Electronic troubleshooting

[RPH]

Number drills are always required in the shop. You can find a piece of steel wire you may be able to clean it up.

 

[Max]

I think we need either a new scriptwriter or a new main character.

The transistor socket leads are too big for the board holes. I was really looking forward to trying them.

I had 3 different NPNs to try out, and none of them worked. 2 PNPs but i haven't tried them since we know the original is a NPN.

I will say my soldering/desoldering skill has vastly improved.

This show may be headed for the chopping block.

I'm going to do another visual once over of all the wires and components.

I tested all of the transistors before I sent them to you, so we know that they are good. And now that you have the new soldering station it seems like you can solder in or remove parts without damage, so the sockets are a nice to have and not a must.

From prior conversations I think that we're pretty confident that we have the base identified, but not as sure on the collector and emitter. So try swapping the collector and emitter on a part - the 2n3904 would be a good one to start with - and see if that changes anything. If it's still DOA at that point, then please post current front and back pictures of the PC board.

I just threw in the PNPs for grins, from the tests you did before the old transistor is a NPN so I would not try the PNPs now.

 

[Snapped-off]

I tested all of the transistors before I sent them to you, so we know that they are good. And now that you have the new soldering station it seems like you can solder in or remove parts without damage, so the sockets are a nice to have and not a must.

From prior conversations I think that we're pretty confident that we have the base identified, but not as sure on the collector and emitter. So try swapping the collector and emitter on a part - the 2n3904 would be a good one to start with - and see if that changes anything. If it's still DOA at that point, then please post current front and back pictures of the PC board.

I just threw in the PNPs for grins, from the tests you did before the old transistor is a NPN so I would not try the PNPs now.

I'll try turning it around later tonight. If that doesn't work I'm at a loss

 

[Max]

If it doesn't work please take the pics and I'll look for shorts/odd things. I'll also finally draw up a schematic and see if anything becomes obvious from that.

 

[Snapped-off]

If it doesn't work please take the pics and I'll look for shorts/odd things. I'll also finally draw up a schematic and see if anything becomes obvious from that.

No problem. The worst part is it was functioning prior to the base lead on the old transistor breaking. So hopefully I just have the leads backwards.

 

[RPH]

While you have the iron hot. Go around checking other devices and joints for defects. You don’t have to cook but a reflow of the solder could catch a poor joint.
Don’t give up! Frustrating, I know but persistence pays off in board repairs. It may be more than just the transistor but still fixable.

 

[Snapped-off]

So removing the transistor this time, the sucker picked up all the pads off the board. I guess I'm getting good at sucking up the solder.

Is that the end?

 

[Max]

So removing the transistor this time, the sucker picked up all the pads off the board. I guess I'm getting good at sucking up the solder.

Is that the end?

Nope. It’s a single sided board so you can just solder to the still ok part of the traces.

 

[Snapped-off]

Nope. It’s a single sided board so you can just solder to the still ok part of the traces.

I scraped off some of the coating next to the hole and got some shiny stuff looking back at me.

I'll get pictures in a few.

 

[Max]

I scraped off some of the coating next to the hole and got some shiny stuff looking back at me.

I'll get pictures in a few.

Perfect! That is what you need. Makes sure to ‘tin” it first with solder so you know you can join to it ok. And FWIW, you may be able to use the socket now as well if the leads are long enough.

 

[Snapped-off]

I mirrored one of the photos so it should be easier to look at.

 

[Snapped-off]

It's not the prettiest but it is attached solidly. I'll remove it if I can get through this testing and get it working. Baby steps... definitely don't want to do electronic repair when I grow up.

 

[FordTruckWench]

No problem. The worst part is it was functioning prior to the base lead on the old transistor breaking. So hopefully I just have the leads backwards.

Transistor pinout is not standardized! It could be that the circuit originally used a transistor with an uncommon arrangement. Thus most replacements might not match the outline shown on the board.

I think we have E and C on the board figured out. The diode in the base circuit really limits the possibilities.

What I find puzzling is that, because of the diode, we should see around 0.7 volts DC on the base trace, even with the transistor removed, when the toy is trying to make sound. An AC measurement should be tiny, but non-zero.

