SA-7800 phono issues

I have had an sa-7800 and a tx-7800 for a month or so now, haven't done anything except deoxy the switches, pots and connectors.

Haven't had a problem until today when I hooked up an old turntable and the volume pot had to be turned to Noon to hear a thin reedy sound.

I tested out another turntable and same result.

These are not mc cartridges or anything fancy (Technics sl-q2 with stanton cartridge and a Denon 80's model as far as I can tell basic MM carts)

So I have downloaded the service manual and although I can't print out that huge schematic--I have looked through the manual briefly but didn't see anything regarding testing out the phono section--did I miss something?

I did an exploratory this afternoon and got the phono board disconnected from the front chassis and with all the tight wires that are wirewrapped in several different directions keeping it pretty tightly in its current position and I couldn't get a good look at the component side.

It looks like there are several test points on the circuit boards but the only test voltages I see are for the bias at the testpoints down next to the heatsink.

What would your first step? Re-capping? I saw something about leaky caps in certain areas reducing the gain of the phono preamp. Don't want to mess up anything that's already working fine.

sorry for long winded run-on sentences

thanks

Daniel

little more detail bout situation

someone asked if sound was thin and reedy in other areas....No it sounds fine with cd, am, and fm coming from the tx-7800.

I'm going over the schematic right now trying to find test points and voltage guidelines.

have a good one, Dan

hey, I'm doing that....

ok, read the incoming power supply voltages on pins 45 (-48v dc) and pin 46 (+48v dc)

the rationale is that to do it to BOTH channels equally, it has to be a power supply issue.

Now, IF you have a double failure the likely culprit is the Q5 and Q6 2sa912 transistors which are run HOT.

Did you ever have the phono work for you? If not, it might not have been an all at once failure, and because the former owner either did not care about the phono, or remember it's failure OR never even noticed the failure, then a progressive double failure is possible.

I replace the 2sa912 transistor with a heat sunk 512-ksa1220xx transistor from mouser.com. On each channel. IF it has taken out the other transistors, the
512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.17 ea
512-KSC1845FTA (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea
are nice replacements.

512-KSA1220AYS to-126 ecb 120/a160 1.2a 20w 155mhz 35-320hfe $0.39 (R=60-120, O=100/200, Y=160/320)

although I have done numerous sa-7800 phono amps yet I cannot recall any specifics to get at the board's components, I suspect I did a lot of wire unbundling.

BE VERY CAREFUL WITH HEATING THE COPPER FOIL PADS when soldering, they have been baked and will LIFT EASILY from the circuit board.

phono

thanks Mark,

The amp and tuner are both "new" to me a month or so ago. I just hooked up the turntable the other day and noticed this problem.

Looks like a lot of info there I will have to take a look at the voltages and get back to you. I appreciate your help

I read on another thread about removing the wirewrap posts from the board instead of trying to unwrap them. I have taken a lot of pictures to make sure I can get this thing back together again

I seem to find a lot of voltages on the schematic but nothing that seemed to be related to the phono section--now that I know what to look for I'm going to look again what page of the pdf were you on?

thanks for your help,

Dan

This manual, 19 meg hi res download.

Page 20 of 28 in the pdf, the GWF115 board.

What ended up happening with your repair Daniel? Did Mark's advice solve your problem?

I ask because I have what seems to be the exact same problem...I plan on following Mark's advice here. Hopefully it works out. I'll post again with an update.

MrAndyPantz said:

What ended up happening with your repair Daniel? Did Mark's advice solve your problem?

Click to expand...

old thread, his last post in it was 10-27-2012, 09:11 PM


danielphanto's Last Activity: 04-16-2014 12:51 PM


don't expect an answer soon....

I get a LOT of threads like that...

Haha! yeah, it was a long shot! I suppose what really matters is that you came back to it

So, as you describe above, I checked my incoming power supply voltages on both pins 45 and 46, and as you predicted, both have failed. I'm about to order the parts you describe above to replace the offending transistors (after a good cleaning of course). I can post updates with pics as I progress.

