Vox VT30 - loud humming

[jondoe]

Ouch!!

Plan 1: get an LM1875 (which in principle nobody cared to fake) from a reputable seller (or bite the bullet and order a couple from Mouser, to justify postage) , it will handle voltage fine and *may* supply enough current for your speaker.

Plan 2: Please post a relatively close up picture showing the other (frontal) side of the heat sink,where the TDA2050 is mounted.
Maybe there is space enough to add 2 TO220 transistors plus one fake TDA2050 or real LM1875 used as a driver, I can suggest a current booster way simpler than the one I showed before: 2 transistors + 1 resistor

Plan 3 ... which I should try first since itīs so simple and quick: *maybe* the fake chips you got so far can "almost" drive your 4 ohm speaker, or they can, but become unstable, you show a nice sinewave, just marred by the spikes, I suggest you add a 2 ohm 10W resistor in series with speaker, I have a hunch it might solve the problem.
If so, get a terminal strip or worst case nail 2 brass nails to the inside of the cabinet to hold the resistor firm but not touching the wood, so it can have fresh air around.

Donīt worry about the "power loss", in fact that speaker has quite more than 4 ohms over most of the range, and you will only lose some around 400Hz ... which is actually good, and will make amp sound "tubier".

I had another quick test of the amp today and I've noticed there is more head room with this IC over the previous IC's, that is, I can get more volume before it starts to spike. Further testing showed the breaking up to be a little inconsistent, so I'm going to spend some time looking over it tomorrow to confirm exactly what's going on. It's lower frequencies that cause the spikes the most, 82hz (closest I can get to E) being the biggest offender.

I was thinking I could increase the two resistor values in the "power level" of the circuit (page 6), so even when the power level is cranked up it's no longer quite putting out the same power, that way I have a working amp, slightly lower powered, but still using the IC.

[J M Fahey]

Make it as simple as possible, just try that 2 ohm 10W resistor in series with speaker.

I strongly suspect the inferior chipamp (I suspect a relabelled TDA2030) protection is working correctly and triggering when it detects the chipamp can not drive that complex load ... reducing drive does not really address that, while making the speaker a milder load goes to the core of the problem.

Try it, itīs inexpensive ans takes all of 5 minutes to test.

Earlier today I was comparing datasheets, TDA2050 is current limited to 5A ; TDA 2030n is limited to 3.5A ... so it can drive an 8 ohm load but not a 4 ohm one with those 24V rails.

2 extra ohms in series may make load pull *just* what it can deliver (simple Ohmīs Law) while altering the drive side is more complex and not guaranteed by any means.

[jondoe]

I contacted the seller of the 2051's and asked if they were authentic, I was told they are "real" new old stock, but they have now removed them from their site Go figure. I pointed out they were useless to me and they haven't bothered responding, not the best customer service, they worked out quite expensive with shipping!

Having played with the amp a little more I've noticed that some amp modes drive the amp/speaker a little harder than others, that and the power pot on the back needs to be over 70% for the glitching to happen. I'm going to button this all up and take it off my bench for a while as it's been hanging around and making me annoyed

Next time I place an order for some bits I'll add a LM1875 and 10w 2ohm resistor and see how the resistor fairs.

I think this is the last Vox VT30 I'll buy and try to fix up, at least the VT50 has a STK404-100S and not a TDA2050!

[Jazz P Bass]

Sadly, the STK494-100S is also discontinued.

It was replaced by the 404-100 SC-E which also has been discontinued.

[jondoe]

Sadly, the STK494-100S is also discontinued.

It was replaced by the 404-100 SC-E which also has been discontinued.

Looks like I'm avoiding all of those Vox's in that case

[jondoe]

Just thought I'd update this thread to say that I took a gamble on an Aliexpress seller, who claimed to have real TDA2050's, the pictures checked out, so I bought two. After the typical long wait they arrived and I was pleased to see these looked real.



After fitting and testing I no longer have the terrible clipping/glitch sound at high volume, proving all the previous chips I tried were fake and this was real

This amp still has a noticeable 100hz hum above about 60% power level, very much so at 100% power, even though I changed the filter caps, but it is at least completely useable as amp.

[jondoe]

Routing of the unshielded cable bundle from the preamp back to the power amp board appears to be crucial to limiting the hum. Ensuring it was held up as far as it can be from the power amp board and as far back towards the back of the amp chassis reduces the 100hz pick up considerably. It's still not silent, but it's much much better

[malachi1313]

Been reading this thread as I just purchased a non working VT50. Only paid $20 for it so no loss if I cant get it fixed. Interesting troubleshooting here to find out what is at fault. I have more of a pulsing issue so I am going to start by replacing the +15 and -15 voltage regulators. I probably will buy them from either mouser or I think the other seller was onn. I believe that the MC7815 and MC7915 have an NTE cross reference #. Didn't realize there are so many counterfeit parts out there. First part I ordered was the dual 6-pin pot that adjusts the power output of the amp. Very similar utube post of somebody having my issue said that part should fix my issue. I was able to put pressure on the POT and the problem would go away. Amp sounds awesome through the headphone jack but nothing comes out of main speaker except a loud non adjustable hum with frequent pops. I re flowed some questionable solder joints, Cleaned the jacks and sprayed some cleaner on some of the pots. I am going to test for DC on the speaker wires to determine if the power IC is jacked up. Hopefully I will be able to get one , as the supply place in georgia mentioned in this thread shows there is only one left. It is the part number that starts with STK. Sorry to Hijack thread and probably should start a new one. I will as soon as I replace the power adjust pot.