CR-2020 transistor substitute for 2SA844

[buglitter]

Hi all,

I'm rebuilding my new-found Yammy CR-2020, which has a power issue and doesn't produce any output. The unit has been tempered that the power supply c. board is missing TR714 and 1 capacitor (please see attachment). I have looked at the service manual, and it specifies a transistor 2SA844 in place of TR714 and a 10uf/35v capacitor; however it didn't specify what type of capacitor. I've looked at some pictures of CR-2020, and it seems that the capacitor isn't electrolytic. I'm trying to find a replacement transistor for the 2SA844, and I'd like to know if a 10uf electrolytic will work. So please give insight!
All best,

 

[avionic]

KSA1015 <---2SA844 I think the pin out is the same . But double check anywy.
As far as the 10 uf cap go with electrolytic.By far the least expensive route.

 

[clinic-audio]

Hello buglitter
Perfect sub for 2SA844 (e,c,b) is 2SA1015 (TOSHIBA) e,c,b, too .
KSA1015 is new production of 2SA1015.

 

[buglitter]

Thank you both for the insightful advice. I have been able to find the KSA1015 at Mouser, but I won't place an order just yet. I'll start working on this receiver when the previous order arrive from Mouser, and will consolidate what else I need into another order to make it worth the cost of shipping.

Again, thanks for your help!

 

[buglitter]

The mentioned capacitor (C719) can be either (from factory):
- not installed at all
- a 10uF electrolytic
- a 10nF mylar (polyester)

All above 3 varieties have been spotted on AK across various CR-2020 threads

In fact, this capacitor must have started out during design as "mimicking" its counterpart in the positive power rail section (C717), providing both a start-up delay and zener noise noise decoupling. This PSU start-up delay of the +/-25V power rails is a gentle way to avoid popping sound during receiver switch on.
The noise decoupling is required to keep the power rails (going to tuner and pre-amp section) as quite as possible: it is certainly required to keep zener-noise from D715 under control.

However, both functions are not required in the section of the negative power rail. To start with, the output voltage of the negative section is actually piloted by the positive rail section (via R731 towards R732/733), and an additional 10uF capacitor would cause an additional start-up delay of the negative power rail AFTER the positive rail has started up. Thus, this would result into a sequential power supply start-up, which is not good practice. Both the negative rail and positive rail should power on simultaneously (for smooth power on of the consumers such as the tuner section and pre-amp/tone control section).

Since there is no zener diode (acting as a reference voltage) in the base circuit of TR704, there is no need for the usual 10uF capacitor for zener noise decoupling

The only reason to keep a capacitor at C719 position is MF/RF noise decoupling of the base of TR704, since any noise present there will directly be amplified in the upstream series voltage regulator TR714/715. But for that function, a 10 to 100nF film capacitor is more than enough, and such capacitor was often installed in the CR-2020. It seems some early models did have a 10uF cap installed...... or....... somebody did that afterwards because the service manual shows a 10uF cap.

In some untouched units, we have seen no C719 capacitor installed at all. A bit brutal, but another 10 cents cost saver for Yamaha.
Just install a 10~100nF film capacitor, and you are fine
But I wouldn't recommend a 10uF electrolytic.

Thanks Oilmaster for a very thorough explanation. I'll replace the installed 10uF electrolytic with a film cap next time when I work on the piece.

 

[buglitter]

UPDATE:

I finally finished rebuilding this Yammy. I was able to get a 2SA844 from a local shop, and replaced the C719 with a 15nF polypropylene film cap. The unit immediately got out of protection. After adjusting the idling current to 10mV on both channels, the unit played without distortion. However, I've noticed that the two newly replaced 3.25W 33ohm and 22ohm power resistors run quite hot. I mounted them on homemade heatsinks and mounted them on the back side of the board to help with heat dissipation. The thermometer registered 50C on the 33Ohm and 45C on the 22Ohm. I replaced the two TR712 and TR715 with the TIP41C and mounted them on modified heatsinks. They also run hot measured at 45C. Other than the heat issue, the unit is performing very well. By the way, I replaced all electrolytic with Nichicon PW and Elna Silmic II, and most Mylar with polypropylene film.
One last thing to do is to adjust the meter sensitivity. They are not responsive at low volume (1/4max) and begins to move when the volume turns past 1/4. Is anyone familiar with the procedures? Thanks!

 

[clinic-audio]

They are not responsive at low volume (1/4max) and begins to move when the volume turns past 1/4.

That's the way they should work ! nothing to touch

 

[buglitter]

They are not responsive at low volume (1/4max) and begins to move when the volume turns past 1/4.