I've just purchased this SA-7800 and it works perfectly, albeit is a little bit dirty inside (had to use my fingernail to scrape the grime off the fuse windows), it also appears that the previous owner may not have used the phono input since he was using it for his TV's audio system :thumbsdn:. So I have no idea about how this failure could have occurred.

Do you think it's "bad style" to re-start this thread? Since it's the same problem, perhaps it's appropriate?

It's best that each piece of equipment has it's own thread.

And if the regulated +/- 48v has failed, is it too high or missing?

Which transistors? The only transistors I talked about are the Q5 & Q6 transistors in the phono amp which have little to do with failed pin 45 & pin 46 +/- 48v power feeds to the phono amp.

In fact I MEANT that IF the +/- 48v was PRESENT, then Q5 & Q6 need to be checked next.

So it's too soon to be ordering parts. Q1, Q2, Q3, Q4 and caps (some are special low leakage Nichicon UKL) are needed as well

Then there's the danger of heat damaged weakened copper foil bonding to the circuit board (lifting foils is bad).

from my incomplete notes:

gwf-115 phono / input

2x 47/25 cea 647-UPW1H470MED 47 50
2x 470/6.2 cea 647-UPW1E471MPD 470 16
4x 47/50 cea 647-UPW1J470MPD 47 63
2x 3.3/25 ceanl (stacked films if possible to fit them in)
2x 4.7/25 bipolar (stacked films if possible to fit them in)

2x 2sa1114 >>> 512-KSA992FBU
2x 2sc1775 >>> 512-KSC1845FTA
2x 2sa912 >>> 1/2 watt dissapation!!! <<< 512-KSA1220AYS + HEATSINKS!!!!!

Click to expand...

q5 and q6 could have hurt q1 - q4 when they failed.

ok. I was measuring from the wrong spot (duh). I'm sure you can tell that I'm very new to this, so you may have to explain certain things like I'm 5...but I'm trying my best and I'm very motivated.

Pin 45's voltage: 46.4
Pin 46's voltage: 47.1

I suppose in that case I'm doing better off than what I posted before But, I did jump the gun earlier this weekend and order (from mouser) Q 1-6 as per your suggestion to Daniel. Those parts were:
2x 512-KSA992FBU
2x 512-KSC1845FTA
2x 512-KSA1220AYS

at least it appears they will be useful.

I haven't detached the board from the unit just yet, so I haven't been able to see if the foil is heat damaged or is lifting from the board. I'll work on unbundling the wire tomorrow and update regarding the foil and the outer appearance of the caps.

Additionally there were a couple parts of your post I didn't quite understand:

2x 3.3/25 ceanl (stacked films if possible to fit them in)
2x 4.7/25 bipolar (stacked films if possible to fit them in)

Click to expand...

I understand that they are capacitors, but I'm not sure I'm confidant in sourcing the correct part on mouser myself. Are stacked film capacitors bigger/different than the current part? If so, what advantage to they have over the current part?

also:

2x 2sa912 >>> 1/2 watt dissapation!!! <<< 512-KSA1220AYS + HEATSINKS!!!!!

Click to expand...

I understood that to mean that 1/2 watt is dissapated in the old part, and the new part has a heat sink built in?

Thanks very much for taking the time to help. It's very much appreciated! I apologise for the stupid questions...this is my first time doing anything like this.
-Andrew

the PC board - first look.

Hey Mark,

I've managed to unbundle some wires and gently flip the PC board over to see the components.

Firstly, the caps all look fine, no leaking, bulging or discoloration. They all appear like new.

However, the one thing that caught my eye was a capacitor that was leaning over and touching pin 31. As you can see it has obvious heat damage. The capacitor code presented on the part is 102J 50 but the designation from the manual is C26, 1500p which (according to page 27) corresponds to 152J 50.

http://i.imgur.com/AzoS41t.jpg?1

I have the transistors ready to replace...do you still recommend I replace them? Or should I focus on taking that heat damaged capacitor out to test it/replace it? Or both?

I also found this product on mouser that may work as a replacement. What do you think?
80-C326C152J5G

Cheers,
Andrew

Checked the "burnt capacitor" it was fine. I replaced the transistors with the ones purchased above...turned the unit on.

The sound is perfect from the Phono input!!!

Thank you so very much Mark!

Best,
Andrew