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Need Help Repairing Pyle Guitar Amp after short circuit--only hums & pilot lite

Started by Roark, August 21, 2018, 02:03:11 PM

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Enzo

Mouser has stock on the LM1875.

I found TDA2040 at bdent.com, a company that has never sent me a bogus part in several decades.

Roark

OK-will correct that! Now with meter set to the 200V range, I get a reading of -5V [DC]
[touching red probe to red speaker lead, or +5V when reversing the probes].

Also with meter is set 20V I get the same readings.

What does -5V DC [offset?] mean?

Enzo

It means the amplifier chip is putting out DC, it is defective.  it doesn't matter that it is positive or negative.


It is possible the input is skewed.  Remove the power amp IC, then meter the input pins to see if it was being fed DC.  Unlikely though.

Roark

I'm about to remove the failed TDA2030 IC Chip.
Are any other tests,etc you think I should do first?

Can I use a screwdriver to discharge the 2 Large Capacitors : 2200 microfarads,25V or is it better to use a resistor? .
Other Capacitors are much smaller: 100 microfarads or less.
Do you suggest discharging them all 30-40 of them?

DrGonz78

I would just replace the chip as it is most probably bad.

You can measure those two bigger filter caps with your DVM to see if they are holding a charge. Chances are the caps discharge rather quickly and probably only millivolts will be stored. Still just put your meter across each one and check the voltage so you know. You don't have to discharge any of the other smaller caps.
"A person who never made a mistake never tried anything new." -Albert Einstein

Roark

Quote from: DrGonz78 on August 23, 2018, 04:14:12 PM

You can measure those two bigger filter caps with your DVM to see if they are holding a charge. Chances are the caps discharge rather quickly and probably only millivolts will be stored. Still just put your meter across each one and check the voltage so you know. You don't have to discharge any of the other smaller caps.

Measurement of capacitor residual voltage:

1]Measurement on the DC voltage scale?

2]With power cable disconnected?

3]This won't discharge the capacitor?

4] or damage my simple meter?

Enzo

1.  DC is what they store.

2.  yes, disconnect the amp from the wall outlet, after all the whole reason you were measuring the voltage was to see if it was still charged up.  If it is powered, then of course it is powered up and charged.

3.  Not likely to discharge the cap much, but even if it did, isn;t that the point?  get the cap discharged?

4.  A meter measures voltage.  VOltage is present or not.  The meter has no way of knowing if the circuit is live or off but still charged.  All teh meter knows is voltage.

Roark

OK,Thanks,Enzo! I ask some [maybe obvious] questions just to be sure I understand the theory,which helps prevent mistakes in general!

Roark

With the 2 biggest capacitors measured at 0 volts & drained:

I'm Checking the D6 Diode [the Diode with the slightly burnt upper leg], in circuit,using the Diode function of the VMM.

1] I consistently get a  reading of 37 Volts, using the probes forward-biased & reverse-biased
Question: Shouldn't the meter say: .37 Volts?---ie: 10x less voltage & with a decimal place?

2] The resistance test gives 37.0 Ohms [with a decimal place!] using the probes, forward-based & reverse-biased, & on 3 different scales, so I tend to trust it.

So,with your experience, do you suggest I get a new  diode? ---or is there more to consider?
______________________________________________________________

PS: I ordered the TDA2050 chip [but it hasn't arrived yet.]
Thanks,DrGonz78 for that suggestion!
& I really appreciate all the help from Enzo! [& tonyharker]


Enzo

Yes, right away.  In my shop I have to ask myself, how much labor time should I use up testing a 5 cent part?

Do you have a good diode of any type sitting around?  measure it with your meter, presuming it is good.  How does that compare to these readings?  37 ohms across a diode is not good, unless the diode has parallel resistance in teh circuit.  SO unsolder one end of the diode and lift it up from the board, now measure it right on its end wires.  Still 37 ohms?

g1

Does your meter explicitly mention volts somewhere when using diode test?
I'm thinking it is probably millivolts instead.  Check a good diode and it will probably give you something in the 500 to 700 range when the diode is forward biased.
So your 37 reading is probably .037V on diode test.

Roark

Quote from: g1 on August 27, 2018, 09:57:53 PM
Does your meter explicitly mention volts somewhere when using diode test?
I'm thinking it is probably millivolts instead.  Check a good diode and it will probably give you something in the 500 to 700 range when the diode is forward biased.
So your 37 reading is probably .037V on diode test.
My manual does say that the diode test reading is in millivolts, so the reading is .037V , as you said.

Roark

I'm now measuring all the other Diodes on the board, to compare/check my D6 result:

ALL Measurements are  in MILLIVOLTS:

D7: 005 in both directions [this diode is right next to D6]
--------------------------
D8: 37 in both directions [this is a group of 4 diodes right next to each other]
D9: 38 in both directions
D10: 007 in both directions
D11: 007 in both directions
----------------------------------------------
D2: 1078 in both directions [this is a group of 2 diodes right next to each other]
D1: 1079 in both directions
---------------------------------------------
D5: 656 in both directions [this diode is by itself]
__________________________________________________________

For each diode, I get the same reading forward-biased & reverse-biased!; just as I did for D6.
Which supposedly indicates a shorted diode.But I doubt that all the diodes are shorted.

So maybe my meter is wrong [I will borrow another]; or there may be no problem with the D6 diode?

I will also find another diode, not in circuit,to test with my meter.

Roark

I now suspect 6 diodes [D6 thru D11] may be shorted [partly based on a borrowed Analog voltmeter with no diode function, resistance tests only, in circuit.]

These 6 diodes are all labelled 4007. Anybody know where to get these? & do I need further identification?
_____________________________________________________________________

Jazz P Bass

That is a most common diode.

You must place a 1N in front of it.

1N4007.