Amplifier blew fuse - big time

Discussion in 'General Electronics Chat' started by Tonyr1084, Feb 4, 2016.

  1. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    I have a RadioShack amplifier. Every time I switch it on it makes a noise like it's drawing very heavy current. I'd spell the sound it makes but I don't know how to spell BOUFFFF. It used to come on despite the big scary noise. The last time I turned it on it did that in a very strong way and never started up. The fuse is blown. Not just burned out with a ball on either end of the filament, no, the fusible element is completely turned into metal balls. I don't know if the transformer is bad or not. No obvious signs of problems, no burns, no trauma, no physical damage. Transformers are not my forte.

    I have the cover off and the drawing depicts the circuit from the plug to the transformer. There are no electronics there except for whatever is under the mystery switch box. It's a circuit board covered with a heat shrinkable tube. I CAN cut it off, but before I start hacking away I'd like to get some information if possible.

    I'm imagining there are probably some capacitors in there. I don't know if any of them can be responsible for the fuse blowout or not. And I'm assuming it's a simple locking push button. There can't be much more to it than that.

    I probably will cut the shrink tubing off and inspect it, and I will post a diagram of the switch.

    Thanks everyone for the help on this.

    [​IMG]
     
    Last edited by a moderator: Feb 4, 2016
  2. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    Here's a diagram of the switch (without the shrink sleeve)

    Keep in mind I am not sure what the blue thing is - I THINK it's a thermistor, one of those thermal resisters that change their resistance with the start-up current heat. On the board it's labeled as C150, so that kind of confuses me too. Is it a resistor or a capacitor? I've never seen a cap with those kinds of markings. AND what's on the device looks like the schematic representation of a resistor. So ? ? ?

    [​IMG]
     
    Last edited by a moderator: Feb 4, 2016
  3. wayneh

    Expert

    Sep 9, 2010
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    Can't see your attachments. The power supply board is covered with heat shrink? That's odd.
     
  4. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    It's only the switch that is covered.

    There are some pretty big drawings in my blogs. Did they not show up?

    Let me see if I can repost them:
     
  5. MrSoftware

    Active Member

    Oct 29, 2013
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    Nothing has shown up yet. Your best bet is to upload them as attachments and embed them in your posts that way.
     
  6. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    Did these files attach?
     
  7. apqo1

    Well-Known Member

    Oct 5, 2008
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    Your diagrams are visible to me. Not sure why wayneh isn't seeing them...

    Do you have a service manual or schematic for the amplifier?

    The transformer is the obvious first thing to suspect, but that would be a petty rare failure, given that they're entirely passive devices and are mechanically robust. Using a DMM in continuity check mode, verify that there's continuity through each winding from end to end, and no continuity between separate windings or from each winding to the transformer's frame. You may need to scrape lacquer to make contact with the frame.

    There doesn't appear to be any inrush current limiting in the design, but if it's a relatively small E-I frame transformer, as pictured in your diagram, that's not unusual. Has the amp always made the "BOUFFFF" noise when turned on (since new) or did that begin after some years in service?

    The blue disc device attached to the switch board looks like a Metal Oxide Varistor (MOV), if your drawing is accurate. If it were a thermistor, it would be shorting the switch and you could never turn the amp off. In any case it's oddly placed in the circuit; a MOV is usually found across Hot and Neutral, and/or Hot and Ground, not across Hot and Switched Hot.

    The MOV (if indeed that's what it is) is there to absorb large voltage transients on the mains input. Their internal structure breaks down a little each time they do their job, so they eventually lose effectiveness, but I don't know what their typical failure mode looks like.

    Even if the MOV has failed to a dead short, it wouldn't explain the blown fuse; it would just turn the amp on continuously. Grab your DMM again and check the MOV for continuity (with the switch off and the amp unplugged!). If it's shorted, it will need replacement as part of the overall repair.

