2018-11-29 Fixing Badly Worn Volume Controls

from FB group Building Transistor Radios 2018-11-30

I’ve done some experimenting on volume controls that are badly worn and noisy. The idea is to substitute some resistance for the badly worn spot. The loud crackling will be reduced and the normal volume point will be moved to a higher point on the control. For a 5k control, I put a 470 ohm resistor between the wiper and the low or common end of the pot. If you can’t find a replacement for a badly worn pot, this is one simple and inexpensive way to get more life out of it.


2018-11-26 Two 6 uH RF Chokes

from FB group Building Transistor Radios 2018-11-26

Two 6 uH RF chokes

The photo shows two different RF chokes, each about 6.4 microhenrys. The one on the left is a ferrite bead, made by Fair-Rite, Cat. # 2643000801. These were about 12 cents apiece from Yes, the wire passes through the small hole only twice and it gives 6.4 uH.

The one on the right is a piece of 3/8″ or 9mm OD Pex plastic water pipe from the big box home improvement store. I bought a short length for a few dollars. I cut off about 32mm or an inch and a quarter, and heated up a straightened out paper clip wire to poke a hole through the plastic for the wire to go through. The wire is 26 AWG solid conductor enameled wire and is about 36 turns, close wound. I will put a piece of shrink tubing over the windings to hold them from moving. But the windings could be secured with clear nail polish or paint.

These will be used for RF chokes which are not critical. The RF interference bead has low copper losses because the wire is only a few cm long. But the ferrite core has some loss at 100 MHz. The plastic tube and air core has very low loss but the wire is much longer and has higher resistance.

The original one, the Z-50, has a SRF of below 100 MHz, also. According to the list it’s 76 MHz.

According to my dipmeter the Pex coil is dipping at 88 MHz. That’s just the coil, with nothing connected to it. The SRF could be raised by reducing the number of turns and increasing the space between turns.


2018-11-25 One Transistor FM Receiver Schematic Part 1

From FB group Building Transistor Radios Larry Daniel 2018-11-26

This is the original schematic posted by Larry. GM is a gimmick capacitor made of two short lengths of solid hookup wire, twisted together for a few pF capacitance.

List of RF Chokes for the ham bands

Larry said he used an RCA SK1008, then corrected that to SK3008. But SK3008 only is good to 45 MHz according to the list, and it’s germanium and very difficult to obtain. So battery polarity should be changed and a 2N3904 or BC547 should be used.

Scott Raschke
I used the inductor calculation tool in Electrodroid and got an RF choke of 36 turns on a diameter of 10mm or 0.394 inch and 20mm or 0.788 inch length. This gave about 6.8 uH, with a core equivalent to air (permeability of 1). The AWG wire table shows 26 AWG or 0.4mm diameter wire should fit on a single layer in that length.

The SRF of the Z50 choke was 76 MHz according to the table. The circuit is operating at FM band frequencies or about 100 MHz. The 7 uH choke has an impedance of about 4400 ohms at 100MHz if its self resonance is ignored, but with an SRF of 76 MHz, the impedance will be lower. It would be best if the choke was lower in uH value and its SRF was higher, 100 MHz or more.

Nothing has been discussed about the tuned inductor. It said ‘See Text’. For a 35 pF tuning capacitor it should be 100 nanohenrys at most. That’s 5 turns 20 AWG wire on a 0.25 inch or 6.4mm form.

Larry Daniel said:

Troops, I suggested this project as a true experimenter challenge. You do not need a lot of parts. In fact you can make most of them and salvage the others. The trick here is by coupling the emitter and collector via minimum capacitance you create an ersatz tunnel diode with a bit of negative resistance. I will let the true experimenter figure out why this might work to detect and demod a strong fm signal. The rfc’s and gimmicks with physical placement of the signal coupling present a true experimental transistor radio. Making this work separates the men from the boys! The cost is very low but the potential to learn is great! Enjoy!

I said:

Larry Daniel
I thought that this circuit was for VHF FM broadcast band. The 2N404 has a fT ten times lower than the SK3008, only 4 MHz. The same for the NTE100. It won’t work for frequencies above that.

The only way I could get the 2N404 cheaply was to buy them from a notorious German dealer. Even then they were more than ‘pennies’ apiece.

For VHFs it is best to stay away from germanium and use silicon transistors.

Larry Daniel said:

“Understand Watson’s point. I just wanted to put out there a simple transistor radio that had some challenge. BTW, I checked my working version and the RCA germanium transistor is, in fact, an SK3008 not the 2N404 as I stated. I will not recommend any parts or values but with a bit of experimenting you can make this radio work! Have fun!”

The SK3008 has an fT of 45 MCHz as shown in the photo of the chart. I think this transistor would be difficult to obtain.


2018-11-24 The Rise Of The PCs

from FB group Vintage Transistor Radios 2018-11-24

Early on, I built and upgraded many PCs when they became popular. Some for myself and friends, but most at work. Before that I was maintaining and repairing terminals and other equipment attached to mainframe computers. It started out small with TRS-80s and Apples but really took off with the IBM PCs and clones. But after many years of competitive bids with some bad experiences we finally settled on Dell PCs. The road wasn’t totally smooth, but we managed to keep hundreds of PCs running despite bubblegum and soft drinks getting into everything. It all came down to economics: It was cheaper to replace with new PCs than attempting to upgrade. Technology changed too fast. No more Ribbon cables, no more floppies to get full of dirt, most of the adapter cards came on the motherboard. Everything changed with USB. And of course there was the LAN and then the Internet. So the whole PC became less standalone and more of a system and our job was more about interconnectivity and less about maintaining and repair. It was some quite amazing changes in technology.


