Yamaha B2 driver board diagnosis

[clinic-audio]

Don't look for 2SC1775A replacement , I will send you PM
2SC1775 C hFE is from 160 to 320
2SC1775 A hFE is from 400 to 800
2SC1775 B same as A

none goes from 750 to 1000

 

[rcs16]

Why is the hfe so important?
many folks have successfully used a ksc1845f to replace a 2sc1775A
2SA810 = 150V 50mA 0.7W, use KSA1220AY
2SC1452 = ? same as above, if so use KSC2690AY
add small heatsinks if they run too hot

 

[clinic-audio]

Hello

Why is the hfe so important ?

Because YAMAHA will not use 2SC1775A but 2SC1775 only if hFE was not so important

 

[belgianbrain]

Well, it has been nearly a month of pouring over these driver boards.

Every component is new except varistor diodes and transistors. Some wise members of these forums pointed out to me that the specs of the original transistors are actually quite untouchable - so all original transistors are tested and returned except TR109 and TR110 on both boards. I got impatient and extracted 2 2SC1755A from my C2 for the time being.

Here is the work of art.

Let's hope they function.

 

[belgianbrain]

It seems I live in a bit of a twilight zone.

Despite having changed 85%+ of the components on these boards and tested all remaining components, the behaviour of both boards hasn't changed much.

I think the problem is most observable at input side of R129 (TR 108/110 collectors). This node should be 0V.

Instead I get 4V on one board and -72V on the other.



Given that neither board changed its behaviour with complete removal, replacement, and testing of ALL components, I'm thinking I'm missing something else - possibly not related to driver boards themselves.

 

[belgianbrain]

One of these boards is close. If I could get one working, then at least I'd have a baseline to compare the other.

I can adjust one of these boards to PG=13.5V, then NG will be about -10V. So they're off balance by about 4V in absolute value.

Interestingly, this is the one where the input side of R129 (TR 108/110 collectors) is about -4V.

What mechanism is supposed to be pulling TR 108/110 collectors to OV? If I could understand this, maybe I could find the problem.

Anyone have insight into this?

 

[belgianbrain]

Don't look for 2SC1775A replacement , I will send you PM
2SC1775 C hFE is from 160 to 320
2SC1775 A hFE is from 400 to 800
2SC1775 B same as A

none goes from 750 to 1000

So what's the difference between 2SC1775A and 2SC1775B? I'm not finding a data sheet for the "B" variant. Yamaha also cites 2SS2705Y, but I can't prove even the existence of this part.

I have a bad TR114 (2SC1775B) and need a replacement. There are none in the C2 I've been temporarily robbing - only 2SC1775A.

The TR114 in the other driver board measures Hfe=600.

I have some KFC1845 hitting about Hfe=450. Is this worth a try, or can I use a 2SC1775A?

 

[clinic-audio]

Yamaha also cites 2SS2705Y
this part (semi conductor ) can't exist ! not with 2SS

 

[belgianbrain]

Yamaha also cites 2SS2705Y
this part (semi conductor ) can't exist ! not with 2SS

Yamaha service bulletin from 1981. Now that I look at it, they probably meant 2SC2705Y.

 

[clinic-audio]

IC270501 is 2SC2705Y

 

[belgianbrain]

Patrice, do you have a datasheet for this part?

 

[rottalpha]

http://pdf.datasheetcatalog.com/datasheet/HitachiSemiconductor/mXwvryv.pdf
http://www.mouser.com/ds/2/408/Toshiba_2SC2705-O (TE6,F,M)-464345.pdf

the difference between the original hfe and the replacent is substantial, hence hfe does not seem that important in this circuit.
use a c1845/a992 and you will be fine. If you still have problems after you put in the c1845/a992, then the issue is somewhere else

http://www.mouser.com/ds/2/149/KSA992-88789.pdf

 

[belgianbrain]

I cherry picked a KSC1845FTA from my stash (found one at Hfe=450) and it worked in this circuit.

The problem with the C1845/a992 family is that the high gain versions of the 1845 are out of stock everywhere.

Does anyone have a suggestion for a high-voltage (120V) NPN/PNP couple of audio transistors that are high gain and of the calibre of the original 2SC1775A? I will need more for other components in the future...

 

[rottalpha]

The problem with the C1845/a992 family is that the high gain versions of the 1845 are out of stock everywhere.

why is the gain important? Yamaha did not think it is when they recommended this sub. You missed my earlier point.

Does anyone have a suggestion for a high-voltage (120V) NPN/PNP

what is your circuit voltage? is it more than 120v across the transistors you are looking to replace?

again, are you trying to solve a problem you do not have?

 

[rottalpha]

TR114 has 84v across so the KSC1845 should more than suffice.

 

[belgianbrain]

I am judging gain by what I am measuring in what I pull out of circuit.

The 2SC1775A and BV I removed from these driver boards were all measuring Hfe 600-1000 on my DCA75.

Anyway, these driver boards are now functional thanks to lots of insight provided by Vint Age and Avionic. I am bouncing back over to my main thread to finish the discussion.

The big problem with one of these boards seems to have been a short between C114 and an adjacent trace. It was very hard to see with the naked eye, but it was there. Both boards also had a transistor or two that was either toast or on its way out.

 

[rottalpha]

I am judging gain by what I am measuring in what I pull out of circuit.

The 2SC1775A and BV I removed from these driver boards were all measuring Hfe 600-1000 on my DCA75.

Anyway, these driver boards are now functional thanks to lots of insight provided by Vint Age and Avionic. I am bouncing back over to my main thread to finish the discussion.

The big problem with one of these boards seems to have been a short between C114 and an adjacent trace. It was very hard to see with the naked eye, but it was there. Both boards also had a transistor or two that was either toast or on its way out.

yes, so the point I made is that the replacement transistors gain was the least of your issues...actually not an issue at all since the lowest rank KSC18545 would have a higher gain than the replacements proposed by Yamaha (2sc2705Y).

 

[belgianbrain]

yes, so the point I made is that the replacement transistors gain was the least of your issues...actually not an issue at all since the lowest rank KSC18545 would have a higher gain than the replacements proposed by Yamaha (2sc2705Y).

Yes, good point. Unfortunately I could not find the datasheet for that one when I was looking.

 

[rottalpha]

Yes, good point. Unfortunately I could not find the datasheet for that one when I was looking.

So what did you end up using?

 

[belgianbrain]

So what did you end up using?

KSC1845FTA