    My next step would be to continuity-check the main board where the transformer secondaries are connected. If, for example, a bridge rectifier has failed, you might find a dead short there, which would explain the blown fuse. Check continuity from each secondary connection point to the amp's chassis ground point as well.

    Others may have more ideas, but this should get you started by eliminating the low hanging fruit.
     
    Last edited: Feb 4, 2016
  8. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    Thanks for the pointers. I believe the drawing is accurate, I've been an electronics inspector for over 30 years and am thoroughly familiar with schematics and blueprints. I have to run out for a few hours but when I get back I'll check the bridges for shorts. And no, I don't have service manuals or schematics. I got it when Radio Shack was closing stores. I got it for $45.00 when its normal price was close to $500. Not out a lot if I can't get it working again, but I'd like to.

    I've owned the amp for a few months. And the noise hasn't always been present, at least not that I've noticed. However, over the past half dozen or so times I switched it on it gave that big jump start sound, like it was trying to power up a 747 jet airliner. A very hard start. And yes, it has been getting worse with every start up. As for the MOV? It doesn't make sense to me the way it's in the circuit but that's the way it's in the circuit. I'll post a picture of it later today.

    I'm a novice electronics home hobbiest and have built many digital circuits and have fixed numerous electronic equipment. I have the shocks to prove it too.

    I'll be back in a few hours. Thanks again all.
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It might also be a bad capacitor on the secondary side. That's the only thing I can think of that might get worse with age.
     
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  10. wayneh

    Expert

    Sep 9, 2010
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    I can see your drawings now. One thing you can check easily is whether the primary is shorted or not. It should measure less than 10Ω I think, but not <1Ω.

    A bad bridge rectifier is a possibility and is also easily checked. Beyond that, you're looking at bad capacitors or even more complex problems. Without a schematic, it will become very difficult. Hopefully you'll find the culprit before too long.
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    In addition to what @wayneh said, shorted output transistors can cause those symptoms also.

    What is the model number of the amplifier?
     
  12. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    Les: it's a Radio Shack 250 watt stereo amp; The "Catalog number" is 3200028. I don't see anything labeled "Model" Just a UPC label, probably a serial number.

    Here's what I've accomplished so far: I've disconnected the transformer from the circuit board (not the bridge rectifier). Put a new fuse in and BOUFFFF (again). Removed the BR and bench checked it for continuity. Sure enough, the positive output is shorted to one leg of the the AC input. Replaced the fuse and retested it. It was a nice quiet start up - but the PCB is still disconnected. I checked the voltages (AC) of the secondaries and they are exactly where they're supposed to be.

    Looking at the PCB, there's an 8 amp fuse that also blew pretty hard. So I'm wondering if maybe something on the board pulled the BR down. Maybe one of those capacitors that have been mentioned before? I don't want to replace the rectifier only to blow out a new one. The PCB fuse appears to be directly adjacent the positive input from the BR, so I doubt it was purely a BR failure; I'm thinking something else pulled it down.

    I'm hoping I don't have to pull the board out of the chassis, there's about a million connectors that will have to be labeled. OK, there's actually 19 two, three or four pin connectors and 8 bayonet connectors. The PCB is labeled J1, J2 and so on. The bayonet's are G1, G2 and so on, AC1, AC2 and so on and where the BR is plugged in - it's labeled V+ and V-.

    So what's next to be checked? If it's a capacitor, how do I test that? I DO have a meter that can check capacitance somewhere in my realm of tools, somewhere in the tool forest.

    Quick EDIT: Do I have to pull the capacitors out of the board to test them? There's two really large 10,000 µf 80v electrolytic's. The BR is powering those caps directly through the fuse (that blew on the PCB).
     
    Last edited: Feb 4, 2016
  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    Go ahead and test for shorts "in circuit". If something reads shorted or very low resistance, then remove it from the circuit and test again. Don't worry about capacitance. Low capacitance doesn't blow fuses.

    Do any of the caps look bulged out?
     
  14. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    The caps appear normal.

