Hi all, having a problem with the power amp board. After checking the caps the big one and a couple others are gone, the guy said the transformer went when he put 240v in the 110v side. With the readings I have for the transformer that seems fine but something is causing the power amp board to produce a burning smell and some smoke at one point and pop the caps. Is this symptomatic of a blown ha1392 power amp chip with all the other symptoms (buzzing speakers when powered on, tape is playing as I reattached the belt but no sound.
My First boombox project is a GF-9000
- Thread starter Obuffalo
- Start date
Have the same GF-535 japanese domestic version.After opened the body something happened to power switch button,the light is remain always on when switching button on and off.What might be the reason?used battery power.Any advice please.
I would check to see if the tape deck power is engaged.Boombox80sJapan said:
Dont bust up this box , put it aside and let it go for now trust me I've thrown vz 2000,s out the goddamn window so I know lol. These sharps can be a motherfuker at times.
The amps are definitely blown. There's no way those chips can handle double the the normal voltage, they usually operate at 15v-18v and because the amps are connected directly to the unregulated voltage rail, there's no protection except for the fusing. If the fuses didn't blow, then the amp will. The datasheets for the hitachi chip specifications states "absolute maximum voltage" = 20V. With a 240v source for a 110v normal input, you are getting 218% of normal voltage, which if we figure is 18V at transformer outputs with 110v, your boombox was then exposed to 39.27v on 240v input. That's double the absolute max rating of the chip. Also, any capacitors in the unregulated rail that is rated at under 39.27v will probably be blown too. You can see the voltage rating printed right on the side of each capacitor. I suspect most of the larger ones will be 16v to 25v caps whereas small ones might be 50v. As for the smoking smell... ok, stop it, seriously. Stop powering it up until you've replaced all the potentially blown components, you are simply doing more damage. When the caps fail, they can open or short. If open, then the cap doesn't work anymore but likely doesn't do further damage. If shorted internally, it results in a huge current draw which causes circuits to cook. If there are regulators involved, they are probably cooking everytime you power up. If the amps are blown, they could also be shorted internally, which will result in voltages present and at levels not normal for the components adjacent (circuit wise) to normally be exposed to.Obuffalo said:
thank you for the reply boomus i went through and replaced caps and replaced the amp chip ha1392 which did make a kind of difference to the type of sound but not to anything else. when i was trying the amp board it wasn't in situ so could only blow the up that board (curiosity was killing me as to where smell was coming from it was the amp chip not transformer). could this have affected further along the circuitry or is it more likely this is the power amp board. also how many amps are there because ive only seen the 1.
Superduper said:
Ok, like I said, when components fail, they can fail open, short-circuit, or partial failure. All can cause unwanted changes to the circuitry and which can also result in cascading failures. Because amp chips are intergrated circuits, internally, there are many connections and if shorted, then some external circuits that normally does not expect direct current might be exposed, and other circuits that are not expecting ground might be grounded, so yes, it is possible, likely even that other components might be affected. Furthermore, like I already said, regulators are another circuit that might be affected because the whole purpose of regulator is to drop high voltage to lower voltage but they are designed only to handle a certain amount of voltage maximum. Simply changing one or a few components and retrying is not a good technique, can result in good replaced components failing again. It is important that you review the circuit diagram and identify all circuitry exposed to the unregulated power rail, and test those circuit components before attempting to power up the boombox again. Also, circuit diagrams usually provide voltage test points. These test points tell you what voltages to expect at certain locations when the circuitry is operating normally. Wherever you see unexpected voltage differences, you should take extra care to investigate and find the cause of such discrepancies. Once you feel comfortable that all is within normal range, then you can power up and try again. Remember, even though the amp chip is on a separate board, that does not mean that the main board is not affected. Voltage regulators are situated all over the place. Additionally, there are fusible resistors too, they should all be changed.Obuffalo said:
The most vulnerable components to suffer damage are:
- Amp chip, and appurtenant components
- Capacitors that may have suffered overvoltage.
