Pioneer SK-550 hum, buzz and burning smell (stuck in record mode fixed)

[Bru87tr]

How would DC get in the left channel if run on AC?

The radio is multi voltage, it also has a DC input too.

The speakers fit fine, they were actually more shallow but a slightly wider magnet. I just had to bend the plate over the transformer to fit the speaker on the left side. It would just catch the edge of it.

I am not qualified to work on this any further, I just don’t have the knowledge or skill. I consider it a total loss, but thanks for your help and I learned a few things. I will upload the service manual I bought when I get it. One doesn’t exist here so maybe those who have the radio and skill with find it helpful.

This experienced really bummed me out. Total $150 loss not a good feeling.

 

[Superduper]

Ok, on this model, it seems to be common for folks who take these apart to put the deck back wrong. On the rear, there is a lever which links to and controls a switch (which we call "record bar".... search archives for info on what this is). This switch is more accurately known as the record/playback switch, and toggles between the 2 functions. You may find that someone has previously opened up the boombox and tried to repair the deck, but put it back without aligning this lever, which could leave the deck stuck in record mode (electrically, not mechanically). If so, the deck will run in play mode but not have sound because it is recording. If this is true, and you put a pre-recorded tape in there, it's possible that the tape is ruined. When you say that the interior is spotless, that's sort of an indication that it's probably been opened and cleaned up before. It's almost impossible to have a boombox with spotless interior unless it really hasn't been used. The unit being operated will generate a small amount of heat which usually vents from the upper slats on the backside of a boombox and draws in cooler air from the lower slats. This convection pumping action over time introduces dust inside the unit.

As for why there is DC, this happens when an amplifier has malfunctioned. Audio by it's very nature is AC. If you were to look at a visible trace of the electrical signal on an oscilloscope, you will see the waveform which will appear very random. On a stable signal (such as a 1,000 hz signal), you will see a clearly defined sine wave. DC is the presence of a steady direct current. It is possible to have AC and DC simultaneously in the speaker outputs. It is known as AC with a DC bias. Your amplifier uses output capacitors. These should filter out any DC and leave pure AC. If the capacitor has failed short, it will no longer block DC. It's entirely possible that your amplifier chip could have been damaged and contributing to the problem or maybe just the cap is bad. But since the amp chip has already been overheated to where you heard humming, it's not worth the risk to leave them in there. Now, don't just go change the amp chip, the output caps and think that it's fixed. It might, but there might be other bad components too. Whenever a component like an amp fails, it is not uncommon to have other defective components which got damaged from or precipitated the damage to the amp so while it's apart, should also be checked. Also, From your comments, it's clear that the science between AC/DC as it relates to audio signals as opposed to input power may be beyond your knowledge but if you wish to learn more, there are a lot of google stuff out there.... I won't waste tons of keystrokes to give any lessons here. Also google DC in amplifier output if you wish to learn more about that too. This is a guess only since I don't have your boombox here to check, just giving you my take based on your symptoms. DC appears to speakers like connecting them directly to voltage. It WILL burn them up slowly (like if connected to 1.5v battery or higher battery). If voltage is large enough, will burn them up immediately.

 

[Bru87tr]

I can tell it has been opened, cause someone was using a screw driver to open the deck door and one long screw in the back is missing. So someone tried something, but it looks so good in there it didn’t look like anyone messed things up.

What you say makes a lot of sense, cause it does take a while to burn up the speaker. Can you answer why when I had my headphones in (wish I didn’t now but they seem to be ok, for now) the heatsink never got hot or even warm to the touch, but speaker and external speaker did? I would have expected the same thing on headphones since it’s not running on a separate HP amp.

You are very smart when it comes to electrical. Maybe if I meet someone here local, they could check it out and see what it needs.

Where is this record bar everyone talks about ?

Look how clean, could eat off it.

As for the stuck in record. The more I played with the functions the better it got. FF, Rew, Play all work (but no sound). It use to get stuck in record mode when you pressed play. Now it doesn’t do that. But it does get stuck in record mode when record is pressed. I am thinking cause the FF reel belt is loose, it can’t FF a tape, sometimes but usually motor just stops. Stop will not disengage record. I have to shut off the radio, spin the flywheel to get it out of record mode.

I do see the record bar now. Looks like it needs to come apart to deoxit. If it’s worth the effort I will do it, at this point I have nothing to lose.

Right now I have it working on the good right channel. Yay mono!

Hey, SD I appreciate your time. I may not get everything your talking about, but the info is good.

BTW: I have practiced a lot, but my soldering skills suck.

Let me know about my headphone question above. I tested the RF speakers and they test fine after hooking them up shortly to the bad channel. Even the external speaker tests fine. But I know damage prob was done at minimal anyway.

 

[docs]

The record bar will be behind the deck more than likely and will have a lever from back side of deck to record bar.

Search around here for record bar and you will learn a lot.

The deck is probably removable, check disassembly instructions in your manual. With the deck out you can see the record bar and it makes it easier to look at the deck, replace belts and I suggest that the issue could be old stuck grease, a dry pivot for a gear which needs degreasing or something along those lines.

