Radio Repair Best Practices

[Yuri D.]

WARNING!!!!! ACHTUNG!!!!!! VERY HIGH VOLTAGES!!!!!!! HOCH SPANNUNG!!!!! LEBENSGEFAHR!!!!!!! DANGER TO LIFE!!!!!!!!


Since we all just rent this equipment until it passes into the hands of other collectors, it makes sense to hand these radios down in better shape than they were before, or at lease in the same shape. Therefore I would like to start a sticky thread about radio repair best practices and about how to avoid destructive pitfalls.

WHEN YOU FIRST GET YOUR RADIO

1. Do NOT turn it on to see if it works.

2. Take all the tubes out.

3. Clean contacts with a good contact cleaning solution and a lint-free cloth.

4. Slowly increase filament voltage and measure on the filament termini.

5. Slowly increase anode voltage and measure on the anode terminus and ground.

6. Test the tubes on an RPG 4/3 (they are not very expensive these days and are usually available on Ebay)

7. Put the tubes back in, then slowly turn up the filament to CORRECT voltage. Filament being too low or too high is not good. Make sure the filament voltage is correct on the termini.

8. Turn up the anode voltage.


There is more freedom to vary the anode voltage depending on desired results/output, but filament voltage must be exactly what the tube specs dictate. If there is one worn-out tube that needs higher filament voltage, and you increase it, then you will dramatically shorten the life of the other tubes.

Yuri

[Yuri D.]

Let's talk about ceramic trimmers used in all Wehrmacht radio sets. These trimmers are 99% all frozen. Any attempt to turn them with any screwdriver will most likely result in a damaged trimmer. The trimmer is comprised of:

• Ceramic base with metal plate and contacts
• Ceramic disk with silver powder coating and soldered screw.

The ceramic disk with the silver powder coating is soldered to the tuning screw on top of the trimmer. The idea is that when you turn the screw with a screwdriver, you can vary the capacitance of these trimmers between approximately 10 and 30 pF. Most of these small trimmers (there are larger ones) have this standard capacitance range. You can significantly improve resonance curves using these trimmers especially at the top end of a frequency range.

So what is the problem? Most of these trimmers are frozen together. My friend was working on these radios in the 1960s and he told me that they were frozen even back then. What happens if you try to use a screw driver on thse frozen trimmers? The second you apply the slightest turning pressure, you hear a "crack" - that's the sound of the solder on the screw detaching itself from the silver powder coating. So now what?

• Your capacitance is now from 10-15 pF
• You CANNOT re-solder the screw to the powder, because you will evaporate the powder and your solder will not adhere to anything (i've tried this :-( )
• You just broke another trimmer that they don't make anymore.

How do you avoid this?

1. If the top disk of the trimmer does not turn WITH YOUR FINGERS - STOP

2. Solder the trimmer out of the radio, apply a very small amount of penetrating oil between the base and the disk.

3. Use Yuri's special trimmer loosening tool to loosen the trimmer by rocking the loop back and fourth until it unfreezes. Use a bit more penetrating oil if necessary and most importantly - BE PATIENT.

4. Continue to gently work the trimmer disk back and fourth until it can easily be turned by hand.

After this procedure, a screwdriver can be safely used to tune the trimmer. (use a plastic, TV tuning screwdriver or your fingers)

Photo of the special tool is below. I just used a tube from a ball-point pen and a thin insulated wire.

[Yuri D.]

Here is the whole tool with the wires protruding so you can tighten them around the trimmer disk:

[Val]

I'd add - use proper tool for screws, especially on the front panel.

[Yuri D.]

I would recommend Wiha brand screwdrivers (German screwdeirvers of course :-)).

http://www.wihatools.com/200seri/266sets.htm

The German screws are made from soft or brittle metals, and it is easy to break these screws in one way or another.

1. Always use penetrating oil when a screw has not been unscrewed recently.
2. DO NOT FORCE A TIGHT SCREW
3. Apply adequate pressure so as to not strip the slot.

Yuri

[Val]

How about keeping a log what's been repaired/replaced and when and if you really need to sell your beloved equipment you pass that repairlog along.

[Tony_Grimwood]

I would recommend Wiha brand screwdrivers (German screwdeirvers of course :-)).

http://www.wihatools.com/200seri/266sets.htm

The German screws are made from soft or brittle metals, and it is easy to break these screws in one way or another.