- Voltage Regulators
- Fusible resistors.
im so grateful for your response, im learning so much from the information along with the research and tutorials i see. its not as daunting as i first thought, just time consuming, good thing i intend to keep this otherwise its currently worth 1000s just in man hours. i will be testing every component individually so my reply may take some while, im still looking for the test points you mentioned in the diagrams from the service manual but will test when/if i find them.
ok first thing ive tested is the Ha1392 chip (after all diodes, capacitors and fuses on the amp board) just because its the first in the circuit but do these seem within a workable range or is something off.
pin number listed voltage / actual voltage
1] 0.6v / 1.0v
2] 0v / 386mv
3] 5.8v / 6.4v
4] 0.3v / 558mv
5] 0v / 033mv
6] 0.6v / 0.68v
7] 7.4v / 8.8v
8] 14.6v / 17.6v
9] / 0.3mv
10] / 17.6v
11] 14.6v / 17.1v
12] 7.6v / 9.4v
ok first thing ive tested is the Ha1392 chip (after all diodes, capacitors and fuses on the amp board) just because its the first in the circuit but do these seem within a workable range or is something off.
pin number listed voltage / actual voltage
1] 0.6v / 1.0v
2] 0v / 386mv
3] 5.8v / 6.4v
4] 0.3v / 558mv
5] 0v / 033mv
6] 0.6v / 0.68v
7] 7.4v / 8.8v
8] 14.6v / 17.6v
9] / 0.3mv
10] / 17.6v
11] 14.6v / 17.1v
12] 7.6v / 9.4v
Superduper said:
Pins 8 and 11 bothers me a bit as the readings are high, but I think that's because the specifications in the schematics are based on 15VDC input. You will get more meaningful readings if you power the board with 15V instead of near 18V since they will more closely match specs. On the other hand, are you sure you are getting 17.6 on pin 8? It should not match pin 10 perfectly, because that seems like it might be shorted. The proper voltage at pins 8 and 11 should be approximately .4v lower than supply. With HA1392 out of circuit, you should be measuring approximately 480kΩ between pins 10-11 and also between pins 10-8 (taken with positive probe on 10, and when tested with negative probe on 10 should read infinity). If testing in-circuit, capacitors might charge up which will skew readings and cause confusion while charging and if the capacitors are bad, you will never get a good reading. If the output capacitors are bad, you could be in trouble... the output capacitors are only 10v rated and could have been subjected to the entire 39v overvoltage condition. If they blow, they may take your speakers with it because your speakers may see full rail DC voltage if shorted. I suggest you change them out of an abundance of caution. Anyways, I suggest you retest to confirm that pin 8 is the same as supply. If yes, your chip may be damaged or defective. Hopefully, there was an error in your measurement. BTW, I am presuming you know to take resistance tests powered down, and not powered up.
Now, if it was me, I would save myself a lot of time at this point and simply confirm whether the amp is working properly or not by injecting an audio signal into pins 2 (L) and 5 (Rt) of HA1392 to see if you are getting audio to the speakers. You can inject directly into the connector CNP7 if it's easier for you. Pin 4 of connector is ground, pin 5 should power left channel and pin 3 should power right channel. Be careful if injecting an audio signal like this to ensure that it is attenuated. There is no volume control and the amplifier will amplify at full gain so it would be ideal if you used an audio signal generator with an adjustable attenuator and slowly bring signal level up. Chips like this usually have very high gain. IF you can confirm that your amp is working properly, then you can move on, but I honestly think you should look carefully at the regulators and the associated zener diodes and associated fusible resistors.
Now, if it was me, I would save myself a lot of time at this point and simply confirm whether the amp is working properly or not by injecting an audio signal into pins 2 (L) and 5 (Rt) of HA1392 to see if you are getting audio to the speakers. You can inject directly into the connector CNP7 if it's easier for you. Pin 4 of connector is ground, pin 5 should power left channel and pin 3 should power right channel. Be careful if injecting an audio signal like this to ensure that it is attenuated. There is no volume control and the amplifier will amplify at full gain so it would be ideal if you used an audio signal generator with an adjustable attenuator and slowly bring signal level up. Chips like this usually have very high gain. IF you can confirm that your amp is working properly, then you can move on, but I honestly think you should look carefully at the regulators and the associated zener diodes and associated fusible resistors.
sorry for the delay ive been reading up and learning over the past few weeks so i could understand what you actually said lol, ive ordered some parts and been waiting for them. i ordered a variable power supply to assist in my endeavours (not just for this). but i will add my results. so far the amp chip seems to be ok it has the same readings as the spares i have (off the board) so doesn't seem to be a short, im waiting for either a voltage dropper i ordered or my power supply to turn up so i can add the readings with an amended voltage going through it. and yes to the resistance test. i will do the sound test but because the readings match up with my spares i feel this will be ok but wont hurt to test. ill use my signal generator on it can use an oscilloscope instead of speakers to check the output.
