Capacitor "Restoration"

[geojunkie]

I have this vintage dual chassis 30B1 Admiral in excellent pre-restoration condition. Haven't yet tested to determine if yoke, flyback, and crt are functional but I believe they will be. This is in the wooden cabinet (bakelite might have been nicer).

My question is whether this is set merits the extra time when recapping to "hide" the new caps within the old capacitor shells? I have yet to go to this trouble in my first couple restorations, but these were later (mid 50s and early 60s) devices. It know it is going to add a lot of time and am not 100% sure it can always be done for every cap. I am looking for feedback on how and when this extra step might be merited and what difference it might ultimately make in the value years down the road. On this set you won't see the caps unless the chassis is turned over, as only cans show up top. What do you folks do?

Dan

[bgadow]

I wouldn't personally do it with this set. Some older radios or tv's may warrant it. I seem to recall that this set makes heavy use of "black-beauties" & I can't imagine it being worth trying to redo these.

[Chad Hauris]

My feeling is that the repairs I make on these old sets are just another step in their history. Back in the 50's or 60's or 70's there would have been no special effort to hide new parts repaired on an old set. You may say that this is not "museum" quality, but many museums just act to preserve historical artifacts in their present state and not use them. There's nothing really wrong with that for some things, but my sets are not artifacts, they are working pieces of equipment, so I don't think it hurts to add in the modern caps. It reasures me to look at a chassis with new capacitors and wire and makes me feel this is something that is continuing, safely and effectively, its useful life.

[Sandy G]

Personally, if I were a collector in, say, 2075 or so, I'd much rather see "modern" caps in there than the original ones-that meant somebody along the way cared enuff to try to save this thing for posterity. Maybe I shouldn't be speculating on what people 75 yrs from now might want, that's just my opinion -Sandy G.

[Rob]

In 75 years I think totally untouched original sets in good physical condition with a CRT that measures good will be worth more than a set that was re-capped with today's vintage modern caps.

[jshorva65]

The product line of CE Manufacturing appears to be expanding, so there are some current production replacements available for certain FP twist-lock cans. Although pricey, they are made using the old Mallory production machinery. NOS units, properly pre-conditioned, may also be usable. In some cases, even the original electrolytics may be salvageable provided that their pressure seals do not show signs of stress or blowout. If this condition is met, it indicates that the unit has not been stressed by prolonged operation under conditions of high DC leakage. Whether a NOS unit or an original capacitor, the procedure for pre-conditioning / re-forming is essentially identical. With original capacitors, all connections must first be removed from the "+" terminal of the capacitor to be re-formed. With a NOS unit, the pre-conditioning is to be completed prior to installation of the capacitor in the chassis. With the capacitor to be re-formed or pre-conditioned confirmed to be out-of-circuit, the process is as follows ...

1 - Connect the "-" terminal of the capacitor to the B- terminal of an adjustable regulated DC supply such as Lambda Model 50, Heathkit PS-2 or equivalent whose output voltmeter calibration is known to be correct.

2 - Set and confirm the output voltage of the supply equal to the DCWV spec of the capacitor under test. Set the voltage with a DMM if desired. Place the supply in "Standby" mode.

3 - Connect one lead of a 20K/5W resistor to the power supply's B+ terminal. Shunt a DC voltmeter (preferrably an auto-ranging/auto-polarity DMM) across the resistor.

4 - Connect the other lead of the 20K/5W resistor to the capacitor's "+" terminal and set the supply to "On" mode.

5 - Within approximately 5 minutes, the voltage drop across the resistor should drop to less than 50% of the applied voltage. Within 30 minutes, there should be less than 10% of the source voltage across the 20K resistor.

6 - It may take 2 to 4 hours or longer to meet this next condition. The voltage drop across the 20K resistor will be less than 1% of the source voltage (0.2% to 0.5% is the preferred range) when the capacitor has been fully re-formed / pre-conditioned.

NOTE - Factory quality-control specs I have read for new capacitors call for leakage currents in microamperes of less than or equal to 1/5 the square root of the product of capacity in uF and voltage spec in volts. Imax=1/5*sqrt(CV). Voltage drop across the 20K resistor will NEVER actually reach zero as some tiny amount of DC leakage will always exist.

7 - Switch the power supply back to "Standby" and unplug the connection from the B+ terminal of the supply. Connect the lead just removed from B+ to the B- terminal such that the 20K resistor and DMM are now shunted across the capacitor and discharge the capacitor until the DMM reads less than 1V across it.

8 - Multiply the capacity spec (in farads) of the capacitor under test by 20,000 (value of the resistor in ohms) to calculate the RC time constant in seconds of the circuit fiormed by R and C.

9 - Reconnect the re-forming circuit so that R and C are again in series across the power supply with the DMM monitoring the voltage drop across R. Place the power supply in "On" mode and note the time in seconds required for the voltage across R to again equal 0.2% to 0.5% of the source voltage. This should be 5 times the RC time constant calculated in Step 8 (+/- the stated tolerance spec of the capacitor).

