CR-2020 Power Supply Conundrum

[w1jim]

I picked up a Yamaha CR-2020 over the summer and am just getting to fixing it up.

Before I even powered it on I performed the appropriate service bulletins - rewiring the switched outlet in the rear, installing the new 33Ω resistors and replacing 2 of the fuse resistors with 22Ω resistors and updating the 2-2SD234 power transistors with new TIP-41C's.
I also replaced any caps that looked to be heat damaged.
There was another 10Ω fuse resistor that I replaced with a 1/2 watt resistor as well.

I also adjusted the bias on the 2 amp stages to factory spec.

Here's my problem - the receiver will play fine for a few minutes and then the audio makes a bit of a "phht" sound and then shuts down.
If I turn it off for a second and then power it back on it will play for a minute or two and then shut down again.

I found that the power output the "-13" goes from -12.8v to +0.77 and the "-25" goes from -25.8v to +9.8. Obviously, mt problem is associated with these voltages - all others show no change.

Also, at the 6-pin molex connector where the power supply board plugs into the board with the main PS caps and diode bridges jumps up in voltage - likely because the load presented by the power supply PCB decreases.
"+NL" goes from +39v to +47.7V and "-NL" (which supplies the circuit for the errant voltages) goes from -40v to -59.9v.

I've replaced the HZ12C at D717 (13v zener), TR714 (2SA844) and checked for cracked joints - with no changes.

Any advice would be appreciated.

 

[w1jim]

The actual PCB - nice and clean with no heat damage as is so common with these.

 

[merrylander]

The +25 and -25 also feeds the tuner board - brown and violet wires at the left rear. They also feed the tone control board you likely have an electrolytic on either board going sick.

 

[w1jim]

Ah, thank you - I'll pull those wires off the power supply PCB and see how the voltages react.
This thing is such a PITA to take apart.

 

[bigbobster]

I recall a similar type problem replacing the series pass regulator transistor. After much suffering a scope look at the power rails revealed that the regulator was oscillating under certain conditions. Touching parts in and around that circuit told me that a parasitic oscillation was happening.

I don't exactly recall what I did to fix the problem. It probably involved a small (22pF cap or similar) type fix around the series transistor.

If you are patient I can unpile the CR-2020 and have a look inside.

 

[w1jim]

I'm thinking it's something in the phono preamp section in the R/R of the chassis.
I pulled the 2 violet wires from "-25" that power the tuner and phono preamp and then reattached the tuner one.
Been playing for a while now with no trouble - I'll see if it keeps going for a few hours before recapping the phono preamp.
Of course part of it is obstructed on the underside with steel bracing.
Don't know why they couldn't have made it a fold out design like the CR-1000, I can only imagine what it's like inside the CR-3020.

 

[w1jim]

I replaced most of the caps in the preamp and tuner sections - tentative success - I'll let it run overnight first.

Next (last?) oddity - I noticed prior to the recapping that when I turn off the OTS (optimized tuning system) the associated LED dims but does not go out completely.
OTS seems to function properly when activated though.

Big time thanks to Rob for getting me this far.

 

[bktheking]

The cr3020 PS board setup is entirely different, much better design.

 

[w1jim]

Well, failed the run test - after an hour or so it dragged down the power supply and shut off the audio output.

I think the next test is to put two individual amp meters in line with the violet feed wires (-25v) for the tuner and the preamp stages and see which one (if any) draws more current when the systems shuts down.

On another note - should the protection relay have a delay? Mine clicks as soon as the power is turned on.

 

[bktheking]

Yes it should IIRC.

 

[merrylander]

Bigbobster's comment about oscillation rings a bell. IIRC replacing the small electrolytic just off the left corner of the 180k sand resistor stopped that,

If you decide to recap the tone board there are only two caps on the board proper. All the rest are on the daughter boards. Rather thna trying to pull the whole board I simply de-soldered the daughter boads and dropped them out.

 

[w1jim]

Progressing - I'm getting the feeling that there's a cap (as Rob suggested) that was problematic that is starting to reform itself after decades of non-use since the amount of time before the unit shut down was increasing.
None-the-less I'm proceeding in recapping most of the unit.

As previously mentioned the protection relay doesn't have a delay to it.
On the backside of the power supply PCB there is a 470 ohm resistor across the base and emitter of TR702 / 2SB544 that I'm assuming is bypassing the circuitry.

*EDIT* - Removed the 470 ohm resistor and the protection delay is still not functioning.

BTW - the clean resistor in the picture replaced the old crappy looking one that was in that position.

Any thoughts on why this resistor is there?

 

[Bubo]

The cr3020 PS board setup is entirely different, much better design.

Dumb question?

Can you lift and transplant the 3020 supply to a 2020, if so inclined?

Or did they have a better, or now more experienced, engineer design the 3020PS??

The 2020 AND 2010 supplies seem to provide endless problems. Given the heat problems on the 2020 it never should have been released, a failure on the part of management, "plain to see it is".

 

[bktheking]

Highly doubtful