 

[Snapped-off]

Transistor pinout is not standardized! It could be that the circuit originally used a transistor with an uncommon arrangement. Thus most replacements might not match the outline shown on the board.

I think we have E and C on the board figured out. The diode in the base circuit really limits the possibilities.

What I find puzzling is that, because of the diode, we should see around 0.7 volts DC on the base trace, even with the transistor removed, when the toy is trying to make sound. An AC measurement should be tiny, but non-zero.

Well based on testing the original transistor, the base is for sure in the middle lead. After getting the socket on the board, I tried them both ways, CE and EC with no results. Also got to test the the S8050 both directions.

Could all this screwing around have killed the chip?

 

[Max]

Well based on testing the original transistor, the base is for sure in the middle lead. After getting the socket on the board, I tried them both ways, CE and EC with no results. Also got to test the the S8050 both directions.

Could all this screwing around have killed the chip?

It’s possible. But we did measure AC on the base earlier so I would not give up yet. Let me draw it up and think a bit…

In the mean time, could you measure the DC voltage at the E,B,C referenced to battery minus?

 

[Max]

OK, I've got a lot of questions:
1. C8, C9, R2 were never there? Or were they removed?
2. I can see the red/blk wires from the battery BT1. But there is another pair of red/blk wires near there. Do they go to a/the battery also?
2.1. If the other red/blk wires are to a battery, is it part of BT1 or a separate battery? How many cells in each?
3. R6 appears to be yellow-violet-gold-gold. Is this correct? If so it's 4.7 ohms.
4. R5 looks like yellow-violet-??. What are the other colors?
5. If the other red/blk wires are to a battery, I am not sure about their solder joints. One or two joints look bad, and once is may shorted. Can you check/re-do them?

Unless I am missing it, I don't see that the diode is connected to the transistor at all...

 

[Snapped-off]

OK, I've got a lot of questions:
1. C8, C9, R2 were never there? Or were they removed?
2. I can see the red/blk wires from the battery BT1. But there is another pair of red/blk wires near there. Do they go to a/the battery also?
2.1. If the other red/blk wires are to a battery, is it part of BT1 or a separate battery? How many cells in each?
3. R6 appears to be yellow-violet-gold-gold. Is this correct? If so it's 4.7 ohms.
4. R5 looks like yellow-violet-??. What are the other colors?
5. If the other red/blk wires are to a battery, I am not sure about their solder joints. One or two joints look bad, and once is may shorted. Can you check/re-do them?

Unless I am missing it, I don't see that the diode is connected to the transistor at all...

I'll get answers when I'm home later. Some spots were blank. I haven't removed anything but the transistor.

 

[Snapped-off]

Red and black from the battery tray go to the board, and red, black, and green are to the board/power switch. The rest of the wires go to switches. The grey rubber standoffs are also switches. I'll get a picture of the entire toy as well, it may help visualize everything better.

Editing: the 2 black above the chip are to the speaker

 

[Snapped-off]

OK, I've got a lot of questions:
1. C8, C9, R2 were never there? Or were they removed?
2. I can see the red/blk wires from the battery BT1. But there is another pair of red/blk wires near there. Do they go to a/the battery also?
2.1. If the other red/blk wires are to a battery, is it part of BT1 or a separate battery? How many cells in each?
3. R6 appears to be yellow-violet-gold-gold. Is this correct? If so it's 4.7 ohms.
4. R5 looks like yellow-violet-??. What are the other colors?
5. If the other red/blk wires are to a battery, I am not sure about their solder joints. One or two joints look bad, and once is may shorted. Can you check/re-do them?

Unless I am missing it, I don't see that the diode is connected to the transistor at all...

From battery minus:
E 0
B 5 mVDC
C 4.58 VDC

C8 is present and accounted for. It's next to the diode. C9 and R2 are absent.

R6 is Y V Go Go

R5 Y V Y Go

As far as the battery/power switch wires, there is no visible short. R1 and the green are touching but they're on the same line of the circuit

 

[Snapped-off]

Underside of the track. The grey rectangles are where the switches are. (Identical to the red one you can see at the bottom. That one closes the switches on the board(grey rubber standoffs))



[Removed picture due to error]

Red dotted line is the diode.

