SX-828 Odd Channel Static Problem

My SX-828 has developed an odd problem over the last few months. Bad static on the left channel when in the FM and Phono 1 modes. There is still music on the channel but the static overwhelms it. Both channels are still perfect when in the tape or AUX mode. (Headphones also have the static.)

I have just finished a fresh Deoxiting and exercising all of the switches with no luck.

I have tried all combinations of the three separate speaker plug connections. I also disconnected the Thorens 150 TT. Nothing helped.

This all started as a now and thing but has now become constant.

I can live with the FM by selecting the right channel to both speakers but cannot live without my vinyl.

Any ideas on why I have a channel problem in some but not all modes?

John

That sounds like one of the transistors has gone bad. Have you replaced anything under the hood? Had the same situation in my 737. In that case replacing the transistors on the EQ board fixed the problem.

Hopefully someone can chime in with a list of replacements for the 828! I bet that is your problem.

The list of troublesome transistors are the usual suspects in the 828
PHONO PREAMP(Board #W21-002). 2sc871 and 2sc458
AF UNIT (AWK-012) 2sa725 and 2sc1312

As you have the crackling in the phono selection and others, I included the phono amp.

Replace the 2sc458/2sc871's in the head amp (w21-02) with ksc1845's.
Replace the 2sa725's on the AF Unit (AWK-012) with KSA992,
Replace the 2sc1312's on the AF Unit (QWK-012) with KSC1845's.

The leg pinout on the new transistors IS NOT the SAME as the old one's. Download the Data Sheet for EACH Transistor. Compare the pinout with the old ones.
READ THIS THREAD on HOW TO Identify PIN OUT's on BJT's. PAY PARTICULAR ATTENTION TO POST #11.

http://www.audiokarma.org/forums/showthread.php?t=43186

There is a cap list here. POST IT to this thread to have it VETTED for current availibility and correctness BEFORE you order. There may be No longer available, updated, or superceded parts. Getting the latest vetting will help make sure you have the best parts. 828 caps list

I thank both of you for your replies. I was almost certain it was the pre-amp board transistors because I knew the cassette deck had its' own preamp. I did forget that if you install an external preamp to a TT then you needed to go through the AUX which means the AUX bypasses the receiver pre-amp.

My preamp board is the B version with 6 2sc1312 transistors so I will order at least 6 of Mouser's KSC 1845fta's.

The replacement caps listed for the W21-002B board are:
647-UPW1V220MDD (22uF 35v) 2 each
647-UES1V4R7MDM (4.7uF 35v) 2 each
647-UKL1E100KDD ANA (10uF 25v) 2 each
647-UPW1V221MPD6 (220uF 35v) 1 each

I will rework the W21-002 board first.

Actually everything EXCEPT the TAPE MONITOR goes thru the W21-002 (Head Amp unit) but the phono goes thru an additional RIAA stage on the board. The rest are just amplified a little (without RIAA) before going on to the AF Board.

So start with the W21-012 board 1st.

Larry

I replaced the transistors and caps on the pre-amp board. All is now well with my baby. Again, I thank all of you.

John

Congrats! It's a great feeling, isn't it?

Now that you've got that done, go thru the whole thing looking for these 3 transistors. These 3 are #1 problem children in PIONEERS. They casue all kinds of problems up to and including blowing the amp from the 1451. It's not a case of IF they go bad, BUT WHEN they go bad.....and THEY WILL GO BAD.
Original......Replacement
2sa725........ksa992
2sa726........ksa992
2sc1451.......ksc3503
Your rig may or may not have them listed as primary parts on the parts lists for the boards. If they are in the parts list, make damned sure they aren't on your boards. If they are CHANGE THEM OUT PRE-EMPTIVELY! The 725's and 726's were used on almost all boards. The 1451's were used almost exclusively on the main amp boards. And the pinouts are different. IIRC the 725 and 726's are on the AF Unit, and possibly on the Main Amp board. the 1451 would be on the Main amp board if it's there. If they aren't there, don't worry about it. But at least check.

Larry, I have the model KUW which has some 2sa725's on the AF board and @sa726's on the main amp board which I will replace. (The main amp board is a PITA to work on.) Fortunately the SX-828's didn't use any 2sc1451s that I have been able to find.

Unclear to me is the hints I have seen about a mod to the protection board using a 1N4004 rectifier.

Experience with two 828's tells me that the final stage TO3's on the main amp board (2sc1080 and 2sa680) are not bullet proof.

Yet again I thank you for your help.

John

All of them with the vertically oriented amp boards are ROYAL PITA to work on.

The 1n4004 "mod" is a "flyback" diode that prevents a reverse voltage to the relay transistor(Q7) on shut down, which over time breaks down the transistor, causing a failure. This "mod" actually became factory installation in later models.

The 828 used 2 different protection boards.

W28-008 board. Use pin 9 and 10. Banded end(cathode) of diode to pin 10. Anode to pin 9.

AWM-025-c Board. Pin 9 and 10. Banded end(cathode) of diode to pin 10. Anode to pin 9

You can replace the outputs with OnSemi MJ21193 and mj21194 . These are about as bullet proof as you can get.

Larry, I have a 1n4004 so will do the mod. Thanks for the tip on the Mj's. They were not on my radar.

John

If the output's are ok, leave them for now, but keep the replacement numbers handy.