IF Transformers

 


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Sometimes there are fixed parallel capacitors across the IF transformer primary and secondary coils.  And they can be internal to the transformer.  That is the capacitors are built into the base. The values, from what I have seen, run from 85pf up to 250 pf.

When these caps go bad it creates a popping static sound similar to Lightning discharges.  The sound goes away with decreasing the volume control (when the volume control is after the IF stages). Below is how to replace these bad capacitors.

 

Silver Mica capacitor Disease  The crashing thunderous roar in the AM band.

 

After hours of burn-in this radio (Blaupunkt Sultan model 2320) developed static.  It started as an occasional pop.  Like the sound you get when some one turns an appliance on or off.  It just progressively worsened.  And the eye tube seemed to be fluctuating with the AM modulation.  So I embarked on the delicate task of disassembling the IF cans, removing the internal mica wafers and adding external silver mica 600 volt caps.  Join me below.

 

IMG_0888.JPG (674635 bytes) Take a photo of the wiring to the IF cans. IMG_0889.JPG (821203 bytes) Get different angles. IMG_0890.JPG (744055 bytes) IMG_0891.JPG (744091 bytes) IMG_0892.JPG (845646 bytes) After carefully marking what coil connected to which lug I unsoldered the fine wires.
IMG_0893.JPG (636612 bytes) IMG_0894.JPG (660761 bytes) The base contains the wafer caps. IMG_0895.JPG (926947 bytes) If the values are not marked on the schematic measure and mark the value on the base. IMG_0896.JPG (874444 bytes) Yes with the low voltage of the meter it will read the mica wafer's capacitance rather accurately IMG_0897.JPG (635271 bytes)Remove the rivet.
IMG_0898.JPG (856094 bytes) Wimpy wimpy IMG_0899.JPG (762965 bytes) POWER! Grunt Grunt Grunt (Tim Allen). IMG_0900.JPG (561606 bytes) Now that rivet is gone. IMG_0904.JPG (685313 bytes) pry it apart. IMG_0905.JPG (606035 bytes) See the caps.
IMG_0906.JPG (608040 bytes) The blackened area is the trouble spot. IMG_0909.JPG (508462 bytes) IMG_0910.JPG (591209 bytes) IMG_0911.JPG (759755 bytes) Snip off the contacts but leave a bit so the lugs do not fall out of the base.  IMG_0912.JPG (629898 bytes) Hot melt it back together. 
IMG_0913.JPG (752057 bytes) IMG_0914.JPG (521939 bytes) Mount your new caps.  250pf and 150 pf.  IMG_0931.JPG (716385 bytes) Installed.    

 

Now for a quicker way to remove the mica wafers.

IMG_0916.JPG (697214 bytes) Take a wiring picture.  Saves schematic tracing.  IMG_0917.JPG (669877 bytes) Move the lower slug out of the way.  You will have to align the IF after all this work for accuracy.  IMG_0918.JPG (394482 bytes) See the caps? IMG_0919.JPG (551794 bytes) Snip the long contacts leaving a bit so the lug stays in the base.  Yes, the coil wires are still attached.  IMG_0921.JPG (524623 bytes) See how short. 
IMG_0922.JPG (566385 bytes) IMG_0923.JPG (512164 bytes) Uncut side.  And Mark the cap values on both sides.  You can not use a cap meter with both coil wires attached.   IMG_0924.JPG (546363 bytes) The schematic has the cap values marked. IMG_0925.JPG (611374 bytes) Replace the cover IMG_0926.JPG (642085 bytes) I use a clam since the hot melt gun is industrial grade heat!  
IMG_0927.JPG (521135 bytes) Shoot some glue in the holes. IMG_0928.JPG (615940 bytes) Move the slug back to where you think it came from.  Not critical since you will be though and do a complete alignment.  IMG_0930.JPG (757029 bytes) Reinstalled.     

The sensitivity all bands increased dramatically.  I picked up several beacons on the long wave band below 350khz.  The eye tube stopped bouncing to the music.  FM was unaffected.  There are not caps in the FM IFs.

 

 

 

Silver Mica Disease. The thunder storm in the radio.  (Zenith T600 Transoceanic)

That is when the EMF (voltage) across two mica capacitors (housed in the IF cans), on a common mica insulator, short together.  Removing the two capacitors and soldering on new ones of equivalent value is an easy fix.  Here is an alternative. 

 

IM000242.JPG (311723 bytes)Archive pic of 1st IF wires.  IM000243.JPG (752813 bytes)The radio plays great.  BUT there is the thunder storm sound.  Silver Mica disease in the first IF. IM000244.JPG (713665 bytes)I first try to repair the IF on chassis. But I had to remove it for bench repair. I marked one  fine wire from each coil and the associated solder lug with red and green Sharpies.   IM000245.JPG (771491 bytes)You can see the round insulator that is covering the capacitors.  IM000246.JPG (122178 bytes) A mica disk and with parallel silver plates applied.  Contacts touch the silver.   There are two caps (parallel plates) on this mica insulator.  One upper right and one lower left. 
IM000247.JPG (633033 bytes)I used some fingernail polish to establish the UP side and how it is placed on the contacts.   IM000248.JPG (864348 bytes) IM000249.JPG (833757 bytes)You can see something black on the upper cap.  That is normally silver migration.  Electrons break over that migration and cause the rushing sound of a thunder storm.  Unaffected by band or frequency.  IM000250.JPG (466832 bytes) Measure and record both cap values.  IM000251.JPG (431007 bytes)Both caps are the same value.  If I can not fix this mica, individual silver mica caps will be soldered on the under side  terminals, this mica will be discarded and the cap contacts cut off. 
IM000252.JPG (337173 bytes)A razor knife scrapes off the back contaminant. I have successfully done this to several IF caps.  At this time it looks like the electrolytic can cap has leaked and contaminated this capacitor.  There is a moisture looking stain on the chassis surrounding this IF leading to the can cap.  It dose not look like silver migration.  IM000253.JPG (136619 bytes)Burnish the capacitor contacting surfaces.  Do a complete job. IM000254.JPG (173447 bytes)Reassembled IF transformer.  See the little bits of insulation on the lead wires (bottom side).  I leave a tiny bit so I know what wire came from where.  And that is one fix for Silver Mica disease. Adjust the IF and the radio sounds great.  