Regarding the Headphones, I think they will have a lower ohm rating than a speaker and until you have done some checks its impossible to say why. My guess would be they could eventually burn out too. I would not switch it on with any speaker or headphones attached in that channel until you can troubleshoot the issue.
And believe me, I’ve learnt myself how to do some troubleshooting, it’s a labour of love and a steep learning curve but the money spent is never lost and one way or another, issues are almost always resolvable so dont give up. When you learn something new and fix it, gold!

If I can view the schematic I’m happy to try and guide you as best I can but you will need the basic tools such as a digital multi meter and soldering iron oh and some patience.

 

[Bru87tr]

The record bar will be behind the deck more than likely and will have a lever from back side of deck to record bar.
Search around here for record bar and you will learn a lot.
The deck is probably removable, check disassembly instructions in your manual. With the deck out you can see the record bar and it makes it easier to look at the deck, replace belts and I suggest that the issue could be old stuck grease, a dry pivot for a gear which needs degreasing or something along those lines.
Regarding the Headphones, I think they will have a lower ohm rating than a speaker and until you have done some checks its impossible to say why. My guess would be they could eventually burn out too. I would not switch it on with any speaker or headphones attached in that channel until you can troubleshoot the issue.
And believe me, I’ve learnt myself how to do some troubleshooting, it’s a labour of love and a steep learning curve but the money spent is never lost and one way or another, issues are almost always resolvable so dont give up. When you learn something new and fix it, gold!
If I can view the schematic I’m happy to try and guide you as best I can but you will need the basic tools such as a digital multi meter and soldering iron oh and some patience.

docs, I want to thank you too. I never thought of checking the speaker with a multimeter. I took an automotive electrical class when I went to school. I aced every class but electrical, I passed with a C. I still had a 3.6 GPA, but it always bugged me I was dumb in electrical.

I have all those tools, but I am terrible at soldering and have tried for years. I just suck at it.

You guys have been great, I hate to give up on it and decided not too.I may not have the skills and confidence to tackle the amp issue, but maybe I will meet someone local here who might be willing to check it out.

So, today I decided to check out the deck again. I can get at the record bar with deoxit, I liberally applied it to all spaces I could get at while moving the record bar. Allowed to dry and powered it up.

Record didn’t work at first and still got stuck, kept trying and now it works, but still no sound and nothing on the level meters while recording from internal mic or radio. I also noticed, the record light doesn’t come on now, good thing is it does not get stuck in record mode and pressing stop disengages it as it should.

I also noticed a spring to the deck was popped out, looks like it was for FF. That didn’t help anything before I tackled what I did today.

Edit: Playing with it more, it seems pressing record is actually making it play. Sometimes I press record then stop, trying again and again. Sometimes the record light comes on and it gets stuck. Spin the flywheel to lower heads and sometimes record button makes it play or record which when the record light does come on it gets stuck. Weird.

Edit 2: The more I play with the functions the more it works. Pushed record, light came on but no movement and stuck as usual. Decided to just move the flywheel lightly, the first time record has moved and the record light on. Not sure if I will get anything back but more and more keeps happening.

Still no sound from deck. I think the record mode is more a belt issue. Every time I push the flywheel along just touching it barely it comes unstuck and starts recording, although nothing plays back after.

If I could just figure out the tape deck no sound playback issue, I’d really be tempted to pay someone to fix the rest. Would be great to get it working even with modern speakers I put in it. BTW: No issues on the right channel, no heat issues, heatsink doesn’t get hot and speaker sounds great.

 

[docs]

I would remove the deck and rebelt it first. I would then check all gears move freely by manually engaging play mode. Once the deck has been proven mechanically, then, and only then, can we move on to why there is no sound. Think of it as a layered approach. We need to fix the top layer then move down to the next layer.

The deck aside, the channel needs fixing. That could be the top most layer before anything else.

 

[Bru87tr]

Rebelt for sure. Is it hard ? Afraid to take the deck apart. I feel like I am so close to getting the deck to run. I came from the door stuck and really only rewind working to all functions working but no output.

The mechanics look good, all the functions look good, using a bright light everything is engaging. Just one belt seems to hold it back a bit, but for sure they all need replacing.

Where on the forum is a good place to look for local bb people? Maybe someone could look at it and fix it for a price?

We have a local electronics guy, but I don’t want to be charged just for them to tell me they can’t fix it.

At this point I would like to find someone capable here to pay. The Rockford fosgate speaker sounds so much better than the old speakers and runs cool.

 

[docs]

State where you are and see if there is anyone close who could help.

 

[Bru87tr]

In Lowell, Ma if anyone local could help me out.

 

[Bru87tr]

I am starting to think someone demagnetized the heads too much ? All functions work now, everything, just no sound. Search function engaged, no R2D2 sound. Nothing. Amazing cause the heads look amazing.

Still hoping to get the left channel looked at.

 

[BoomboxLover48]

The extending lever at the back near the motor of the deck is the one moving the long record switch.
I looked in my pictures and forgot to take a pic of it after I removed the deck.
~Royce

 

[Bru87tr]

You know what I don’t like about that lever? It sits really close the the board right above some board connections. Mine isn’t bent either and it looks way to close to me.

 

[BoomboxLover48]

It has to stay close to the board next to a white nylon bar of the record switch. That lever pushes on the record switch we talked about.
~Royce