EXAMPLE: Suppose we have a 40uF/450V capacitor. Further suppose that we observe 1.35V (0.3% of 450V) across R at Step 6. The circuit time constant is 0.000040 farads * 20,000 ohms = 0.8 seconds. Suppose charging in Step 9 takes 3.8 seconds and the capacitor is rated +/-20% tolerance, we therefore have successfully re-formed our 40uF capacitor.

10 - Discharge the capacitor and reconnect it into the tv circuitry.

PRECAUTION 1 - Insert a 1/2A fuse in series with the B+ feed from the 5U4 filament in sets using a single 5U4. Use a 1A fuse for applications where a pair of 5U4 tubes are operating in parallel. This will help protect the set's power transformer and other costly parts in the event of an overload at some time after the set has been restored.

PRECAUTION 2 - If the manufacturer did not fuse the set's B+ Boost as part of the original design, insert a 1/4A fuse between the damper cathode and the junction of the Horiz Lin coil and first B+ Boost filter capacitor. GE models 800C, 810 and other similar models did not include a B+ Boost fuse. RCA, Fada, Dumont and many others included a B+ Boost fuse in nearly all models. Fusing the B+ Boost will help prevent a blown flyback in the event that the Horiz Output, Vert Output, or Damper tube becomes "gassy" or in the event that severe HV arcing or some other such condition should occur.

PRECAUTION 3 - A slow-blow fuse rated the next standard rating above the normal line current specified in the set's Sams folder should be added in series with the set's AC line circuit. For example, GE model 810 draws 2.2A and therefore should have its AC line circuit protected by a 2.5A slow-blow fuse.

WARNING - NEVER under ANY circumstances attempt to re-use any electrolytic capacitor whose seal is "blown" or bulging or for which the voltage drop across R in Step 6 remains greater than 1% of the capacitor's DCWV spec. Such a capacitor is excessively leaky and will overheat during operation, causing its leakage current to increase even further and is very likely to eventually overload the set's circuitry severely.

[jshorva65]

With regard to hiding an SBE "Orange Drop" or Mallory "150-series" cap in the old paper shell, I don't recommend it unless a material other than wax but with similar appearance and superior sealing properties is used to "pot" the thing. Remember that a less-severe form of the same problem that "killed" the wax caps will eventually return if wax is used as potting material for the new caps. What happens is that the wax absorbs moisture from the surrounding air and any impurities that the water vapor may contain. Eventually, that moisture-laden wax (even in a "stuffed" cap, it's still touching the leads) begins to act as a resistance in parallel with the capacitor inside. The elimination of the paper dielectric limits how low the leakage resistance can go for the "stuffed" cap, but even a megohm of unwanted DC resistance across a capacitor can change its behavior in the circuit enough to impair overall performance.

Although I doubt that a "stuffed" cap is likely to short and "pop" as some (particularly AC line or screen grid bypass) paper caps were known to do, leaky coupling caps in a video amplifier make for an awfully crummy picture and may cause tubes to wear more quickly than normal due to running at a higher bias point. The change in the slope of the circuits' "DC Load Lines" from what the engineers had intended them to be can cause anything from distortion of signal waveforms to reduction of tubes' life expectancies.

Consider a two-stage RC-coupled amplifier using cathode-resistor bias. Normally, the coupling capacitor should pass the AC component of the signal while isolating the vastly-dissimilar DC levels of plate and grid circuits. If we introduce unwanted DC resistance into the model in parallel with the coupling cap, we now have a voltage divider fed by B+ through the preceding stage's plate load and the capacitor's leakage resistance with its "output" dropped across the grid resistor of the following stage. The result will be that the grid will be biased to a potential more positive than normal and the tube will draw more cathode current than under normal conditions.

For two halves of a 6SL7, 6EU7, 12AX7 or similar high-mu triodes as V1 and V2 in a two-stage audio amp, let's assume B+ as 300V, plate loads of 100K, grid resistors of 470K, cathode bias resistors of 1.5K, quiescent plate currents of 1.5mA, quiescent plate dissipations of 225mW and 0.022uF coupling capacitors (a very common circuit in guitar amps, but also a great generic two-stage RC-coupled amplifier circuit example). All voltages referenced to "ground" or B-. Grid voltages should normally be 0V, and we would observe about +150V at each plate and +2.25V at each cathode. If interstage coupling capacitor C2 developed a leakage resistance of 1M, things would change slightly in the first stage, but drastically in the second. The leakage and the second-stage grid resistor would become a DC voltage divider so the DC level of the grid of the second stage would become 1/3 the voltage at the plate of the first stage (150V*0.5M/1.5M=+50V). The second triode would wear very quickly and the plate and cathode resistors will almost certainly overheat and drift in value. For this example, the effect on the first stage of the 1M resistance of C2's leakage in series with R4 as an additional 1.5M DC load resistance is less than 10%. The effect on the circuit of V2, however, would be significant even if C2's leakage resistance were 10 times greater (10M). For this condition, we find that the grid voltage for V2 becomes 150V(0.5M/10.5M)=+7V.