MD1502U

 

This article is aimed at beginners who, like me, take the first steps in the restoration and repair of tubes radios and they have a minimum theoretical knowledge of radio technic.

The story started when I received a small Mediator 1502U radio, which I immediately turned on to see if and how it worked. When I turned on the radio for the first time, with a test bulb of 100W and then without it, everything worked well. The next day  when I turned on the radio again I saw smoke coming out of the radio and a resistor incandescent: I still have not understood what it happened.

When I opened the radio, in the burned area, there were some things that I could not understand and reconcile between what I was seeing in the radio and what  was instead indicated in the wiring diagram.

There was something a bit strange among the power resistors, it were too many, I could not able to determine their values and, unfortunately, I did not find in Rmorg and everywhere a picture that shows the internal of the radio for checking, so to clarify my ideas, I schematized what I saw on the radio and how it was originally intended.

Thanks to this, I found that the lack of correspondence between the radio and the wiring diagram, was due to the fact that in a previous repair, concerning the burned resistor R2 + R3 + R4 (180 + 200 + 430 Ohm), which was not been disassembled to be used as a support for the separate assembling of the new two R2 of 150 Ohm and R3 + R4 of probably 200 + 430 Ohm, unknown Watt.

 

Original  Old repair 

So I asked for help in the RMorg Forum (Power resistor) to know the power of the burned resistor, and Marc Gianella that at this point, I can call "the friend" Marc, listened to my help request and he has followed me from the start to the end of this experience, also through direct contacts, giving me many good advice and even for free electronic components, to get back into operation the radio.

Furthermore I have to mention also all the explanations he gave me, much more than I could understand, but that, to try to keep up with him, have stimulated in me the desire to document me, to better understand what we were doing as, as I said, it was the first time I faced these problems.

First of all, Marc advised me to think about security in various terms:

- MY SAFETY, since the radio is a model without transformer and for this reason it is very dangerous to work as it is directly connected to the electricity grid, so first of all he told me to equip me at least with an isolation transformer ( trenntransformator  )

- OF THE RADIO, using a test bulb to prevent possible short circuits could cause damage to the radio ( vorschaltlampe ). Furthermore, to protect the output transformer the capacitor C30 instead to be connected to the chassis has been connected to the output transformer.

- OF THE PERIPHERAL DEVICES connections (turntables, MP3 players, iPhone), replacing some capacitors with those of type X or Y: the capacitor on the power line with one of type X (C32), while for the connections with the outside, replace the existing paper capacitors with Y-type capacitors (C3, C34, C35, C36).

Before     After 

Performed these preliminary operations, we worked to put the radio back into operation, so we replaced the fall resistor which, as I said at the beginning, was burnt in my hands (R3 + R4) with a resistor of 680 Ohm 25W and all others "paper" capacitors probably at a loss, (C12, C20, C27, C29, C30, C31, C38)

Replaced parts 

At this point we made some measurements of the resistors to determine the defective ones and (R16, R21) were replaced

         .

After these operations the radio  worked again !!! but it heated a lot that was because the tensions were generally too high (on average + 15%) so probably we needed to readjust the values of the fall resistors R2 that Marc has calculated around 280 Ohm.

In fact, after some attempts with different resistors, adding a resistor of 150 Ohm 11W to R2, the measured voltages corresponded to those declared in the wiring diagram and finally I could close the radio again because the work was finished

At the end 

 I do hope that this experience of mine can help others like me who want to dedicate themselves to these activities

 

Still many thanks to Marc for the support during all these activities and many thanks for your attention.

Roberto Licandro

 

Roberto Licandro, 18.Jan.18

Weitere Posts (3) zu diesem Thema.

Dear Collegues,

I need to ask  your  help to know th power of the resistor of the Mediator 1502U-10 named in the scheme (R2+R3+R4), i suppose it should be around 5 Watts but before to replace the burned one I think it is better ask to someone more expert than me.

Many thanks for your attention

Roberto Licandro

Pls. Find the attached image

Allegati

• Mediator 1502U-10 Resistor (R2+R3+R4) (229 KB)

Roberto Licandro, 30.Nov.17

Weitere Posts (6) zu diesem Thema.

Dear Collegues,

after my previous clarification request I continued to study the case and at the end I think I have found the explanation to the inconsistences that I saw.

The previous owner repaired the radio adding two new resistors, one of 150 ohms (instead of 180) in the schema indicated as R2 and another one I suppose (because is burned) of 630 ohms as R3+R4.

The problem that caused the discrepancies between the schema and the radio, is that the old burned resistor (R2+R3+R4) WAS NOT demounted and used as assemblage support, so the immage I have seen, with of all resistors burned, was different to those in the electric schema.

In any case thanks for your attention tha case may be considered closed.

Yours Sincerely 

Roberto Licandro

Allegati

• Correct implementation schema (156 KB)

Roberto Licandro, 28.Nov.17

Weitere Posts (2) zu diesem Thema.

Dear Collegues, 
 
I would like get in touch with someone able to give me some clarification about the radio Mediator 1502U-10 that I have just received as a present. 

The resistors of rectifying circuit are burned and unsolder and I would like to try to substitute them, please consider that i am a beginner in radio reparation.

I have noticed some difference between the electric schema published in RMorg and what I have seen in the radio: there is an additional resistors that does not appear in the schema.

Unfortunately, among the photos already published, there are not those concerning the chassis and the cablage of the components, so i can not check what is the implementation on another radio; please find in the attached picture, where I have tried to rebuild an immage of my model that I do hope it is correct. 

 Many thanks for your help to my repair attempt  and do not exitate to contact me for any information you need. 

Yours Sincerely 

Roberto 

 

Allegati

• Chassis Photo (282 KB)
• Electric vs Rebuilded Implementation schemas (174 KB)

Roberto Licandro, 27.Nov.17

Weitere Posts (2) zu diesem Thema.