Potato Cakes

I found I was missing a jumper for the GR Zero pot and now the meter goes to 0 when I power up. However, when I was going over and fixing this and a couple of other connections revealed I there are other issues going on. I'm getting very low output and the attack knob in the OFF position does not disengage the limiting that I am seeing with a volt meter on the output. The meter itself stays at 0 even with gain reduction taking place. My fear is the issue is on the rear of the bottom board which is very inconvenient to access with everything wired in place. The contacts from the input to phone jack to input attenuator seem fine and the same is true on the output. I did notice on the schematic there is a Hi/Lo Gain connection but I don't know if it's relevant to this build or if it was something they incorporated to connect to various pieces of gear at the time like the old Collins modules. I know it's a mistake somewhere, but I don't think I can stare at this schematic anymore tonight. Suggestions on where to look are definitely welcome.

Thanks!

Paul


Potato Cakes

What switch do you use here ?  I found out today, that I somehow forgot to order the ratio switch ...

I just saw this. Did you find the correct switches?

Thanks!

Paul

Potato Cakes

It works!

It turns out the biggest culprit was that Test 1, 2, and operate functions are backwards for me. I am using a 2x6 switch since Pole 3 on the wiring diagram for S3 is not being used. I checked my wiring twice and confirmed that I didn't have a stop pin incorrectly installed and it still behaves this way. Maybe the wiring is meant to be shown on the other side of the PCB. All I know is when switched to Test 2 I get the full functionality of the unit.

I also had a number of errors on my part that I found when troubleshooting:

- Missing ground connection for C10
- Wires for Pb and Pc on the ratio switch were backwards. This gives you an incorrect voltage for V4 on Pins 1 and 5 according to the schematic. This also made me realize that I wasn't getting compression before but just attenuation from the output transformer as the different windings were being selected.
- Solder bridge on between bottom solder pads R19 R20 and the trace below it. I may have done this after initially thought I was finished when I was trying to reflow solder to make sure the rivets were properly connected. What was annoying was that I couldn't resolve this with some wick. I had to take a razor and cut away material between the trace and the solder pad to ensure they weren't shorting together.

There are some other things I found to be incorrect with the wiring diagram which I noticed during initial signal tests.

First was the wiring of the output attenuator. I am using the Bourns T-Pads from Jeff at CAPI and when wiring the In/Out according to the diagram, turning clockwise decreased output signal, swapping those connection fixed that. See below

On the output Venier, CW and the wiper are shorted together which mirrors the input Venier control. This decreases the signal when turning CW. Swapping those connections to match the input Venier fixes this.

For S4 as related to the switched potentiometer that I have, the wiring is also backwards. With the attack turned to the off position, the center solder lug for S4 should be shorting to ground. Again, this maybe because I have a different switch from what was modeled in the drawing, so one needs to check which solder lug is shorting to the center lug when in the off position and make that the ground connection.

I still need to verify the attack and release controls function properly with the current wiring.

One thing to be aware of is when applying solder to the rivets to ensure a proper connection is you may have turn up your iron a little hotter than normal. I usually have my iron around 750F and at this temperature what I found was as I applied solder it wasn't heating up the rivet quickly enough to create a bond, pushing solder to the other side forming large blobs which I had to remove. This was what caused the solder bridge mentioned above. When it turned the iron up to 800F it seemed to resolve this.

My last tip is before you install the boards, double check that all the of rivets are properly soldered to the traces and verify connections and that you do not have any shorts between points that are close but are not supposed to touch. Pulling these boards out after they've been wired into place is no fun.

Now I just need some knobs and this goes in the rack. I had high hopes for neatness of the wiring, but during the course of troubleshooting it starting to get a bit messy.

On to the second one!

Thanks!

Paul
« Last Edit: July 17, 2019, 12:03:31 AM by Potato Cakes »

Potato Cakes

One more.

I forgot to mention for the plate and cathode balance, I adjusted their respective pots till I got equal voltages on CW and CCW referenced to ground. Don't know if this is correct and I await the release of calibration documents for this project for the correct settings.

I must say again a huge thank you to Martin for this project. Getting this made from a couple of photos off the Internet and a schematic must have been quite the endeavor. I thank you very much for letting me be apart of this project. I hope I am able to answer any questions and help others complete successful builds.

Thanks!

Paul
« Last Edit: July 06, 2019, 01:04:50 AM by Potato Cakes »

Phrazemaster

I haven’t started my builds yet - and won’t for quite awhile.