Superduper said:
hi finally received the my parts so first time ive been able to sit down with the box, i tested the way you described and after soiling myself when the signal generator was switched on the speakers and amp chip seem to be working. i had evrything connected up except cnp7 and cnp8, i put the signal generator through pins 3,5 of cnp7 and a speaker on pins 5,4 of cnp8 and a very very loud signal came through. pins 8-11 have some continuity between them on the meter but this seems the same as on the new spares i have, new readings with the 15v input are ( in cuircuit but disconected from the rest of the components. what output capacitors do you mean, which number are they because all the caps on the amp board seem to be fine but i have a new meter coming and can recheck them again. i can look at the zener diodes as soon as you explain what and where they are. 
1 - 0.71v
2 - 0.05v
3 - 5.84v
4 - 0.48v
5 - 0
6 - 0.65v
7 - 7.80v
8 - 14.89v
9 - 0
10 - 15.30v
11 - 14.88v
12 - 7.9v
1 - 0.71v
2 - 0.05v
3 - 5.84v
4 - 0.48v
5 - 0
6 - 0.65v
7 - 7.80v
8 - 14.89v
9 - 0
10 - 15.30v
11 - 14.88v
12 - 7.9v
Ok, it's 2, 3 months later. I've no idea what I'm looking at or what advice I already gave, and I'm loathe to spend the time and research to do it all over and reacquaint myself once again with the whole thing. And why are you checking continuity between 8-11, did I ask you to do that previously, and how did you do that, details matter, they really do. And I can already see that the measurements you gave this time around are different than last time, which is that your pins 8 and 11 voltages are about 1/2v lower than supply (pin 10) which is what I mentioned to you last time was suspect since you gave the same readings for pins 10 and 8. And did you remove and reinstall the amp chip? Or else how are you comparing the old chip and the new spares? One in circuit and the other out of circuit? And if you are getting audio out, then that generally indicates that the amp chip is fine, but that is a dual channel amp and you are only checking one channel. Are you just going to presume that the other channel is good because one is good? Sometimes, amps act up after they've had a chance to heat up, so keep that in mind too. Oh and if you don't know what a zener diode is, then you are going to have to educate yourself on that. There is no way that I'm going to be able to give you an education on every single component in your boombox, nor am I going to study, research and tell you where each and every component is located. You are going to have to do some work. The zener diodes are different from regular diodes because they restrict reverse current (like any regular diode) up until the zener voltage, in which case, then it breaks down and allows current to flow. That's why when you check the voltages (based on schematic specs), it will tell you alot and this characteristic is also why they are frequently used in voltage regulators. On the schematic, it looks like a regular diode except instead of a straight bar perpendicular to the arrow, it has a squigly one. As for output caps, they are exactly what I described them, which is that they are directly in series with the speaker outputs. In general, they are typically 1000uf or thereabouts. Large ones. Oh and did you check for DC at the outputs? The outputs are the 2 leads from the amp that are connected to the speakers (outputs). If you put a DC voltmeter across those leads when they are operating, they should not have any significant DC. If they do, you are likely going to toast any speakers connected to it. Do not measure AC, because it will show a reading. I'm concerned only with DC. They should read low mv if anything. If more than 50mv, I would replace the amp module and any associated electrolytic capacitors in amp circuit.
Now with all that being said, I will just give you some general troubleshooting tips. IF you conclude that the amplifier portion is working fine, then you need to work backwards using the schematic as a guide. Map the signal path from (i.e. aux input jack) to the amplifier input (pins 3/5 of connector 7). Work your way backwards probing point by point until the signal breaks in which case, you have likely found the problem circuit source. Keep in mind that it is normal for volume to change, possibly increasing as you work your way backwards, especially before/after a transistor which may be amplifying the signal.
