Need some tech help with my Mark IV (I.C.)

[Enc3f4L0]

Some of you might remember my amp busted a few months ago... It smoked... Turned out to be some caps... The tech replaced'em and the amp was working fine again (except for channel changing).

It seemed like the channel switching with the FU-3 was what caused the whole short that started the problem...

I payed the tech the equivalent to 350usd at the time, and that included everything fixed... The amp was supposed to be perfect. The problem now was he needed parts from Mesa Boogie to continue his job.

I then took the amp home to play with what I had... I was unable to switch channels both with the footswitch and with the "Mode Select Switch" on the back of the amp. ***this decided to go bad as well... no matter where the knob was, the amp would stay in a led-less channel state (almost as if there was a 4rth, clean, louder channel on the amp)***. I could only play on 1 channel IF i had an external footswitch attached and engaged.

Okay, so I emailed Mesa Boogie a couple of times and got no reply.
I then called them and spoke with Mike Taylor. He seemed like a nice, helpful guy. He gave me his email so I could send he the parts I needed (and to avoid long-distance calls). I did as we arranged and sent him the parts...

Weeks go by and no reply... I call him again, and this time I give him the parts by phone. He said the Intergrated Circuits I mentioned weren't "proper part names" and he had to check with a tech and get back to me (with pricing/shipping info so I could make the payment).

It's been 5 more weeks now and no news...
I won't even need the hassle and expense of international shipping anymore, I'll be in the US in 3 weeks and have a local address.

Enough of the briefing... I now come to you experts in need of some help.
I'm hoping someone will know what I should order if I listed the parts my tech asked for...

- 100k Audio Taper Pot (My R1 master volume has dead spots)
- Mode Select Switch (FT.SW-RHY1-RHY2-LEAD-EX.SW selector)

Integrated Circuits: (this is exactly what my tech specified)

- 74L922
- 74LS14 (x2, one for the amp, one for the FU-3)
- 74LS138 (x2, one for the amp, one for the FU-3)
- 7805

I'm sort of in a hurry now... Like I said, the amp is broken for months. My trip will be in 3 weeks and this has to be sorted out by then.

If you've come this far reading my problem, thank you.

I'll appreciate any help.

Thiago.

 

[screengrid]

Hi, sorry to hear about your trouble with your mark IV. I have a MK IV combo from 1990 ( bought it new in january 1991 ) and I have the complete schematics, I got from the German Mesa Boogie distributor in 1992.
According to this schematic, I can say the following:


switching matrix section 2: 1x 74LS14, 1 x 74LS138
FU 3 footswitch: 1x 74C922, 1x 74LS14, 1x 74LS138 and 1x 7805

100k pot's for R1, R2 and LD master volumes


did you try the R1-, R2-, LD- extern jacks already?
actually I don't believe, that the mode selector switch is bad also. You should also have a look on the four switching transistors Q5 - Q8 and the diodes D1 - D5.


hope that helps a little

kind regards
Arno

 

[Enc3f4L0]

Thanks for taking the time screengrid.

It seems like the ICs the take asked for are correct (except for the L/C typo in the 1x 74C922 ?)... Funny how Mike Taylor at MB says they aren't part names.

The external jacks are working fine. That's the problem... It's like the Mode Select Knob is stuck in the 'EX. SW' position.

Since the only pedal I have around is the one that came with my Behringer V-Amp 2, and it only keeps the signal active if you're pressing it down... Imagine the hassle... To be able to play I have to press the footswitch against some surface so it maintains pressure, enough to have the channel always engaged. If I wanna change channels, I have to physically change jacks on the back of the chassis. Not very useful in a live situation...

actually I don't believe, that the mode selector switch is bad also. You should also have a look on the four switching transistors Q5 - Q8 and the diodes D1 - D5.

I don't understand much about amps... Just what my tech told me. Is this something I should comment with him? Are these 'replaceable'? Should I consider ordering more parts from Mesa Boogie just to be sure?

 

[fatboy135]

- 74L922
- 74LS14 (x2, one for the amp, one for the FU-3)
- 74LS138 (x2, one for the amp, one for the FU-3)

:lol:
this ic are very common ic, I could even send you some spare of them, you could get them from any electronics shops, this components are latches, flip flops and boolean logical ic that could be find on all digital equipment since a DVD till a Nasa machine.
they are very common and regular.
Also the 7805 is a voltage regulator from 0-5 volts looking into the ic´s components and the 7805 I could imagine that your pedalboard uses the classical TTL digital family that were used before we were born.
So don´t be saddy man, this trouble is a very little trouble with a easy solution. And you can even repair it by yourself.

 

[Enc3f4L0]

Thanks for the nice words fatboy. I do hope this is a minor fix... It's not a nice feeling to have your precious boogie screwed up.

I'll leave the fix to the tech though... My mission is hard enough as it is just ordering these parts, hehe.

Is it always this difficult to get replacements from MESA?

 

[Tuna141]

Like Fatboy135 mentioned, these are common electronic parts. I wouldn't even bother ordering them from Mesa. I'd simply purchase them from an on-line electronic distrubutor - should only cost you a couple of bucks. Radio Shack may even carry most of them.

I don't know where you are located, but here are some distributors to check:

www.digikey.com

www.mouser.com

 

[Enc3f4L0]

Right now I am in Brazil.

I'll be in the US in 3 weeks, and stay for 10 days. That's why I wanted to order all the parts I need prior to my going. A friend of mine kindly offered his store's address to receive any parts. Then he can forward'em to my hotel.

I'll check out those links. Thanks.

 

[fatboy135]

the spare parts are very easy to get, think that electronics components are only electronics components, there aren´t special or mighty mystic. Even think that the components of boogie were special manufacured for them, then the rk for example could be 10.000$, too expensive for an amp.
So the electronics are electronics, a resistor is a resistor there isnt anything special on boogie resistors or in NASA computers resistors, they are all the same. So I recommend to you to get a good electronics dealer, like mouser, farnell o rs and all your troubles will be finished. :wink:

 

[Enc3f4L0]

Even the EASY is hard for me... I went to mouser's site and searched for one of the IC... There were like 10 matches, of different pricings...

I wouldn't know where to start on chosing the correct parts. lol

 

[fatboy135]

give me a couple of days because I´m a bit busy and I will do the shopping list for you...

 

[Enc3f4L0]

Update:

I got all the parts!

Michael Taylor emailed me a couple of days before my trip to arrange everything. He was awesome!

Now I need to take the amp + the parts to the tech... Let's hope these are enough to get it to 100% again.

 

[fatboy135]

sorry I forget completely you nice that you gets the ic

 

[Enc3f4L0]

No problem at all. You meant to help.

Everything is cool now.