UA 2-610 Troubleshooting

[Nishmaster]

Hey all,

Kinda stumped with a fix on my bench. I've got a UA 2-610 where one of the channels will suddenly drop immensely in volume and then become terribly distorted. If I crank the gain knob enough, BAM! Music comes through loud and clear and I can return the gain knob to the original value for a while. Eventually it will cut out again, especially if there is no signal going through it for a while.

I'm not hugely familiar with this circuit (although it looks fairly simple, 1 12AX7 and 1 12AY7, seems like the gain knob is after the first two stages, which are the 12AX7), and I'm not seeing any usual suspect so far like cold or cracked joints. UA has replaced the tube multiple times, but it always reverts to this behavior eventually. Strangely enough, it's only channel 1, the other channel has never had any issue. Sadly UA will not provide a schemo and I'm not going to send it back to them just to swap out a tube yet again.

Anyone have any quick thoughts?

-Matt

 

[Stagefright13]

I had A problem once on A 610 solo... The tube sockets looked like they were soldered fine. But had to reflow them anyway to get one of them to work.

John

 

[mitsos]

this is supposed to be a rev eng of the original, vintage, 610 micpre.  John Hinson worked for the new UA at the time, and did the rev eng for them.  I understand the output trafo was changed (he says by the accountants) possibly the output circuitry but I don't know exactly what, never had one open.  John Hinson sometimes posts here under Winston O'Boogie, maybe he'll chime in. But since you got the thing open, maybe you can confirm if this is it or even close?

 

[dustbro]

this is supposed to be a rev eng of the original, vintage, 610 micpre.

Has anyone ever worked on an original 610 strip? I've seen a lot of UA data, but I've NEVER seen this circuit in any original UA design.
JH is a strange dude, so I'd like to see pics and confirmation from another source before I trust that schematic.

 

[mitsos]

well, I saw someone post pics of one he had, can't find it now, but I don't think the guy posted much more about it. could be wrong though..  MrClunk started buildng one but I think he had some issues and stopped, not sure where he left off.  seems there might be some errors in the circuit.

 

[Jean Clochet]

I'd like to see pics and confirmation from another source before I trust that schematic.

The schematic is correct.  There is a large Universal Audio blueprint which is the same circuit as this but has a bit more info for how the module wiring is brought out to a McMurdo connector for wiring it into a desk.  I have seen a copy of the blueprint within the last few years (it's a large format document) but I don't have a copy myself.

I think (don't know for sure) the reason we may have not have seen too many original 610 docs is because the modules weren't released in the same way as most of the other Universal stuff.  I don't even know that Universal sold pre-built desks that had the modules in them but only sold the modules by themselves for us old-timer desk DIYers.  Again, I could be wrong on that but, regardless, I don't think you should worry about the posted PDF being correct as far as the original 610. 
Don't know how it fits in with the 2-610 though.


Cheers.

 

[Jean Clochet]

seems there might be some errors in the circuit.

Hi Mitos,
As I said, I don't have a copy of the old blueprint but I just too a look another look at the PDF you posted and I don't see anything that could be a mistake.  Of course, I don't have part values memorised* but everything looks in accordance with what would be appropriate for a circuit like this. 
It says on the bottom that it's copied from document C-10068 so that's the one to look for in case you're still worried. 
I see that it also states the connector was an Amphenol whereas I said it was a McMurdo.  That's just me jumping to a European-centric conclusion  8)

*JH actually had/has the knack of memorising complete schematics (quite complex ones) after just working on a unit once.  I've only known one other person who could do that.

 

[mitsos]

Actually Jean, should have been more careful with my choice of words. I am not the one to judge a circuit like this, but it's been mentioned a few times that this may not be the actual circuit, or that it might contain errors.  I still plan on making it, and, if it doesn't blow up, futz with it until it sounds good.

here is one of those comments:
http://www.groupdiy.com/index.php?topic=38943.msg481212#msg481212

If Abbeys gain comment is correct then instead of shorting the resistor, maybe UA might have replaced it with a variable resistor which would then become the gain knob. Am I talking out my @$$?  How else might have they have gotten variable gain? 

thanks for chiming in!

