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Post by mcirish on Feb 2, 2023 9:40:55 GMT -6
I picked up a second hand Stam 1073MPA from a forum member a few months ago. It sounds good when it's working. I do have a problem with one of the channels. It took me a month to finally track down what piece in the signal chain was causing this. I thought for the longest time that the 76D was the source of the problem, but now I'm pretty confident that it is fine.
1) path: CM7 - 1073MPA - 76D - Lynx Aurora
2) I will be recording a vocal and suddenly the level completely drops out. Probably down 32dB
3) At first I thought it was the AS 76D - took apart and cleaned - even replaced the input attenuator (supplied by AS- those guys are great) - the 76D was not the problem
4) Took apart the 1073MPA and cleaned all connectors, including the 1/4" jack - didn't have any problems
5) In a session a couple days ago, I changed the input attenuation on the 1073MPA from 35dB to 30dB - the signal dropped out 24-32dB (a guess)
6) Tried changing switch settings, no luck
7) Plugged in a 1/4" plug and then unplugged it = back to normal operation
I have not had time to dive into it yet but from what I remember, the normalizing pins of the 1/4" jack are not being used. Three wires go to the jack. It's wired as a stereo jack. I assume the ring and sleeve get shorted when a cord is plugged into the 1/4" jack. Not sure why/how the volume of the mic pre would drop when there is no 1/4" plug inserted. I sent a note to Stam asking for a schematic of the 1/4" input circuit so I could diagnose further. No reply so far.
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Post by svart on Feb 2, 2023 9:55:24 GMT -6
I picked up a second hand Stam 1073MPA from a forum member a few months ago. It sounds good when it's working. I do have a problem with one of the channels. It took me a month to finally track down what piece in the signal chain was causing this. I thought for the longest time that the 76D was the source of the problem, but now I'm pretty confident that it is fine. 1) path: CM7 - 1073MPA - 76D - Lynx Aurora 2) I will be recording a vocal and suddenly the level completely drops out. Probably down 32dB 3) At first I thought it was the AS 76D - took apart and cleaned - even replaced the input attenuator (supplied by AS- those guys are great) - the 76D was not the problem 4) Took apart the 1073MPA and cleaned all connectors, including the 1/4" jack - didn't have any problems 5) In a session a couple days ago, I changed the input attenuation on the 1073MPA from 35dB to 30dB - the signal dropped out 24-32dB (a guess) 6) Tried changing switch settings, no luck 7) Plugged in a 1/4" plug and then unplugged it = back to normal operation I have not had time to dive into it yet but from what I remember, the normalizing pins of the 1/4" jack are not being used. Three wires go to the jack. It's wired as a stereo jack. I assume the ring and sleeve get shorted when a cord is plugged into the 1/4" jack. Not sure why/how the volume of the mic pre would drop when there is no 1/4" plug inserted. I sent a note to Stam asking for a schematic of the 1/4" input circuit so I could diagnose further. No reply so far. The 1/4 most likely has a shorting switch so that the signal passes through it when nothing is plugged in. The contact points are probably tarnished and can make a high-ohmic connection if any small amount of current is passing through them. Probably need some Deoxit sprayed in there. |
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Post by mcirish on Feb 2, 2023 10:19:33 GMT -6
It doesn't have any shortingbon the jack. I also did use deoxit on them as well. It's pretty intermittent but I can always get it to stopped screwing up if I plug and unplug a 1/4" plug. Very odd.it acts like it's a jack issue but I don't see the signal going though it, visually. I guess I will crack out the scope
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Post by svart on Feb 2, 2023 10:33:35 GMT -6
It doesn't have any shortingbon the jack. I also did use deoxit on them as well. It's pretty intermittent but I can always get it to stopped screwing up if I plug and unplug a 1/4" plug. Very odd.it acts like it's a jack issue but I don't see the signal going though it, visually. I guess I will crack out the scope I'd be interested to see what you find. It was common to insert a shorting jack and a resistor between the input transformer and the switched attenuator to add DI to neve modules that didn't have it, which would cause this exact problem you're seeing. |
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Post by mcirish on Feb 2, 2023 10:33:52 GMT -6
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Post by svart on Feb 2, 2023 10:45:05 GMT -6
What's intersting is that there are 3 wires going to that 1/4. Instruments only need 2 wires.. And that's a shorting plug, but they don't have it hooked up like i'd expect for a shorting plug. It's almost like they're using the Ring to trigger something else when it's grounded. Do you hear a relay click when you plug/unplug the 1/4? |
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Post by mcirish on Feb 2, 2023 10:50:39 GMT -6
I didn't notice a relay sound but you could be right. There are three relays on the PCB. One might be used for the 1/4". That would make sense. Odd that changing the input attenuator would occasionally cause the relay to flake out. I can get a replacement from Digikey and see if it fixes it, assuming the relay is in the signal path.
