Differences in Resistance values (Newbie question)

[Rublyow]

Hey everyone,

First off, I am an absolute newbie when it comes to electronics, so apart from a bit of exposure to circuitry in a physics class in college and to soldering up a billion guitar cables in my lifetime, I know next to nothing about circuitry or electronics.  The Quickie is my first DIY project, but so far so good! 

Everything is assembled and soldered, and all of the voltages roughly check out.  The only thing that has me a bit concerned are the resistance readings on terminal strips T1 and T5.  The readings are roughly identical up to about 20K, at which point the reading at T1 is lower than T5 regardless of the position of the volume know.  This difference becomes greater as the volume knob is turned up.  A few readings are:

T1=46.9k, T5=49.3k (knob about half way up)
T1=77.9k, T5=86.7k (knob about 2/3 way up)
T1=93.3k, T5=103.0k (knob at max volume)

Is this something to be concerned about, and if so, what might be causing it?  As I mentioned, all of the other resistance readings check out, as well as the voltages.  Unfortunately I can't hook it up and listen to it yet, as I'm waiting for a set of interconnect cables to arrive in the mail.

Thanks for the help!

[Paul Joppa]

The potentiometer is made of a carbon composition track, and carbon composition is rarely a precision material. Inexpensive ones are usually +/-20%, so the max setting should be between 80K and 120K. Your values are well within the spec.

The other question is tracking; the potentiometer is a voltage divider, so at the midpoint it will be 50% resistance if it is a linear pot. 46.9/93.3=50.3%; I get 47.9% on the other side. You can do the math for the other setting, and you see that you are actually quite a good match between channels.

[Rublyow]

Great!  Thanks for the quick response.  As I mentioned, I'm still waiting on some cables, and it will be another couple weeks before the finish on the casework is done, but I'll post back and let you know how things are sounding (especially if I run into any more questions).

[Rublyow]

Just quick follow-up question, I installed the PJCCS upgrade for the Quickie, and the voltages on A6/B6 are a bit low, about 15 volts as opposed to the stated 19-21 volts.  The voltages for IA/BA look good (about 38.5) as well as for A5/B5 (about 1..  The batteries I'm using have only been used for testing, but I wasn't sure if this would be enough to affect the voltages, or if something else might be going on (or, as with my last question, if these values are actually within the acceptable range and nothing to worry about).

Sorry again for the really basic questions! I'd rather just be a bit more cautious in the beginning until I get a better grasp of this stuff.

[Paul Joppa]

Probably the 1.5v batteries are reversed. It's a known error in the manual. I can never find a reference, maybe someone has bookmarked it?

[Rublyow]

Hi Paul - I switched the D cells and the values are now a bit high:

A2/A4/A6 = 23.4 volts
B2/B4/B6 = 24.2 volts

I also have lowish readings on T2/T2 and A5/B5 (around 1.85 volts), similar to what was given in another thread where you commented they they seem low. I checked the orientation of the two bypass capacitors as you had recommended in that thread, and the striped side leads are both correctly inserted into T3.

Any thoughts?

[Paul Joppa]

Those numbers look OK to me.You can measure the voltages at each of the three pins of the MJE350 transistor which would give me a little more information. I am not certain whether the PJCCS manual voltages were measured with the correct filament battery orientation or not.

I should probably acquire a Quickie myself (and a few different 3S4s), just to do some measurements on.

[Doc B.]

We have a couple at the shop. Just grab one next time you are over.

[2wo]

Me too? Kind of a safe bet as I am on the East coast...John 

[Rublyow]

Paul - good to know the values at least look reasonable. 

If it helps, the values on the MJE350 pins are 37.0, 23.4, and 36.2 volts for pins F,C,B, respectively.  The only difference between the readings on the two transistors is that the middle one of the other transistor is 24.0 volts, but the other two pins are the same.

[Paul Joppa]

Excellent! Thanks for posting; the values indicate that everything is wired correctly, and the voltages look much closer to my theoretical expectations. The 0.8v difference (36.2v to 37v) explains the reduced current which had worried me a bit; it's actually closer to ideal than the original calculation, though I may eventually revise the PJCCS component values slightly to reflect this voltage.

I have posted the corrections to the D cell wiring, and Doc says he will make that post a sticky. In a month or two, I will have acquired a Quickie and made enough measurements to provide corrected voltage measurements, and hopefully we'll revise the manual for future production.

[Rublyow]

Great - thanks for all the help!  Now I just need those cables to arrive so I can hook it an and give it a test-run.  I'll let you know how it sounds.