Thanks for that clarifying answer by Tassieviking!

Thanks for your reply, joecool85!

To clarify: When the amp is cold & with the Guitar Plugged In: I hear both the HUM AND the PROPER SIGNAL. The Hum disappears eventually,but may return.

Have you ever heard of [with the Guitar Plugged In] a HUM problem,in particular  [I mean vs CRACKLING or STATIC] as a symptom of a Cold Solder of the input jack?

I guess this is a shorting/switching/shunt input jack & I wonder if that is somehow the problem?

It's a Fender Frontman 15G Amp, which is useful to me because it's small enough to fit on my desk, for when I give lessons. There's a pretty loud Hum from the Amp, mainly when cold & whether or not anything is Plugged In. This is with the Amp Volume set to zero.
The Hum usually Stops after the Amp has warmed up for about 5-10 min. For awhile the Hum was eliminated by [repeatedly] unplugging & replugging the guitar cable from the input jack of the amp.But now the problem is getting progressively worse & the Hum returns, even after the Amp has warmed up [& now there's even other noises: distortion?,crackling?,static?].

I'm asking your advice to confirm whether this Hum is likely to be caused by a cold solder joint of the Input Jack?... or something else?

To isolate the problem I eliminated: the Cables, A/B Box [Splitter], Guitar & Effects Pedal I normally use.

The problem appears to be in the Input Jack because I noticed I used to be able to eliminate the Hum by pulling the cable plug/tip part way out of the input jack,to the point where the plug/tip first touches the contact in the jack ; & then pushing up & down on the plug/tip [jiggling a bit]: I  would hear the hum stop completely & there was a click sound!  But this doesn't work any more.

Quote from: DrGonz78 on September 05, 2018, 02:40:06 AMI would be guessing that the power transformer is 24vac w/ center tap and it results to a split power supply of -/+12vac. 12 * 1.41 = 16.92vdc. Check out your voltages on your amp to be sure.....
Now confirm your power supply voltages to make sure. Which if a 2030 chip suggests -/+18vdc supply then I would think its very close to that indeed.

-------------------------------------------------------------------
The label on the Transformer says:
"TDB-15L-1,   
OUT: 12VX2,     
ACIN: 120V"

Is this the "split power supply of -/+12vac" that DrGonz78 guessed? [so I don't need to measure the Transformer voltage]?
----------------
Many Thanks to DrGonz78, again, for that detailed analysis!!

Quote from: DrGonz78 on September 03, 2018, 12:45:19 AM
With the TDA2030A chip running 36v power supply or rather a -/+18v supply it will be max rated at 20 watts as per data sheet. So now you will have the TDA2050 in place that can handle 32 watts at 8 ohms or the claimed 50 watts with 4 ohm load. In those examples though it is with a power supply capable of delivering 50v or -/+25v from the power transformer. Your amp will still have the -/+18v power supply and will have a new chip that is highly overrated to your amp's power supply. Honestly looking at the TDA2030 data sheet I think it's really only putting out 14 watts RMS with a 4 ohm load. I would be surprised if the 4 ohm speaker caused any trouble for TDA2050 chip.

Here's what jlassen36wy [EBay seller]  said:

"The limitation is the power supply circuit.
As far as thecircuit becoming unstable at 4 ohms, that depends on the circuit design.
The TDA2050does not begin to taper off power at 18 watts.
So, the power supply may become loaded down trying to feed the chip.
It is a design issue, and I have used TDA2050s in all sorts
of TDA2030 and TDA2040 circuits successfully,but I can't guarantee it as a substitute.
[PS: TDA2030 is 18 watts into 4 ohms. TDA2050 is 32 watts into 4 ohms]"

Does that agree with DrGonz78 said above?
DrGonz78 seems to have more information [that I have a -/+18v power supply] & sees no instability problem.

Thanks for the analysis,DrGonz78! ---That's a relief for me!

I was going to follow these instructions:
"Turn the amp on for about 2 seconds then off again. Then put your finger on the plastic part of the chip amp see if it has heated up really quickly. If it is ok for 2 seconds double the time and try again - repeat until you have reached 1 min." "[Amps usually recover if they are load unstable but they will run much hotter than they should so watch for this when you start using the amplifer.]"

To replace my TDA30 chip I bought a TDA2050  from jlassen36wy = Amp Repair Houston, EBay.
After ordering I noticed the TDA2050 webpage says:
"NOTE: Placing this TDA2050 32 watt integrated circuit chip into a 20 watt circuit (i.e. replacing a TDA2040) will not increase the power output of your device and may cause instability especially with a 4 Ohm load."

I'm worried I got the wrong chip because My speaker is 4 Ohms!
But My amp is a 30 watt amp & has "Rated Power: 45 watts Peak".

So maybe this chip is OK?

What I don't know is : How many watts is my original TDA2030 chip?..... & Is my circuit 30 watts? 

[TDA2050 is a 32watt chip ; TDA2040 is apparently 20watts? ]

To replace my TDA2030 chip I bought a TDA2050  from jlassen36wy = Amp Repair Houston, EBay, :

I just noticed this today on the webpage:
"NOTE: Placing this TDA2050  32watt integrated circuit chip into a 20watt circuit (i.e. replacing a TDA2040) will not increase the power output of your device and may cause instability especially with a 4ohm load."

But my speaker is 4 Ohms. Is this a problem?

Quote from: Enzo on August 30, 2018, 05:47:46 PM

Mouser charges for shipping, but if I recall, if you prepay, they don't, is that correct?

Thanks for all the info,Enzo! But Mouser DOES charge for shipping: prepaid it's $9.60,total, for 10 diodes.

Question for DrGonz78 & others : Have you bought 1N4007 Diodes from jlassen36wy?,[EBay company you suggested for the TDA2050 chip]


PS:MIC Brand is Chinese.

Quote from: Jazz P Bass on August 30, 2018, 01:55:06 AM
That is a most common diode.

You must place a 1N in front of it.

1N4007.

Have you used this seller?:
hstbeto Bellflower,CA https://feedback.ebay.com/ws/eBayISAPI.dll?ViewFeedback2&userid=hstbeto&&rt=nc&ftab=AllFeedback
-- who has many  xlnt reviews for:10pcs 1N4007 Rectifier Diode 1A 1000V - $1.60- US Seller - Free Shipping,

listed as MIC Brand,apparently made by:  https://mklec.com , in Ohio 

or who do you recommend?

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?
_____________________________________________________________________

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.

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.

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]