AudioControl Richter Scale Series Three (Repairs/Mods)

[Cybershaman]

Wow! Great job! I'm very envious of both your knowledge and the test equipment you have. I wish I could drop one of my RS units off for you to massage. They obviously deserve better than I could ever do for them. :/

Which lamp did you end up using again? I want to make sure so I can get a couple on order to install in my units. I would like to get one of my RSs into my audio system and don't want to do that until I can actually read the meter.

BTW, I've been messing around with a pair of Polk Monitor 70 Series II speakers with their passive crossovers removed for a while now. Truth be told, it's been more of a crossover learning project and the unusual "2 1/2-way" design Polk used with these speakers has fascinated me for some reason. It's actually only been fairly recently that I really got them dialed in (using a MiniDSP 4x10 HD digital crossover). They sound pretty nice for a pair of fairly inexpensive speakers. I'm driving them with two vintage Carver TFM-15cb amps (the lower two and upper two woofers) and a two baby kit built S-5 Electronics monoblock tube amps (for the tweeters) that sit on top of each speaker. I use the MiniDSP to crossover to my subwoofer, too. I think I've got it dialed in pretty much where I want it right now, so I'm going to start soldering up a few resistor arrays around the value I'm using to plug into the RS. I plan on using the RS crossover because...

,,,I just bought a pair of DCM TFE200s that I just couldn't pass up. I used to have a pair of Timeframe 700s a long time ago (I lived in Ann Arbor, MI where they were made) and was always sad I sold them when I went on "walkabout" as a young lad. I know for a fact that the TFE200s "have complicated crossover problems" and I am planning on eventually doing to them what I've been doing with the Polks for the last (checks calendar), oh, jeeze, almost eight years now. I seem to have an addiction to forming obsessive relationships with complicated gear.

Anyway, if you could tell me what lamps you got, I'd really appreciate it. I'll keep following your progress and can't wait to hear how things go when you finally get them into your system. As always, the more details the better! If I can't work on my RS units like you can, I can at least live vicariously through you! (Seriously though, I wonder if there are any people out there who would work on my units? I wouldn't shy away from paying some good money to have them inspected/tweaked properly by someone who knows what they're doing and has the equipment. Know of anyone?  )

[Smoky]

Hi Cybershaman,

I come on EEVblog asking for help 99% of the time

Now about the lamp. My Richter Scale is setup for the lamp to run on 28VAC. Later versions of the RS use one half of the transformer winding and use a 14v lamp.

The transformer winding measured 27.5v actual so the #6838 wired lamp that I used does run a little dimmer than it should, and since the original lamp discolored the back of the meter from heat, I'm fine with it not being so bright.

I thought about adding a little rectifier board to power an LED but I'll wait and see how well the 6838 lamp holds up.

As for your 14v circuit, you may need to experiment. The only 14v lamp I see with a low candle-power and current requirement is a #9388. Again, you may want to measure the actual voltage because I see a couple 13.5v lamps listed too.

I think that AudioControl realized that the 28v lamps were running a little hot so they switched to a 14v circuit in later models 

Speaker crossovers are a different animal. I have a friend willing to sell me his ADS L1530's but they are long over-due for the capacitors in the crossovers to be replaced. Plus the drivers are most-likely needing of a re-cone job too so the deal may not happen (too risky).

I'd be happy to go through your units but I want to wait to see if my HP 35660A is able to run a frequency response test on them. That way, I can see how well all of the adjustments work.

I did check the ripple current on the 14.70v rail in the rectifier circuit. It measured ~28mV, sort of high IMO. I ordered some 560uf /35v caps (in place of the 470uf) to experiment with to see if it'll bring the ripple down a tad and not effect the voltage all that much.

Again, I'm just practicing/playing with this thing. Btw, here's a copy of the schematics uploaded by HalfApt on AudioKarma too.

[Cybershaman]

I'll try to measure the voltage for the lamp leads on my RSs soon. I just got new batteries for my multimeter, so I'm good to go now, measurement-wise. I just have to wait for a a good day when my pain issue isn't being that much of a bother. :/ I think I'll go ahead and order up some lamps, though.

