

Running the bias cold (high collector voltage = low collector current) causes soft clipping which will increase the 2nd & 3rd harmonic content. Biasing is usually a matter of personal taste.

A 10K trimpot will probably get you there, but 20K is also ok.