OK so this is the analoge multimeter I'm using it doesn't have diode check function so I'm using ohms reading this is what base to emitter reads
Sharp GF-575E Audio Distorting pick up failure
- Thread starter Christopher1389
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I was referring to the fusible resistor that you initially posted a photo of, where we confused where the reading was 0 or 1. Why are you using a diode check function and what do you mean base to emitter? Are you checking the transistor again?
Ok, the steps you need to follow is that you 1st need to determine the status of that fusible resistor. Because if it's open circuit, then you can't get live voltage readings on that transistor, which is what I indicated to you that we need. Not your diode checks that you are apparently performing in-circuit and for which I don't have confidence in. The live dynamic readings of the B/C/E leads is the best source to see what is going on there. But for us to get those readings, the supply (fusible resistor) needs to be passing current.
So...
#1, make sure that the fusible resistor is passing current. You can either take it out of circuit and measure it using the ohms function on the lowest scale.
#1A, you can also see if that resistor is passing current by testing it live, with meter set to DC volts, set to measure up to 25 on the scale, and with black probe at ground, probe both sides of the resistor leads with the red probe. Note voltage. If similar voltage is present on both sides, I think we can move forward with -->#5. However, I still recommend that you ascertain whether the resistor value has drifted from factory.
#2, if that resistor is open circuit, then it MUST be replaced before we can proceed any further.
#3, if that resistor has continuity and is reading other than the factory spec +/- 10% or so, it should be replaced regardless.
#4, however, if that resistor is reading lower than 20 ohms or so, we can still proceed with a live voltage check of the regulator transistor.
#5, to do that, place the meter in DC voltage function, scale to measure up to 25v, and with black probe at a suitable ground, use the read probe and read voltage from all 3 leads of that transistor. This will tell us if the regulator is working properly.
As for my other suggested test: testing for audio output at the boombox line-out jacks, have you tried that yet?
Tinman
Member (SA)
It looks like R660 is supposed to be a 15 ohm resistor so the 200 ohm setting should read it.
He remarked that it only reads something when he's on 2000k setting.
Is it a 15 ohm resistor?
OP might want to look up it's color band code online.
When referring to diode check function I was testing the transistor base to emitter.
And testing the resistor I was using ohms reading at 200 ohms setting but I get open circuit
And in post #28 I mentioned that the line-out jacks have the same static sound
Ken
Member (SA)
This is quite a long way to go to find an open resistor. As you have pointed out several times, sound amplification that starts normal and fades away sounds like an IC overheating problem. Without proper cleaning and re-insertion of that IC, including carefully dealing with the pin corrosion, those issues should be noted for revisiting.
Didn't someone once post a basic test sheet here that had columns for component, test, and results so that all test parameters and results could be charted and not written in standard English conversational style?
There is a right way and a wrong way to use a multimeter to check a transistor, depending on its type. I’m not sure testing this board while powered up is the best way forward quite yet, but you’re in charge here and it’s quite a shock seeing you with more patience than you used to have and I with less. I’m really, seriously, trying not to interject here and failing miserably. He said the sound fades away, and if he has signal at the line out jacks then correctly re installing that IC and replacing that resistor should do the trick minus a cap problem that it appears he has already eliminated. Now it’s just a matter of not blowing anything up and avoiding smoking the board while testing it, and I haven’t even glanced once at the schematic. Thats my contribution, be careful, don’t do that, because I have and it sucks. Like dead power supplies, a dead amp is a dead amp is a dead amp and if you have ever melted a circuit board in your life you know what I mean.
Ahhh, the glory days of replacing 100v original power supplies with 120v ones and completely re capping entire boomboxes…. NOT, lol.
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I’m actually losing my patience because there could be dozens of tests and checks which should all proceed quite quickly, yet we keep getting stuck on simple steps (as you say, checking a resistor) and if we can’t move forward or with confidence, then I will probably give up here since I can’t physically do the test myself and my instructions aren’t helpful. However the OP has just confirmed that he’s also getting static from the line-out jacks which suggests that the issue is before the power amplifier. Therefore this potential regulator issue is quite possibly related. The OP has only basic meters available to him and sometimes, checking live is the quickest way to diagnose. With a live circuit, I can tell in a manner of minutes (the time it takes to probe 3 leads) if that regulator circuit is working properly and or which component may have failed. For example, if one side of that fusible resistor is reading 15v and the other side is reading 0, there’s either a dead short to ground on one side, or more likely the resistor is toast O/C.
Ken
Member (SA)
Yes. That is exactly the correct path forward. My only concern was the OP level of expertise in troubleshooting a live circuit. I checked and found that while this DIP IC is available in Europe at 3 for $5, and in HK for $1, it is more conspicuously available on Ebay for $10 apiece.
I need to spend more time interacting on the site, I find research and sourcing relaxing and my patience comes back. I could upload some basic test equipment tutorials, they have profligated on the internet, but I don’t imagine the interest is there. it was, at one time, members were interested in becoming more proficient at the basic skills required for boombox repair. I’m not sure we have the membership now to accomplish the level of assistance that used to be available from our members, nor if it’s even needed any more. It is a credit to you that you seem to be shouldering the burden of that.
By the way, I checked out WikiBoombox last weekend. Congratulations, you were just getting that off the ground last I saw. It’s quite an accomplishment. I’ve already used it a few times as I scanned through Ebay for the first time in years.
Im just going to say it, never measure resistance with the diode test function on your meter. You are actually injecting voltage when you do that, and while 0.3 and 0.7 volts may not be much, you can’t measure resistance that way, at least with a Fluke as the meter resets to a voltage scale In diode test. Measure continuity, yes, resistance, no. It also gets you into the bad habit of checking signals with the wrong dial setting. That can be disastrous for your meter and having one recalibrated is not cheap, although I haven’t had to send one out for over 20 years.
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