Yamaha C-4 Preamp Recap

[Jake O Bob]

Remove D308 and D312 -- test them out of circuit. Better yet -- swap them out with a couple 1N4148's.

D308 and D312 are this 1S1555 diode. Do I replace each one with 2x 1N4148's in series? or do I replace them 1:1?

 

[avionic]

replace them 1:1?

 

[avionic]

Measure the emitter voltages of TR626 and 627. Looking for + & - 30vdc..

Re-measure these voltages please.

 

[Jake O Bob]

D308 and D312 replaced. Turned on and immediately saw something spark. R263 resistor burnt up on the function board. I was able to replace this since I had a spare. This is the second time a resistor has burnt up in that area since I had replaced R261 along with TR197 a few weeks back.

Turned on again to take measurements, only to have smoke come up from the same area. Not able to pinpoint exactly where. Either R263 is burning again or the clear/silver capacitor (C187 ?) might look like it's melting next to there

 

[avionic]

clear/silver capacitor

Polystyrene -- easily damaged by heat including soldering

 

[avionic]

Pull and check/re-check TR195 & 197

 

[avionic]

I had replaced R261 along with TR197 a few weeks back.

What part # did you use for TR197.

 

[Jake O Bob]

What part # did you use for TR197.

https://www.mouser.com/ProductDetail/onsemi-Fairchild/KSC2383YTA?qs=JV7lzlMm3yJNKQVlvFWlyg%3D%3D

 

[Jake O Bob]

Pull and check/re-check TR195 & 197

Since I'm new to this, I'm including photos of my measurements. Think I'm doing this correctly. Following this link for transistor measurements with MM in diode mode:

https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-0700

TR195 seems bad. Doesn't make sense why I get open for B to C. And why I get a voltage reading on C to E. Based on that, I'll say it's bad. What's an equal replacement part?

TR197 seems good since measurements go along with what the link says. It should be good anyway since I just put this in recently

 

[avionic]

2SA673 sub


https://www.mouser.com/ProductDetail/onsemi-Fairchild/KSA1013YTA?qs=ljbEvF4DwON6KcAiegZDyA== sub

Is your 2SA673 shaped like and " outhouse " ?

 

[Jake O Bob]

2SA673 sub


https://www.mouser.com/ProductDetail/onsemi-Fairchild/KSA1013YTA?qs=ljbEvF4DwON6KcAiegZDyA== sub

Is your 2SA673 shaped like and " outhouse " ?

Shaped like this with a rounded back

 

[avionic]

Shaped like this with a rounded back

These may be ECB instead of BCE...

 

[avionic]

If you had something like a DCA-55. It would identify the pin out for you. I think those cheap Chinese rigs do the same.

This is what I currently use.

Peak Atlas DCA model DCA55 | Peak Electronic Design Limited

 

[Jake O Bob]

These may be ECB instead of BCE...

Going off the service manual (unless it's misprint ?)
TR195 is 2SA673 = BCE
TR197 is KSC2383YTA = ECB

I've seen those PEAK tools before. Prob a good idea

 

[avionic]

Going off the service manual (unless it's misprint ?

The schematic illustrates and " outhouse " style TO-92

 

[avionic]

First page... Identifies ECB for the half round TO-92 transistor..

Datasheet Archive: 2SA673 datasheet by Hitachi Semiconductor

Preview and download the 2SA673 datasheet by Hitachi Semiconductor on Datasheet Archive.

www.datasheetarchive.com

 

[Jake O Bob]

I picked up one of the cheap transistor testers. Just not sure what the screen result means since it didn't come with a manual.

This is the TR195. Should I assume if it does NOT say it's broken, then it tests OK?

 

[avionic]

I picked up one of the cheap transistor testers. Just not sure what the screen result means since it didn't come with a manual.

This is the TR195. Should I assume if it does NOT say it's broken, then it tests OK?

Looks good to me...

 

[Jake O Bob]

Pull and check/re-check TR195 & 197

Anything else I should be pulling to check? These 2 are testing good with the new tester

 

[avionic]

Anything else I should be pulling to check? These 2 are testing good with the new tester

The rest of the transistors 199,201,203 & 205 in that circuit.