Audio Pro A4-14 Mk2: Ready for new age!

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Hi Guys,

I had wanted to do it a long time ago but.....anyaway I'm finally on vacation and I have a litte bit time to put in place some modifications, substitutions and calibrations on my old active loudspeakers Audio Pro A4-14 Mk2 that I have bought more or less........thirty years ago. :)

I have replaced the suspension rings of the woofers, Sipe "AS.130 42148" with new foam rings, I have replaced the two old electrolitics capacitors on the DC power line with two electrolitics on high capacity to improve the current request on transient, I will clean the potentiometers and the switches with a specific spray, I will substitute the critical tantalium capacitance on the signal line.
After these activities I will calibrate again the bias of both power amplifiers and the ACE Bass circuit like indicated on the Service Manual.

Below the first one photo concerning the capacitors substitution on PA circuit, more photos will follow.

Kind regards
franco:)
 

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You can see below some photos concerning the position of Tantalum condenser and the indication for Bias and ACE Bass adjustments, by the way here I have added the voltage value (not indicated on Service Manual) that must be set to -5.5dB.
These simple notes I hope that can help someone to recalibrate well the A4-14 Mk2 active loudspeakers system after the necessary substitutions.

Kind regards.
franco
 

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Only one important thing if you want to readjust newly the bias current!

In fact to avoid the possible broken of the final transistors, I strongly sugget to substitute the original trimmers (for both amplifier they are 470 Ohm, but it's better to use 1K) with most reliable mult-turn trimmer and to set it before on max value of resistence to avoid a possible initial high current that could be destructive for final transistors.

In any case, you can substitute the BDW93B and BDW94B final transistors. with the "C" version that it's better because allowing 100V of VCE instead of 80V.

Good luck!
Franco
 
Hi Guys,
You can see some photos concerning a litte modifications on the electronics like the electrolitics capacitors substitution (all), in particular on the Power Supply of ampliers, the trimmers to adj the bias currenta and the particular of the woofers with new foam suspensions.

I want to say that with the modifications, aboveall with now capacity to respond to high request of current, in fact the electrolitics capacity its increases to six time respect of the original, the "old ladies" are now even more faster with amazing bass response, braked and even deeper.

Kind regards.
Franco
 

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A4-14 MK2 vintage new life

Hi!
I am new to this forum and I am looking for some help and guidelines in renovating my Audio Pro vintage speakers.
I have a pair of A4-14 MK2 that I bought (new) almost 35 years ago. Two years ago I fixed the woofers ( new foam rings) and the speakers got a new life 😊
However a few months ago I stared to notice that one of the speakers midrange driver was breaking up (made a rasping sound after approx. 10 min on power up). The rasping sound continued for a month (always after +10 min). Now the speaker went silent and after a check I noticed that two fuses F602 and F601 (servicemanual, 2A F fuse) are blown. I replaced them but when power on they blew again. (see picture)
I checked the transistors and diodes with a multimeter/tester and I am getting an OK reading. I looked at the capacitors (electrolyte) and non seems bulgy or leaking. However based on the vintage age of the speakers
I still suspect that something can be wrong with the capacitors. Nothing seems to be burned on the board.

I have also tested the speakers (Bas, mid, treb) and they all are showing approx. 4 ohms resistance (ie don’t believe that I have a short circuit in the cones)

I haven’t done any repair or built any speakers in 35 years but intend to try to pick it up again 😉
I have just ordered a ARCELI LCR TC1 ESR Tester (Transistor Inductance Capacity Resistance ESR Meter) to try and test the capacitors in circuit and will cross check the transistors.

Do you guys think that you can point me in a direction to isolate the faulty part (attaching a picture)?
Also when I am at it, should I replace any parts (unsolder) to give this old lady (A4-14) a new life and perhaps do the same with her partner, the A4-14 that is still working)?


Thanks for sharing!
/Peter
PS
And must say that you did a terrific you with your vintage speakers. I hope that I can do the same with mine.
 

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