 

[Jean Clochet]

here is one of those comments:
http://www.groupdiy.com/index.php?topic=38943.msg481212#msg481212

Thanks for pointing that out.

Beware, the schemo is flawed.
The Lo Gain/Hi Gain switch should just short R11 and C14 go permanently to junction of C4-R11.

The schematic states that this is an internal link, NOT a switch.  There's a difference here and a link does exactly as it's  supposed to for gain change, which is  short out R11.

I have never seen an old 610 with a gain switch and I don't think any of them had one so this must have been something that was set once internally, put in the desk and left alone.
As for the 2-610 and a gain control, I could see how just using a log pot or a switched set of resistors would be the way to go.  You'd possibly be looking at trying a bigger range of feedback R change than the old schematic though as it looks like the Hi/Low difference is only anout 9 dB on the old module.

I seriously doubt C15 going from V1B cathode to V2A cathode. I don't know where it is supposed to go (there are several possibilities).

Hmm? Well I'd have to ask abbey why?  I don't see any reason to doubt it's as was on the original doc.  I also remember there being a discussion on here about it at some point and JH posting a schematic snippet showing a later revision or alternate version of the old 610 that indicated omiting this particular cap but adding some other part or parts en lieu.  It doesn't make sense to me that direct mention was made of this cap being exactly where it is shown and then saying it was changed out in version xy or z. 


Anyway, I don't know anything about the new 2-610 and maybe none of this is helpful to the OP, in which case I'm sorry for the diversion

 

[Jean Clochet]

I looked through the old posts by JH and found the thread I remembered. 
I'll re-attach the schematic of changes to this thread but the old thread I pulled it from is here:

http://www.groupdiy.com/index.php?topic=13038.msg419479#msg419479

Hope it helps out,

Cheers..

 

[mitsos]

Stupid me, I have that file somewhere (see me below that post?)  : 

I don't know if the original is like this, but I think I'll try to get access to a modern 610 and see what's been done.  I'm gonna try to wind that output trafo myself for now and see if I can't get this thing going again.

Nishmaster, hope this info actually helps you in some way. 

 

[Jean Clochet]

Stupid me, I have that file somewhere (see me below that post?)  : 

I missed it but now I see you plain as day 

It looks like the 610 traffo (original) has a 50:1 impedance/7:1 turns ratio and is also the load for the 12AY7 valve.
Not too much of a difficult job.
I don't know where you're located but there is probably a suitable part already available from someone such as Sowter or Lundahl.
I'm not as familiar with US manufacturer's parts but I daresay they have something too that would work.

Of course, I understand the option or satisfaction of winding your own if that's what you prefer. 
I don't know that I'd be that good at it myself although I've wound a few EL34  power amp units in my time. 


Any road, yep, I hope Nishmaster gets something out of this too.

 

[Nishmaster]

Thanks for the info all. When I can, I'll put some photos up of the unit.

The 2-610 has two different gain controls: one is a rotary switch labeled "Gain" with the following settings: -10, -5, 0, +5, +10. Then, there is an additional large knob labeled "Level" which is a pot. This one has no dB markings, just 0-10. My assumption is that the "Gain" switch sets the gain of the first two stages as in the internal link in the schemo, but expanded. Also assuming that the "Level" pot is just a voltage divider before the 12AY7.

Additionally, the LF and HF switches have a much expanded control set, with the values going from +9 to -9 in 11 steps. There is also a 3 position switch that changes the center frequency.

I'll try and reflow the sockets and see where that gets me.

The unit is described here: http://www.uaudio.com/hardware/mic-preamps/2-610.html. They also have a guts picture on that page, but it's hard to see everything.