RF alignment later in the week. 

 

These two IFs are from a Hallicrafters S-38

IMG_1295.JPG (33594 bytes) Make a diagram of the wires. IMG_1298.JPG (105736 bytes) measure all caps and record results. NOTE:  the coil has been removed.  IMG_1300.JPG (86928 bytes) Remove the mica with the silver cap plates.
IMG_1306.JPG (587175 bytes) This IF has three caps in the base.   IMG_1311.JPG (559274 bytes) Inside the second IF with three capacitors. The schematic has the cap values marked. One of the capacitor silver plates has separated.  IMG_1312.JPG (616063 bytes) Trim the contacts so the remaining metal holds the solder lugs in place with the replaced insulating disk. 
IMG_1313.JPG (93650 bytes) This transformer has the replacement caps installed with in the metal enclosure.     

 

 

See this "Boat Anchors" site for a great example of a Zenith IF can restoration.  Say a Hi to Bob while you are there.

 

Read more about Silver Migration:

http://www.gdsiswitches.com/silver-migration.aspx

http://www.ul.com/pwb/silver.html

http://www.sjsu.edu/faculty/selvaduray/page/papers/mate234/kimvu.pdf

 

I occasionally receive questions about crackling or static in a radio.  Before proceeding to open up your IF transformers consider a full restoration.  Here is what I replied to Charles.  He has already replaced the electrolytic capacitors in a Zenith 6s128.  Charles writes:

 

Paul,

My name is Charles (named changed) located in Southern part of Ohio and your site is great I must say, Great Job.

I had a question about a Zenith 6S128 that I am working. I have went thru and replaced all of the the electrolytic and have installed a new drive belt and have the radio up and running and have deoxit all the controls and cleaned all the tube sockets.

My problem is I am getting a static almost like a control is dirty when you turn it.  But its not a control.   Any ideals as to where to focus on or search...

Any advice would be greatly appreciated. 

Thanks,

 Charles

 

Charles,

My first suspicion is silver migration of the IF transformer capacitors.  But you have separate variable trimmer capacitors in the IF cans as seen in the schematic.  So there should be no silver migration problems in the IF cans.   

Check the speaker for voice coil problems.  Carefully push the cone in and out and listen.  If you hear scratching there may be bad voice coil.  You can sub a PM speaker across the voice coil wires.  Those are the ones going through the paper cone.  The Field coil must remain in circuit.  If the static clears....have the speaker rebuilt or do a PM speaker and resistor (1000 ohms >10 watt, Pls calculate it) substitution.  If you used larger electrolytic hum should be minimal. 

Next I would think a resistor or other capacitor is breaking down.  All of the waxed/paper capacitors MUST be replaced C2-4, C6-13, C15-19 and they reuse the resistor numbers on the schematic (i.e. there are more than one C7's). Then all of the carbon composition resistors (they reuse resistor numbers too).  The big wire wound resistors if there are any should be good.  

C20 I believe is a low inductance capacitor.  There probably are flat metal tabs or woven wire leads.  I have used modern caps with the shortest possible leads in this position.  And some times clipped the flat leads at the capacitor body and tacked the new cap in place. Try and not replace this cap at first.  It can throw off the dial scale.  If you have to replace it to render the radio functional then there is little choice. 

If any of the Domino looking capacitors are labeled with "Micamold", they are paper caps and must be replaced.  

You may have a Candohm resistor R11 riveted to the chassis.  That resistor can and often goes bad.  It should be replace with discrete resistors. The wattage of the discrete resistors can be calculated with P=IE or P= [E(squared)] R  then double it and order the next higher wattage resistor. 

Any of these parts in their aged state can cause the crackling and/or static.  Unfortunately, this includes all parts like the power transformer or audio output transformer. 

Good Luck and keep me posted! 

Happy New Year, 

Paul.

 

These two 460KHz IF transformers are from a Canadian GE C600,1,2 AA5.  The one red dot has an intermittent contact to the mica wafer.  The blue dot transformer is converted to discrete silver mica caps just because it was there.

 

IMG_3296.JPG (901059 bytes)Make a diagram of where the caps and coils hook up to the leads.  IMG_3297.JPG (397224 bytes) Try not to break the wires.  I measured the cap wafer in the base.  So I had to disconnect one lead from each coil.  A pain in the butt.  Clip the spring contacts so they do not short with the mica wafer caps removed.  I used a Dremel tool with a tiny saw/grinder blade to cut the melted weld.  
IMG_3298.JPG (278616 bytes)Solder the new caps inside the can if there is room.   IMG_3302.JPG (402534 bytes)I measured the capacitor values using the two little short lead banana jack probes.  Much easier not to deal with the fine multi strand wires.   IMG_3303.JPG (232803 bytes) Hot melt glue the base together.

I soldered a 37k ohm resistor in series with the coil, put a scope across the resistor and a signal generator across the series combination.  The scope looks at the voltage drop across the resistor.  I peaked the coils to 460KHz. This will change a tad with the tube's grid and plate impedances.  This radio should play with the current adjustments.  Just peak up the IF with the transformers installed on the chassis for maximum output.  

 

 

 

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