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Post by mcirish on Feb 3, 2023 12:21:43 GMT -6
Quick update. Found the specs on the relay and also found I had the equivalent Omron relay. Installed and will test a bit later. Can't think what else it might be off the top of my head. Looks like the coil gets activated via a bipolar transistor which is controlled via the 1/4" input jack when a plug shorts the ring to sleeve. It was so intermittent that it was hard to track down. Stam never came through with even a reply to my email, so I'm hoping I got it. Otherwise, I will trace out the circuit further.
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Post by svart on Feb 3, 2023 12:56:47 GMT -6
Quick update. Found the specs on the relay and also found I had the equivalent Omron relay. Installed and will test a bit later. Can't think what else it might be off the top of my head. Looks like the coil gets activated via a bipolar transistor which is controlled via the 1/4" input jack when a plug shorts the ring to sleeve. It was so intermittent that it was hard to track down. Stam never came through with even a reply to my email, so I'm hoping I got it. Otherwise, I will trace out the circuit further. Sounds likely at this point. I had a guitar amp that had weird cutouts similar to this and I replaced the relays in it and it fixed the issue. |
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Post by mcirish on Feb 3, 2023 22:53:37 GMT -6
So far, no issues with the new relay. It was so intermittent that it may take a while to be positive but I'm thinking it's fixed.
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Post by mcirish on Feb 5, 2023 10:41:29 GMT -6
well, bummer. It acted up again. Still, pluging and unpluging a 1/4" seems to bring it back. Very frustrating. Stam has still not responded to any email on this. I guess when I get some time, I will have to trace the circuit. I'm not feeling good about this preamp now. It's failed in three sessions. Unless I'm paying close attention, I didn't even notice the channel went out. Makes me look bad in front of clients. What a drag. I guess I will change the vocal chain back to the Hardy M1 instead of the Stam.
Mine is one of the older units. I see the new ones have a line switch as well. Not sure if that just cuts the gain for line input level or if that switch controls the input relay now. Sent another note to Stam. Hopefully, I will hear something soon.
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Post by svart on Feb 8, 2023 9:51:58 GMT -6
The other possibility is DC offset building up. Could be a bad coupling cap or something allowing the circuit to drift and the sudden plug/unplug could be discharging it. Just a spitball idea though.
It's a weird one. Thought the relay would fix it as it fit all the symptoms.
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Post by stam on Feb 9, 2023 1:15:16 GMT -6
well, bummer. It acted up again. Still, pluging and unpluging a 1/4" seems to bring it back. Very frustrating. Stam has still not responded to any email on this. I guess when I get some time, I will have to trace the circuit. I'm not feeling good about this preamp now. It's failed in three sessions. Unless I'm paying close attention, I didn't even notice the channel went out. Makes me look bad in front of clients. What a drag. I guess I will change the vocal chain back to the Hardy M1 instead of the Stam. Mine is one of the older units. I see the new ones have a line switch as well. Not sure if that just cuts the gain for line input level or if that switch controls the input relay now. Sent another note to Stam. Hopefully, I will hear something soon. This is a very old unit, I'd be happy to assist when we have some time next week My email is stamaudio@gmail.com |
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Post by mcirish on Feb 20, 2023 9:46:10 GMT -6
I had it pulled and sitting on the bench for a few day last week. No problems. I put it back in the rack and ran it for a few more days. It showed the problem three times over this last weekend. When it started acting up, I pulled everything out of the path, so I could be sure it was the preamp. I even swapped cables and input channels on the Lynx Aurora. The problem always follows the first channel of the preamp. I now don't think the relay was ever part of the problem. When the preamp lowers it's gain, I notice that the output trim pot acts as if it has DC on it. Once the problem clears, the output trim pot acts normal again. I also noticed that driving the input gain up full, gets it working again. That would explain the relay 'seemingly" being the problem. When you plug and unplug the 1/4" input, it sends a spike through the circuit, which clears the problem. Acts kind of like a bad connection, but I've checked them all. So, something in the circuit is failing, but what it is, is uncertain.
It's back on the bench today. Hopefully we can get it to mess up so we can find the defective component. It's so intermittent that this will be hard to trace. Using heat and freeze spray today. Schematics would be a big help. It doesn't exactly match any Neve schematics I have.
