Bush RP50

[kirstyd]

Hi all
the amp in this was completely dead. Investigating further i found this resistor was open circuit. According to the service sheet its a 180 ohm 1/2 watt so i put a new one in [new one is 1 watt ] and it came to life and after fiddling with the turntable i have had it playing records for about half an hour and it sounds ok for what it is [i have one of these so i know what to expect] the thing is the new resistor and the one i am pointing at are getting hot not burning just hot. Is this normal or could there be something wrong. I am fully aware of the live chassis.

[TrevorG3VLF]

The UCL82 will take about 45mA and this can be checked by measuring the voltage across that HT resistance. Use an analogue meter to get an average. The current will be in pulses so the power will be much higher than a simple calculation will give.

The UCL82 bias should be about 16V, a leaky grid resistor will drop this bias and give excessive UCL82 current. A leaky cathode capacitor will also give excessive current.

Measure pin3 the grid, should be about 0V.
Measure the cathode voltage, should be about 16V.

[ms660]

It would be normal for the one on the left to get warm but at some point it will get hot, hotter and hotter with possibly disastrous results if you don't replace the red capacitor on the right hand side in the photo.

The large resistor being pointed to will run hot, it's the rectifiers surge limiter, I would replace it with a 5 watt wire wound type, it should run a bit cooler with that type fitted.

The red capacitor I referred to should have been the first component to have been replaced, it's the audio output control grid (g1) coupling capacitor.....+ve g1 is bad news..

Lawrence.

[kirstyd]

will change that red cap tomorrow .I presume its just a normal cap [not an electrolytic] service sheet says its 0.047 400 volts .When you suggest i test the cathode voltage would that be pin 9?

[kirstyd]

service sheet says that cap is foil and plastic

[Tim]

Assuming the third picture has the base of the UCL82 on the left, then the new resistor appears to be the cathode resistor(Pin 2?). I would expect a cathode current of around 40-50 mA, therefore a cathode resistor of APPROX 320 ohms.

[kirstyd]

yes thats the UCL82 the resistor is between pin 2 and the chassi .the service sheet says it should be 180 ohms 1/2 watt

[TrevorG3VLF]

Here are details of the UCL82 according to Mullard.

[kirstyd]

thanks trevor