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Yamaha CR-220 Noise and popping after unplugged

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DoctorDoug

New Member
I could use a bit of help on this as I work on repairing this CR-220 I have have since about 1980. A year ago, I heard a pop from the right speaker in the middle of the night that woke me up. The unit was turned off when this happened. I noticed on follow up that it would pop occasionally when I turned it on. But it also made like a crackling hiss after it was turned off and even if unplugged. It was mostly in the right channel and the noise was also present in the headphones. I went through the Deoxit bit, which did not really help but it did a little for a short time.

I replaced the power switch and spark killer. That worked for a short time with no popping on the right channel. Finally, I put the CR-220 aside until my frustration subsided. I tried again last week. The strange noise from the right speaker was there and I noticed it in the headphone right channel and slightly on the left channel. I opened up the CR-220 chassis again and started looking at capacitors because nothing else made sense why the unit made noise through the right channel hours after unplugging it. I found 5 leaking larger capacitors: 2 x 4700 mfd 50v, 1x 1000 mfd 50v Elna, 1x 1000 mfd 35v, and 1 x 10 mfd 16v. After lots of searching and reading spec sheets, Mouser to the rescue. The 10 mfd 16v tested ok. The 1000 mfd 35v sprayed electrolyte on it and on the board.

I installed the new caps. The noise was gone, but there was an occasional right channel pop. I also noticed the left channel had very low volume through the speakers and was distorted. I was going to replace transistor TR713 (2SA659) before but did not. I purchased two of them. The reference voltage for TR713 was in spec, so I did not change it. I found that test point TP1-TP2 had 0 volts between them. TP3-4 was a bit high, so I adjusted it to spec. After further investigation, I found that the voltage to TR717 and TR718 had 0 volts on two leads. Since they were 2SA659, I pulled them and replaced them. The pulled ones had 0 hFe. Then I checked the power amp transistors. The left side pair had partial voltage and one had 0 volts. They are 2SA771 and 2SC1986.

I am getting a bit tired of replacing components and adding more cost. Now when I turn the unit on, I get a gravely whooshing sound briefly from the right channel even with the volume knob at 0. When I turn it off, I get the same sound briefly with a sharp cutoff. Will replacing the left channel 2SA771 and 2SC1986 fix the left channel, stop the gravely whooshing sound when turned on and off, and provide voltage to test points T1 and T2? The distortion and low volume is also on the headphone left channel. It is like I am close to fixing the CR-220, but I don’t know. Should I replace the 2SA771 and 2SC1986 transistors on both channels, even though the right channel has good audio except for the whooshing sound at power on and off? Some of the challenge is the poor quality of the schematic showing the transistor voltages. I am going to look at TR715 (left channel) because like TR717 and TR718 are part of the circuit tied to the 2SA771 and 2SC1986. I will probably look at TR716 for the right channel also.
 
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avionic said:
What is your measured ..
Emitter voltage ?
Collector voltage ?
Base voltage ?

For those two output transistors ?
Click to expand...

Thanks for the question Avionic. I just went through the CR-220 and compared voltages to the reference voltages in the service manual for several transistors. I did voltages from left to right per the actual board layout. For the TO-220, that should be Base-Collector-Emitter. The left channel TR719 2SC2986; B=3mV, C=24.4V, E=5mV. The left channel TR721 2SA771; B=123.6mV, C=23.5V, E=5.6mV. Then I checked the working right channel: TR720 2SC1986; B=0.588V, C=25V, E=7.8mV and TR722 2SA771; B=0.616V, C=23.4B, E=27mV. My guess is both left channel transistors are bad.

Then I checked both transistors on the opposite side of the heat sink rail (TR709 and TR710) and attached to the rail with heat sink compound. TR709 (L channel) voltages were 6.6mV, 12.9mV, 4.2mV and TR 710 (R channel) voltages were 0.625V, 0.47V, 1.23V. The left channel TR709 looks bad. Also TR715 and TR711 voltages looked bad too. I am tempted to replace all five of the listed left channel transistors and see what happens. I don’t know if that will resolve the gravely whoosh at power up and power down however. I have gone so far, I would rather not give up.
 
you could turn the speakers to off ..i know it wont fix it but it should stop making noises in the speakers with the plug out . :D
 
avionic said:
Check D701 and 703 -- Can be replaced with 1N4148's..
Click to expand...