Yeah, AC really messed up on those lamps. On the series 1 and 2 units, they only turn on when you engage the Analyze button. I guess throughout the three series they kept trying to up the cosmetic appeal of the units and figured that having the lamp for the meter on full time would make it look nicer, even though you hardly ever really use it. I guess they just used the same ol' bright/hot lamps they'd been using and, man, I haven't seen a single listing for a used Series 3 RS come up where the lamp wasn't burned out.

I'm fascinated by speaker crossovers. Now that am getting the DCM TFE200s to play with, I'm thinking of making some nice external passive crossovers for my Polks that will look nice enough to display. I am inspired by a guy who for a while was making these insanely expensive crossovers for nicer Polk speakers (and a very few other makes/models) for people. He went by VR3 Mods and, OMG, his crossovers were works of art. Worthy to be displayed. Unfortunately, he (and I guess he had a few others helping him) stopped doing that sort of thing. He's still active on the Polk forums. I dug up one of his posts that have pics so you can see the sort of thing/level of quality he was shooting for here: https://forum.polkaudio.com/discussion/174528/polk-audio-sda-srs-2-3-crossover-modification-thread He helped me out a few times. In fact, he modified some of his binding post plates to make it so that I could have three pairs of them on each plate so I could tri-amp my Monitor 70s, along with another plate for my Polk CS2 center channel speaker that I redid the crossover for as a fun little side project. So, I want to do something like that for my Model 70s. Just so they are "plug and play" instead of having to re-setup the digital active crossover. Plus I could maybe sell them. They sound pretty nice now, IMHO. I just can't decide if I want to replicate the stock Polk crossover (easy) or try to replicate my tri-amp setup.

I'd be honored to have you look at my RS units. Let me know when you get some free time and I'll send them to you. I will make it worth your while. We can talk via PM when we get to the point where you're done with yours.

I went to your other thread on Audio Karma. I've just been having a hard time lately physically, so it's been a bit of a challenge to do anything fun with my electronics/sound system. :/ I have to spend a LOT of time just sitting in bed and the pain makes it hard to concentrate. Hopefully things will improve. We'll see. I take life as it comes and try to live each moment to the fullest. Carpe diem is my motto! Take care and talk to you later! (I'll try to chime in at Audio Karma. I don't think I misread that resistor on the trim pot, did I. Check the pic for the Rev C board I posted. I'll double check the one on the Rev. B board. That one was shoddily soldered and kind of mushed down on the board. I must defend my honor! heheheh )

[Smoky]

As for that resistor/potentiometer combo, I don't believe that the resistor value is too critical as long as it is below 300 Ohms or so. HalfApt on AudioKarma measured his combination in circuit and he read ~450 Ohms. I think each unit had its pot adjusted on the bench to best match its microphone. The 1KOhm pot adds quite a bit of adjustability. I installed a 100 Ohm resistor in mine.

When you find some time, locate the two solder pads on the back of your boards that correspond to the resistor/pot combo and publish what you measure on your DMM for resistance. It'll be interesting to see where your units are set at.

Anywho, I'm making some test leads for my signal analyzer. I own some Harbour Ind. RG-316 coax but I'm also going to try some RG-400 (double-shielded) to make the BNC cables. I ordered some Amphenol ends and the proper crimping tool too 

[Cybershaman]

Sorry I haven't got back with you. Been having a rough go of it physically/medically these last couple of weeks.  I'll try to get that measurement for you soon.

[Cybershaman]

Rev. B unit: 672 ohms
Rev. C unit: 648 ohms

Does that sound like I measured it right?

[Cybershaman]

PS: I ordered some lamps...

Eiko 6838 T-1 Wire Terminal Halogen Bulb, 28V/0.024 Amp
Eiko 2182 14V .08A T1-3/4 Wire Terminal Halogen Bulb

I'm not sure about the 14V bulb. Like I said, I've been having a rough time lately and I ordered them with my phone after I went to bed. I was, like, "I'm ordering some lamps!"