Honestly, I'd just sell this and buy something different, but my conscience won't allow that. I feel I need to solve this problem before moving it along to another user. I kind of wish the guy who sold it to me would have done the same.
The hunt continues.
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Post by svart on Feb 20, 2023 10:34:02 GMT -6
As mentioned before, it really could be DC building up.
You could let it fail and then measure DC at various places and compare between channels.
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Post by matt@IAA on Feb 20, 2023 22:08:15 GMT -6
That sat in my rack for a couple years. I fired it up and tested it before it left. Sorry I didn’t use it long enough to find an intermittent fault.  I read over the thread and don’t have any brilliant ideas. Are you able to probe to find where the signal loss happens while the unit is in the fault mode? |
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Post by mcirish on Feb 21, 2023 14:46:15 GMT -6
That sat in my rack for a couple years. I fired it up and tested it before it left. Sorry I didn’t use it long enough to find an intermittent fault. I read over the thread and don’t have any brilliant ideas. Are you able to probe to find where the signal loss happens while the unit is in the fault mode? Yep Matt, we have it on a bench with a scope on it just waiting... for it to mess up. But of course, it's not displaying the problem yet. I've swapped out cables and stripped away all the gear around it in the studio and it still had issues over a couple days. As I said, the hunt continues. I think I will swap out the other relays for that channel and touch up soldering on the board as well. I worked as a tech for 20 years before I got into IT work. It's a bit baffling. Maybe it doesn't like what's around it in the rack? That would be a first for me, but I may try to move it to the only other free rack space and run it some more. The worst part about it is that I just can't trust it during a session. The second channel has always worked just fine. On a side note; I'm very sorry if it sounded like I tossed you under the bus. My frustration got the better of me. I'm sorry my tone was harsh. I figured it hadn't had any problems for you. |
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Post by matt@IAA on Feb 21, 2023 19:31:11 GMT -6
All good man I genuinely feel bad about it.
Could it be related to the gain switch position? That adjusts NFB and turns on an extra amplification stage at higher gains …?
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Post by svart on Feb 22, 2023 7:47:42 GMT -6
All good man I genuinely feel bad about it. Could it be related to the gain switch position? That adjusts NFB and turns on an extra amplification stage at higher gains …? I wonder if it could be oscillation then. Something much higher than can be heard. |
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Post by mcirish on Feb 22, 2023 10:45:44 GMT -6
All good man I genuinely feel bad about it. Could it be related to the gain switch position? That adjusts NFB and turns on an extra amplification stage at higher gains …? I wonder if it could be oscillation then. Something much higher than can be heard. I'd see the oscillation on a scope. I will try setting it to various gain levels and let it sit for a while at each. It is pretty crazy. If it would just act up, I'm sure I could find it quickly. I'm on day three of it sitting on the bench with a scope. No problems so far. |
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Post by EmRR on Feb 22, 2023 12:11:38 GMT -6
Reminds me of a Gates SA-39 I had on the bench for weeks like this. Once I saw a problem similar to what the owner experienced, but it was a different problem they hadn't seen! I fixed that one. We punted and rebuilt a lot of it. They got it back, the problem was still there, I never once saw it. Good luck.....
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Post by matt@IAA on Feb 22, 2023 12:27:52 GMT -6
If it is instability, capacitive load on the output could potentially be involved….
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Post by mcirish on Feb 27, 2023 16:31:51 GMT -6
Finally got it to show the problem today. The first channel is going into some high frequency oscillation. It can be seen on the output trim pot as well. Used some freeze spray and it went away when we hit a particular transistor. Ordered some BC550B transistors and will further test once they come in.
So, I'm not crazy... I'll let my wife know.
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Post by svart on Feb 27, 2023 17:12:53 GMT -6
Finally got it to show the problem today. The first channel is going into some high frequency oscillation. It can be seen on the output trim pot as well. Used some freeze spray and it went away when we hit a particular transistor. Ordered some BC550B transistors and will further test once they come in. So, I'm not crazy... I'll let my wife know. Possibly a bad bypass/stabilty cap around it. Doesn't necessarily mean the transistor is bad. |
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Post by mcirish on Feb 27, 2023 17:26:43 GMT -6
Finally got it to show the problem today. The first channel is going into some high frequency oscillation. It can be seen on the output trim pot as well. Used some freeze spray and it went away when we hit a particular transistor. Ordered some BC550B transistors and will further test once they come in. So, I'm not crazy... I'll let my wife know. Possibly a bad bypass/stabilty cap around it. Doesn't necessarily mean the transistor is bad. Thought the same thing. I have to check out the caps and see if they have markings as to what their value is. Most are polystyrene. |
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