D701 and D703 in circuit give 24-25 mV in either direction and there is a tone from the meter. For a switching diode, sounds like both are bad. I checked D702 and D704 in circuit and got 583 mV one direction and 1038 mV the other direction. All were done with diode setting on DMM. I see a trip to Orvac Electronics in Fullerton tomorrow. Maybe changing D701 and D703 will fix the 0 volts idling voltage from V703 at TP1 and TP2. I will follow up.
 
avionic said:
"In circuit" testing is usually inconclusive..
Click to expand...

Thanks. I couldn’t justify removing them from the board to test and then solder them back in, so I tried it in circuit. However, unless there is a pricing error, I am getting a bag of 100 1N4148’s for $3 and will test the new ones before installing them.

This goes along with a prior problem with one of the power supply diodes being bad and that caused the pilot lamps to have a short life about two years after I purchased the CR-220 new, and for the next 15 years I kept replacing them every couple of years. I finally measured the voltage and found the bad diode and replaced all four diodes. Haven’t had a lamp blow since then.
 
avionic said:
"In circuit" testing is usually inconclusive..
Click to expand...

Here is the update. I purchased a 100 pack of 1N4148 diodes and did the board-level repair. Inconclusive was split. D701 was removed and tested ok. Replaced it anyway. Did the same on D703. The original tested bad with voltage both directions out of circuit. Replaced it with new. Checked the voltage at TP1-TP2–still zero and no change with adjustment of V703. I tested V703 and it works. With power off and large capacitors discharged first, the transistors that were noted as bad before with comparison to reference voltage, all tested bad in diode mode with the DMM.
 
avionic said:
Have you tested TR707,709 and 711 out of circuit ?
Click to expand...

No, but I concentrated on TR711 with power off and caps discharged and with power on checking against the reference voltages. One pin had 0.1 volts and the other two had 0V in diode mode. Powered on it was 13.1mV, 3.9mV, 13.4mV left to right. That one is rectangular shaped with a bevel facing the back of the board. I don’t know the pinout with that shape.

TR709 is up against a vertical divider heat sink and has heat sink compound holding it to the heat sink, so I did not remove it. I had to check it from the bottom of the board. Its diode check with DMM failed and power on voltage was wrong.

I checked TR707 power on against reference voltages on schematic. Left pin had 21.2V, center pin had 4.9mV, and right pin had 21.8V. I believe the collector should have 1.2V but the copy is poor and it could be 12V. The flat side of the transistor is facing the back of the board. It’s a 2SC1175 TO-92, so the collector is in the center. Base on Left and emitter on right, I believe.
 
There are posts in other threads with comments about fake transistors on e-Bay and from China. I have spent more than 3 hours searching for what appears to be bad transistors in my CR-220 and have not found good substitutes by comparing specs that are stocked at Mouser or BDent. The following are the ones that are testing bad in circuit:

TR715 and TR707 that are 2SC1175
TR711 2SA844
TR709 2SC1918 — Mouser has KSC1845 Epitaxial stocked that except for the bent pins looks good
TR719 2SC1986 — on e-Bay in the U.S. and China
TR721 2SA771

Do you have any suggestions?

Thanks


I still have a hard time believing there are fake transistors because there is little profit.
 
DoctorDoug said:
Thanks. I will take a look.
Click to expand...

Here is the latest on the CR-220.

The above original transistors were removed from the left channel of board and replaced with the above replacement transistors. Bending the pins on the TO-220 power transistors to pass through the holes in the new Aavid mica thermal interfaces took some ingenuity. I found a DMR radio mounting bracket the right thickness to provide 0.58 inches from the center of the screw hole to the bend and used it as a bending form. The bracket was clamped in a Panavise and the transistor body was held against the bracket with the pins on top. I used a flat knife sharpening stone and rocked it over the pins to bend all three pins to a right angle. It worked perfectly and the transistors fit perfectly. It also helped that I have several micro torque wrenches and was able to torque the two power transistors to 4 inch-pounds to sandwich the mica between the transistors thermal paste top and bottom and the heat sink.

The results are almost perfect. The left channel sounds fine through the speakers and headphones. However, there is still 0 volts measured at TP1-TP2 and VR703 has no effect. I have not replaced any resistors. The transistors in that circuit and the left channel were replaced. The small electrolytic capacitors do not show any leakage. I don’t know about the ceramic caps—they look good.