    On board, there are two 8 amp fuses. One on the negative leg and one on the positive leg. It's also the positive on the BR that is shorted. Since the fuse on the PCB (pos leg) blew, I'm betting that something drew massive current (greater than 8 amps) (Likely a lot more than 8 amps because that fuse was also obliterated).

    As for very low resistance readings, how low is too low (close to short - or shorted - which is 0Ω)?
     
  15. Lestraveled

    Well-Known Member

    May 19, 2014
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    Depending on your meter, caps will start low and then increase resistance. If a cap reads 10 ohms and doesn't change, it is probably bad.

    I say check your output transistors for shorts.
     
  16. wayneh

    Expert

    Sep 9, 2010
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    Another trick is to put a lighbulb in series with the board's power. Even if the amp is shorted, it can't draw more than the wattage of the lightbulb. This will prevent your fuses or BR from blowing again during a quick power-up test.
     
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  17. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    After a wonderful Super Bowl Game (and grand children all over the place) I'm back to the problem at hand.

    wayneh, I actually like the idea of the lightbulb. I'm considering that in the place of the fuse.

    So far all I found was the shorted BR. The capacitors seem fine, the output transistors also seem fine. I'm not sure why the BR went, but maybe that's all it was, just the BR going bad. CORRECT ME IF I'M WRONG: if the BR shorts pos to AC, can the back rush from a charged capacitor blow the 8 amp fuse? (the 8 amp is on the board - the original 7 amp fuse is the main power in before the transformer) It was the 8 amp fuse on the positive line.

    Shack Amp BR.png
     
  18. #12

    Expert

    Nov 30, 2010
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    No. You're looking for a short besides the BR. First place to check is the big capacitors. They would explain a noisy start-up on the way to complete failure.
    If they are good, you will suspect the output transistors as second most likely to fail.
    Even if the big capacitors are bad, the shorted BR could have nuked the output transistors.
    This is what I call a cascade failure. Caps short, BR shorts, AC hits the power transistors, they fail as a short.
     
  19. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    Checked resistance across all output transistors

    (plus and minus represent the probes)
    B+ to C- = 12.21 to 12.35 meg Ω
    C+ to B- = 9.87 to 10.92 mΩ

    B+ to E- = 12.29 to 12.38 mΩ
    E+ to B- = 9.9 to 10.9 mΩ

    C+ to E- = Ever Rising Resistance, quickly into the KΩ range. ~ 5 sec = ~ 40KΩ
    E+ to C- = Ever Rising Resistance, quickly into the KΩ range. ~ 5 sec = ~ 40KΩ

    NOTE: When checking the capacitors, using the DMM on the Ω setting, touching the leads (pos to pos, neg to neg) the resistance showed steady increase. Reversing the leads showed resistance drop to zero then start back up again as the capacitor discharged and reverse charged.

    The BR was heat sink mounted, but the thermal grease didn't appear to be well applied. Could heat have caused the demise? I want so much to put this back together and try it out. I just would hate throwing money into it only to blow my money up.
     
  20. Tonyr1084

    Thread Starter Active Member

    Sep 24, 2015
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    Also checked the MOV (if that's what it is)(that blue thing on the power in side - and yes, that's how the board is assembled, with the MOV across the switch). Shows completely open (or beyond my meters range).

    [EDIT]

    I've removed the caps; charged them through a resistor up to 12 volts DC. The caps show (on the scope) a steady rise. When power is disconnected they hold power for a long time with no sign of diminishing. So I don't think the caps are bad either.

    I'm really confused. What to do? The transistors don't seem to be shorted and the capacitors also seem to be fine. No idea why the noise has grown worse over time unless there's something I'm missing. I have eight power transistors on the heatsink. There's two small transistor looking devices but the board doesn't indicate what they might be. And when I check across each possible combination I get mostly in the mega range resistance. Only in one direction do I find resistance starting out low and increasing the longer I hold the probes there. IF they were shorted I wouldn't be seeing those kinds of readings. NO?
     
    Last edited: Feb 8, 2016
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