mediator 1502U-10: Power Resistor

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

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

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Hello Mr. Licandro

Unfortunately I don't speak Italian.

Safety first
Please consider that the radio has no mains transformer, therefore it is always connected to the mains. DANGER when taking measurments or when connecting audio sources. Even wet hands may cause an electric shock because the screws of the knobs may be connected to the live wire/phase. The use of an isolating Transformer is highly recommended!

The heaters of all tubes are series conneced and drop 116 V, the heating current is 100 mA.
The lamp drops 19 V 97 mA.
Thus
- R4 drops 43 V and emits 4.3 W 
- R3 drops 20 V and emits 2 W

The UL41 Anode pulls 50mA.
The other "high voltage" consumers drop 13.5 V over R1, therefore the current through it is 25 mA.
Thus the UY41 must deliver 75 mA.
R2 therefore emits 1 W.

Usually the power rating of resistors includes some reserve. 

The capacitor C32 should be removed. If you want to replace it, use a X type.

C29 should be replaced before operating the radio next time. According to the list, some more paper caps remain to be checked and replaced.

One reason for the burnt resistors may be the radio has been set to 110V and was connected to 220 V.
The two NTC resistors may be damaged as well as at least one of the heaters
Somewhere in the Forum the replacement of such NTC's has been discussed.

Good luck!
Kind regards
Marc Gianella

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Dear Mr. Gianella,

first of all many thanks for your interest , the very fast answer and the precious  suggestions about the security aspects.

In relation to  my question about the  resistors R2,R3,R4 I would like to take advantage of her patience asking if  I can combine R3+R4 resistence with a new one having 680 ohm/5W or 10w? Which is the tollerance allowed in terms of ohms and watts?

 That because i have some problems to find the right component, where i live there are not specialized shop and in on-line shopping (distrelec.ch, alibaba, ...) I did not find the right one: if you can suggest me where buying it would be wonderful.  

Frankly is not clear because the resistor burned, i checked the volt setting and it was correct.

Still many thanks for you help and excuse me if I bored you with other questions.

 

Yours Faithfully

 

Roberto Licandro

 

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Dear Mr. Licandro

Of course you can combine R3 and R4. Probably they had been separated for models with other mains tension (127V, 150V) or in order to not to exceed a certain power dissipation.
The permissible tolerance is given by the specifications of the tube heaters. Common tubes are usually specified to 10% tolerance. More information is given in the respective datasheets. In the present case it will be important to meet the specified 100mA heater current.

One 680 Ω Resistor covers partially the higher mains voltage of 230 V and emits 7 Watt. Take the next higher power dissipation class, such as VITROHM KH 216-8 (11W).
This will meet the specs and the resistor itself is not fully charged.

Some suggestions:

KH 216-8 at Distrelec

KH 216-8 at Grieder

This might be an interesting looking alternative, Danotherm GRF 10/63L 25 Watt 

or this, Welwyn W23 10.5W

 

Kind regards

Marc

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Dear Mr. Gianella,

many many thanks for your help and your patience, thank of you I have learned a lot of things. 

Just now i have placed the order following you suggestions, when I will repair the radio  I will publish the results in the Forum and that I do hope wll be all right.

By the way i noticed that the lamp mounted in the radio was not 19 V but 6.3 the resistence burned could depend of  this ? 

Your Faithfully

Roberto Licandro

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Dear Mr. Licandro

The wrong dial lamp was hardly the reason for the burnt resistor. It increased the heater current about 20% at the most.
By the way, what current is it rated, 100 or 300mA?
A suitable lamp isn't easy to find. For testing you can use a resistor of 180 - 220Ω 3W. 

Besides the probably wrong wiring of the set, some defective capacitors (C1, C29, C30) may be the reason for the burnt resistor.

I add a picture of the schematic with all the paper capacitors to check marked yellow. From experience most or all of them have to be replaced.
The most important ones for security reasons are marked red. They have to be replaced in any case before operating the set.
- C32: Remove it and replace later by X2 Type
- C3, C34, C35, C36: Replace by Y type
- C29: Causes UL41 overload, replace with at least 400V type
- C30: Causes damage of the output transformer, replace with 630V type. Switch replacement over transformer instead to zero.

The following steps are dangerous. Maybe you should ask an expert for help.
Could you tell me what equipment you have? (Measuring devices, isolating transformer, Variac, ...)

 

Kind regards
Marc Gianella

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• capacitors to check (241 KB)

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