Bottom left yellow is R1 and the respective wires for the power switch.

 

[Max]

That answers one thing that I wondered about - whether the two switches were mechanically coupled. They had the same number on the PCB so I wondered. Are the switches momentary, or do they ‘click’ each time you press one? If the latter they may have gotten out of synch with all of the handling…

In looking at the circuit board I really need to understand which traces are power and ground. How does the power switch work? It looks like 3 positions, so on, off, and ? Or is it just two positions? Since there are two black wires, where exactly does the center black wire go?

 

[Snapped-off]

The track switches are momentary. Press one and music would play for 5-8 seconds. Different melody from each switch.

The furthest left are directly from the batteries. The other 3 go to the power switch.

 

[nadogail]

I would like to help because I am a retired Chief Electronics Technician, ET A School at Treasure Island was 1961 & 1962. They took my ET tools away from me in 1970 when I made Chief. We were taught with vacuum tubes and transistors were just coming into the Fleet so they added a week to our A School to teach us Transistors and Magnetic Amplifiers. Most of my career was managing ET’s and teaching them stuff that was not included in the Navy schools. I had a lot of job assignments that were not electronics related.
Reflowing the PWB with a eutectic alloy may well be your best fix. You will not find eutectic solder at Home Depot, go to an electronics store that serves HAM Radio Operators that build their own equipment. Computer repair technicians also use it because of it’s low melting point.
GOOD LUCK, that branch (Component Level Repair) is a specialized skill.

 

[FordTruckWench]

Unless I am missing it, I don't see that the diode is connected to the transistor at all...

I believe it is in series with the 220 ohm resistor. One end of the stack is grounded. The other goes to transistor base.

In looking at the circuit board I really need to understand which traces are power and ground. How does the power switch work? It looks like 3 positions, so on, off, and ? Or is it just two positions? Since there are two black wires, where exactly does the center black wire go?

What I see in the pictures: The switch is SPDT. When "on", it connects battery + to the circuit positive power rail. When "off", it grounds the positive rail via the 10 ohm resistor.

I have a hunch - which needs the OP to take a voltage measurement (w/ power on, and meter measuring relative to battery negative): What voltage is on the end of the yellow-violet-yellow resistor closest to C8? What voltage is there when the toy should be playing music? (Note, the opposite end of this resistor goes to the positive power rail and should always show full battery voltage.) I ask because this resistor only goes to the COB. I have a feeling that transistor base current comes from this resistor...

 

[Snapped-off]

I made a doosy in the last picture showing the diode for what it's worth. Correct path is this dotted line.


@FordTruckWench voltage on the left side of the resistor is 4.58 and the right side is 0.58, and 0.68 when pressing a switch. Checked it several times. Let go of the switch and voltage drops back down to 0.58 VDC.

 

[dogdog]

Hope you know that transistor is not acting as a switch, but it is a single transistor audio amplifier. Not that I remember anything.

 

[Snapped-off]

Hope you know that transistor is not acting as a switch, but it is a single transistor audio amplifier. Not that I remember anything.

Beats me. I didn't understand what a transistor was prior to this thread. I did read they are used as both solid state switches and audio amplifiers.

I've been wondering if I can solder some leads onto the original transistor since it was working prior to the base lead breaking.

 

[nadogail]

Beats me. I didn't understand what a transistor was prior to this thread. I did read they are used as both solid state switches and audio amplifiers.

I've been wondering if I can solder some leads onto the original transistor since it was working prior to the base lead breaking.

Because of the fragility of the internal connections in a transistor and their relatively low prices I recommend you get a new one and replace the component with the broken lead.

 

[nadogail]

Because of the fragility of the internal connections in a transistor and their relatively low prices I recommend you get a new one and replace the component with the broken lead.

 

[Snapped-off]

Because of the fragility of the internal connections in a transistor and their relatively low prices I recommend you get a new one and replace the component with the broken lead.

