Marshall AVT50H Issue

[zorgan]

Trying to troubleshoot a Marshall AVT-50H guitar amp

No sound from headphones or outputs, no effects return etc

the tube is glowing and I've also swap tested the tube with another AVT-50 amp

The main large board I have ordered and replaced along with the other board with IC1 chip but the parts sent were different, not AVT50 but AVT50X and AVT275 the board sent is slightly bigger than the original (the smaller one on the heatsink that is) just wondering if these are actually interchangeable?

The amp is powering on, the fan is running, when I put the headphones in I can hear no noise at all.

I've checked all connectors time and time again, but yet no sound at all, what else can I check? I do have the right boards installed also for the AVT-50H ? the boards have AVT-50X on them as you can see in the attached image.

[g1]

How did you verify that the original board was bad?
When you put a signal to the input, is there signal at the FX send jack?

[Mick Bailey]

Marshall only gives only one schematic for the AVT50 and no variants. The power amp board you have is the AVT275 and this is the same drawing for the AVT50.

The headphone out comes off the power amp output. You need to make sure your FX loop sockets are passing a signal - link the send and return with a patch lead and try the amp again. There's another test; because the CD input feeds into the power amp, try feeding a signal there to check the power amp is working properly. You should get speaker and headphone output.

If this doesn't work you may be missing a voltage. The power amp runs at +32V (CN1 pin 1) and -32v (CN1 pin 2) and the rest of the amp is +/- 15v which can be checked off ZD5 and ZD6. All voltages measured to ground.

If you're not familiar with troubleshooting mains operated equipment you're best leaving it alone. In addition, what transistor amps lack in voltage is amply made up with current - a slip with a meter probe can leave you worse off.

[zorgan]

understood Mick, thanks for the reply, there's more of a story attached to this repair, and I put my hands up, my own fault totally.

The original issue was the dreaded R58 issue, after replacing the input jack and the R58 resistor I had accidently connected up connectors wrong inside, frying the old board and the power amp. I guess we learn from our mistakes, so these two boards are totally brand new from Marshall and after replacing I thought everything would be fine, the connectors I got the wrong way around blew a couple of resistors near the headphone socket on the old board and the power amp board frying IC1 chip in the process and god knows what else. I've been beating myself up quite a bit and was a costly mistake I will certainly not repeat as the boards were £80+ to purchase, I'm just really not wanting to throw any more cash at this amp, I will test as directed and report back asap.

[g1]

Not sure about this model but some of these will blow the power amp chip if you do not discharge the power supply caps. before connecting the harness to the new output board.

[zorgan]

both boards were replaced at the same time

[g1]

Just wanted to let you know as you will probably be disconnecting your amp board again during troubleshooting.

[zorgan]

Just wanted to let you know as you will probably be disconnecting your amp board again during troubleshooting.

Yes I've tested connected at the moment, I'm a little nervous at disconnecting with the caps charged, I will leave connected for now but if I do need to disconnect, I will take both boards out and use a 10k resistor and short the caps to ground (which I've not done before but with the 10k resistor between the + on the caps and the other going to ground this should be okay?)

Big thankyou to Mick Bailey for the directions for troubleshooting, so I have followed the instructions and being extremely careful I was testing the voltages as directed.

15v both ZD5 and ZD6 +/- TEST OK!
34.7 and 34.6 (CN1 pin 1) and -32v (CN1 pin 2) are also present TEST OK!

With headphones connected, I tried FX loop with a patch cable, also tried CD Input, not getting any hum hiss (it's acting like I have no headphones at all plugged in) i.e. no ambient sound or anything.

[zorgan]

Anyone able to help with further troubleshooting steps?

[Mick Bailey]

The signal path for the CD input inserts after the preamp via C40 to IC5b (pin 6). You need to make sure that the output on IC5b (pin 7) is present. This then couples to the power amp IC via C13, so check the other side of C13 as well. If there's a signal then you have a problem with your power amp circuit.

You can use a scope, or if you don't have one an audio probe works fine. I have one rigged up to an old beginner amp.

[zorgan]

The signal path for the CD input inserts after the preamp via C40 to IC5b (pin 6). You need to make sure that the output on IC5b (pin 7) is present. This then couples to the power amp IC via C13, so check the other side of C13 as well. If there's a signal then you have a problem with your power amp circuit.

You can use a scope, or if you don't have one an audio probe works fine. I have one rigged up to an old beginner amp.

Unfortunately do not have a scope yet (but may be getting one in the next few days hopefully)

I've made an image as I need to verify the pin-out of this chip, if I'm correct the signals your referring too measured using an audio probe should all be at line level?

If I make an audio probe I can hook it up to a mixing desk I have here, then use the headphones to monitor any output signals?

The C40 Capacitor also I measure also? here is a picture of the chip and C40 capacitor, are the number for the pins correctly labeled? (if not i`ll edit the image, as I do not want people finding this on google images and blowing themselves up )

one other question, I measure this whilst the unit is powered on? or off?

[52 Bill]

The C40 Capacitor also I measure also? here is a picture of the chip and C40 capacitor, are the number for the pins correctly labeled? (if not i`ll edit the image, as I do not want people finding this on google images and blowing themselves up )
[ATTACH=CONFIG]28788[/ATTACH]

Please edit your photo, the pin numbers are incorrect. Pin #1 is closest to the small circular indentation at the lower left side of the chip.

So they should be numbered
8765
1234

[Mick Bailey]

Just one more thing; first check you have -15v on pin 4 and +15v on pin 8 of IC5 (measured to ground).

You don't need to check C40 at this stage, but you could make sure the CD input signal appears on pin 6. A mixer with headphones should be fine. With any fault condition the signal can have DC or be swinging much greater or lower levels than expected, so keep the volume right down to begin with. Line level will be OK, but make sure you don't probe the +/- 15v supply to the IC.

When you've made your audio probe, test your setup with your signal source (CD/MP3 etc) to check it works before exploring the circuit.

Checks are with the circuit powered.

[Jazz P Bass]

You can also use a DVM set to read volts ac.

Look at the output pin to see if you have any Vac signal.

[zorgan]

Sorry for the late reply as i've been away on holiday,

Okay so it's looking good, made an audio probe today, tested IC 5 and it definitely has audio coming from Pin 7 (the output)
-15v and +15v are present too! (tested as directed with the multimeter)

again thanks Mick for the great instructions, what should I be checking next?