grundig: Grundig 7063W/3D volume control & capacitor questions

The first photo shows what I believe to be the volume control on the schematic for this model. The problem that I am having is that a single connection with 3 components has become detached from the edge of the wafer assembly on that control. The thing that is baffling, is that the wafer does not appear to have any type of lead or electrical connection coming off of it where that connection would have been soldered to. Could it have been a common area for that connection with no electrical significance? Or is there  a small trace somewhere on that wafer that would served as the connection? I can't see it and it's not visible in the photos. The schematic does show what I believe to be the components that are connected to the volume control, which are the ones that make that single connection which appears to have broken away; the 25n cap, 15k resistor and .2M resistor. Does anyone have some experience with this this control and how this may work? The last volume control photo is the front side of the component.

Also, a second question the last photo shows a couple of blue capacitors. These are throughout the set. I'm in the process of replacing all the wax and paper caps and wanted to know if these should be replaced along with the others, or if they are reliable enough to leave in the radio? Thanks in advance for your assistance.

 

David

Attachments:

• Volume control schematic (45 KB)
• Volume control broken wafer edge side 1 (91 KB)
• Volume control broken wafer edge side 1 close up (60 KB)
• Volume control broken wafer edge side 2 close up (49 KB)
• Blue capacitor (152 KB)

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Quite a few radio sets had a "tap" on the volume control track. So it can look like it goes nowhere as a small conductive coating connects the rivet to the main track the wiper moves on.

The theory is that it is automatic "loudness" compensation. The frequency response of our ears changes with average sound level. If music is played much more softly  than intended it will sound "thin". Hence 1960s to present amplifiers may have a "loudness" button that essentially changes the bass response (on some it may change treble too).

The tap would have resulted in a bass boost at low volume control settings.

So there are two main things you can do.

1) replace the control with a compatible "new old stock".

2) Set fixed bass response by either ignoring it or changing the circuit a little.

Search: volume control tap

You'll see a myriad of suggestions if you can't replace it.

Repair Option?

You could drill a fine hole close to broken part and twist a solid wire through and solder the components to it. Put a little epoxy at edge to stop it moving. Then use PCB repair silver paint to connect the inner part of wire clear of epoxy to the 270 degree track near the broken rivet.

 

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Thank you MIchael for your thorough explanation. I will do a search as suggested as well. One question. When you say "270 degree track near the broken rivet", do you mean the inside part of the board where the rivet was once attached?

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The main body resistor is  probaby a printed coating on the wafer of about 270 degrees for the wiper of the volume control to connect.

Older "pots" use a separate track riveted at the two ends (and tap if made) to a base. The wiper can be a pair of plated sprung blades or a small carbon brush on sprung arm.

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Does anyone have any thoughts on the blue caps in the link I provided in the previous post? I believe them to be polystyrene. If that's the case, they should probably still be OK, not needing replacement. Do you have any experience with these? Thanks in adavance.

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