Marshall AVT150 channel switching issue

[Arbutt]

Hello all
I have a AVT150 with a channel change issue.

At switch on it comes on OD1 and if you press OD2 both LEDS are on if you then press OD1 again, it switches back to Just OD1
( if you press OD2,, then OD1 LED stays on ) unsure if that is correct or not ? )
But if i press the Acoustic sim or Clean button it lights up then returns to OD channel
Its the same on the Foot switch

If you try the amp its working ok on OD1 or OD2, If you press and hold clean or acoustic buttons it changes channel until you release the button. = same as the foot s/w
Cleaned and checked the button switches, ( Not latching type ) ok
As i had them i replaced IC2 and IC3

My thinking now is its not on the front pcb its on the main pcb something up with the foot switch circuit
I would like to change IC5, but i ain't got any

Hope i have explained it well
any help would be welcomed

marshall_avt150_avt275.pdf
many thanks Mr A

[Jazz P Bass]

As IC 5 appears to be related to the FX section, I would think that that is not the culprit.

I would look at IC 3.

[Arbutt]

Thanks jazz p . I have replaced ic2 and ic3. With no change

[Arbutt]

hello all
i'm on this one again
with this fault described in the OP and changing IC 2 and Ic3 still not switching channels
amp comes on with OD1 and if you press OD2 both channels are up which is wrong
If you press Acoustic or Clean buttons they light up and as soon as you release the button
the channel goes back to OD1
If you hold the clean or acoustic button in it works ok - does the same with foot s/w

https://www.vishay.com/docs/70040/70040.pdf DG212 pin out and data

this amp came to me like this
IC 2 on
pin 16= 4.6v if you press clean channel button it drops to 0v
pin 9= 4.6v if you press clean channel button it drops to 0v
pin 1 and 8 = 0v
pin 12 = 4.6v

Ic 3
pin 16= 4.6v if you press clean channel button it drops to 0v
pin 9= 4.6v
pin 1 =0v
pin 8 = 1.67v
pin 12 = 4.6v
i will confess not to understanding the switching circuit, But i'm beginning to think its being caused but the foot s/w circuit

15v +/- is there and all working correctly apart from this stupid channel change issues
thanks again
Mr A

[Jazz P Bass]

What I can see is that you are looking at the logic inputs.
That is good.
But you have not looked at the outputs.
Simple switch. Open or Closed.

[strato56]

That 1.86V on pin 8 from IC3 is weird. Should be a high (4.6) or low 0v , I guess.
The logic signals F-OD1, F-OD2, F-CL1 and F-CL2 come from the back PCB. Can you measure the signals DIGOD2, DIGOD1 and DIGCL2.

[Arbutt]

Morning all

Please bare with me i'm writing this as im testing
Ok this is what i got on ic2 DG212BD ( pin 13 / 4 = 15v +/- Pin 12 = 5v )

With OD1 on i have
pin 16 =4.6v
Pin 15 -14 Closed
pin 9 = 4.6v
Pin 10-11 Closed
pin 1 = 0v
Pin 2-3 open
pin 8 = 0v
Pin 6-7 open

Press OD2 ( this is with both OD1 LED and OD 2 LED on
Pin 15 -14 Closed
Pin 10-11 Closed
Pin 2-3 open
Pin 6-7 Closed

On IC3 DG212BD on OD 1 ( pin 13 / 4 =15v +/- Pin 12 = 5v )
Pin 15 -14 Closed
Pin 10-11 open
Pin 2-3 open
Pin 6-7 closed

Press OD2 ( this is with both OD1 LED and OD 2 LED on
pin 16 = 4.6v
Pin 15 -14 Closed
(pin 9 = 0v)
Pin 10-11 open
pin 1 = 0v
Pin 2-3 open
pin 8 = 1.67V ??????) this is with od1 or od2 on, when you press acoustic sim the 1.67v rises to 4.6v then rtns to 1.67v
Pin 6-7 closed

30 mins to the testing i'm now getting 0.86v on Pin 8 of IC3, and with CNF7 removed i have 0.86v on the connector

with CNF7 removed still not working .... with od1 and od2 on and clean and acoustic only working when pressed on

Before i sign off for the weekend
IC2 and IC3 have been changed for DG212BDJ supplied by marshall IC2 had a BDJ in from original and Ic3 had a CJ in from original. I can't see any difference

Hope this makes sense... it don't much too me
I will return on monday

[Arbutt]

Update
I managed to spend a little more time decided to removed IC2 and IC3 and test without them
I'm having the same issue i feel fairly sure that this issue is on the back board
Checked all the connectors all going to correct location

[Arbutt]

Job done. I was testing ic5 which I suspected from the start. There was a few strange voltages. Only had a used one. Fitted it and working correctly.

Now I can focus on my other pain job. Xr8600
Many thanks to you all
Mr a