Monday, August 19, 2019

Dipping my toe into Maxwell's Law and FS Discriminator

The law states the E field is represented by a vector. The vector length represents the strength of the field and its has direction. The vector can be rotated 0 to 360 degrees to show the direction of the field.
Here we have some examples of vectors representing fields.
There is also a law for magnetic fields which says we can represent B (magnetic) fields with vectors. There is a little difference between E and B fields. E fields extend to infinity or until they are absorbed by a particle of opposite polarity. E fields are radiated by positive charged and absorbed by negative charged particles. Magnetic fields form closed loops. There is a three way relation between E fields, B fields, and motion. E fields in motion create B fields. In other words current in a wire will create an electromagnet. A conductor moving in a B field will cause a current to flow in the wire. These factors will always be present perpendicular to each other. We analyze their interactions using vectors. The arrow above the E says use a vector to represent it.
Some use FBI to remember the left hand rule. The thumb is F the direction the wire is Forced to travel. B is the direction of the magnetic field. (north to south) I is the direction of induced current.
The rule being applied when the wire is fed a current while resting in a magnetic field.
A current carrying conductor will have a field around it. This illustrates the field polarity. The E field will be radiating perpendicular to the wire surface. As I said before the E field, B field and motion. In this case the motion is current flow.

This brings us to the left hand rule for coils.
This is how to find the total inductance when coils are in series and parallel as long as the fields do not interact.

Take a look at Ohm's Law.
Put your thumb over the unknown and the formulas is what's left. Look at (A) two or more inductors 1/Lt = 1/L1 + 1/L2 + 1/L3 and assume E=1. Using I = E/R we find the current in each path. Adding give the total current and using the assumed 1 volt for 1/Lt gives our total. Anywho, I went through all that to get here.
Using vector analysis we can see how the discriminator works.

Sunday, August 18, 2019

magnetic properties

How many magnetic properties are there? I made a little oscillator using a pill bottle for the coil former. I tested the frequency with an air core for the reference. Inserting zinc, ferrite, copper and aluminum gave these results. Remember increasing the coil size will reduce frequency.

With air core it is 803.
The zinc is a roll of Zinc pennies. The frequency increased indicating the zinc lowered the coil's inductance.

The ferrite lowered the frequency because it increased the coil inductance.
The copper raised the frequency.
The aluminum raised the frequency too. The test demonstrates that we could use copper, aluminum, or zinc to tune a circuit and raise the frequency when inserting the core. The question is how can this be?
Here I have two disc suspended in magnetic fields. One is attracted the other repelled. One is aluminum and one is copper.

Troubleshooting a receiver part 3

Before jumping into the FM a couple of points to make.
I let the post speak for itself.

The ratio detector is also used in commercial radios. We can look at them more but a search should find the theory behind them.

The set passes AM and FM through the same amp. The top transformer feeds a ratio detector. The lower transformer feeds the AM detector.
Notice the bottom secondary feeds the two diodes in the EBF80.  The line marked Audio is going to the volume pot. The FM audio comes off the top secondary and down to the same line. Since AM works and FM does not we could do some theoretical analysis about the common components.
I'll save that until some test are run.

Troubleshooting a receiver part 2

First check the plate voltages. Ap is good A,Tr is zero volts.

The triode plate is fed through R17. The screen grid of the pentode is good, we know this because the plate is good. An open screen grid circuit would put the pentode into cut-off.
R17 is open. After replacing R17 and applying power the new R17 starts smoking. Checking the components connected to R17 finds C6 is shorted. Replacing C6 and R17 repairs the audio section. The AM radio is now working but the FM is still dead.

Friday, August 16, 2019

Troubleshooting a receiver part 1

First step check the plate voltage. If it is abnormal check the screen and grid voltage. Next post will address what these reading tell us.

Silver plated Vcap with Ceramic insulator

Vcap at Fair radio
They are in the August sales flier.
30 - 320pfd
silver plated
ceramic insulator
Look for Fair Radio on Ebay.

Tuesday, August 13, 2019

BC546B amp update

I modified it for a gain of 50 in each stage and the new build is stable.
EDIT:  The 22 Ohm resistors provide degenerative feedback. This reduces gain but improves stability.

Sunday, August 11, 2019

Vacuum Tube Amp in SPICE

You can download LTSpice IV  and this zipped file. Install Spice and extract the zipped file to the folder of your choice. For example I used a folder labeled \amp.

The circuit to examine

This will give us access to a functioning circuit to examine.

Load the schematic in LTSpice and run it. Close the program and click on the diagram will place the voltages as I have done. The teacher says first adjust the DC biases and then the AC. Look at the suffix on the voltages. The grid voltages are in pV. (10^-12)

Take a close look at U1. The screen grid and plate are close to B+. The grid is basically 0. This is normal for the circuit.
Possibly the most common problem with the old electronics is the paper capacitor aging and leaking current. I simulated that problem here by placing a 10 meg resistor across the coupling cap. You can see the grid is high positive and the plate and screen grid are drawing high current. (the series resistor are dropping more voltage and reducing the plate and screen grid voltages.

U2 plate voltage was effected too.

U1 voltage is riding 6.8 volt reference. Most likely it plate is glowing a cherry red.

I "fixed" my leaky capacitor and the grid is at 0V.

U1 grid is swing positive and negative. (The 0Volt reference it should have)

This is what U1 plate voltage and current should look like.

Be aware the screen grid can drive a tetrode into cut-off. I increased the value of R3 which reduced the screen grid voltage. If you remove the screen grid voltage it will turn off the plate circuit.
You can download the program and test the circuit actions with different faults. This is intended as an introduction to troubleshooting with more to come. Whether you have only a DVM or a shop full of test equipment you can find faults and fix them with some practice and understanding of how the circuits function. Some people suggest the shotgun approach. This can be used but be careful about changing parts in the tuning circuit and in reflex circuits. You can change bypass and coupling components and expect no problems.

Wednesday, August 7, 2019

vintage vs new components - the good, bad and ugly

My components are group B. With a Beta of 200 - 450 a couple of them should may a high gain amp.

That's a good gain alright with some negative feed back it should be stable. This is with a 2k headphone.

The circuit would drive an earbud too.

Look at the high frequency response up to 30 Mhz.

I tried some different transistors and the HF band is still high gain. Why does this matter?

In the early days of radio we had some component limitations working to our benefit. The transistor high frequency response being 1Mhz or less limited the set to TRF but it simplified the filtering requirements. With all the transistors I've shown so far the frequency response is into the shortwave range. If RF comes into the input it will be amplified and radiated by the earbud leads. When I hear a squeal I can move the earbud leads and "find a station". "Hey I built this cool regenerative receiver." Well not really but you will find this type set being built. In a radio we use filters to keep RF away from the headset to prevent this. This is one reason I like the old Germanium audio transistors. They are low gain and have a low frequency response but they don't pass RF like the later transistors. Let's look at the circuit with a germanium transistor.
It is not a high gain amp but the response drops off below the short wave.

This is my circuit as built. It will drive the earbud with my AF generator set to minimum. It will also squeal with a test lead connected to the input. The question is , "how to keep RF out of it?".

Sunday, August 4, 2019


These screen shots were taken from The designer in training manuals . If you find them interesting you may want to download the books.