Kr-720 outputs running hot, too hot?

[tom3]

Have the Kenwood Kr-720 receiver, bought new in 81. No real problems since new except for cleaning the tuning cap now and then. Gets dusty and the digital readout and centering indicators go haywire. Had the top off to give it a cleaning today and noticed the output transistors really running warm. Both sides at about 120 degrees. A couple good sized resistors on the left side (rightchannel?) read at over 150 degrees. Those resistors are right next to one of the outputs too. Idle current check was right on the money, DC measured at speaker terminals was low but jumped around quite a bit. So, is something going wrong here? How hot is too hot? Don't usually see this on vintage stuff at normal volumes. (Is this vintage?) Receiver is working fine, good sound both channels. Anyone have experience with this or a similar unit?

 

[Blue Shadow]

120 is below the idle temps on some of my gear, a nice warm transistor with a heat sink attached. If it doesn't get way too hot in use, might be no problem.

You can check the voltage drop across the resistor and calculate the watts being dissipated and if close to the watt rating of the resistor, look for issue that might cause it while replacing the resistors that have been stressed with a bit higher wattage part.

 

[transmaster]

You also might consider replacing the thermal paste Under the power transistors it might not make any difference but it eliminates one possible cause.

 

[tom3]

Thanks for replies. The way these transistors are located it would be tough to clean and repaste them, not TO-3 units. Located on a big metal plate that is real warm too so I'd guess they are transferring heat ok. I will check the voltage drop on those resistors, look to be 2 watt or so sized. I'm probably worrying about nothing here.

 

[avionic]

Have the Kenwood Kr-720 receiver, bought new in 81. No real problems since new except for cleaning the tuning cap now and then. Gets dusty and the digital readout and centering indicators go haywire. Had the top off to give it a cleaning today and noticed the output transistors really running warm. Both sides at about 120 degrees. A couple good sized resistors on the left side (rightchannel?) read at over 150 degrees. Those resistors are right next to one of the outputs too. Idle current check was right on the money, DC measured at speaker terminals was low but jumped around quite a bit. So, is something going wrong here? How hot is too hot? Don't usually see this on vintage stuff at normal volumes. (Is this vintage?) Receiver is working fine, good sound both channels. Anyone have experience with this or a similar unit?

Has anything changed in your system lately. Like say 4Ω speakers ?

 

[avionic]

If both channels are the same....Either its normal or you have and identicle problem in both channels..

 

[avionic]

A couple good sized resistors on the left side (rightchannel?) read at over 150 degrees.

Large wattage resistors are large for a reason. To handle more heat.

 

[NnionzngEMF]

Can someone with the KR-720 verify that when selecting and tuning on AM that the Tuner Indicator lights and Signal Strength meter does or doesn't operate. Figured I'd ask to save some troubleshooting time.

Mine operates on the two FM selections, though I do have a Tuner Indicator Lamp that is burnt out. I think I'm going to replace the "grain of wheat" lamp with an LED(s) and resistor. Thinking that once that is replaced the Power Meter will start working again since I see the output of the lamp feeds into the transistors related to the Power Meter. I have to read into the schematics more, though figured I'd ask around.

Thanks in advance for your time!

 

[Boyum]

Have the Kenwood Kr-720 receiver, bought new in 81. No real problems since new except for cleaning the tuning cap now and then. Gets dusty and the digital readout and centering indicators go haywire. Had the top off to give it a cleaning today and noticed the output transistors really running warm. Both sides at about 120 degrees. A couple good sized resistors on the left side (rightchannel?) read at over 150 degrees. Those resistors are right next to one of the outputs too. Idle current check was right on the money, DC measured at speaker terminals was low but jumped around quite a bit. So, is something going wrong here? How hot is too hot? Don't usually see this on vintage stuff at normal volumes. (Is this vintage?) Receiver is working fine, good sound both channels. Anyone have experience with this or a similar unit?

Hello from Pennsylvania, I have the same issue with a 720. Did you ever resolve the heat problem. I have four resistors that get hot as heck. R171 and 172 off the top of my head. The other two are on the other side by the heat sink below a output transistor. Not looking at the board now.

 

[tom3]

Haven't done anything to it since posting. Still working fine. I've read these receivers have some sort of problem with oscillation in the output section. I'm not smart enough to even understand that. But maybe that could be generating some additional heat? After 40 years I'm surprised it still works at all. Untouched except for cleaning the tuning cap and a drop of oil now and then.

 

[Boyum]

Thanks tom3, I'm poking around with my meter and replacing lots of resistors. Wish me luck.