Interlude
What do the removed capacitors measure up to? I used my multimeter for capacitance and a simple debounced switch combined with my oscilloscope to estimate equivalent series resistance based on voltage at the 500 nanosecond mark from the step input. The result I get this way is a blended figure of ESR from DC to about 10MHz instead of ESR at a specific frequency.
If you were wondering what sort of rig I am using to measure ESR, the circuit I used in my SL300 repair was only an SPDT microswitch paired with a simple Set-Reset latch (CD4013) for debouncing running off 12V. After putting some more thought into it and not being satisfied with how lazy the CD4013's rising edge was, nor the way it drooped for several microseconds afterwards from driving about 6mA into the capacitor under test and resistors, I decided to improve it by adding a 74HC08 as a buffer and paralleling the outputs for lowered output impedance. Why use a 74HC08? Mainly because I happened to have some on-hand, but also because HC-series CMOS gates are able to drive their outputs within millivolts from either rail under light load, even at 5V.
How much improvement do I get out of buffering the 4013's output through the gates? With a 5V supply voltage, 1kΩ resistor and a 1200µF FM capacitor providing output loading, the CD4013 takes 240ns to rise to only 3V (60%) and becomes current-limited beyond that point (the chip has weak output drivers at 5V). With the AND gates acting as buffers, the test signal rises from 10% to 90% in 19ns and 0% to 100% in less than 40ns. Not bad for a $0.40 part and five-minute tweak.
In principle, voltage across an ideal 10µF capacitor with a 1kΩ charging resistor using a 5V source should be 250µV after 500ns, which can be considered negligible for my ESR evaluation purposes, since it is close to the noise floor of my test setup. With 5V across the 1kΩ resistor, the test current can be considered constant at 5mA for any remotely reasonable amount of ESR. Therefore, the ESR can be estimated as the voltage difference between initial turn-on and 500ns later divided by 5mA, which translates into 0.2Ω of ESR per millivolt. Using this approximation, we are talking about an approximation error in the order of 1% on 10Ω ESR, less for lower, more reasonable values of ESR. Since I am working with a noise floor of about 1mV, my ESR measurement resolution is limited to 0.2Ω while using this method. It's useless for measuring good caps but good enough to spot low-quality or significantly degraded ones. I could achieve better resolution by using a lower-value resistor, but then I would also need much stronger drivers than a 74HC08.
I am not going to post a full set of oscilloscope screen grabs this time around. However, here is a comparison of a fresh Panasonic 16V 1200µF FM as the good cap reference and the first 1000µF Teapo capacitor on the 5VSB output as the bad one. The Panasonic has a specified ESR of 0.018Ω and, as expected, my measurement attempt shows effectively no change in the noise level before and after the step since the ESR is an order of magnitude smaller than my measurement capability. Noise at turn-on is just parasitic inductance causing momentary oscillation due to extremely low ESR, while the 0.5mV bump afterward is caused by wiring resistance in my test rig and is the same with my test leads shorted out. The Teapo, on the other hand, lost nearly 99% of its nominal capacitance and instead of its already passable 0.29Ω specified ESR for “general” applications, the 440mV rise implies an ESR in the neighborhood of 88Ω and the charge curve implies capacitance much worse than the 11µF reported by my meter. In any case, anything beyond 10Ω worth of ESR is already so far gone that worrying about accuracy is like beating a dead and buried horse.
So, how did they fare?
| Capacitor | Measured Value | Estimated ESR |
|---|---|---|
| Teapo SEK 50V 22µF (photocoupler output filter) | 333nF | 1128Ω |
| Teapo SEK 50V 100µF (auxiliary supply filter capacitor) | 43µF | 35Ω |
| Teapo SEK 10V 1000µF(first 5VSB output filter cap) | 11µF | 88Ω |
| Teapo SEK 10V 1000µF (second 5VSB output filter cap) | 79µF | 7.2Ω |
| Fuhjyyu TNR 10V 2200µF (first 5V output filter cap) | 31µF | 2.4Ω |
| Fuhjyyu TNR 10V 2200µF (second 5V output filter cap) | 59µF | 1.8Ω |
| Teapo SEK 16V 1000µF (12V output filter cap) | 990µF | Less than 0.2Ω |
| Teapo SC 10V 3300µF (first 3.3V output filter cap) | 3250µF | Less than 0.2Ω |
| Fuhjyyu TMR 10V 4700µF (second 3.3V output filter cap) | 3040µF | 1.4Ω |
| Panasonic FM 16V 1200µF (just because I have a bag of them) | 1199µF | Less than 0.1Ω |
As predicted, all capacitors connected to the standby/auxiliary transformer are well and thoroughly thrashed, especially the 22µF cap that used to be next to ZD4. I am surprised to see there are still two seemingly serviceable capacitors on the main output rails, and even more surprised that one of them happens to be the general-purpose Teapo SEK on the 12V rail.
Why does the FM get a result of “Less than 0.1Ω?” Because I fiddled with my test circuit some more after I finished the repair to find out how much I could lower the test resistor before seeing significant droop on the HC's output. By halving my test resistor, I double the test current and double the ESR sensitivity to a much more useful 0.1Ω/mV.