I will also tell you that if the amp is working while disconnected to everything else, but you smell smoke or burning when it is connected back up again, something is definitely not connected right. Or is your speaker outputs shorted? Check the speaker harness too to make sure it is not shorted. The service manual has a wiring diagram and it is quite tedious to check each and every wire hookup, but that's something you will have to do if it was messed with before and can potentially be mis-wired.
Now with all that being said, I will just give you some general troubleshooting tips. IF you conclude that the amplifier portion is working fine, then you need to work backwards using the schematic as a guide. Map the signal path from (i.e. aux input jack) to the amplifier input (pins 3/5 of connector 7). Work your way backwards probing point by point until the signal breaks in which case, you have likely found the problem circuit source. Keep in mind that it is normal for volume to change, possibly increasing as you work your way backwards, especially before/after a transistor which may be amplifying the signal.
I will also tell you that if the amp is working while disconnected to everything else, but you smell smoke or burning when it is connected back up again, something is definitely not connected right. Or is your speaker outputs shorted? Check the speaker harness too to make sure it is not shorted. The service manual has a wiring diagram and it is quite tedious to check each and every wire hookup, but that's something you will have to do if it was messed with before and can potentially be mis-wired.
Just want to say thanks and I appreciate your help even if I don't get it working I've learnt lots, Ok before I go through what you have said and respond I have a query about the c545 capacitor, while it's on the amp board it's not going infinate on the tester it's stopping at 1348 and 643 but when it's off it's reading correctly, I thought it had blown so removed it to replace then found it ok all other resistors and capacitors on the board test ok. Also I mentioned before that the tape is playing FF RR ect(no sound). The led for the power indicator does not light up when powered on (works fine when tested on the meter). when I power on the box I get the quiet thud sound through the speakers like I do on my hifi, I get a small amount of feedback through the speakers when I touch the volume controls but not on any others. I will study the schematics because I'm trying to follow the power to see why it's going to the tape motor but not to the power indicator led when the button itself is working as it should. If I have to test every component I will, I presume the next place to look at is the main board after the suggestions above. The readings are different as I had to put a voltage dropper in-between the transformer and amp board to regulate it to 15v as it was nearer 18v before. I tested the speakers with the signal tester through the connector that plugs into cnp8 just incase they'd been fried and the signal was fine and came through there correct speakers.
Ok, when you test a capacitor, it may not test like you expect when it's in-circuit. The reason is that when it is in-circuit, you will be testing the capacitor and everything else connected to it too. Electricity doesn't take (only) the path through the cap simply because you are looking at it and put probes on it. It will go everywhere it is able to, and if it is in circuit, that means it will take whatever path the components in the circuit will allow. When I test a cap in circuit, I'm merely looking to see if it is shorted and verify if the meter counts up. The speed at which it counts will depend on it's size and experience really helps here to know what is normal. Other than that, it's merely a quick check and does not really determine whether it is within spec, rather I would be looking to see if it might be the potential cause of an acute issue and a shorted cap definitely will cause that.
I mention about checking speakers because I was under the impression that the speakers had been blown before, or maybe I am confused about a different thread/boombox.
As for different measurements, you have completely misunderstood. If you look back, I mentioned to you that pins 8 and 11 should NOT have the exact same measurements as pin 10, which is the power pin. I specifically mentioned that pins 8/11 should read ~1/2v lower than pin 10 but pin 8 was reading the same voltage (in your previous reading). In your more current reading, it was guess what, 1/2v lower. So my comment is not that the voltages are different. Rather it was an observation that pin 8 should read 1/2v lower but did not previously, but currently does.
I need you to provide the voltage measurements (running) of Q501 area. This will tell if that ripple filter is blown and which may prevent the pre-amp circuitry from functioning. I really wish you had mentioned that the power led was not functioning in the beginning. The first thing I do when I evaluate a boombox that isn't working properly is to look at all the symptoms. All. If you look at the schematic, you will see that there are a very large number of sub-circuits. Most higher end electronics are fairly complicated. Rather than checking every single circuit, evaluating the boombox and looking at all the symptoms usually gives clues of where the problem area may be so we can narrow down the potential areas to troubleshoot.
