Peculiar Case Of Secondary Wire Found Open In 2.1 Multimedia Speaker System Model Deltron
This speaker system was brought by a recently added regular customer with the complaint that it had some problems due to which it was kept aside for quite some time now and wanted to check up whether it could be brought back to working condition again. When I powered it up, I could hear a thud sound from the woofer, but the front panel was not lit and there was no display.
As usual, the first job is to always open and clean, which I did. The front panel is to be pried open with a flat screw driver taking care not to damage the wooden box. The rear cover had the power supply and amplifier section attached to it. Let us have a look at the inside:
I checked for the 5V that should be present to the front section. It was missing. Looked for any shorts in that section and found two LEDs open and burn marks at the legs. So, replaced the two LEDs. Did not find any other defects including the push to on switches. Then traced the 5V wire to the main board and found that there was no input to the 7805 regulator IC. The input was coming from a separate tapping of the mains transformer. I could get a 7V AC from the common ground, which was fed to a IN4007 smoothed by a 1000/25V cap. There was no DC. In order to check whether the regulator IC was loading, I removed it. Even then there was no DC. I removed the cap but there was no change. So replaced the IN4007 thinking that sometimes such odd behavior will be shown by diodes and then put a new 1000/25V cap. But the problem was still there. I removed the AC input connector that comes to the section, lubricated it and reinserted it. But still no DC voltage at all! I removed the AC input connector once again and checked the continuity! I was surprised to see that the reading was in Meg ohms, which is very much abnormal. Evidently, the wire was open somewhere deep inside the winding. I put back the 7805 after checking for any direct shorting using the analogue multimeter. I took a 0-9, 1A transformer and wired it there to check whether the front panel area was ok.
Then switched on the set and gave 230V AC input to the 0-9 transformer and found that the front panel was lit and display was on and checked whether all functions were working. I found the remote control malfunctioning and informed the customer to get a replacement, as my attempts by opening and cleaning it failed.
Fit the second transformer in between the main transformer and RC sockets, for which I had to take out the antenna wire. Drilled a separate hole for that wire and took it out to the rear. Let us have a look at the addition to the family:
While testing, I noticed that the output of Left and Right speakers were not in proper place. So I had to cut and rewire it so that the outputs come to the points where ‘Speaker Outputs’ was marked. That is what you see in the second picture above. Took out all connectors and lubricated it. Also lubricated and cleaned the two potentiometers in the front panel section.
On further checking, I noticed that ground of one of the RC adaptors was open inside. So, bypassed it with an external wire, picture of it shared below:
Then assembled everything back and tested the set for sufficient time for any flaws and then moved it for delivery to the customer.
Mission accomplished with new experience, because of which satisfaction got entry into the collection bag!
P.S. The above is a shot from a video taken and that is why it is not sharp enough. My apologies!
This article was prepared for you by Parasuraman Subramanian from India. He is 76 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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Yogesh Panchal
May 16, 2026 at 6:20 pm
Good Job!Sir
Still i found linear power supply supply are more reliable then SMPS.
SMPS Circuits having more protections still failure rate is high.
Albert van Bemmelen
May 16, 2026 at 6:56 pm
Glad that you were able to fix this speaker system with partly defect transformer by adding a second 9V AC transformer to feed the 7805 regulator to be able to create the previously missing 5V DC. Instead of adding a new transformer another option would be to add a small 5V 3A module that can easily be fed by inputting 230VAC. This affordable ones: https://www.youtube.com/watch?v=ihUAA20oxik
They are not expensive, and light weight and about the same size as a small transformer so would make an easy fit.
I bought a couple of these to make safe replacement power supplies for the well known famous Commodore C64/128 computers. The power adapters I made with them require both a 9VAC transformer (9VAC/1A) and a 5V DC voltage (at least about 2,5A DC) but the modules are 3A max.
And good to know is that the 5V/3A modules that are sold on Aliexpress are a bit bigger why those on Amazon are to be prefered!