1975 Allen & Heath MOD2 Rebuild Project
- Thread starter ampegbassplayer1969
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Balijon
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Balijon
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Balijon
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musika
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Theo, Many Thanks,
You are quick with the photos.... I am certain that this will help me alot! Do you have the documentation? PM me if you would like it and I will send mine via email.
David
You are quick with the photos.... I am certain that this will help me alot! Do you have the documentation? PM me if you would like it and I will send mine via email.
David
musika
Well-known member
from Balijon:
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.
I think the 100R is the resistor used to give 12V (from 24) for the relays on the submaster output modules. It may not be shown on the drawings except on the power/wiring page.
David
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.
I think the 100R is the resistor used to give 12V (from 24) for the relays on the submaster output modules. It may not be shown on the drawings except on the power/wiring page.
David
Balijon
Well-known member
musika said:
David, I did some more trace checking on my board today against the schematics you send me (great set, THX!).
The burned out resistor on ampegabassplayer1969 picture is the 100R series/fuse resistor that feed the two monitor amps (2x 2004E schematic). The 220uF (on my board) is the stabilizer capacitor behind it tied to ground. The resistor to the left of the burned-out resistor is R3 and is 33k.
Theo
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musika
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Nice work... I see it now.
When a circuit is burned like that, can it affect other parts as well or does it stop destroying parts at the capacitor prior to the circuit ground?
When a circuit is burned like that, can it affect other parts as well or does it stop destroying parts at the capacitor prior to the circuit ground?
Balijon
Well-known member
When the 100R blows/burns-up, it isolates everything behind it (where the fault is) from the rest of the desk, so the rest of the desk is not affected and will still work.musika said:
Most likely there is a damaged part in one of the 2004E monitor amps that is behind the 100R fuse-resistor.
Candidates are the transistors (very likely, as one has been replaced before with another type), also the big capacitor (220uF/25V in my module, 47uF/25V in the schema) can die due to age and short-out (on my module it was replaced too).
Theo
musika
Well-known member
Theo,
In your posted image numbered "0833", I see a couple of resistors that are added immediately following the main 10 pin connector that look to be 12k's (I cannot really see the color bands) and they are added on the underside. They seem to be part of the left and right stereo signal path. Do you have any idea why they are have been soldered there?
Also, I tried to see what amplifier is behind the failed 2004E on your module, and I would guess that there are dual left and right 2004C amplifiers that are shown on the Aux Monitor Module Board B schematic. Do you agree that this is the faulty amp / transistor component?
I'm trying to get a better handle at reading circuit flows in general, since I am a novice at all this even though I've been at this for a few years...
David
In your posted image numbered "0833", I see a couple of resistors that are added immediately following the main 10 pin connector that look to be 12k's (I cannot really see the color bands) and they are added on the underside. They seem to be part of the left and right stereo signal path. Do you have any idea why they are have been soldered there?
Also, I tried to see what amplifier is behind the failed 2004E on your module, and I would guess that there are dual left and right 2004C amplifiers that are shown on the Aux Monitor Module Board B schematic. Do you agree that this is the faulty amp / transistor component?
I'm trying to get a better handle at reading circuit flows in general, since I am a novice at all this even though I've been at this for a few years...
David
Balijon
Well-known member
Yes they are 12k. My desk was last used as a FOH-live desk, I think that these resistors may be the feedback resistors that are marked 4k7 in the schema of the 2004C monitor bus amps. Maybe they changed it to create more amplification.musika said:
This is clearly one of the modifications not coming from the factory. I will do some more trace checking.
My modules are fine (I think) the burned-out resistor pictures are from 'ampegabassplayer1969', not mine.
I marked the 2x 2004E parts on the schema of the 'B' board. The output of these amps (yellow and orange wire in my module) go directly to the monitor stereo output jack.
So currently ampegabassplayer1969's monitor outputs will not work because the 100R resistor is blown, the cause is in the 2004E monitor amps.
Theo
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ampegbassplayer1969
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Thanks for all of the information and help so far guys. I'm going to replace all of the capacitors on this strip and then I'll get back to you when I test it!
ampegbassplayer1969
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If I were to replace a few transistors on the monitor amp, what would you recommend?
musika
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I've gotten mine here (England):
http://stores.ebay.com/LittleDiode?_trksid=p4340.l2563
A good source.
http://stores.ebay.com/LittleDiode?_trksid=p4340.l2563
A good source.
ampegbassplayer1969
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- 11
Cool, I'm not sure how to find out the values of the transistors I want to replace.. All of the transistors are the same on the monitor amp, but I can't find "c732" on the web. Any idea what to replace them with?
MagnetoSound
Well-known member
ampegbassplayer1969 said:
Replace any that don't work, or that you suspect are noisy. There is no point in replacing good ones.
musika
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...on replacing transistors, I have been using an alligator clip on the component side (as a heat sink) when heating these up. Can you tell me if this is necessary ??
musika
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Theo,
I tested this today. I would judge that the 12k resistors that were added in the stereo buss path boost the signal by ~20db. No doubt they were using these for the house speakers. You were right on.....
How is your work on the board progressing?
I'm down to debugging the cue system, the solo buss, and a problem with one side of the monitor speaker amps. Any ideas as to how to fix the monitor amp? I am going to first try a continuity test, then replace the two transistors. Any thoughts? I do wish that I had extender boards for this console! I've searched the planet for McMurdo connectors and have come up with nothing (nothing that looks like these).
David
Balijon
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musika said:
David,
I have been on the road for a couple of days, I picked-up a beautiful Midas PR05C studio console from '81 for my Midas 'Big-Sisters-Collection'.
I have to rig-up a suitable PSU before I can start testing with the MOD2, as the original PSU is MIA.
For testing your monitor amps: I would take out the transistors of the faulty channel and test them first, most likely one is dead. Another thing you could do is measure the voltages at the transistors of both channels and compare, this will provide you a good indication where things go wrong.
I have been looking for connectors too, to build an extender, no luck so far. I have seen people racking channels, maybe we could contact them for some spare connectors.
Theo