I mention about checking speakers because I was under the impression that the speakers had been blown before, or maybe I am confused about a different thread/boombox.
As for different measurements, you have completely misunderstood. If you look back, I mentioned to you that pins 8 and 11 should NOT have the exact same measurements as pin 10, which is the power pin. I specifically mentioned that pins 8/11 should read ~1/2v lower than pin 10 but pin 8 was reading the same voltage (in your previous reading). In your more current reading, it was guess what, 1/2v lower. So my comment is not that the voltages are different. Rather it was an observation that pin 8 should read 1/2v lower but did not previously, but currently does.
I need you to provide the voltage measurements (running) of Q501 area. This will tell if that ripple filter is blown and which may prevent the pre-amp circuitry from functioning. I really wish you had mentioned that the power led was not functioning in the beginning. The first thing I do when I evaluate a boombox that isn't working properly is to look at all the symptoms. All. If you look at the schematic, you will see that there are a very large number of sub-circuits. Most higher end electronics are fairly complicated. Rather than checking every single circuit, evaluating the boombox and looking at all the symptoms usually gives clues of where the problem area may be so we can narrow down the potential areas to troubleshoot.
i honestly cant believe i hadn't mentioned the led with the symptoms i had to go back through just to check, the running voltages of q501 are 0.74 0.78 0.37 so nowhere near the 14 -13 sugjested . i only noticed the led issue to be fair when i put them in the slots as they wernt when i got it, when i got it, it had been "worked on" and whilst stripping and rebuilding it i realised the leds power didnt derive from the power board. my bad im a dumbass
You didn't mention which voltage measurements go to which lead (B/C/E, which corresponds to base, collector, emitter) of the transistor. Oftentimes, it matters, and I usually don't have much patience for guessing. Remember, details matter. In this case, I can already tell that R513 is probably blown. Check and verify that it is blown, replace it with a new fusible resistor if possible, then recheck. Hopefully that brings the preamp back up as well as your power LED. If not, repost new voltage measurements after replacement. Also, check R510 while you are at it too.
Q502/D502 is another voltage regulator (8.3V) that needs to be looked at but right now, you won't be able to test it since R513 is blown and chances are, it was probably saved by the R513 fusible resistor blowing. I suggest that you do not hook up the speaker output connector yet until you can verify all voltages are back to spec first (after you replace those fusibles). If you can not find fusible resistors in that value, then use standard metal film resistors and be sure to stand them off from the board. In other words, don't mount them flat against the PCB. Instead, stand them off the board about 1/4". That way if they overheat, they won't scorch the board. Finally, if you find fusibles but they are larger than the old ones, (for example, 1/2w instead of 1/4w, or whatever is called for), DO NOT use them, they will not blow when they need to and will subject other components to the risk of failure. After voltages are back to spec, you can reconnect speakers and hopefully things are back to normal, normal being pre-blowed up normal.
hi thanks for that, i found fusible 4.7ohm 1/4w ones nut couldn't find the 12ohm 1/4w ones in fusible not that seemed worth the elevated price, so i brought the ones you described. the original ones were elevated and covered with what i presume is a flame retardant material so i will use the to elevate and encapsulate the new ones. i hope so, i only paid £40 and a 200 mile round trip off Facebook marketplace for the box as it was sold blown so getting it working would be a dream as i couldn't afford to buy a working one. the bits will be here by wednesday so ill update you then.
You can probably get 10Ω flameproof resistors which will likely work fine, and a better choice than 12Ω regular resistors or even use a pair of 2+10 ohm in series, but unless you plan to plug the unit into the wrong AC mains again, should be fine. However, I'm not sure what you mean when you say encapsulate. If you mean the leads, yes. If you mean the body, NO. Putting shrink wrap, or any other coating besides ceramic will trap the heat more and probably turn a resistor that normally would not flame up into a stinky smoky fireball. Heatshrink tubing or any other plastic tube is almost certain to be combustible. Just stand the resistor body off from the PCB and you should be fine.