1. Always use penetrating oil when a screw has not been unscrewed recently.
2. DO NOT FORCE A TIGHT SCREW
3. Apply adequate pressure so as to not strip the slot.

Yuri

... And ALWAYS use a screwdriver with the correct tip profile - i.e.that fits the screw head snugly - for the screw you wish to turn. There is no "one-size-fits-all" screwdriver.

[Yuri D.]

When you first get your radio

1. Do NOT turn it on to see if it works.

2. Take all the tubes out.

3. Slowly increase filament voltage and measure on the filament termini.

4. Slowly increase anode voltage and measure on the anode terminus and ground.

5. Test the tubes on an RPG 4/3 (they are not very expensive these days and are usually available on Ebay)

6. Put the tubes back in, then slowly turn up the filament to CORRECT voltage. Filament being too low or too high is not good. Make sure the filament voltage is correct on the termini.

7. Turn up the anode voltage.


There is more freedom to vary the anode voltage depending on desired results/output, but filament voltage must be exactly what the tube specs dictate. If there is one worn-out tube that needs higher filament voltage, and you increase it, then you will dramatically shorten the life of the other tubes.

Yuri

[Val]

Visual inspection is also in order in the beginning, when you receive equipment - see that nothing is shorted (short circuited), nothing is broken, nothing is bend, nothing is loose inside (especially metal things that could cause short circuit), no wires unconnected, no visual damage. Only after careful visual inspection turn on the power. Also pay lot of attention to any robust and clearly non-factory soldering and repairing - for example it had happened, that before selling someone just soldered loose wires together so that equipment seems to be complete.

[fieldgear]

I'd also suggest plenty of pics. Digital camera's are cheap, & there is nothing like having a "before" & "duing the restoration process" set of pics. Its a valuable record of whats been done, & an invaluable resource if you have that one wire left that just doesn't seem to go anywhere!

Best,.>Rob

[Yuri D.]

New discovery about loosening old rusted screws.

I just discovered this and am very excited to share this information. On my new T8PL39 receiver, the screws were significantly rusted on several of the controls, and after 2 weeks of applying penetrating oil, I could not get them to budge. Then I used an old watch-maker's trick and heated the screws with a soldering iron. I applied the soldering iron to the head of the screw for a period of 3-4 minutes. Lo and behold, I was able to unscrew every screw, no matter how frozen. I think also the earlier penetrating oil application aided the process by being heated up in the rust and liquifying it to a certain extent.

For frozen/rusted screws:

1: Apply lots of penetrating oil, and if that doesn't work,

2: Apply a 45 watt soldering iron at full strength to the head of the screw for 3-5 minutes.

3: Use a good screwdriver to turn but make sure to apply plenty of pressure against the screw as you turn.


Be careful not to burn yourself on the hot metal around the screw.

Yuri

[LA6NCA]

Decupling capacitor test, Resistance in the tube socket.

I have created a probe that makes it unnecessary to solder out the capacitors for test.
I use a network analyzer to measure the frequency response of the capacitor.
My probe has two pins.
One is GND.
The other is connected to the signal from the analyzer via an 56 ohm resistor.
The signal is transmitted to the input of the analyzer from the same pin via a 56 ohm resistor.
In this way, I can measure the reactance in the capacitor wiry accurate.
The rest of the circuit connected to the capacitor influencing little on the measurement result.
A signal generator and oscilloscope can be used instead for an analyzer.
Frequencies from 1 MHz to 10 MHz should be used.




More info at my KwEa page here:

http://www.la6nca.net/kwea/index.htm


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One problem I often see in German WW2 radios is that connection from tube sockets to GND is bad.
There may be corrosion in the tube base and screws.
To find such problems, I use the following method:
I add a DC current of one ampere to the tube base.
Then I measure with an mV meter the voltage between tube socket’s and GND.
Here I measure 2.2mV. That is 2.2 mOhm.
It's good.
In one of tube socket’s I measured 100 mOhm.
This is wrong and had to be fixed.




LA6NCA

[Yuri D.]

Helge, this is very useful. Up to this point in time, I've had to solder out all capacitors prior to test to isolate them from the circuitry network of the device.

[Val]

Also, you definitely need the right tools for repairing. Especially when you hit the corner and need some break.

http://www.eeggs.com/items/39244.html

[kobieras1939]

hi i looking for panel from radio pazner mw ec does anyone have it?
thansk for help