FARFISA ART-482/1 door entry intercom repair
In the building where I live, among the various installed systems therein, there is a Farfisa door entry intercom. The system was put in operation from the beginning of the life of this building, namely it exists for 23 years up to now. At the end of last May there was a system failure. Nothing was working. No picture of the entry door camera, no sound and worse than anything else no electrical control on the entry door opening mechanism.
The building manager, knowing about my profession of course, asked me to take a look on it. The case was really difficult due to the fact that the system was already very old and when the building manager asked the official service center of this company for help, they informed him that the system was characterized as obsolete and therefore it was beyond any technical support any longer.
I answered the manager that I will see what is going on with it and try to repair it. The system has its PSU installed separately in the basement of the building. From there leaves all the cabling to the front of the entry door where the camera box is installed, along with the intercom audio amplifier and the panel with the calling buttons which energize each buzzer of each respective apartment of the building. Moreover this PSU controls the distribution of the video-audio signals coming from the camera box towards each of the internally installed video monitor- handset located in each apartment. Of course the system is already very old and its video monitors are based upon 5” B/W CRT displays technology. Nevertheless its picture quality was still very good.
You can see the PSU box below:
And the internal video monitor-handset below:
Well, having myself seen the symptom of this failure it was easy to find out where the trouble was. No doubt it was a PSU failure.
So, I went to the basement of the building to check it. In that place there was a strange smell like burned paper, dispersed all over the area. Very soon I realized that this smell was coming from the PSU box…
I opened the front cover of it and verified that the smell source was the power transformer. Bad news therefore…Next, checking the fuses of the PCB, I saw first one of them wrapped with copper wire, which already was melted. This fuse is rated at 1,6A and is a slow-blow one. Upon pressing the “open the door” button of the internal video monitor units, it supplies the electromagnetic lock of the entry door of the building with 12V A.C, which in turn pulls the lock blocking lever, releasing the lock and allowing thus access on door opening.
Apart from that above, one of the (two) transformer primary protection fuses was also blown.
You can see below the internal parts of the unit. The two missing fuses are: just above the big 5V regulator the first one, rated 1,6A and used for the protection of the electromagnetic mechanism of the entry door along with the secondary winding of 12V A.C of the power transformer which feeds it. This secondary winding is connected with the two grey color cables in the PCB. The second one, rated 1A, protects the one half of the primary winding and is located just below that regulator, at its left corner. Also the one at its right side has the same rating as the previous one and protects the second half of the primary winding. There are no more fuses on this PCB.
For better view of the first fuse’s place you can see the picture below:
And for better view of the second fuse’s place you can see the picture below:
Furthermore, with blue-red is shown the one half of the primary winding, while with blue-brown is the second one. Each one is rated at 127V A.C and the brown-blue connection is their middle connection which is not connected to anywhere else, as the two half primaries are connected in series in order to meet the European voltage standard of 230V. In a 110V system they would be connected in parallel. This flexibility makes the use of this unit universal in terms of utility voltage.
Note as well that the manufacturer took in account the fact that this transformer is intended to work round the clock and therefore they made its primary capable to withstand 2×127=254V instead of the nominal one of 230V. The net practical result of this provision is that the transformer is working at a much lower than its normal operating temperature while being under its nominal load. This results in turn in extended longevity of the transformer.
The yellow-yellow-green secondary winding unfortunately had no voltage information on neither the PCB or on the transformer’s body. Now the picture was quite clear, not even needing any fuse replacement and retest the unit afterwards…I removed the entire unit for repair.
First of all, I tested the power transformer in unloaded condition, using again the well known lamp trick. I connected in series with the primary windings a 230V/40W test lamp. Upon powering the transformer the lamp was on with full brightness as I expected…
Now what? I already knew from the building manager himself that the device had no technical support any longer from the official service center and I hadn’t all the necessary information about voltages and currents of the transformer’s windings in order for me to order a new one with these characteristics from local transformer manufacturers…
On the other hand the information about replacing the entire system with a new one was that it would cost around 4000€ (material and labor, for new equipment of equal quality). Under the current circumstances…no way for such a replacement. So, I had no other choice but to open the transformer and assess the vital information about its parameters by measuring the count of turns of each winding and the wire diameter of it as well…
The transformer is shown below:
In order to make the long story short, this took me one full Saturday in working hours. The 12V secondary winding was melted and I could hardly remove it from the bobbin. Dismantling the transformer’s core was also impossible as its impregnated laminations were toasted and stuck with each other. The odyssey had begun. I started removing and counting the turns one by one until I had the full drawing of the transformer windings available along with their wire diameters. Then I performed all the necessary calculations to verify that the voltages and currents I assessed represent the original parameters of this transformer. After this verification, I ordered a new transformer. Meanwhile the static tests I performed in the rest of the discrete circuit components didn’t show any defective component. This left the question “why this transformer’s 12V winding was found melted” an open issue (apart from the stupid heavy bridging of the fuse with copper wire by whomever did that).
After some days I had the new transformer in hand. It had almost the same base with the original and so the mechanical rework I had to do was easy. Only two new holes in the chassis. Then, I reassembled the entire unit and reinstalled it in the basement.
Upon powering it, the 12V A.C fuse was instantly blown. So that open question I had before, came to its answer. I removed the burned fuse from its holder and then ran an Ohms test in the remaining circuit. The result of it was zero Ohms! There was a short circuit somewhere in the entry door locking circuit.
I went at the entry door and dismantled the locking mechanism. I performed a new Ohms test of the electromagnets’ windings resistance. Zero Ohms! The culprit was found. I opened the assembly just to satisfy my curiosity. The (two) internal electromagnet windings were melted.
I went at once to buy a new lock from a nearby shop and replaced the old one. Then I performed a new power up of the PSU after replacing the lock fuse again. All perfect. I performed all the voltage tests and found them as expected. I verified that the calling function to each one of the apartments was functioning properly and asked the people there to check and report to me if they had any problem with the system. Everything proved to be in order.
Then I left the front cover of the PSU open for two hours more and then I checked the transformer’s core temperature. This was also fine. No feeling of overheating by touching it. Next, I closed the unit putting its front cover back in its place. The repair was over.
All these four months passed from the date of its repair, the entire installation operates in proper order. And I am very happy using it many times every day, remembering the little odyssey I suffered during its repair.
Nevertheless it was really worth it.
This article was prepared for you by Paris Azis from Athens-Greece. He is 59 years old and has more than 30 years’ experience in electronics repairs, both in consumer and industrial electronics. He started as a hobbyist at the age of 12 years and ended his professional carrier as a senior electronics technician. He has been a specialist in the entire range of consumer electronics repairs (: valve radio and BW TV receivers, transistorized color CRT TV, audio amps, reel and cassette tape recorders, telephone answering and telefax devices, electric irons, MW cooking devices e.t.c) working in his early stages at the official service departments of National-Panasonic first and JVC afterwards, at their premises in Athens.
Then he joined the telecoms industry, working for 20 years as field supporting technician in the sector of DMRs (: Digital Microwave Radio transmission stations), ending his carrier with this subject. Now he is a hobbyist again!
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Note: You can check out his previous repair article below: