1975 Allen & Heath MOD2 Rebuild Project

[ampegbassplayer1969]

A few months ago I met a musician who had a 1970's studio that he hadn't used in over twenty years... the equipment sat in a moldy basement ever since. He told me I could take all of the gear, and we found some real gems. Here's one piece that I received - it's a 1975 Allen & Heath MOD2. Check out my photobucket album: http://s50.photobucket.com/albums/f309/ampegbassplayer1969/?start=all

With the help of Riggler from GroupDIY, we were able to rebuild the power supply for this console... Next up, I need to repair the channel that powers the board. It looks like there was some sort of blow out around the potentiometer for the monitor volume control, and it took out some electronics around it. I've already contacted Allen & Heath, and they don't have a schematic for this console.

I'm not really sure what I can do besides hope that someone else has one of these boards... I would need the values of the two resistors that burned out. The potentiometer reads "Radiohm - Made in UK" "LIN A"  "4K7" - a google search sent me to a  Radiohm PDF, which led me to believe it's a 4.7k dual pot.

The channel is upside down - the monitor pot is above the red buttons.

I'm new to electronics repair, and have very little experience. Any help or a point in the right direction would be a great help! If there's any more information you need do not hesitate to ask.

Thanks!

 

[r2d2]

hope this help you

http://www.allen-heath.com/UK/Products/pages/OldProducts.aspx

http://www.allen-heath.com/UK/Downloads/Pages/Download.aspx

http://www.allen-heath.com/UK/Support/Pages/endoflife.aspx

peace
r2d2

 

[shot]

I love this console! I have MOD3!
It took a lot of time, but now I have all the documentation (or at least everything I could find) for MOD3 and MOD2. There is a small difference in those two models, but not significant.

I will upload you user manual with schematics somewhere. I'll let you (all) know where I've uploaded it.

Here is the picture of my MOD3 in the attachment

 

[r2d2]

you can also upload here :
http://www.groupdiy.com/index.php?board=19.0

peace
r2d2

I love this console! I have MOD3!
It took a lot of time, but now I have all the documentation (or at least everything I could find) for MOD3 and MOD2. There is a small difference in those two models, but not significant.

I will upload you user manual with schematics somewhere. I'll let you (all) know where I've uploaded it.

Here is the picture of my MOD3 in the attachment

 

[shot]

I've sent all the documents I have for those two models to admin (a) groupdiy since files are larger than 1Mb
Hopefully it'll be soon in the Technical Documents section

 

[ampegbassplayer1969]

Thanks guys! Shot, what differences have you noticed between the MOD2 and MOD3?

 

[Balijon]

I have a MOD2 exactly like yours. It is currently in storage and waiting for time to restore.
I will have a look tomorrow and take the master-module home.

Looking at your picture, it appears that there are two identical buffer-amps that are mirrored. It looks that one transistor has been previously replaced (the one in the circle, not marked 'GR'). I would suggest to test both transistors if they are still OK.
The burned out resistor is likely the resistor that supplies the +V to both amps and to the large capacitor. It acts as a fuse if something shorts in the circuit that is behind it. Likely the value will be between 50 and 100 ohm and is not very critical, I will check the value on my board. As the resistor acted as a fuse, I expect that at least one transistor is gone.
Measure the other resistor and check it against the same resistor in the other amp (I can't read the value as the red and brown wire runs across).

Theo

 

[shot]

Thanks guys! Shot, what differences have you noticed between the MOD2 and MOD3?

I haven't looked much into details, but from what I saw transformer on mic input on MOD2 is Sowter and on MOD3 is ETAL (??)
Mic input on MOD3 to me sounds fantastic, so I guess MOD2 should also be top.

 

[Balijon]

Thanks guys! Shot, what differences have you noticed between the MOD2 and MOD3?

I haven't looked much into details, but from what I saw transformer on mic input on MOD2 is Sowter and on MOD3 is ETAL (??)
Mic input on MOD3 to me sounds fantastic, so I guess MOD2 should also be top.

I have attached a picture of my MOD2 input, the transformer is unmarked. Can you tell if this is a Sowter or ETAL? Do you know which type-number?

Theo

 

[shot]

I really don't know. It looks, considering the size, very similar to ETAL (Electronic Technologies Anglia Limited...duh what a name!), but I don't know what is Sowter 3397 supposed to look like.

I've attached the photo of ETAL transformer from my MOD3...

 

[shot]

Here is the input section schematic for MOD2

 

[shot]

...and the input section of MOD3

 

[ampegbassplayer1969]

Cool, do you happen to have a schematic for that whole master channel that directly receives power from the psu? I will have to test the resistors later. Thanks again!

 

[Balijon]

I really don't know. It looks, considering the size, very similar to ETAL (Electronic Technologies Anglia Limited...duh what a name!), but I don't know what is Sowter 3397 supposed to look like.
I've attached the photo of ETAL transformer from my MOD3...

Looks like ETAL to me too. I will send Brian Sowter an email to ask more details on the 3397.

Theo

 

[Balijon]

I'm not really sure what I can do besides hope that someone else has one of these boards... I would need the values of the two resistors that burned out. The potentiometer reads "Radiohm - Made in UK" "LIN A"  "4K7" - a google search sent me to a  Radiohm PDF, which led me to believe it's a 4.7k dual pot.

I dug out the monitor module today.
As I already expected, the burned-out resistor is 100R, the resistor on the left is 33k.
The big cap (220uF/25V) has been replaced on my board, I would suggest replacing that one too, please use a higher rated version like 220uF/35V.
Another thing I noticed is that it looks like the outputs on your board are not connected (the yellow and orange wire on my board).
If you need anymore info, please let me know. I am not able to test or measure anything yet, as the console came without a power-supply..

Theo

 

[Balijon]

My cat (Cisco) likes the MOD2 too..

 

[shot]

Cool, do you happen to have a schematic for that whole master channel that directly receives power from the psu?

There is no shematic of the master section available on the net, as far as I know. The best there is, is a flow diagram that contains elements of the master section. If you combine it with shematics of the DOA's you can maybe get somewhere. But there is no complete master section schematic around..
And this is for MOD3... i honestly can't tell if it differs MOD2...

Here si this flow diagram:

 

[musika]

ampegabassplayer1969,

I have a complete set of MOD2 documentation in pdf form that I can send to you - send me a pm with your email address.

I have been struggling with my monitor section for quite a while as well, and currently have mine apart.  This module can be a real challenge because of the connections between the boards. I have owned mine since the late 1975 and I believe that all MOD2's came with Sowter 3397 trannies. It is marked on my schematics.  They (the trannies) are unmarked.  The MOD3's came with a different vendor ELTAL as Shot has said..  It is an awesome sounding board.  Mine is marked Serial number 020.

Let me know if you need more.

David

Can someone please post a photo or two that shows the wires that connect the MOD2 master module board "A' with board "B"??  Unlike the MOD3, the MOD2 pcb's had no values printed on them or markings of any kind, except a red highlighted number that signified the board itself.  Mine has "103" stamped on it.

 

[Balijon]

Can someone please post a photo or two that shows the wires that connect the MOD2 master module board "A' with board "B"??  Unlike the MOD3, the MOD2 pcb's had no values printed on them or markings of any kind, except a red highlighted number that signified the board itself.  Mine has "103" stamped on it.

My board is stamped 104. I have no clue how original it still is, as I have seen some modifications and the board was used as a FOH mixer. Also a fader is missing in this module.
Here are some pictures, there are three groups of interconnections:
- The top multiconnector: 2 wires, bridging the Power feeds
- The middle section: 3 wires, bridging Cue, Solo, ?
- The bottom multiconnector: various resistors: bridging talkback (Slate) to the groups

Theo

 

[Balijon]

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