Pete Ditmars
New Member
Hello, I am new to this forum. I have enjoyed reading many of your previous posts and I have learned a lot in the process, but I am missing some critical info that I need to repair my amplifier.
I have a Yamaha CR-820 from 1977. It worked well for 40 years and then gave a loud POP and no more sound from any source. I would like to repair it myself because a) I want to learn how to do it b) I am nowhere near a shop that could repair it. I have some serious time to devote to the project. I am hoping that someone out there can point me in the right direction.
First thing that I did was look at the fuses: tested all ok. Next I looked for obvious burning on the PCB: Nothing. Checked all the resistors that I could see (did not remove the transformer): no burning. Ditto for the capacitors: no obvious swelling or damage.
Found no voltage at the speakers, so I checked the speaker protection relay. It appears that the speaker protection circuit functioned as designed, since the relay is not sending power to the speakers and one channel of the amplifier has 44.1 vdc on it (measured at the relay). The other channel has .0004 vdc measured at the relay.
I then let the amplifier warm up for 10 minutes and started checking voltages with input = aux, nothing connected to speakers or headphones, vol =zero, tone controls = mid, filters = off.
The transformer output voltages are: 35.9 vac (orange to orange) and 17.8 vac (orange to blue). The main power supply transistor (TR806) measures base 29.3, collector 47.1, emitter 28.7).
The amplifier test points read as follows:
TP1: (44.0 vdc to chassis ground). If this is emitter of TR725, collector is 44.0, base is 36.4
TP2: (44.1 vdc). If this is the emitter of TR723, collector is 44.3, base is 44.0
TP3: (.0006 vdc). If this is the emitter of TR726, collector is 44.0, base is .58
TP4: (.0014 vdc). If this is the emitter of TR724, collector is 44.1, base is .63
Based on what I have read, it is likely that one or more transistors in the right channel amplifier (TP1 and TP2) is toast. There may also be capacitors that need replacing.
So. Now what should I do?
1.) Am I correct in saying that the TP1 and TP2 outputs are a problem and that TP3 and TP4 are reasonable (i.e., left channel ok, right channel screwed)?
2) Does it seem reasonable that I am looking for bad transistors? Or capacitors? Or something else?
3) The circuit diagram lists what I assume are test voltages at various points on the right channel amplifier. The left channel amp is almost (but not exactly) the same as the right amp, but no test voltages are shown Do the same test voltages apply? Partial schematic attached.
4) How can I identify which transistors/capacitors are faulty? When I tested the voltages in circuit at various transistors, I sometimes got readings that matched fairly close (but not exact) to the circuit diagram, and sometimes they were WAY off. But everything is so interconnected I don't know where to look for the root cause of a bad reading, nor do I know how much variance in the readings to allow for. Do I have to remove each component and test it individually?
5) Maybe someone else has had this exact problem and there is a thread that goes into what steps to take to diagnose it?
Thanks for reading this long post, but I wanted to include everything relevant to save time. Looking forward to fixing this sucker!
I have a Yamaha CR-820 from 1977. It worked well for 40 years and then gave a loud POP and no more sound from any source. I would like to repair it myself because a) I want to learn how to do it b) I am nowhere near a shop that could repair it. I have some serious time to devote to the project. I am hoping that someone out there can point me in the right direction.
First thing that I did was look at the fuses: tested all ok. Next I looked for obvious burning on the PCB: Nothing. Checked all the resistors that I could see (did not remove the transformer): no burning. Ditto for the capacitors: no obvious swelling or damage.
Found no voltage at the speakers, so I checked the speaker protection relay. It appears that the speaker protection circuit functioned as designed, since the relay is not sending power to the speakers and one channel of the amplifier has 44.1 vdc on it (measured at the relay). The other channel has .0004 vdc measured at the relay.
I then let the amplifier warm up for 10 minutes and started checking voltages with input = aux, nothing connected to speakers or headphones, vol =zero, tone controls = mid, filters = off.
The transformer output voltages are: 35.9 vac (orange to orange) and 17.8 vac (orange to blue). The main power supply transistor (TR806) measures base 29.3, collector 47.1, emitter 28.7).
The amplifier test points read as follows:
TP1: (44.0 vdc to chassis ground). If this is emitter of TR725, collector is 44.0, base is 36.4
TP2: (44.1 vdc). If this is the emitter of TR723, collector is 44.3, base is 44.0
TP3: (.0006 vdc). If this is the emitter of TR726, collector is 44.0, base is .58
TP4: (.0014 vdc). If this is the emitter of TR724, collector is 44.1, base is .63
Based on what I have read, it is likely that one or more transistors in the right channel amplifier (TP1 and TP2) is toast. There may also be capacitors that need replacing.
So. Now what should I do?
1.) Am I correct in saying that the TP1 and TP2 outputs are a problem and that TP3 and TP4 are reasonable (i.e., left channel ok, right channel screwed)?
2) Does it seem reasonable that I am looking for bad transistors? Or capacitors? Or something else?
3) The circuit diagram lists what I assume are test voltages at various points on the right channel amplifier. The left channel amp is almost (but not exactly) the same as the right amp, but no test voltages are shown Do the same test voltages apply? Partial schematic attached.
4) How can I identify which transistors/capacitors are faulty? When I tested the voltages in circuit at various transistors, I sometimes got readings that matched fairly close (but not exact) to the circuit diagram, and sometimes they were WAY off. But everything is so interconnected I don't know where to look for the root cause of a bad reading, nor do I know how much variance in the readings to allow for. Do I have to remove each component and test it individually?
5) Maybe someone else has had this exact problem and there is a thread that goes into what steps to take to diagnose it?
Thanks for reading this long post, but I wanted to include everything relevant to save time. Looking forward to fixing this sucker!