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

[Bru87tr]

Hello to any who will help, first thread Here.

I was looking for a different boombox on eBay and instead found this one. Great looks!

I found an old thread here on how to disengage, but I am afraid to force the back cover off to work on it. All the screws out and looks to want to come apart, does this need more force ? I didn’t want to break anything so stopped.

Anyone know how to get it out of record mode ? Someone said spin a wheel to get it to neutral and once turned on should dIsengage. I notice the pin that rests on the pinch rolled doesn’t move, from my cassette days I remember this moving to move the tape along. I am thinking the motor is dead?

Even stuck in record mode, ff and rew will work. The heads look really clean too like it wasn’t used much. Anyway, any help would be much appreciated.

Thanks,

Ralph

 

[docs]

See if there is a service manual to see instructions on disassembly. Sometimes the deck door needs to be open. In this case you need to try and see if there is a small hole in the front where you adjust the heads and get a very small screw driver in there and try and push the heads down to open the deck door.

 

[Bru87tr]

I got the deck door open, can’t get heads down.

 

[Radio raheem]

Some of these pioneers you need to remove the outer door like sk909 but im not sure about this one

 

[Bru87tr]

A service manual was posted the other day and jumped on it.

I just figured someone experienced with this bb could get me started before it gets here. Not sure what’s holding the back of the radio fr9m coming off. Not sure what screws from the front cassette come out. I got the door ejected and off.

 

[soundsyztem]

Make sure you get two screws from the front of the box out. They should be accessible once the deck door is off.

Good luck

 

[Bru87tr]

Going to wait till the SM gets here. Not sure which screws.

 

[Bru87tr]

Ok,so got the bb open. I can’t see the wheel to spin to lower the heads into neutral.

BTW: This bb is super clean inside.

Well, figured out how to spin the flywheel and heads down. Plugged it in and pressed play. Heads stuck up again in record mode.

Anyone have an idea what the issue is ? I don’t see any broken be.ts and all other functions work.

 

[JVC Floyd]

It could be the main drive belt is worn and probably needs to be replaced.

 

[Bru87tr]

So, if I hold the play button it plays but no sound. The belt doesn’t look to have slack when playing, but won’t stay in play mode unless I hold the button. Good thing is when a cassette is in there it doesn’t get stuck in record mode. Only with no cassette.

But why no sound while playing ? ff and rew fine.

It could be the main drive belt is worn and probably needs to be replaced.

It looks like it may have some slack, slightly. Also, shouldn’t I hear sound while holding play down? Looks to play fine when holding it but no sound from cassette. Pre recorded.

Ok, got it to stay in play mode. Damn, just no sound.

The more I use it now it doesn’t get stuck. Bummer no sound.

How do I go about deoxit the volume levers? Just put a little right along the tracks lightly ?

 

[docs]

Have you had to disconnect anything, connectors or wires etc?
It could be that the deck is out of alignment in terms of the gears and the record bar inside is engaged which is why no audio?
You could put it on radio and try recording, or just pressing play and see if you get something on the tape from the radio.

 

[Bru87tr]

Have you had to disconnect anything, connectors or wires etc?
It could be that the deck is out of alignment in terms of the gears and the record bar inside is engaged which is why no audio?
You could put it on radio and try recording, or just pressing play and see if you get something on the tape from the radio.

I didn’t try recording, but no sound from cassette. I have a bright bike light, it looks like the head is engaging fine.

So..... the radio fun didn’t last long. Played for an hour and a loud hum, shut it down fast and could smell burning. I checked and didn’t see any leaky or bulging caps before I put it back together to play finally.

Nothing looks burned, just a very hot PS and the speaker magnet is very hot, hopefully that speaker isn’t toast now. The fuse now looks cloudy a bit, didn’t before and it didn’t pop.

So I guess I am looking for a new PS now. Anyone know a good source for new and is that prob the issue ? Nothing else felt hot or burned.

The speaker that got hot does sound like it got some VC rub now.

It’s the speaker that smells burned.

Should I try DC and/or batteries ?

All I did was deoxit it the volume bars and lower the heads on the tape deck so it wasn’t running all the time anymore.

Was sounding great for an hour.

Ok, tried a dc switching power supply, same thing.

Figured I’d try turning the multi voltage selector. It has 120 (was already set there and correctly), 220 and 240.

Not sure if coincidence but put it back to 120 after turning it a few times, plugged it in and radio came on, not long after that started making hum/static/popping noise again. Immediately shut it down, just trying to figure out of it’s a total loss as pretty bummed.

So, last I need to try batteries. Anyone have an opinion what might be wrong? Would the amp chip cause this if it went ? I thought I heard someone say the chip in this is weak and distortion can cause failure. Although I never had this louder than 4 on the volume.

 

[Superduper]

When you said it was "stuck" in recording mode, you really need to be very specific about what you mean. Do you mean you set it to record and it stayed there? Do you mean you attempted to play but it was recording instead? Do you mean the record button is stuck in the down position? The thing is... the symptoms will kind of indicate whether this is a mechanical issue or an electrical issue.

