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.
Now for a quicker way to remove the mica wafers.
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.
These two IFs are from a Hallicrafters S-38
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:
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:
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.
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,
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.
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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