The problem is the original type is unknown. The numbers on the transistor can't be read. The closest guess was a S8050 NPN, however it did not work. (I ordered a new one)

What is known about the original, it is a NPN with the base lead in the middle.

 

[Snapped-off]

I'm just more so curious if I could hack the original transistor back together if this toy will work again. Then I can get back to the hunt for the correct transistor to replace it with.

The original still tests good, but all 3 leads are broken off.

 

[jbfsr]

Do a search for single transistor audio amplifier. You may find a number of a transistor that may work.

 

[Snapped-off]

Do a search for single transistor audio amplifier. You may find a number of a transistor that may work.

Now that I've got the transistor socket I can test them easily. I just don't know what specs I need. I'm surprised it doesn't work with any of the transistors I've tried.

 

[Max]

I gave Snapped-off 3 different general purpose NPNs to try. One was a 2N3904 and he’ll have to tell you the others as I did not write them down. The ‘3904 us a very common and high use part - maybe the only other higher use part is the 2N2222.

If you can solder to the old part, I’d give it a go. You are so close to the die that you may kill it when you solder it, but you’ve really got nothing to use.

 

[Snapped-off]

One question I have is wouldn't something at least happen with any of these other transistors?

Sure it'd either over or underpower the speaker.. but wouldn't something happen? I'm just thinking along the lines of changing out an amplifier in an audio system. So it's probably not even remotely the same thing.

The best I can make out of the original is Sx050 - x being unknown.

 

[RPH]

Try to solder the legs on. Good practice for neural surgery. It’s dead as it sits so hard to kill it anymore. It may just Frankenstein it to work.

 

[Snapped-off]

Try to solder the legs on. Good practice for neural surgery. It’s dead as it sits so hard to kill it anymore. It may just Frankenstein it to work.

I tried. Couldn't get the solder to stick to the transistor. Maybe if the leads weren't broken off flush.

 

[FordTruckWench]

@FordTruckWench voltage on the left side of the resistor is 4.58 and the right side is 0.58, and 0.68 when pressing a switch. Checked it several times. Let go of the switch and voltage drops back down to 0.58 VDC.

Yeah! It's alive! Alive!

Um, er, but not what I expected.

What I expected: The COB has a 1 bit D-to-A converter, i.e. a digital pulse stream that when averaged (filtered) is the desired analog value. This pulse stream is fed to the Gate of a FET (Field Effect Transistor - which is a totally different type of transistor). This transistor's Drain is connected via a resistor to V+. The Source goes to the Base of our external transistor. In simple terms, the FET is an on/off switch that (when on) allows a little bit of current to flow into the Base circuit. And the external diode bleeds it away.

Q: You say it drops back to 0.58. Does this happen right away, or a few seconds later?

Q: Are you sure you probed the right side of the Yellow-Violet-Yellow resistor? Not the Yellow-Violet-Gold. Not C8. Not the Red-Red-Brown resistor. Not the "vertical" stack of three solder points.

If you actually saw 0.58 to 0.68 volts on the base of the external transistor, that could be very good news. That means the device is trying to turn in on with the 0.68 volts.

Thinking out loud (this is directed more to Max): What if the transistor is a Darlington?

 

[Snapped-off]

Q: You say it drops back to 0.58. Does this happen right away, or a few seconds later?

Q: Are you sure you probed the right side of the Yellow-Violet-Yellow resistor? Not the Yellow-Violet-Gold. Not C8. Not the Red-Red-Brown resistor. Not the "vertical" stack of three solder points.

If you actually saw 0.58 to 0.68 volts on the base of the external transistor, that could be very good news. That means the device is trying to turn in on with the 0.68 volts.

Thinking out loud (this is directed more to Max): What if the transistor is a Darlington?

I'd say the voltage change is instant. Definitely less than 1 second.

Left gave 4.5 and right gave 0.58/0.68.

 

[FordTruckWench]

I'd say the voltage change is instant. Definitely less than 1 second.

Left gave 4.5 and right gave 0.58/0.68.

Oh well, this was probably a red herring then. The voltage change might be explained by changes in load in parts of the chip.