But I want to thank you Paul for your hard work, openness, and sharing your experiences with us. It’s going to make it that much easier. You and JLStudios are blazing the trail that Martin laid out.

Thanks to all of you!

Mike
***********************
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*****
** * Kablooie!

Potato Cakes

Does anyone know of a source for the correct style knobs stateside? I know Don Audio has them but it would be faster to get them on this side of the pond.

Thanks!

Paul

rainton

Does anyone know of a source for the correct style knobs stateside? I know Don Audio has them but it would be faster to get them on this side of the pond.

Thanks!

Paul


You can get the original knobs at www.studioelectronics.biz


Potato Cakes

Got it. Thanks for the link.

I found some NOS versions for like $37 a piece. I don't need mine to be that vintage looking.

Thanks!

Paul

Congrats on the build! Paul.
I will try and post calibration.In next day. Busy..But Its fairly simple.......
Attack and Release mid point.
Attenuators full clock wise.
Test position 1
Plate balance trim pot for min deflection ( In either direction )
Cathode balance trim pot min deflection ( In either direction )

Those are the basis steps......Will post the doc in a bit.

I would like to see the pcb,s thicker 1/8" thick so they do not flex.
Being longer then the original and thinner they bend to much!
The traces should be of higher quality silver so they take the solder better.
I have a feeling old lead solder might be better to use.....

Other then that Its a great kit! But It can be even better.
John

Potato Cakes

Sounds good. Looking forward to seeing the info you have. I'll wait till I complete the other one and do them together (hopefully this weekend).

Years ago I bought a bunch of Sn/Pb lead that I still use. The lead free stuff seems to create more problems than I normally cause for myself.

Thanks!

Paul


Potato Cakes

This is probably a silly question, but is the area around the PSU board supposed to generate a noticeably high amount of heat? I've never used one the original units but the fact the PSU resistors have such a high wattage rating I am guessing this is normal for this circuit.

Thanks!

Paul


Yes Yes and Yes...:-) This design  gets VERY Hot on the Power resistors!
The original Sprague Koolohm are some of the best!!! As in the originals..Look at the originals and see ALL
of the Burned/Brown circuit boards! Thats how hot they get.
As long as your voltages measure correct......Its right........
John

Potato Cakes

Yes Yes and Yes...:-) This design  gets VERY Hot on the Power resistors!
The original Sprague Koolohm are some of the best!!! As in the originals..Look at the originals and see ALL
of the Burned/Brown circuit boards! Thats how hot they get.
As long as your voltages measure correct......Its right........
John

That's what I figured. The voltages are measuring well with the tolerance of the components being used.

I've yet to do any actual listening tests yet. I'm waiting on the T-Pads to come in for the second unit (tomorrow I hope) and then test them both.

Thanks!

Paul

Potato Cakes

I got the second one up and running. I manage to swap the same two of pole connections on the ratio switch as before, even while trying to avoid making that mistake again! I also had a rivet not making a connection to it's related trace, but fortunately it was on the side facing inward and was easy to spot.

Now I have one issue that is seemingly minor but mind boggling that I would like to try to sort out before putting in the rack. It seems the input Venier control has no affect on the signal. I can measure the same resistance changes between the input terminal and OUT connection on the attenuator on both units. The input controls work the same as well. What is confusing is if the potentiometer is reading correctly and wired the same as the working example (as per the wiring guide), then they should behave the same as they are an inline with the input transformer and everything else is functioning properly. I certainly don't see how I could be getting the same measurements but one is a faulty part. I'll poke around some more when I am about to get back to this.

Thanks!

Paul

Potato Cakes

I went to do some more testing and listening on these units today and found some more errors. I first thought there was an issue with the wiring diagram in regards to the output pot. For some reason during my initial tests I thought it should be backwards, but it is indeed correct on the wiring layout. The Output Venier does need to have the strap swapped so that the wiper and CCW are shorted together. The input Venier on the second one still baffles me.

On the first one that was built, it seems to work great. I do get a buzz when the output control is turned past halfway, but keeping it at 11 O'clock is more than loud enough. The buzz does not change with adjusting the input control. I am wondering what is an acceptable level of noise on these units. The attack, release, and ratio controls all seem to respond correctly. I feel the noise could be less, but I wasn't plugged into the balanced power I use for my studio, so maybe that will help. I do have issues with EMI from somewhere as when tracking guitars, the player has to sit on a stool and rotate to find the spot where it cancels out, but I don't know if this is the same thing.

