P700 Transistors H705 / 706

[camaronut]

When I restored my 2270, I decided to use the ZTX694B transistors as a replacement for H705 / 706 as listed in the parts list found in a another thread. I made sure I bent the leads correctly to maintain the ECB as on the board. Everything sounded great.

I noticed on other threads where KSC1845's were used, so I decided to get a few and make the swap, being that it sounded like the norm to use with all restorations.

So I plugged everything back in like normal and the phono sounded horrible. No bass, hardly any separation....sounded like a transistor radio.

Went back to the ZTX's, and it was back to normal.....excellent sound.

Has anyone else had the same issue?

 

[quaddriver]

odd. on paper, the 1845 should be superior. its on voltage is significantly lower, less loss, and its output capacitance is 1/3 that but both are too low to rolloff.

very odd.

 

[wlhd1610]

KSC1845FTA?

 

[camaronut]

KSC1845FTA?

Yes...straight from Digikey.

 

[catrafter]

I've never had a problem with KSC1845FTA as replacements for 2SC458. Could it be that you installed them improperly?

Tom

 

[MBuras]

Given the pinout and visual differences between all three transistors I’d bet money @catrafter is on the right trail.

 

[Golanr]

I have an 2275 so the board is called P400 but it looks the same. I recapped what you call C701-C704 and C713-C715
I replaced H701-H706 with ON Semiconductor KSC1845FTA. The originals are really bad.
I also replaced H709 (VD1212 Diode) with 2, in serial, 1N4148

I agree with Tom, check for proper install of these NPN and make sure they adhere to the board B.C.E layout. Use my photo to compare



Use my photo to compare

 

[camaronut]

I have an 2275 so the board is called P400 but it looks the same. I recapped what you call C701-C704 and C713-C715
I replaced H701-H706 with ON Semiconductor KSC1845FTA. The originals are really bad.
I also replaced H709 (VD1212 Diode) with 2, in serial, 1N4148

I agree with Tom, check for proper install of these NPN and make sure they adhere to the board B.C.E layout. Use my photo to compare

View attachment 1159722

Use my photo to compare

Absolutely installed them like yours in the picture. I made sure of the correct orientation. I had the data sheet next to me when I installed them last night. That's what has me baffled.

 

[Golanr]

You may need to adjust the trimmers (per the service manual) do you have a scope and frequency generator?
Or, you could just put the ZTX694B and be happy. I know I wouldn't and try to find the issue with the 1845s

BTW, it's a remote option but one or more of the new 1845 may be bad. you can test them outside the board. Or replace the ZTX694B one by one and test until you see what fails. Takes time, but it's fun

 

[rBuckner]

I've used the KSC1845's dozens of times in this style phono preamp and they always worked great.

 

[camaronut]

You may need to adjust the trimmers (per the service manual) do you have a scope and frequency generator?
Or, you could just put the ZTX694B and be happy. I know I wouldn't and try to find the issue with the 1845s

BTW, it's a remote option but one or more of the new 1845 may be bad. you can test them outside the board. Or replace the ZTX694B one by one and test until you see what fails. Takes time, but it's fun

Nope...don't have a scope or freq generator...I wish I did.

I tried again last night with another pair of 1845's....same thing. No bass - and doesn't sound right at all. Sound seems like its contained between between 1-6Khz....

but.......There are a few 2SC1775A's (Marantz's service bulletin) on Ebay I could try...