[Cybershaman]

BTW, I took a look at the diodes in the rectifier circuit of my Rev. B unit and they're 1n4002 diodes. Thinking of doing what you did and swapping in some UF4004s and the caps while I'm at it. (I have some nice 470uF Nichicons looking for a home. ) While it might not make a HUGE difference, it would be something to do just for fun. I need to stay busy and distracted from the other stuff I have going on. I can't just sit around my house/in bed all of the time. It's really not healthy.

At any rate, can you think of any reason NOT to? Oh, yeah, I'll swap in a 100 ohm resistor like you did for the lamp, too. I definitely don't want to replicate what AC did when they first produced these units.

After your done with yours, I still wouldn't mind you taking a look at one or both of my units. Again, I'd make it worth your while and we could work something out via PM. Maybe do one at a time since I need to get one into my system mostly for subwoofer crossover duty. Anyway, hope all is going well with your RS. Can't wait to hear how it works in your system! Take care!

[Smoky]

Hi Cybershaman,

My policy has always been, if I pull a 30 year old electrolytic capacitor out to test it, it doesn't go back in, even if it's good. The reason I tested the four 470uf caps in my Richter Scale is because one was bulged on the top. Before you decide to replace any of the capacitors in the rectifier circuit, you need to measure the level of ripple. I'd like to see what your waveform looks like too.

I did not use a 100 Ohm resistor in the lamp circuit, I used a 270 Ohm. I used the 100 Ohm resistor in tandem with the 1K Ohm pot in the microphone circuit which gave me some adjustability (100 Ohm to 1.1K Ohm). I'm going to make a small rectifier board and use an LED if the lamp doesn't last. Btw, the amperage draw on the Halogen lamp you bought for your 14v circuit may burn hot. Did you check its candle power?

Your resistance measurements prove that each unit was aligned individually. HalfApt isn't sure what the bench reference is though or how to perform it.

I wish I could spend more time on electronics but my day job is kitchen and bath remodeling and it's been long hours lately!

[Cybershaman]

Hey! It's a good thing you have steady work! But, yeah, that does cut into fun time, though. :/

- That 14V lamp is 1 watt. Yeah, I had a feeling it might be too bright when I bought it. I don't really know where to get tiny lamp bulbs, so I got those from Amazon. Where do you recommend to get them from?

- So, you think they dialed in each mic for each RS it was packaged with, eh? Well, I better remember that when I finally get one of mine set up. I just have the one mic between them. That does seem pretty labor intensive when your shipping, what, thousands of units to customers. Wouldn't they have had to use some sort of (tiny) anachoic chamber or something? Maybe they just had a reference resistor circuit or something and just plugged that in to dial them in on the assembly line?

- I'll have to get back to you with the rectifier circuit ripple when I next have a relatively pain free day. I don't have a scope for the waveform, though. Just my DMM. I've actually been thinking of getting one. Suggestions?

- I was going to replace those caps and diodes just for something to do. I have the opposite situation from you; I have too MUCH time on my hands! (Pain/medical issues willing, of course. ) I do have to be careful to not get too carried away sometimes. Especially when working on old equipment.

Anyhoo, let me know a good place to get lamp bulbs and I'll see about ripple soon.

[Cybershaman]

PS: I just remembered that AC's Phase Coupled Activator Series 3 had a programmable 18db infrasonic filter. I THINK it was built into the crossover circuit, which also used a 16 pin resistor array to set the frequency just like the RS series 3. It was 14 pin with 6 resistors (there's no resistor in the middle slot). You calculated the frequency just like the crossover module. I could be wrong about where the filter resides, though. It might actually be in the bass restoration circuit. It's not shown on the simple signal flow diagrams in the manual. :/ Anyway, I just thought that was interesting. Peace!

[Smoky]

Here's a quote from HalfApt on AudioKarma:

"I've had a thought to adjusting the mic sensitivity without a calibrated signal:

    AC instructions say to turn the volume control up until the meter reads 0dB.
    instead, pre-set the volume control to your normal listening level, then adjust the trimmer to get 0dB. Best for your system."