On the second one, there is a masssive increase in gain and the buzz is at a very high level. Somewhere at the input past the attenuator where signal is feeding into V4 is where I am suspecting as the gain reduction is much more sensitive than the other one and the throughput gain is wildly different. Like the other one, the input control does not change the noise level. I do seem to be getting correct voltages, but that may require double checking as well. The only thing that my mind keeps going to the Hi/Lo settings as shown after the input transformer, but followed the wiring diagram the same for each build, so I am wondering where a short or open connection to cause the gain to be wildly different. The ratio settings seem to be correct as I do hear and see on the meter the behavior I would expect at the various ratios.

I don't think I can look at it any more tonight. I'll circle back to this tomorrow and see if any bright ideas come to mind.

Thanks!

Paul

I will say this again......
Mine works perfect!!!!
If you use anything other then what was used in the original...Asking for problems. IMHO.
The 300 ohm Verniers must be wire wound.....
Centralab WN or WW 300 are the PROPER ones. 
Langevin where used in 90% of the 176,s  600/600 ohm. ( Some Daven) I have seen in person and on 100,s of pictures of actual
176,s......
I copied mine down to the exact! Screws etc.......
My only gripe with this kit is the PCB,s are to THIN! And to long......They need to be shorter and thicker.
And better quality silver traces.
Check every solder joint!!!
My Attenuation works perfect on my unit.....Just like my 175b.......At 12:1 ratio sounds JUST like my 175b........
« Last Edit: July 17, 2019, 07:17:39 PM by jslstudio »

rainton



I'm pretty sure it's minor issues Paul. Great to hear that for the most part your units are up and running!

Back then when I built my prototype I spent days to trace the signal through the circuit even though that included finding errors in the original pcb layout and schematics - at least they were present in the original service manual.

I have to admit it's always worth to double check the solder joints at turrets of the pcbs while tracing the signal. I have been talking to Livingnote about the pcbs and they will be thicker next time and we'll also try to find ways to improve them otherwise.

Back then, after realizing how crucial it was, I made sure I added some solder to every single fork turret and rivet from both sides.

My unit has been working perfectly ever since - even though I used 500ohm pots with a resistor in parallel for the verniers, so your choice of pots there is not as crucial as long as you use good quality pots.
I eventually replaced my "regular" vernier pots with switchable ones as I modded my unit along the way and it still works great.
I used push-pull switched pots for both verniers - the input one switches the unit to hi-gain when pulled (optional as per the original pcb) and the output vernier switches the meter from GR to OUTPUT when pulled...

There are some more mods - even ones that were offered by UA directly back then. Such as the 600 Ohm T-Pad option for input and output attenuators which - according to the service manual gives you an additional 6db ea.

The optional t-pad attenuators were Langevin AT-601 and the actual model number of the 176 would vary depending on wether input, output or both were equipped with the t-pad option.

The models were: standard 176, 176T or 176TT depending on what was in there - and BTW the list price for the base model was $480 back then, while the 176TT was $520.50

Yes 500 bucks for a 176  hahahaha :D
Last time I checked now was 12K... :o

Whatever model the unit actually was, the front panel obviously always just said "176"

The standard model mostly used DAVEN LA-353G attenuators as per the original schematics (attached below)

In my unit I had  DAVEN LA-353G for both - input and output, but I eventually swapped the output attenuators for a 600 Ohm T-pad - so my unit is actually a 176T.

Anyway - back to topic. The unit shouldn't be too noisy actually - at least with my unit I was surprised how quiet it was in terms of noise or hum. If the hum is noticeable double check your ground wires and routing. In this circuit pretty much every ground wire is routed to that one star ground left of the PSU board.
But if your unit was not plugged into the balanced power and you have problems with interferences or hum in your studio anyway that might very well be the culprit for hum being introduced.

The gain being wildly different on the second unit IMHO is pointing towards a cold solder joint.

Also check your 6BC8 tubes - the first one I tried in my unit was gassy - even though it tested well on the tube tester, and also could be balanced during calibration, the signal distorted pretty easily and the meter would drift wildly in GR setting when turning the release pot...

...another 6BC8 fixed all of that.



Potato Cakes

Did you have to use shielded cable anywhere?

For the Venier controls, the 500R carbon track ones with a 750R  resistor parallel is working fine except for one of them where it doesn't seem to doing anything. I may have a bad pot and not know it. The Bourns T-pads are also working well and have a very nice tactile feel to them. I'm not sure what advantage electrically the Daven stepped T-pad has unless there is a shielding component which I am not aware. The same for the wire wound 300R pots.