That's the way they should work ! nothing to touch

Quite different when compares to Sansui 9090. The only things left to do is the jumper mod and the cabinet restoration. I also need to investigate a possible grounding issue. I've noticed that the R. meter jumps every time when I touch the tuner knob. The L. meter is unaffected. If you have a clue, please give a few tips. Thanks

 

[clinic-audio]

Hello Buglitter

is it only when your speaker out /signal Q switch is on ?

Sorry not tip there , just read your owner's manual .
If you watch carefully inside ther meter you will see two scales
- first one for power output (same on the left meter)
- second one (only on the right meter) for signal Q
Simple no ?
Signal Q mean "signal quality"

 

[buglitter]

Hello Buglitter

is it only when your speaker out /signal Q switch is on ?

Sorry not tip there , just read your owner's manual .
If you watch carefully inside ther meter you will see two scales
- first one for power output (same on the left meter)
- second one (only on the right meter) for signal Q
Simple no ?
Signal Q mean "signal quality"

View attachment 959647

Hi clinic-audio,
I'm familiar with the meters as well as their function, but I really appreciate your explanation. Apparently, my issue got some things to do with the grounding. Leave alone, both meters behave normally. But when I started to touch the tuner dial knob, the red pointers on both meters dropped dead to zero. Letting go of my hand allowed the pointers to resume their normal functions. Perhaps there have been some solder bridges left behind when I rushed through the recapping process on the entire system. As for now, the unit plays very well; hence I just leave it until the next time I take it apart for another tune up. Truly, this unit really cost me a lot of time to fix, but the end result is very rewarding.

 

[avionic]

Touching the dial knob is suppose to switch the SP OUT/SIGNAL Q meter to the SIGNAL Q function.

 

[avionic]

Touch the tuning dial and dial in a good strong FM signal. Do the FM TUNING and SIGNAL Q meters respond ? Does the Left channel SP OUT meter work as it should ? Might take a little volume for the meter to register.Releasing the tuning knob should engage the right channel SP OUT function of the meter.

 

[buglitter]

Touch the tuning dial and dial in a good strong FM signal. Do the FM TUNING and SIGNAL Q meters respond ? Does the Left channel SP OUT meter work as it should ? Might take a little volume for the meter to register.Releasing the tuning knob should engage the right channel SP OUT function of the meter.

You've nailed it right on the head! Touching the dial knob indeed causes the meter to switch from SP OUT to Signal Q. I didn't notice this behavior because my FM reception was quite bad. Following your advice, I hook up an external antenna and tuned to a strong station, and it became crystal clear. Thank you, avionic for this helpful tip. Without it, I probably wouldn't be able to sleep well until I solve this very last issue. I'm pleased that this receiver is now singing very nicely. Now I just need to find some one to send the cabinet for a complete reveneering. Do you know anyone? Thanks

 

[avionic]

LOL Get some REM sleep tonight.

 

[avionic]

. Do you know anyone?

No I don't. But I'm sure if you do a little searching there are probably dozens of people here on AK that can point you in the right direction.

 

[buglitter]

LOL Get some REM sleep tonight.

Haha no kidding!

 

[buglitter]

No I don't. But I'm sure if you do a little searching there are probably dozens of people here on AK that can point you in the right direction.

Definitely will. In a worst-case scenarios, I may just do it since the original veneer looks uninteresting to my eyes, and I can't make it look any worse lol.

 

[buglitter]

Wow, it has been more than 4 years since I started this thread. It is time for an update of what really happened.

The unit has been sitting quietly on a shelf over the last several years until I had a break from school to finish the restoration. As it turned out, the unit wasn't completely fixed. Several issues arose after warming up including relay clicking, idle current bouncing , pass-through transistors run hot etc. And of course, the ugly wood case was poking my eyes every time I look at it. So i decided to finish it all up. Below was a list of things I have done to get the set up and running nicely.

Replaced all diodes on power supply board with modern 1N4148 and UF400x. Replaced bridge rectifiers, D713, D801 with UF400x vertically mounted on homemade PCB. Replaced power supply ceramic caps with WIWA PP. Replaced TR702, 706, 709, 710. Replaced R751 100ohms 5w resistor. Replaced main board (drive-amp) ceramic caps with WIWA. I also went against expert's advice to replace the notoriously noisy 2SC458 used as a thermal tracking tranny TR608 with KSC1845 to correct the bouncing idle current . Since replacement, the idle current was statically set at 10mV without bouncing. Both pass-through trannies are mounted on a bigger heatsink away from the original locations on the PCB with proper insulation (MICA and plastic washers}
I also managed to finish the case with oak veneer by myself. It wasn't perfect, but it surely looks better than the old ugly case. The unit has been heavily tested for more than 10 hours non-stop playing without any issues. +NB ~38V, -NB ~39.7V. All +/-25 and 13V are within spec. I'm still waiting for the main filter caps to arrive so that I could finish up the restoration, and the parts are not available until July, 2022.
I am attaching a few pictures of the unit for you to appreciate.