Since I know that a fixed 200 Ohm resistor was installed in my unit, that's where I'll set the resistor/potentiometer combo for starters.

What I'm looking forward to is the comparing of the Hi and Lo output crossover points on the DSA.

About speakers, I've been following Troels Gravesen of Denmark. He builds custom speakers and sells the crossovers and drivers for each of his designs. The drivers he uses are state of the art, amazing stuff. I plan to build a pair:

http://www.troelsgravesen.dk/Diy_Loudspeaker_Projects.htm

He also created a subwoofer enclosure using ScanSpeak 30W/4558T00 subwoofers as one of the options. I have four new in the box from a trade-deal two years ago to put to good use! Woodworking tools, I have plenty of

[Cybershaman]

That's an interesting idea for adjusting the sensitivity.

I'll peruse Troels Gravesen's website. Looks pretty interesting. Thanks for sharing!

I would like to try and build a set of these someday: https://www.linkwitzlab.com/LXmini/LXmini+2.htm Partially because it looks like something that would be a little easier to build from a wheelchair standpoint. We'll see. That's a ways off in the future, though. I really do need a new subwoofer. I'm currently using a BIC Acoustech PL-200. They have a PL-200-II version now which moved the port from the rear to two in the front. Anyway, it's alright. But being in my small apartment it's standing wave central. I really need to find a new house soon so I can let my system breath a bit. Especially since I just got a pair of mint condition DCM Time Windows (build date: about 1983 or 84). I got them mostly for sentimental reasons. They really need space around them to sound properly. Anyway, I'm getting a bit off topic!

Should I just move over to the Audio Karma site for further RS discussion? Or is this your main thread? I'm not sure when I'll feel well enough to dig back into my RS project. :/ Hopefully it will be soon. Talk to you later!

[Smoky]

Things are going well Cybershaman!

I mostly post here on EEVBlog. I'm geared more towards improving my audio equipment that I have while I'm in the process of remodeling my house I'm planning to have it all come to completion around the same time

Anyway, I spent over six hours ordering parts for the two electronics projects I'm working on! This vintage gear takes some time to find the best fitting parts. My goal was to replace all of the Polyester caps with Polypropylene. I had to go through Digi-Key, Mouser, Arrow, and Newark to find them. Pin spacing had to be measured and crossed-over to make sure lead-length was sufficient on each part but they also needed to fit the space. I also wanted the Brand to be uniform and no distributor carried all of one series. Lots of fun though.

I ordered RG-400 cable, Amphenol BNC's ,and an Amp Pro Crimper, to make test cables for the analyzer too. I'm in no hurry.

Troels knows his stuff too. Click on one of those speakers and check out the drivers and tweeters he uses in his builds. Incredible.

[Cybershaman]

I can feel your good vibes, buddy! I'm happy your projects are coming along. Yes, I can relate to trying to find That One Kind of component. I am always amazed how places can stock all of these tiny things of such a wide variety of such and such a specific specification. And it seems like half the fun is hunting for the ones you need.

I might hop in the other thread once I feel better. Hopefully soon. I'm sorry I keep bringing up my physical condition. It's just that it's such a big obstacle for me. Playing with my hifi, both working on it and listening to it, is one of the few things that makes me happy. I just have to keep going, one foot in front of the other. The alternative just isn't an option for me.

Anyway, I will get those measurements soon.

OH! Speaking of measurements! I am getting a pair of mid-80s DCM Time Windows speakers (they are in near perfect condition, in original boxes, from a collector, and the price was just too good to pass up AND he's going to deliver them to me himself because he comes to my area regularly! ) and the seller also just happened to have a Speco Model SA-1000 analog multimeter. He's going to have his tech check it out and make some leads for me, too. He wants just 18 bucks for it. I have NO idea what to look for in an old multimeter. Does that sound reasonable or is it a waste of 18 bucks? Any advice would be greatly appreciated!

Anyway, that's it for now. Oh! I'm going to go ahead and put that 14v lamp in my Rev. C RS. I tried it out by just touching the leads to the solder pads behind the existing lamp, and while it hot warm, I was still able to hold it in my fingers for a while without it hurting.

