KR-7400 Burned board

ilikeguitars

New Member
Recently picked up this receiver from the local goodwill and it seems to be in working order, both left and right speaker outputs are fine. Upon opening it up i noticed the burned looking area on the power supply board. the resistors are 330 ohm 2W and seem to run way too hot. I have little experience with restoring vintage receivers and wanted to know if i could possibly change these resistors to higher wattage and if it would help at all. Also can anyone point me in the direction of how i would change the lamps to LEDs, if its not to much work. If anymore information is need hopefully i will be able to supply it.

Front
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Back
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Service manual diagram red area is of concern
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parts list
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PDF of service manual
 
looks like they put those in because of a heating problem. They equal 165 ohms. 2 330's in parallel. The KR9600 has a similar problem cooking the pc board. You can go up in wattage on the resistors but a better fix is to get a 25Watt metal case one that can be mounted to the chassis in place of the 2 330's and jumpered into the circuit. This will cool that down considerably.
 
looks like they put those in because of a heating problem. They equal 165 ohms. 2 330's in parallel. The KR9600 has a similar problem cooking the pc board. You can go up in wattage on the resistors but a better fix is to get a 25Watt metal case one that can be mounted to the chassis in place of the 2 330's and jumpered into the circuit. This will cool that down considerably.

So should i go with a 330ohm 25 watt metal case resistor or a 165ohm? where would be the best place to order one of these?
 
...a better fix is to get a 25Watt metal case one that can be mounted to the chassis...

That's the kind of engineered solution I like to see. THEY knew it was going to get warm. YOU know it does get warm. Make it a non-issue forever. I'm guessing there is sufficient space on the adjacent rear bulkhead of the receiver to place such a device. Noise/inductive shielding shouldn't be a problem considering it's right after the bridge rectifier in the circuit anyway and there's a 220µF cap across it.
 
I have a question about this recommended repair with the 25W resistor.

Would it be prudent to measure the voltage drop on this resistor pair to see how much power is being dissipated or is this something that is going to vary considerably based on usage making a measurement a one-time look?

If the voltage drop is going to be consistent and known would a ceramic resistor be an adequate replacement?
 
Okay i was able to find a local store that stocks chassis mount resistors. are either of these viable options? First and second, would it be bad to go with a 200 ohm instead of the 150 ohm?
 
Ohm's Law is pretty clear in that regard. If the existing resistance provides the correct voltage on the downstream side, you can use that voltage in the formula, then use the resultant current for calculation of different resistance values and see what happens to the voltage.
 
Can one explain how to measure a voltage drop or the voltage of the resistor in laymen's terms, please.

So, were you able to fit the 25W metal case resistor in the chassis? I am asking because I recently acquired the same receiver model and noticed those resistors get so hot that after 10 minutes the air starts smelling like frying electronics, so I should do the same hack... guess it was very poor design on an otherwise great receiver ;-)
 
Eventually I was able to buy some bigger resistors to replace the factory ones (2watts) on this board. I got 2 330ohm 7watts and 1 1Kohm 5watts, they still get hot, but less than the old ones, there is no more smell of cooked electronics in the room, that's good enough for now.
 
The other advantage is that even though the same total energy is being dissipated the larger resistor should tolerate it better - Possibly indefinitely.
 
I'd remove the stock resistors and go with a 165 ohm 25W aluminium cased resistor as suggested. The aluminum cased resistor will have have mounting holes so you could drill a couple holes in the outside of the metal riser bracket shown in the photo. Then mount it on the outside of that riser bracket using some thermal compound and a couple of sheet metal screws. Run some jumper wires from the resistor to the in and out of where the two paralleled resistors use to reside on the pcb and you have a permanent fix.

In any case those existing solder joints are roasted and I wouldn't power up until something is done.
 
Interesting I've had the same problem. I did check the power through the resistors, and it turns out it was around 2.2 watts. The twin resistors are 1 watt, and together they dissipate the 2.2. The result is the same burned board appearance. I replaced them both with one 150 ohm 10 watt Wire wound.

You might also check R2 and R11? I found those pretty toasted as well, but are buried lower on the board and hidden behind some other parts. They too are marginal for handling the power dissipation. Just as a precaution, I'm replacing Ck4 and Ck6 (two 220uF caps) which are in the circit. Those two are both in the paths of the fried resistors and may be a tad bit leaky.

I haven't found any other problems downstream, but will keep you abreast?

I'm so glad I found your posting. I was getting stymied myself!

Dave
 
Hi, A 25W resistor seems a bit of an overkill, I would replace them with a 5W 150 ohm wire wound cement type in the original position, mount it off the board for better air flow, it will get hot, that's normal, clean up the solder pads with fresh solder/flux also replace the capacitor near it you can see its been stressed.
Good advice from Dave, but A 10W resistor is physically much bigger than a 5W.
 
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Same issue

A lot of these have this issue it appears. I just recieved one that has two 330ohm 2watt tied together also, the service docs call for 150ohm 2watt in Rk1 spot. Mine had same burn marks as in pics and it burt off traces, also had a toasted 220uf cap also. Replaced all caps on board, traces for R1 on mine were burnt off but I was able to scrape off a new portion in which the leads were able to reach. I think I will be ordering a higher wattage 150ohm resistor before I do much else to it. Also one channel on tuner is lower but traced that to the low pass filter on MPX board, tapped it and weak channel became strong. Thread is a little old but ...
 
A lot of these have this issue it appears. I just recieved one that has two 330ohm 2watt tied together also, the service docs call for 150ohm 2watt in Rk1 spot. Mine had same burn marks as in pics and it burt off traces, also had a toasted 220uf cap also. Replaced all caps on board, traces for R1 on mine were burnt off but I was able to scrape off a new portion in which the leads were able to reach. I think I will be ordering a higher wattage 150ohm resistor before I do much else to it. Also one channel on tuner is lower but traced that to the low pass filter on MPX board, tapped it and weak channel became strong. Thread is a little old but ...

Did you end up getting a different higher watt resistor. I am going through the same thing with my 7400 and sourced this at Mouser. Copy this into their search 756-W23-150RJI
Very high dissipation and heat tolerant to 350C.
 
Did you end up getting a different higher watt resistor. I am going through the same thing with my 7400 and sourced this at Mouser. Copy this into their search 756-W23-150RJI
Very high dissipation and heat tolerant to 350C.

Sorry for bumping and old thread, but I have the same problem with my 7400 and have a question regarding clarification of the replacement advice here.

If I go with the recommended replacement above, would I only be purchasing 1 of these 150 ohm 10W resistors to replace the previous 2? If so, after removing the previous two 330 resistors, on which of the two spots would I be soldering this new one?

Thanks
 
Hi
I also replaced the resistors to my 7400.
i put two 330 ohm, 5 watts each and they still get hot (verry hot)
So, i would recomend that you instal a 25 watts - 165 ohm resistor.
 
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