2018-11-22 Homemade 9 pF Gimmick Capacitor

Also posted to FB group Building Transistor Radios

Watson’s 9 picofarad homemade “gimmick” capacitor

This homemade capacitor can be used for circuits were the capacitance is not critical, such as a wireless microphone for the FM broadcast band. The wire can be obtained from the cable used to connect telephone outlets. Solid (not stranded) plastic insulated hookup wire can also be used.

I used about 300 mm or 1 foot of 24 AWG or 0.5mm solid (single conductor, not stranded) plastic insulated wire.

I cut it into 6 pieces, each 50 mm or 2 inches long. I stripped the insulation 9 mm or 3/8 inch off one end of each of them.

I took 3 of the pieces and twisted the bare wires together and soldered them. I did the same for the other 3 pieces.

I took one wire from each 3 wire piece and twisted them together tightly using my long nose pliers. I made sure the insulation was not damaged. I did the same for each of the remaining wires. I then had 3 twisted pairs, and two bare leads to connect to.

I checked to make sure there were no wires touching on the insulated ends. Then I twisted the 3 twisted pairs into a tight bundle. I measured the capacitance with an LC meter and I got 9 picofarads.

If the capacitance needs to be reduced I can use only 4 wires or 2 twisted pairs. Or I can make them shorter to begin with, or cut them shorter afterwards. I can also untwist the wires to adjust the capacitance and then twist them back together.

As I said, the pF value is not stable because it depends on how much twists, and how much give there is in the plastic after you build it. If you put it in parallel with a variable cap, then you can compensate for changes over time.


2018-11-20 Solder V. IDC Punchdown

from FB group Bell Telephone…

2018-11-22 Happy Turkey day!

I use solder all the time, and I trust it as long as it’s done properly.

But one thing I learned in my decades of telephone work is that the wiring systems the telcos developed were tested and proven before they were deployed, and they were an optimum combination of cost, reliability, effectiveness, and probably most important, conservation of labor. I can see soldering the thousands of pairs in a CO where it’s permanent, but in the field where things are changing, it is much faster and less expensive to use IDC (punch down) connections. In ’98 I brought up the new HQ, three floors worth of crossconnects in under a week until I had callouses on my fingers from handling the crossconnects. Same with several weekends of a major MPOE move. Lots of memories of those. 😊 I couldn’t have done it without my punchdown tools.


2018-11-19 Depth of Loop Antenna

from FB group Building Transistor Radios 2018-11-19

I shared my loop antenna – see photo, and it is somewhat 2 dimensional. The arms are 762mm across but the depth of the windings is only about 35 mm. Compare that to the ferrite loopstick at the other extreme, it’s 10 mm diameter but 100 mm long. I’ve noticed that the loopsticks are quite directional, I can null out a strong station effectively.

I was thinking that if I wound another loop antenna using a square cardboard box, I could cut the box down so the windings have little depth, only 50 mm or 2 inches. This gives a wide ‘figure eight’ pattern with minimal directionality. But should I wind the coil over a deeper box, about the same depth as the width or length? This would give a narrower figure eight pattern with more directionality.

This would take more wire, because spreading the windings over a wider area gives less inductance so more turns will be needed to get the same inductance. Would it be worth doing this to get a narrower pattern? The loop antenna would be bulkier, taking up more room.


2018-11-17 Cause Of Bleedover From Radio Stations

from FB group Building Transistor Radios 2018-11-19

I used to work for a guy who did radio engineering and would troubleshoot problems such as this ‘bleedover’. For convenience some radio stations colocate their transmitters near or at the same site. Because the signal strengths are very high, the two stations can cross modulate their signals and give sum and difference signals. This can be caused by rust and/or corrosion on metals especially if they are dissimilar metals. Copper oxide was once used for rectifiers. So if the interference is heard on more than one receiver, it could be the radio stations or something nearby is causing the problem. This has been called the Rusty Bolt Effect. If this can be verified, the radio stations should be notified so they can take remedial action.


2018-11-16 Low ESR MLCCs

From FB group Building Transistor Radios 2018-11-19

Garry Heard
I’ve found that a problem with using electrolytic capacitors to filter out the RF noise is their internal ESR, effective series resistance. They don’t filter out the RFI as good as they should. I found that MLCCs do a better job, if I used some multi microfarad MLCCs. I got some ‘106’ or 10 uF, 10V MLCCs which have a very low ESR. They’re better than ‘lytics. You can also use tantalums, but they’re somewhat more expensive. Also some RF chokes that are high current and low resistance.


2018-11-15 ESD and Weather

from FB group Building Transistor Radios 2018-11-19

Bob Johansen
I worked at a company that had ESD mats on the workbenches, about 1980. We had wrist straps with a 1 Meg resistor in series with the cord to prevent shock hazard. I found that the leads of the resistors were broken but the heat shrink over the resistors hid the breaks.

It gets somewhat dry here but not as bad as where there are real winters with snow. Most of the time we get zapped when we walk across carpet and touch metal objects.

I’ve read that the problem with very dry weather is that static can build up on equipment and discharge without any external influence. So it’s prudent to put a multi megohm resistor to ground to allow discharge.

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