Nakamichi Receiver 1, Error in PCB?

[Erika_DV]

After having had a bad experience with my R-1, blew the left channel, I just had to dig deeper into the problem.
I found a difference between the schematic and the actual main board.
The component in question is VR301 R and L. (Bias control)

Here is the question: which of the two situation is correct?
My guess: VR301L is wrong (as tinkering with it blows the whole left side MOSFETs).
I don't have enough knowledge to come to the right answer.
Who can help?



First photo is part of the schematic to situate the VR301
Second photo is part of the schematic what is actually on the PCB.
Photo 3 and 4 the back of PCB as it is now (the flux has been removed...)

 

[northpaw]

I've never had or worked on an Receiver1, so what I can say is pretty limited, but since you asked via PM for me to comment, I'll say what I can.

I agree that there is a difference between what is on the schematic vs the PCB for VR301L, such as the PCB appears to show the wiper of VR301L connecting to R320L (I looked at the layout diagram of an Receiver2 and it shows the same), but the schematics for these units do not. Whether the doctored schematic you show is correct, I can't say. But it is not uncommon for the PCB as manufactured to differ from the schematic.

In this case, the layout diagram and the PCB are in agreement, and both differ from the schematic. So I would not try to modify the PCB, if you were contemplating doing so. It was a produced and functioning unit, and should work properly in the delivered state if there are no failed parts.

As to why the output transistors (the 2SA1492/2SC3856 are BJTs, not MOSFETs) are blowing when you try to adjust the bias via the VR301, I can only guess. Perhaps they could blow if you seriously overbiased them, or if you have a short somewhere. Sometimes the bias pots fail, and it is possible a faulty pot has placed an inappropriate voltage on the transistors. You could pull the offending one and check it. Also, the schematic you show has many voltage markings, which should help with tracking down problems both around the bias pot, and elsewhere.

Good luck.

 

[Erika_DV]

Thank you Northpaw for your comment.
May I ask if you can explain the principle of the Idling Current circuit and specific what happens if the variable resistor malfunctions eg central contact looses connection.
I would like to understand the function of the variable resistor in the Idling Current circuit.

 

[northpaw]

I can share my rudimentary understanding, but for more info you will have to read books on amp design and/or search the web. The output transistors have to be kept in an On state in order to provide linear output and low distortion. This means the audio signal needs to ride on top of a DC voltage that is sufficient to turn the output transistors on. Bias provides this. The bias pot is part of network of resistors and other transistors that provide a minimum DC voltage on the base of the output transistors to keep them turned on. In the typical design used for audio amps like this, the bias is defined as the quiescent current flowing through the emitter resistor with no music signal. For convenience, bias is usually measured as the voltage across that emitter resistor, rather than a current. Running bias higher than the minimum amount needed to turn the transistor on minimizes crossover distortion (no crossover distortion = Class A), so there is some sonic benefit to running higher bias than the manufacturer states, but it also creates more heat, and if the cooling ability of the heat sinking is not up to the task, the transistors will blow. As to what a failure of the pot in the Receiver 1 would do to the bias, you would have to analyze the specific circuit.

I saw your previous thread from 2021 on blown transistors (it was a link at the bottom of the present page). When output transistors fail, it is standard practice to also replace the next transistor up the chain from the last broken one, even though it tests ok, as it may be compromised. For example, driver transistors Q307L and Q308L are the next ones upstream from the output transistors, and the bias transistor Q306L is the next one after that. It is possible that the issues you are seeing are related to these transistors.

 

[Erika_DV]

Thank you for your response.
I now understand the principle. I am still in the dark why the left channel is made different from the right channel - are they?- and why the PCB is not what is on the schematic.
I have some research work to do....
I will also check which transistors I changed...
To be continued...

 

[northpaw]

I will also check which transistors I changed...

This brings up an important point. Replacements should be obtained from long-established, trusted suppliers to industry. In the US that includes companies such as Digikey, Mouser, and Newark, and in the UK/Eu, Farnell, for example. Transistors obtained from ebay, Alibaba, Utsource, etc., especially obsolete ones like these Sankens, run a high risk of being fakes. There are many reports of such fakes testing OK on simple component testers, but failing in circuit under higher load. If your replacements were fakes, it might explain their failure while attempting to bias them.