Pignose Hog 20 No Sound

[Archie Speed]

I've happened across a Pignose Hog 20 that has seen better days and thus far I cannot get any sound out of it at all. No hiss, no hum. The previous owner had used an 'incorrect' wall wart & blew a few things up.

So far, I've replaced blown resistors R11 & R1, ZD2 & D9. 3-wire bi-color LED has the front half blown off & I don't have a replacement yet (3-wire/bi-color).

Everything thus far has pointed to a bad TDA7240AV, 20W Bridge amp, which is an obsolete item that I happened to have 2 spares of. Tried both & no joy as far as getting any output from this IC. It is getting 12.9VDC supply voltage & I've stuck a signal generator at the input & it traces straight thru to the TDA IC, where signal stops. Checked speaker & tests ok on another amp. Checked headphone jack & no prob found there either. Testing resistors on-board all check ok. Leery of pulling too many components on this as the PCB traces seem to want to lift at just over the temp of melting solder. Really thin & frustrating.

Any thoughts on this? I'm getting too old & feeble to get my head around this sucker. A bit different than the usual culprits. The link below is to a previous Forum post on a Hog 20 that includes the schematic.

https://www.ssguitar.com/index.php?t...21280#msg21280
Regards,

Archie

[J M Fahey]

Sorry, clicked that link and could not see the schematic, nor in Rolyīs suggested link either.

IF you can see it, post the direct image link here.

[Archie Speed]

My fault for posting the wrong link. Corrected link in 1st post & attached schematic.

[Archie Speed]

ST TDA7240AV data sheet attached.

[g1]

What is the voltage on the 7240 standby pin, #2 ?

[Archie Speed]

Hi G1,

7240 pin#2...I get somewhat erratic readings on #2. On power up I can read zero vdc one time & 1.04 vdc 10 minutes later. I see that this is a Standby/SVR point on the TDA7240A. Momentarily grounding it resets the potential to 'o' vdc. Removing the ground doesn't elicit a response. I'd like it if the data sheet for the 7240 had an internal logic breakout. I understand that the voltage should be above 1 vdc to get this out of stby (?). All testing thus far has been with the amp assemblies separated but interconnected, batteries, charger & all.

I'm going to setup a test circuit on a breadboard, sticking as close to the data sheet as possible for values & see if I've got a series of 'bad' 7240 IC's or something I'm missing on the PCB.

Thanks,

Archie

[g1]

Did you try replacing that cap on the standby pin, C12 ?
Where did you get your 7240's from?

[Jazz P Bass]

If I am not mistaken, the Standby pin is held to 1 volt (or such) internally.
You ground the pin to Mute the IC.
C12 merely pulls pin 2 low at start up (mute) and gradually rises to whatever voltage the IC internals put on that pin.(unmute)

In other words, for testing, you could remove C12.

[Archie Speed]

Thank you for the input gentlemen.
G1,
I have replaced C12 with no change. No idea where these 7240's point of origin was. I've had them that long. And I'm leery about where to purchase obsolete IC's. I'm looking.

JPB,
Thanks for the explanation of C12's function & how the stby/mute works. I'll try yanking C12 & seeing what happens.

I mocked up a breadboard model of the basic TDA7240A test circuit with nothing connected to the output (no Zobel Ntwk...nothing). Fed signal to input & got output signal on pins 5 & 7 of the IC. So...I'm going to re-verify IC voltages on the mock up and the PCB. I'm thinking that I'll remove a few components to open up the TDA outputs and seeing if this PCB follows the mock ups behavior in getting output. The fact that the same 7020 IC seems to function in the mock up model but same chip chokes in the PCB layout, I'm leaning towards thinking that the 7020A is ok. The original 7040A was literally blown up (bulged, cracked & ugly). Only have 2 spares. Roll them too.

I'll keep updating this project. Didn't think this ckt would be such a PITA (for me, at any rate).

Regards,

Archie

[g1]

If there is something funny on the board, the chip could be going into a protect mode.
With the chip out there should be approx. 4 ohms from pin 5 to pin 7.

[Archie Speed]

G1,

I'll pull the IC tomorrow & measure as you noted & report findings.

Thank you for your time.

Archie

[J M Fahey]

You are doing it vright, pity information is somewhat obscure and the chip is notbthatcommon:
1) they donīt detail Standby conditions as you found.
Not even basic whether chip is ON with switch ON or with switch OFF, expected voltages, etc.
Fine if it works, bad if you have to troubleshoot.
2) I remember helping somebody long ago with one of these, faintly remember the Zobel design was cheesy, using what-datasheet-says restored operation, so please do.
3) measure DC voltage to ground at each output; they should be halfway between +V and ground (so about +6 or 7V to ground)
4) just for kicks measure voltage on input pin 3
They donīt show internal circuits any more but imagining 2 possible input implementations I would expect either half +V , basically pin 5 or 7 value *or* , if itīs a TDA2003 grandson, 0.6 or 1.2V (1 or 2 "diode drops" indicating "1 or 2 Vbe").
What I would NOT expect there is either 0V or close to +V .

[Archie Speed]

G1,

Took a quick read while I had the IC in mock up w/o anything on the outputs & it did, in fact, read 4 ohms (3.98). Took it out this AM & I read open line. Weird.
Started to take my original spare out of the PCB & the trace from IC 2 to C12 lifted, and I'm using a station set pretty low.

This amp has a dual color/3-wire LED setup that lets the user know when power is on and when the battery level drops to an unusable level. As this LED literally blew up, the traces on the board joined in and lifted and/or disappeared. I am guessing that removal of components associated with these indicators while troubleshooting won't hurt anything. Give me a chance to repair the traces to at least functionality.

No electronics supply stores around here, so I sent an order to Mouser today for some components to refill my dwindling stash. When I get them, I'll implement the data sheet Zobel Ntwk.

JMF,

Give me a day or two to get my guano together & I'll have some good readings to post after I get some things back together again. I don't even have spare 2.5A fuses. Limiter yes...cure for clumsiness, no.

I'll post the requested measurements as soon as possible.

Regards, and again, thanks for the help & time.

Archie

[Archie Speed]

Success! It breathes. Finally got my resupply parts from Mouser AND managed to locate 2 ea 7240's that are purportedly NOS. These were the horizontal versions but bending the pins got the orientation right w/o damage.

During the wait for parts, I tried a couple of times to get voltage readings as requested but was stumped because each reading I took was different. Poked & prodded around the PCB & components & couldn't get anywhere. But when I removed the IC to replace it, the eyelet associated with pin4/GRD came with it. Apparently there was a very fine hairline crack in the PCB trace, right at the point where solder flow stopped. Meter probes & wooden sticks probing apparently applied enough pressure to make/break the contact & yield screwy readings, just could not see it with a magnifying glass. Now I can go check the other IC's I suspected as 'bad'.

I'll get to reworking the Zobel ntwk as per the data sheet after I play with this a bit & see what other surprises it might have.

Thank you all for the support, assistance & willingness to share your wealth of knowledge.

Archie.