I did start out on this build trying to acquire all original parts, but I could not seem to find any of the Daven/Langevin T-pads with the correct values nor the 0.5uF oil can caps that had the proper dimensions. So I had to decide if I wanted to keep spending time scouring the internet for parts that are hard to find and command a high premium or if I just wanted to get the units built with equivalents that are of high quality. I did use original UTC input and interstage transformers as well as much quality NOS tubes that I could find. The 12AX7 and GZ34 are modern Mallory tubes. For 0.5uF I used nice 0.47uF film caps.

So circling back to my post above, the noise I get when the output gain on the "properly" working unit is above halfway is more of a buzz and not a hum that one might experience from AC heaters. The noise is the same type on both builds. The input gain issue I know I'll find and sort out as it shouldn't be hard to track down, it's my laziness which is the biggest obstacle. Is there anywhere in the build where shielded cable is necessary? When you crank open the output on either of your units do you experience any type of noise other than the natural noise floor of this circuit? I will check solder joints again but I did test continuity with all the solder points and their respective traces on the both boards before I installed them. Not saying that I didn't miss anyone of them, but it is less likely than some other error of mine.

I will say that with the properly gained unit of mine, it does sound very nice just testing is with program material. I'm down to finding a way to get them quieter.

Thanks!

Paul

Potato Cakes

Hi, Martin,

I'm pretty sure it's something easy/silly regarding the gain issue. Most of my build issues usually are.

I'll double check with the balanced power source. Other builds I can initially test with regular wall power and be just fine, but they usually have a well regulated PSU.

$500 for a UA176 is also as crazy to think about as dumpster diving for Fairchild 670's in the 80's.

I did have my star ground moved closer to where the power comes into the unit as that is what seems to recommended in modern electronics and I have used this on every other build of mine with no noise issues. I'm not sure if this would matter in this instance as well.

I don't seem to have any sort of issue with the 6BC8 tubes as I'm not getting any distortion or strange meter behavior but it is good to know that having those issues could be a bad tube.

I'll report back when I sort out the gain issue then I'll move on to the noise issue.

Thanks!

Paul

Potato Cakes

Got back at it tonight.

The gain difference was due to having missed a rivet that connects R3 and R4, which was where I was suspecting to be the vicinity of the problem. All the venier controls are working properly now. Also, both units seem to behave the same with matching control settings.

I connected the first unit to properly balanced power and +4 gear and the noise floor is significantly lower. I do still hear the buzz when I crank on the headphone volume and the output attenuator, but I get so much level at that point that it nearly clips the converters. At the halfway mark it is much quieter and I still get a very usable level. The second unit still has a noise problem even though the gain issue is resolved. Through a happy accident whilst doing other tests, I found that I am getting 2.4VAC between ground and either hot or cold connections on the XLR immediately on power up with no signal present. The sound is the same buzz as before at a higher level than the other unit with what sounds like a 60Hz hum. There is no voltage between hot and cold themselves. On the other unit I am getting 0.06VAC between audio pins and ground. This is also taking place with both the GZ34 and OB2 pulled. So I thought maybe the power transformer might have an issue with the center tap so I pulled that and the voltage dropped to 1.8VAC, which means that maybe somehow the one yellow lead on the power transformer that ties to pin 8 of the GZ34 is somehow leaking voltage across C11D to ground. For fun, I pulled all the tubes and I get 1.1VAC immediately on power up (with no tubes I quickly turned it off to prevent the H+ from going too high from having no load). So I'm not exactly sure how this much voltage is getting leaked to the ground. I'm afraid I'm going to have to pull it all apart and ruin my fairly decent wiring job.

So this brings me to some more questions:

What is an acceptable noise level for this circuit? Is there a knowable S/N ratio I can use to compare to my quieter unit (1kHz tone at 0dBu with input and output controls set to halfway and compressor turned off)? I also noticed that there are two ground connections on each of the PCBs. Is it necessary to use separate connections or can they be tied together and then sent to the star point? I will admit I did combine grounds on each PCB on both builds. This means I can rule out the combined grounds for mystery voltage on the one unit, but I am wondering if separating them would make the other unit quieter.

On a side note, I did manage to find an error in the wiring guide. In the layout, Pb of the ratio switch, which connects all of the color/white leads from the output transformer, is connected to C8A and Pa, which connects to the ratio resistors, is connected to C7. On the schematic it is reverse of this and this is how I have mine wired which gives me compression. The other way does not compress.

I probably still have a bad solder joint on the noisy unit that I haven't found or even missed a connection. I just don't want to unnecessarily undo a bunch of work if it's not needed.

Thanks!

Paul


 

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