preemptive prefilter procedure

Fonofilm Industri A/S, Ortofon cutters, amplifiers, and processors.
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preemptive prefilter procedure

Post by Fonotec » 22 Mar 2016, 16:49

The GO741 cutting amplifiers, each, receive + and - 40 VDC, regulated power, from the GE741 dual power supply. The dual power supply receives mains power, in the USA, these days, at ~ 120* v, alternating 60 cycles per second. (The power transformers are able to be "strapped" for 210-240 vAC (requiring a 4-Amp fuse) or 105-120 vAC (requiring an 8-Amp fuse).

* I usually limit the full 120 v from the wall outlet to about 112 vAC, by inserting a 10-Amp General Radio Variac between the surge protector and the GE741. This is to minimize unnecessary heat and also to have the Variac's Ammeter for charge-flow reference.)

To smooth the bridge-rectified mains power, there are two 4,700 microFarad capacitors for the positive DC and also two such capacitors for the negative DC, on both sides of the power supply. So, there are a total of 8 stud-mount caps in one dual power supply. (A 5W 1 kOhm "bleeder" resistor is soldered across the relevant lugs of the parallel capacitor, of each pair of supplies, that is closest to the bridge rectifier. Parallel capacitance sums, so there are about 9,400 microFarads for smoothing each leg of the dual supply.) The exact output voltage for the four signals regulated (+40.000 V, - 40.000V, + 40.000V, and -40.000V) can be fine-tuned with a small trimpot on each (Crowbar circuit-protected) regulator circuit board, within the chassis of the dual power supply, that corresponds to one of the legs of each channel's power signal. {Each GO741 cutting amp has an internal "Power Unit," which amplifies the current available from the GE dual supply, so that it can provide about 550 Watts of cutting power on demand.}
GE_741_Manual_excerpt.JPG (18.25 KiB) Viewed 2147 times
By now, the dual power supply's prefiltering capacitors are almost 40 years old, since these amps were built in 1978. These capacitors are an electrolytic type, and were made in Germany, by Frako, having the additional part name, Elko, for "Elektrolytkondensator." Each of these old caps still have acceptable ESR readings (around 130 milliOhm), and all are still showing capacitance within +/- 20% of the target, though some are measuring a little higher than the others, within that range, but those with higher capacitance, are all at the same amount, as if they are from the same batch. However, in spite of the good build quality of these Frakos, since they are now at an age where failure is considered normal in most electrolytics, and in light of the "potential" hazards facing the load circuit in such an eventuality, I decided preemptively to renew the prefiltering capacitors in the studio's main drive-package's dual power supply.

Below is a picture of the original (1978) prefiltering capacitors, after they had been removed from the dual power supply chassis. I will keep them in storage for now.
Elko_Frako_4k7.JPG (152.73 KiB) Viewed 2220 times

...and here is a picture (below) of the new (2016), stud-mount, screw terminal (with new machine screws and lugs) capacitors, made by Kemet (special-ordered, with a 5-month lead time from Mouser), taken right after they were installed. I tested this dual power supply and found all four power signals to be stable and fine-tuned them for the exact, target voltage with a tweaker. Incidentally, there are a couple of smaller Frako electrolytic capacitors on each regulator board. I may renew those soon, as well, but they are in good physical and electrical condition, still.

The red cable goes to the positive terminal of the relevant capacitors; blue cable, to the negative terminal... The black cable is from each power transformer's center-tapped ground and, therefore, goes to the negative terminals of the capacitors that smooth out the positive-going fully-rectified voltage on each channel and to the positive terminals of the capacitors that smooth out the negative-going fully-rectified voltage... The two yellow cables going into both bridge rectifiers are for the alternating current (60 cps, in USA) mains voltage.
GE741_Kemet.JPG (169.86 KiB) Viewed 2220 times

(Note: the GE741 chassis, like those of the GO741 amps, has been updated with a very quiet (16 dB SPL), high flow fan, by Thermaltake.)

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