Ok. That's REALLY it for now! Take care and keep the reports on you progress coming! Peace!

[Smoky]

Here's another update since tons of parts came in. I revisited the new Nichicon UHE 470uf 35v capacitors I installed in the rectifier circuit to replace the original Paccom caps. The measured ripple current seemed high with the new caps since the rails are only 14VDC. The first two pictures show the DC voltage and the AC ripple of the new 470uf caps.

I went ahead and ordered a bunch of Nichicon UHD 560uf 50v caps to experiment with. I matched four of the new 560uf caps right at 520uf actual. That's about 10% higher in value from the original 470uf's. In they went.

The last picture is of the new 560uf UHD caps installed with one of the 470uf UHE caps sitting loose for comparison.

[Smoky]

And here are the results from the new 560uf UHD caps.

The voltage of the 14v rail increased from 14.61 to 14.80 VDC.

The ripple dropped from 27.8mV to 19.4mV

So increasing the value of the capacitor 10% caused the voltage to rise 1.5% yet dropped the ripple 30%!

I also noticed that the voltage readings with the 560uf caps remains very stable compared to the 470uf capacitors, where the 14v rail jumped around 2-3 hundredths of a volt DC.

I think when choosing a capacitor in a rectifier circuit, shy away from using a miniaturized cap if possible. Ripple current capacity rules and size matters

I also read the data sheet of the LM4136CN IC's scattered all over the PC board. The minimum supply voltage for them is 12v, typical is 14v, and their maximum is 18v.

The crimpers, BNC's, and crimp-on RCA jacks came in so I made new RG-400 cables for the HP 35660A to compare the frequency response curves of both channels. I measured the custom resistor module and found that a few of the eight resistors didn't match very well so the analyzer should be able to show the irregularities.  I have also been told that there are eight 22nf capacitors around the LM4136CN IC's that can cause the response curve to vary too if they are out of tolerance amongst each other.

[Smoky]

The AudioControl Richter Scale has one input and two outputs for each channel. One of the outputs is the "Low" signal and the other is the "Hi" signal based on the crossover frequency.

I connected the HP 35660A to the Richter Scale one output at a time. At this point, I didn't know how to set the marker's position but it didn't really matter because the graphs looked obviously different. Here are the results for each output. The factory crossover resistor module is installed and makes the signal split at ~80Hz. Here is the order of the pictures:

Left channel Hi-out.
Right channel Hi-out.
Left channel Low-out.
Right channel Low-out.

Notice how the roll-offs don't quite match when compared to the opposite channel.

The Left channel Hi-out even appears to climb higher in dB than the other three.

So, definitely, something is wrong

[Smoky]

There are eight 22nF "feedback" capacitors in the crossover network. I removed and measured them. Opposing pairs varied ~7% in value so I replaced them. I installed eight PP caps that all measured .0215uf.

I also made a new resistor module using eight 100K Ohm resistors. All eight measured 99.12K Ohms. This will also shift the crossover point to 73Hz. I used a new Aries 16 pin header and cover.

While I was installing the new 22nF caps, I noticed that pin 14 of an LM4136CN in the crossover circuit appeared to be open. Just a little bit of solder was on the pad.

I wonder if this is why the Left channel Hi-out had a higher dB level?

Wait until you see the new results from the HP 35660A

[Smoky]

Here are the Left and Right channel Hi-output graphs.

I learned how to set the marker to a fixed 100Hz. Take a look at these beauties now

The difference in the channels is reflected by the marker's "Y" dB-down reading.

The Left channel now fades off at the same level as the Right!

[Smoky]

And here are the Left and Right channel Low-out graphs.

I adjusted the span of the DSA to move it more towards the low frequencies and set the marker to a fixed 50Hz setting.

Are we Rock'n or not

The last picture is the schematic to the Linkwitz-Riley crossover circuit. I put red dots next to the eight new 22nF "feedback" capacitors and the green dots are next to the eight new 100K Ohm matching resistors that were installed.