When the CR-220 is turned on or off there is a short 2-second sizzling sound like dripping water into a frying pan of hot oil coming from the right channel speaker and headphones with the volume at zero. I had it running for about an hour and when I turned it off there was about 7 seconds of a rumbling sound with a sharp cut-off from the right speaker. I am guessing there is a bad capacitor in the right channel. I also purchased a set of the same transistors as the left channel for the right channel. TP3-TP4 voltage was adjusted and is within specs without installing any new transistors.

I am out of ideas to repair it. I am considering going to the oscilloscope and inputting a 1k audio tone through AUX on the CR-220 to try and trace it.
 
mbz said:
Measure dc voltage between TP1 and GND/chassis, note sign +/-
EDIT: Use the oscilloscope.
Click to expand...

I had previously cleaned TP1 and TP2 just to rule out any corrosion. Before I powered up the unit, I checked the resistors in the circuit around VR703–they were all in spec. The two transistors in that circuit were already replaced. I then powered up the unit. TP1 had 0.00 volts to ground.

After repositioning the positive probe on TP1, the meter briefly flashed a minus sign before 0.00V. Hmm. I switched to older minigrabber probes and connected them again to TP1 and TP2. The DMM showed 11.3mV. That was better than 0.00. Perhaps I was lucky. I was surprised when I was able to adjust the voltage to around 33 mV on the left channel with VR703. Success.

Next, I double-checked TP3-TP4 for the working right channel. The voltage was not stable and was bouncing around from 30mV to 35 mV. I readjusted VR704 to between 32 mV and 34 mV. It would never stabilize but was close to 33 mV. I then checked transistor voltages in the right channel. They were in agreement with those shown for the corresponding transistors in the left channel as shown in the schematic. Is it OK to have a wandering voltage between TP3-TP4?

I am still having about 4-7 seconds of warbling white noise from the right channel upon power on and more of a rumbling sound on power off. Turning off the speakers is not an acceptable fix, even though it prevents the noise. I had already replaced the large electrolytic caps and six transistors in the left channel including the power transistors. I did purchase duplicate transistors for the right channel just in case they were needed. I can’t locate the source of that right channel noise. The caps all look good.
 
TP voltages look ok, you don't have "crazy" voltages like 35Vdc. The bias drift/bounce should be revisied once you sort out the
audio issues. Far from dangerous however it may be an early wanring sign, might be simply mains bounce...

Try using the oscilloscope to track down the source of the right channel noise.
Start with right speaker posts noting "wave" and p-on and off. Does wave extend into positive and negative scale.
Capture pix if possible.
Next test at TR708base, compare with speaker trace, it will be lower in amplitude so need to change "volts/div"
but looking for same shape as spkrs. Next test at TR716b and TR718b.

The amp does not have any protection/mute circuit so power on/off transients are not unexpected however these normally
present as a "pop". The power off noise does read a bit like a power down transient.

DoctorDoug said:
Turning off the speakers is not an acceptable fix
Click to expand...
Try and track down the problem however it might leed you "nowhere" (=startup transients) in which case the other option
is to fit a basic protection circuit/kit from ebay/china.
 
mbz said:
TP voltages look ok, you don't have "crazy" voltages like 35Vdc. The bias drift/bounce should be revisied once you sort out the
audio issues. Far from dangerous however it may be an early wanring sign, might be simply mains bounce...

Try using the oscilloscope to track down the source of the right channel noise.
Start with right speaker posts noting "wave" and p-on and off. Does wave extend into positive and negative scale.
Capture pix if possible.
Next test at TR708base, compare with speaker trace, it will be lower in amplitude so need to change "volts/div"
but looking for same shape as spkrs. Next test at TR716b and TR718b.

The amp does not have any protection/mute circuit so power on/off transients are not unexpected however these normally
present as a "pop". The power off noise does read a bit like a power down transient.


Try and track down the problem however it might leed you "nowhere" (=startup transients) in which case the other option
is to fit a basic protection circuit/kit from ebay/china.
Click to expand...

Thanks. Just a quick follow up before I go to the scope another day.

I replaced TR718 initially when it tested bad before I replaced the other transistors. I did not document the voltages of TR708 but checked them and found them matching the schematic voltages. I also checked TR716 before I ordered the replacement transistors. Its voltages: Base 1.14V, Coll 24.8V, and Emit 0.588V. I will recheck all three at the time I use the scope and look for changes. I believe I have spares for TR708 and TR716.

Also, the noise has changed since I adjusted VR704. Power on is now a pop followed by a shushhh tapering off. Power off is now a pop-crackle-pop-crackle-shushhh tapering off.
 
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