grundig: Polystyrene intermittents
I have just restored a Grundig 2540U that has been in my extended family since it was bought around 1965.
Most of the restoration consisted in cleaning tube socket and switch contacts with Deoxit.
The power switch was stuck, but some wd-40 free it up, and it works fine now.
I thought I was all done, until the FM would come in and out,intermittently. It would get very soft, but not dead, then it would suddenly get loud again.
Some circuit tracing narrowed the problem down to the ratio detector transformer. I used the schematic for the 2420U,2440U as a guide because these models are similar and I could not find a schematic for the 2540U
This cap is normally installed inside the Ratio transformer can, but was actually visible through an opening at the bottom of the can.
The cap is shown here with one leg already disconnected, and partly removed from the can.
I used a pair of long scissors to avoid taking the transformer appart to cut the cap end open.
The can you see in the photo is the bottom half of the IF transformer shield. The top half slips over the top. The bottom half can't be removed without disconnecting all the transformer wires.
After disconnecting one leg of the intermittent Polystyrene cap, I tucked it back in place, to document it as the original.
The ceramic trimmer was soldered directly on the legs of the 39pF mica cap before wiring to the bottom terminals of the transformer.
I realigned the new trim cap by recentering the ratio detector voltage, which was very well centered when the polystyrene cap was working.
The ratio detector voltage is centered, or nulled, when the difference between the detector audio output voltage and half of the voltage across the electrolytic cap in the detector is zero. The half voltage of the electrolytic cap is usualy obtained by isntalling a couple of 100k resistors, but in this case a series pair of 15k resistors was already wired across the electrolytic cap.
This is the third intermittent PolyStyrene capacitor I have had to dig out of the IF transformer cans of German AM-FM radios. The other two cases were the middle FM IF transformer of a Grundig 87-USA, and the middle FM IF transformer in a Olympic-Kobold 5720W.
All three of these sets showed the same sporadic drop in FM output, that could sometimes be brought back by switching bands. The band switching would cause an electric spike that would bridge the connection temporarily.
I have never personally got into troubles due to intermittent polystyrene capacitors in old radios, but i have often heard old repairmen warn against them and recommend replacing all such capacitors before attempting alignment if a radio or TV IF amplifier should seem to be out of alignment.
However, when i was in the TV repair business in the late eighties, we had a case where brand new polystyrene capacitors had a very bad reliability. It was the frequency determining capacitor for the horizontal oscillator in some cheap 14" and 20" color televisions, i think it was a Daewoo chassis. When this capacitor failed, the frequency rose from 15625 Hz to about 500 kHz. Of course, the horizontal driver stage did not work well at such a high frequency, so the horizontal output transistor was neither fully saturated nor switched off, and hence got very hot and failed short circuit. As the power supply of those sets, a kind of buck converter built around a Sanken thick-film circuit, was not short circuit proof, the Sanken part and often some other parts in the power supply also failed. An expensive repair due to a unreliable capacitor! Good luck for the customers that the sets usually still were under warranty when the capacitor failed. The importer of those sets managed to get free spares and a rework kit for the power supply from Daewoo, and the repairs did not take many minutes when a number of sets had been fixed.
Thanks for sharing your experience with the polystyrene cap reliability. Now polystyrene caps are on my list of things to check, especially with intermittent problems. As a matter of principle and priority on preservation, I never start a radio restoration by replacing all the caps, instead, I test them, and do the usual gentle monitored power-up to reform electrolytics.
Contact problems are not surprising, considering that the Aluminium foil and the tinned wire are dissimilar metals, which invite corrosion under moist conditions. The failure rate also suggests that the metal bond was not sealed against moisture. Perhaps an oil or grease would have made a good seal. Other types of foil caps also have dissimilar metal junctions, but apparently not the reliability problem.
Too bad about Polystyrene reliability, aside from the limited temperature range up to 85oC ABS-MAX, they are one of my favorite types for very low dielectric absorption for the generation of very linear ramps.
How do these intermittent polystyrene capacitors behave when using a digital capacitor checker?
Do they show an accurate reading similar the value that is printed on the outside of the capacitor, or are they shown as, "open" on the meter?
Assuming that the cap remains open during the test with the digital meter, it should read open because the intermittent is in series with the entire cap. Keep in mind that the polystyrene material is quite soft, so the intermittent connection may not show up after the cap is removed for measurement.
"intermittent connection may not show up after the cap is removed for measurement. "
Does this mean that these capacitors look good on the checker, but when they are connected, in circuit, with voltage going through them, they exhibit problems in the IF?
It is very common that a film capacitor which has gone open circuit, will heal temporarily if it is touched or subjected to heat from soldering, but then it may get open circuit again after some hours, days or weeks.
The questionable polystyrene capacitors in the IF were taken out of circuit for testing and all were within their posted values on the cap checker. Would the next step be, to just replace with silver mica dip capacitors of same value?
Do the polystyrene capacitors behave like the film capacitors for intermittents?
Polystyrene is just one of many different plastic dielectrics that can be used in so-called film capacitors. The question of reliability from open-circuit failures does not have much to do with the kind of dielectric, but with the method used for connecting the metal foils to the wires. In the past, many capacitor manufacturers had proprietary patented methods for this connection.
Some brands of capacitors may be perfectly reliable although old and using polystyrene dielectric, some others may not.
I recommend that you dont replace any polystyrene capacitors that work well in practice, as an open circuit in most radio applications does not cause any consequential damage. If you have to replace them, use either modern polystyrene capacitors or ceramic capacitors with C0G (older designation: NP0) dielectric. Silver mica is overqualified for use in a conventional broadcast receiver IF.
Silver mica types are by far the best choice to replace styroflex tuning capacitors, when found defective. NP0 types works fine but, unless you find military hermetic types (CKR or similar), they are highly hygroscopic and might change their capacitance value in the time.
To remove the question mark only...