Let me tell you, that HP 35660A is bad-to-the-bone

[Smoky]

But then, there's the $1,000,000 question

How accurate is the resistor module in relation to the actual crossover frequency being set in the Richter Scale?

In the manual, there is a formula. Divide 7234315.59 by the wanted frequency to get the required resistance of the needed 1% resistors.

So, since I used 99.12K Ohm resistors (actual), I plugged in 7234315.59 and divided it by 99,120. The result is 72.985Hz.

Now how can I confirm this?

I set up the Siglent waveform generator to a sine wave at 100mVrms. I sent that to the Left channel input of the Richter Scale. I then attached two BNC to RCA cables from the Left channel's Hi and Low outputs of the Richter Scale into channel 1 and 2 of my TDS420 oscilloscope.

I turned the frequency dial of the Siglent until the amplitude of both channels of the oscilloscope were equal.

Guess what? The frequency on the oscilloscope reads 74.424Hz.

So, 74.424Hz minus 72.985Hz by AudioControl's calculation means something is off a whopping 1.439Hz

[Cybershaman]

Hey, Smokey.

Been having a rough time of late, but I've been trying to follow your progress. Yes, something is off on AC's formula. It's interesting that they later just said to use 7200 instead of 7234315.59. If you use 7200 with your 99.12K resistors it comes out to 72.64 (rounded) which is 1.784Hz off. Clearly something in the equation needs work. Is this after you swapped out the resistors and caps?

I had another question for you concerning how you are hooking your RS into your system. I ran into trouble when I realized that with the series 3 they did away with the dedicated warble tone frequency output and lumped it into the main output. That kind of sucks with newer systems these days that don't have tape loops. So, if you want to use the warble tone generator you have to manually swap the main output on the RS to another input on a modern preamp in order to adjust the volume until you get a reading of 0db on the meter @ 250Hz. That's pretty inconvenient for someone like me who is regularly swapping in different speakers. :/ It's really hard for me to swap cables when I'm sitting in a wheelchair. Anyway, I'm curious how you are integrating your RS into your system. Do you have your RS installed in a tape/processor loop or are you going straight pre-out to the RS and then to your main left/right inputs on your amplifier?

Anyway, that's it for now. I've been having a very rough go of it lately, so I haven't been able to do much work on my system. I actually just got a Emotiva MC-700 which does all of the bass management very nicely. HOWEVER, I am also expecting a pair of DCM Time Bass subwoofer modules soon and I think I will have a job for the RS when those arrive. Stay tuned.

[Smoky]

Hang in there Cybershaman!

I think AudioControl realized that the value of the resistor module wasn't going to move the crossover point in every RS the same. It probably varied between units so the formula is just a general one. As you can see, you need to test your unit to verify the crossover point.

I plan to make several more resistor modules for my unit but that won't be until I build my speakers and measure their responses.

My stereo system consists of a Cary Audio SLP-98P preamplifier which, Btw, has a monitor switch. The amplifiers I have are two Cary Audio CAD-211FE mono tube amps for the Hi-outs going to the main speakers and a Nakamichi PA-7 for the Low-outs going to the two sub-woofers.

I'm about to move the cables of the DSA to the Richter Scale's Main Rcvr/Pre-amp connections. There is a known issue that HalApt identified regarding a dip in the 20Hz area and who has found a fix for it

[Smoky]

The cables from the HP DSA are now plugged into the input and output of the Left channel Main Rcvr/Pre-amp connections. I did measure both channels and they are the same so here is the Left channel graphs to show the issue.

The first picture is with the signal passing through the Richter scale without the Equalizer Program or the Subsonic Filter activated.

The second picture is with the Equalizer Program activated.

The third picture is with the Equalizer program and the Subsonic Filter activated.

And the fourth picture is just the Subsonic Filter activated.

Notice the dip around 20Hz without anything activated and then it gets even worse when the EQ Program is turned on. A member of AudioKarma, HalfApt, believes it's caused by incorrect values of several capacitors and three pairs of resistors in the infrasonic circuit when none, one, or all of the filters are energized.