SX-770 Blown Amp

[paulcalif]

Bought a (another) SX-770. This is another one that should probably have been put out to pasture, but I love a challenge.

I broke my own rules on this one. I started with repairing the wooden cover (that should be last)

Put it on the DIM bulb, seemed ok, played music through Aux, but only left channel worked. Ass-u-me'd it was dirty controls.

Moved on to the lamps because only one was working. Another step in error.

Finally got back to testing. Cleaned all the pots, switches etc. Still no right channel. Osilliscope lead me to the main amp. Fuses were good. Clipped all the leads and pulled it out.

Found several burned components, but the board is ok.

Two of the power transistors were toast and I think they are either EOL or exspensive. I saw a snippet where someone replaced the originals with TIP41C's (which I had in inventory), totally different foot print.

So, I did a little modification to the heatsink, drilled the holes for the leads larger, drilled the mounting holes on the TIP41C's larger so I could used the existing mounting hardware and insulators.

Broke off the collector leads. Collectors are now connected through the "insulated from the heatsink" mounting bolts in the same manner as the originals.

Added heat shrink to protect the leads from the now enlarged pass through holes in the heatsink.

The one thing I missed was that two of the original transistor's collectors connected to the board by the top screws and two used the bottom screws, since I was only using the top screws, I had to run two jumpers.

Now before anyone gets upset that I'm butchering a classic receiver, note that this thing is in really bad shape. Bunch of corrosion all over the chassis. This is more of a science, proof of concept, project.

Waiting on the replacements for the burned up parts that I don't have.

I'm hoping it works once it's all back together. If I had it to do over, it might just be a donor.

 

[Bruno Primas]

The bias trimmers in this era of Pioneer MUST ALWAYS be changed, as you can see.

Nice work on the transistors. That seems to be the correct method of repairing these.

 

[paulcalif]

Thanks! The 2 500 ohm trimmers are ordered. I didn't find trimmers in the correct orientation for the 50k trimmers. Think I'll see if it works before worrying about those.

 

[Bruno Primas]

Measure across the 50k with the power off and make sure they are reading somewhere around 30k-ish.

I have restored about 25 of this era of Pioneer. I believe 12 of them had bad outputs due to the bias trimmers. I have never seen on of the balance trimmers be bad.

 

[paulcalif]

Bruno,
You may not realize this, but you saved me a few bucks. I read the 500 printed on the back of a trimmer, and assumed it was 500 ohms. The worst part is, I went through this once before and should have remembered.

I had already ordered new 500 ohm trimmers from Mouser before I saw your post. It didn't even dawn on me right away but then it did. 50 OHM not 500. I was able to cancel my order and put a new order in with the right value trimmers, so thanks again.

By the way, is there a particular trimmer you like for the 770? I couldn't find a multi-turn that I liked for this,g so I just went with a single turn.

I don't have near the experience you have but I'm learning. I started restoring strictly Pioneer receivers from the 70's in January this year and now have well over 20. Most are complete, but I find it's much more fun to hunt for them and repair them than it it to sell them

 

[Bruno Primas]

Good catch. I believe all of the early solid state Pioneers, (except the 1000 and 2500), use 50 ohm bias trimmers.

I buy the 25 turn Bourns multiturn trimmers. I "tie" the inside leg to one of the outer legs and the spacing for the two outer legs fits perfectly in the holes provided. I bend the legs in a 45° so that when the board is mounted, I can adjust them easily. I then measure the trimmers outside legs and set it to 30 ohms to begin.

There is a post on this site where someone mounts a new trimmer inside the old trimmer which looked very nice, though I have never done it.

 

[EastPoint]

Take the old trimmer, remove the guts, solder the new trimmer to the frame of the old one. It’s often pretty easy.

 

[paulcalif]

The amp is back together and installed. I don't know how to adjust it, service manual isn't any help. There was a post on this site but I didn't quite understand stand. Can someone please post a quick walk through for an intermediate novice? (Bruno?)

On the plus side, the 100W dim bulb didn't even light and DC voltage at the speakers is very low millivolts

 

[Bruno Primas]

I believe it should be set to 15 mv. There is a thread about this. Markthefixer details it in post #5.

E

Thread 'Smoking resistors on a Pioneer SX-770'

May 8, 2021

Hello,
I was doing a simple recap of my beloved sx-770, everything was going well until I got to the main amp board. Just to reiterate, all the other boards were recapped and tested functional. When I replaced the caps on the main amp the left channel had no signal so started trouble shooting and seemingly made things worse. There was smoke coming from the driver transistor 2SC904. So I ordered all new transistors just for fun, I replaced them all on the left channel and now the white 0.5 ohm 2w resistors are burning up and there is a LOT of smoke.

Here's the transistors I used as...

• Ethan Pierce
• Replies: 8
• Forum: Pioneer Audio

 

[paulcalif]

Update:
Man, this thing has been a real challenge. I've been blowing TIP41C's like they are going out of style.

Finally got the amp working perfectly after weeks of troubleshooting. Was close to giving up several times. One of the little ceramic caps had a bad connection causing oscillation, increasing bias to the point that the TIP41C's were burning out, it's a long story, fixed now.

With that working, I tested aux which worked great. Moved on to phono, sounded horrible. Rebuilt the phono board and now that's sounding great.

Tape input works great.

On to the tuner, FM was all distorted crackles and it was in sync with the FM stereo lamp, which stayed on with any input selected. Replaced the transistor for the lamp, it's off now, but still just crackling badly.

Replaced all of the transistors and electrolytics on the MPX board, a tiny bit better, still crackling.

Moved to prior board 002, signal was very weak. Replaced all of the transistors and electrolytics, crackling gone but still weak and no stereo light.

Traced the weak signal from the output all the way back to the tuning capacitor box where my scope can't pick up a signal on this 770 or another working one that I have.

So, I ordered a more sensitive scope that's due tomorrow.

My guess is it's something in the tuner capacitor/gang, won't know until I get that scope.

I'm getting pretty good at working on these things but the FM section is above my skill set, and I don't have the equipment to do an alignment. My question is, can I take the output from another receiver's tuning cap and jumper it to this one using small coax just to test it. I have a lot of Pioneer receivers. I would cap couple it of course.