About the speakers getting hot.... I would immediately "ohm" the speakers, both of them, and see what they are reading. If the hot speaker is showing really low ohms, it IS shorted. Do not use or turn on the boombox until you replace that speaker. If you do, you will damage (or have already damaged) the amplifier.

About your comment about the amplifier being weak and that's why the speaker is hot... the amplifier is plenty for this boombox. The only time the amplifier output puts stress on speakers is if you are overdriving the speakers (they aren't rated for 500 watts you know) or if the amplifer is driven to output a square wave instead of sine wave. This won't happen unless you drive it to clipping and you will know because it will be distorted and sound like crap. Lots of people like to run their boomboxes until the music turns to noise. That is distortion and I don't recommend using it like that. Unfortunately, boomboxes are calibrated in such a way that they can and do output distortion if cranked up too far. Yes, it's louder like this, but don't do that. Sound will be distorted, listener will get hearing fatigue, and it stresses everything from PS to speakers.

About the power supply.... if the fuse is smoky, it might be blown or overheated. It might just have saved the PS, that's what it's for. It should be replaced but don't bother to do that until you have resolved the reason for the over-current condition which is the cause of the fuse issue. They never blow or overheat for no reason. If you simply put another fuse in there, and keep powering it up to "try" and see if the condition magically got better and the problem gone, I hate to be the one to burst your bubble but that isn't going to happen. FIX the short or else the short will kill your boombox.

 

[Bru87tr]

It indicated by a light it was in record mode. When I got the heads lowered and pressed play without a cassette, it indicated record mode (and heads got stuck). Then I had to spin the fly wheel to lower the heads again. When a cassette was inserted and play pressed, it played (didnt go into record mode) but no sound.

Only one speaker got hot and my guess it was because it sits right in back of the power supply which was also really hot. But now after what you said, I am thinking the speaker shorted and the heat caused the PS plate to get hot.

I only read online someone saying the amp chip was weak. So was just asking about it.

The fuse wasnt blown, I took it out and replaced with same fuse and no difference.

Ok, so what your saying is the noise from the left speaker that smells burnt is causing this noise and probably shorted ? So, If I run only the good speaker it may play fine as a test ? I almost did this yesterday but got tired and bummed about the issue.

Thanks for your help and recommendation. I will test the speakers with an ohm meter tonight.

 

[docs]

Put an ohm meter across the speaker terminals and check both speakers against each other. It will be marked on the speak what its raring is. This will tell you if the speaker is blown.

 

[Bru87tr]

Yes, it is marked and think it said 3.5, expected 4. Not home now but what I remember

 

[Bru87tr]

So this is what I found.

The left speaker and tweeter both tested at 1.5 and 1.7 on the multimeter.

The right speakers tested within range. So, I decided to hook up the right speakers to the right channel, all good.

I hooked up the right speakers to the left channel and sounded fine, kept it playing the radio for 10 min then started smelling a smell again. The speaker magnet was getting pretty warm and the heat sink was getting pretty hot. Unhook!

So, I decided to plug in an old desktop speaker (4ohm) with a 3.5mm connector to the external 3.5mm input. Tried the right channel, played great for 15min, heatsink stayed cool. Plugged it into the left channel, after 5 min the heatsink was hot, disconnect.

Headphones run fine. Speakers disconnected, no load, heatsink stays cool.

I am no electronics guru as you guys already know. What do you think ?

 

[docs]

Well, as SD said, something in the left channel is over voltage so its probably time to check the schematic and voltage check from power amp IC(s).

 

[Bru87tr]

Over my head. I did replace the speakers with some 4ohm Rockford Fosgate 6” car speakers I had laying around. They bolted up perfect, just in case I get it figured out or someone can help me it will be ready to go. Sounds better with the RF speakers in it. Didn’t run it long, but didn’t get as hot as quickly.

Doesn’t look as good as the vintage speakers though, but nice looking.

You should see the inside of this thing. Not even dust, super clean inside.

Not a very good experience with my first vintage boombox. Thinking I should have waited for a Panny 5050.

I did learn some stuff though.

 

[Superduper]

The left woofer and tweeter are blown. DO NOT use them again. The right speaker was OK, but now since you decided to try them on the potential bad channel without first confirming actual problem and exposing them to the bad channel, it might also be bad. The amp is likely blown or damaged. I highly suspect that you have DC in the left channel. This is a huge no-no, DC will kill the speakers. Speakers are only intended to be used with AC. It can be measured with a multimeter on the DC scale. If you read anything other than tiny millivolts, any speaker connected to that channel will be killed.

You can keep trying new speakers, but you'll keep killing them. FIRST fix the problem before exposing speakers to that circuit.

At the minimum, I would change the amplifier chip, and the 2200uf output capacitor on that channel, and any other cap appurtenant to that channel. Make sure all the resistors are in spec too that are part of the output circuitry. There aren't that many. For best balance, if one cap is changed, the sister cap for the other channel should also be changed so the 2 channels will have balanced sound. Capacitor replacements, especially those in the audio signal path often imparts tonal changes that might be noticeable.

As for the replacement RF speakers, make sure that they aren't going to protrude further back than the stock ones. Replacement speakers like that often have bigger magnets and deeper cones. There isn't a lot of room behind the speakers and they could short out other circuitry if they contact something when you bolt it back together.