Yamaha CR-450 protection circuit - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

cgehansen · 25th Jul 2025, 5:34 pm

Quote:

Originally Posted by G6ONEDave

What transistor is in TR801 location, if it's still that TIP41, that could be the problem with voltage drop on TR803. Also make sure that TR802 is not heavily conducting.

Dave

Thanks for the reminder. Yes TR801 has already been replaced with a TIP31C.

The voltages on TR802 are as referenced on the circuit diagram and the transistor tested OK off the board. TR802 is a 2SC944. I have a bag of 2SC828s but not sure that would be a suitable replacement?

cgehansen · 26th Jul 2025, 1:26 pm

Quote:

Originally Posted by Marconi_MPT4

Have rectifiers D803, D804, D805, and D806 been checked particularly the latter two since these are part of the +17V supply to collectors of TR704 and TR705 via transformer centre tap and R706 (22R).
Rich

Thank you. I've removed D803 to 6 from the board and tested them. D805 appeared to be faulty. I've replaced both D805 and D806. However the voltage did not recover to 40V.

I then replaced C804 and C805, tested R807 and R806 which are OK, and also tested D801 off the board and it appears to be fine.

The voltages at the base (32V) and emitter (31V) are as per the circuit diagram.

I've noticed if I disconnect R808 at the leg connecting to TR803's emitter the relay activates. The voltage at the collector also recovers slightly to 33V. The emitter to ground resistance of TR803 is 0.4kohm but with R808 disconnected it rises to 6.5kohm.

I'm guessing that signifies an issue downstream of R808?

cgehansen · 27th Jul 2025, 3:28 pm

It seems there's something odd going on in the tuner section. Disconnecting R803 allows the voltage at the collector to recover to 39V.

With R803 connected and moving the selector knob when AM or FM are selected the voltage to drop further to around 26V.

When FM is selected there's an odd hiss or buzz coming from the power supply end of the board which sound very unhealthy. This doesn't happen with FM muting selected unless there's an FM signal recieved.

All the problems started when I replaced TR801 in an attempt to get the radio working. The protection relay stopped working after than. Before then the AUX input worked fine but there was no sound from the radio (AM or FM).

27th Jul 2025, 3:28 pm	#23
cgehansen Pentode Join Date: Sep 2017 Location: London, UK. Posts: 130	Re: Yamaha CR-450 protection circuit It seems there's something odd going on in the tuner section. Disconnecting R803 allows the voltage at the collector to recover to 39V. With R803 connected and moving the selector knob when AM or FM are selected the voltage to drop further to around 26V. When FM is selected there's an odd hiss or buzz coming from the power supply end of the board which sound very unhealthy. This doesn't happen with FM muting selected unless there's an FM signal recieved. All the problems started when I replaced TR801 in an attempt to get the radio working. The protection relay stopped working after than. Before then the AUX input worked fine but there was no sound from the radio (AM or FM).