Crate GX-40C weirdness

[yldouright]

@The Dude
Without being able to spec all the parameters, it's hard to know what's fake about them. I've been burned by parts from "reputable" vendors on op-amps before (LM3886 specifically). To make matters worse, the schematics call for both the TDA2040 and TDA2050H. The vendor you provided sells TDA2040V, how do I know which is the best suited for the supporting electronics of the circuit? Even if you compare the spec sheets, we don't know how close they match the spec because of what I mentioned before about more modern wafer masks.

[The Dude]

I've bought from Audio Lab of Georgia. They are a reputable seller and do not sell fakes. If you'd like to try the eBay IC's, go for it.

Edit:
"To make matters worse, the schematics call for both the TDA2040 and TDA2050H. The vendor you provided sells TDA2040V, how do I know which is the best suited for the supporting electronics of the circuit?"

You'll want the 2 sides of the amp to match, so it's simple. Just look at what's in there now and order one of those (if that is indeed even the problem).

[Enzo]

Wet side refers to the side that gets the variable phased signal from the chorus, not the reverb.

If it seems you have a bad power amp, then find the input to the power amp stage and apply a signal. ANything pass? TDAxxxx power amps are as simple as it gets - most of it is the IC.

Does anyone have the GX40C+ drawing? Mine are in a storage unit across town.

[The Dude]

There's a schematic link in post #14. Here's a direct link for the download page.

https://supportloudtech.netx.net/lou.../#/asset/17533

[Enzo]

Oh, well there it is, your headphones are driven from the main signal, so it works. Plugging into the phones jack, activates the four power amp mute JFETs, Q12,13,14,15. One of those transistors fails and you get no sound there.

But look even simpler - the speaker connections. The speakers in the amp are wired in SERIES with the external speaker jacks. A dirty cutout contact on an external jack will turn off your speaker. Leave the speakers connected and plug an external speaker into the extension jack. Wakes up?

Is the speaker OK?

Other than that, the TDA chip is the whole power amp.

[nosaj]

I've bought from Audio Lab of Georgia. They are a reputable seller and do not sell fakes. If you'd like to try the eBay IC's, go for it.

Edit:
"To make matters worse, the schematics call for both the TDA2040 and TDA2050H. The vendor you provided sells TDA2040V, how do I know which is the best suited for the supporting electronics of the circuit?"

You'll want the 2 sides of the amp to match, so it's simple. Just look at what's in there now and order one of those (if that is indeed even the problem).

+1 on AudioLabs of Georgia. Excellent outfit. Would not hesistate to buy from them at all. Preferred over the Bay for sure.
nosaj

[g1]

The concern with the chinese counterfeits is usually when the chip or transistor is obsolete. Why would there all of a sudden be stockpiles of old stock in China? Or new stock when the labelled manufacturer is not making them anymore.
Usually they are on ebay and shipping from China. Those 2 factors for any obsolete components are a big red flag.
We call them counterfeit, but often they are just a lower spec part that will sometimes work, but usually blow when asked to do it's full potential.
For example, a 2030 might be re-labelled as a 2050. It's an actual working part, but not what you think it is. For some people it would work, for some it will blow at higher volumes.
A transistor that is supposed to be a 10amp type might only be 2amps.
Just examples but that is the general idea.

[Jazz P Bass]

The LM1875 will work just fine.

[yldouright]

@Jazz P Bass
Assuming the pinout is identical, the LM1875 is a much faster chip=more parasitic effect and potential oscillation. This is also a concern when you shrink the same chip to a smaller die.

@g1
Agree, out of spec, overspec and differing parasitic parameters all have the potential to create havoc in the original circuit.

@Enzo
I promise to report the result when I get the amp back.

You'll want the 2 sides of the amp to match, so it's simple. Just look at what's in there now and order one of those (if that is indeed even the problem).

Agreed but that is what might make changing both chips less problematic. It takes out the potential for a mismatch by increasing the odds the chips are from the same manufacturing process. Even if you get a vintage chip, some are oriented vertically (TDA2050V) and some horizontally (TDA2050H). does anyone know if the pinout is still the same when the chip inside is rotated 90 degrees?

[Enzo]

Don't overthink this. LM1875 is a common 5-leg power amp IC used in the exact same applications as the TDA20xx series.

The circuit in the amp is simple and right out of the data sheets for the ICs.

The V and H versions of the IC are nothing more than how the legs are pre-bent. The V version is for mounting sticking up so the rear legs stick straight down and the front ones are kinked forwards. The H version is for mounting the IC flat on a board. SO the legs all stick out then band back to poke down into the board holes. It is the exact same part, you can unbend the legs, but I rarely see the H version anyway.

A lot of transistors come in pre-bent form. Like small TO92s. You can get then with three in a flat row - most common - or with the center leg kicked forward, or with it kicked back. All depends on your board layout. The parts are the same though.


You want the two amps in your unit to be the same. They don't need to be matched whatever that means here. You just don't want a far flung audible difference. SO just the same type number is good enough. This is just a guitar amp, the circuits are very stable by design. And the freq capability of the IC won't matter much to a circuit that rolls off by 5kHz into speakers with even less range.

[yldouright]

@Enzo
Thanks for clearing that up, I thought the H/V designation could have meant they were layed out differently in the package. In your advice, it would seem a TDA2050H or TDA2051H would also be suitable replacements for the TDA2040 that could be in there now. Again, I'll know more tomorrow but why wouldn't changing both chips to the same lot be prefereable to seeking out an obsolete chip to match?

[Enzo]

For the cheap price of two chips, I don't care which you use. I just wanted to point out that having both ICs from the same batch or something is pointless precision. If they are both TDA2050 that is close enough.

How are your existing ICs mounted? Flat against the pc board? Or sticking up vertically from the board?

[yldouright]

@Enzo
I ordered 10 TDA2050's from China. If I ever mod this circuit for more power with a higher voltage trafo, the 2050 has more headroom than the 2040. I plan to replace both chips for consistency. The chips lie flat on the heatsink so I'll have to bend the leads to mimic the "H" version. It's confirmed now, the right channel doesn't work. Inserting an exteral speaker on the output jacks shows the left channel is operational and the right channel is not. By the way, the internal speaker continues to work when the external jack is plugged in on the left channel. I expect this is also true of the right channel. Both sets of leads from the board to the speakers are fine. I note evidence of dried liquid in the amp so even though it's unusual for an amp chip to die, my money is on a dead chip and not the JFETS but we'll see when I get the chips.

[J M Fahey]

Wish you luck

That said, you might find this test report video interesting:

[yldouright]

I received the Ebay TDA2050 chips I ordered and unlike the photo on the listing, they're similar to the ones described in @J M Fahey's post above with the indents in the center. I'm going to test a few in a jig before I put them in the amp. I can't say I wasn't warned