-Matt

 

[mitsos]

Jean, thanks again. I'm in brazil and have access to a very rudimentary (hand tensioned wire) machine, and someone to run it for me.  We've become friends by now, he's woudn a bunch of stuff for me,  including some protos for the pultec I've been working on for ages.  Of course, he's not doing the trafos in the "kits" those will be done in the US on CNC equipment, but that kind of stuff is only really available for larger quantities.  Sowter is out for me, they end up very pricey when yuo factor in the GBP exchange rate! Same with Lundahl. Plus I'm american, and believe we have the best iron kidding aside, I can get them made in the states, and if I were to do nickel that's what I'd have to do (no nickel lams available here) but I can find M4 (actually I'm not sure if that's what it is, it's a bit thinner than M6 though) in EI 625, 75, etc.  I'm getting kinda stoked about this preamp again, I think I'll try it out finally.

Nish, looks like the 2-610 migh be closer to the original than the mono one,which I believe has a pot for gain. but it must be doing the same thing I guess. 

let us know if you get to the bottom of it.  While you've got it out and can compare a bit to the schemo, let us know if you find any glaring differences. I'd be interested to know what they're doing at the output, since that's where JH says they changed it. thanks in advance.

 

[Jean Clochet]

I'm american, and believe we have the best iron

  Maybe so. 
I suppose a septic* piece of iron is more in keeping anyway since it's a septic piece of kit you're building 

Given when this was first built (the original), I'd bet the output transformer didn't contain nickel although there's nothing wrong with using Ni in a modern iteration. 


Nishmaster, thanks for the info about the 2-610.  Hopefully you'll be able to fix it where Universal failed.  It seems madness for them to have just replaced the tube multiple times when, obviously, after the first time around the issue was elsewhere. 
Let us know how you go on with the reflow.

* rhyming slang;  Septic = septic tank = yank.

 

[mitsos]

that's part of my thinking. Had a talk with Tom Reichenbach a few years back about this trafo, he too expressed a bit of disdain at UAs choices in output trafo design, but didn't have details off the top of his head.  He did recommend a gapped trafo, but I'm not going to make it that easy on myself.  Probably just going to build a monster truck sized trafo and call it a day.

Septic?!? Wow, I looked that up, it's actually true!  bastards! 

Thanks again for the input, this project has been on my mind for a while, gonna have to get it done now!

good luck Nishmaster!

 

[lassoharp]

If the online schemo for original is correct, 55H pri sounds like a $$$ part.

mitsos, if you wind a gapped OT of that spec that is good for high current please put them in the WM - I will buy.  ;D

 

[rickc]

this sounds like a tube whose grid doesn't have a path to ground.

get the thing to act up, then probe each tube grid with your DMM. Chances are it has a low enough input resistance to let the tube restore its bias conditions. when you find the right spot, the thing will pass signal again. Then you need to poke around that part of the circuit to figure out what is going on.

you could also just put a 1meg resistor on a clip lead. ground the clip lead, then use the free-end of the resistor to probe the tube grid (pins 2, 7 on a 9a-based tube).
9a is the designation given to the familiar dual triodes; 12a*7, 12bh7, 6072, 5751, etc. It's a handy way to instantly know that something is socket compatible.

--rickc

 

[mitsos]

If the online schemo for original is correct, 55H pri sounds like a $$$ part.

mitsos, if you wind a gapped OT of that spec that is good for high current please put them in the WM - I will buy.  ;D

Well, I don't know that I'm going into the trafo-winding business, but I need to make this for me, so... maybe we can move this to the drawing board and come up with a suitable design.  Do people have use for a 7:1 output trafo though? Back then Tom R suggested I go with 4:1, which is what some others have also said, but I'm stubborn, and a stickler for little details ..

 

[Nishmaster]

To be honest, I would be hard pressed to find a reason to clone this thing. At the studio we've got API, Daking, Manley, Focusrite, and this one is probably my last choice. I do use it on mixdown for bass often, but that's about it. It's kinda murky, with very low headroom. Dynamic or loud vocals are out, don't even try to use it on any drums, and definitely not acoustic guitars. When it distorts, it's not a terribly warm crunch; it's actually quite fizzy.

It does see some occasional use as a keyboard "de-harshing" device, although the tube side of our Manley TNT does the job better. Could be it's just not my sound preference, though. I also don't know how different it is than the original, but the basic circuit doesn't appear to be much (or at all) different from my cursory glance.

-Matt