AudioControl Richter Scale Series Three (Repairs/Mods)

[Smoky]

This AudioControl Richter Scale crossover/equalizer came along with a pile of audio gear I bought three years ago.

I want to replace the burnt-out lamp and test a few of the electrolytic capacitors since this thing was made in the mid 1980's.

Not only that, this thing has a custom resistor bank used to define the crossover frequency. Since I plan on building new subwoofers, the crossover frequency will be adjusted.

I also want to test it to see how balaced the channels are by frequency and amplitude and to fix whatever else inside needs fixing 

[Smoky]

The first thing I checked were the fuses and for anything burnt. Sure enough, a small resistor that is tied to two green wires of the power transformer and two twisted blue wires going directly to the lamp was burnt.

Looking closer at the resistor I could only read the first band. It is blue (6). The two holes next to the resistor are where the two green transformer wires are connected to the PC board.

The blue pair of twisted wires called out to me being AC voltage going to an incandescent lamp.

[Smoky]

Here's a picture where the twisted pair of blue wires attach to the back of the dB meter's PC board. The lamp is the type where it has just wired leads soldered directly to the PC board next to the blue wire's connections. The PC board looks like it has seen some heat and, maybe, this lamp isn't the correct or the original one?

As for the transformer's wires, I set up the DMM and measured the voltage of the green wires (~27.5 VAC).

[Smoky]

Here's a little info on how the crossover frequency is tailored.

Inside of the chassis, there is a 16 pin socketed pad that is fitted with 8 metal film resistors. It easily unplugs from the PC board to be exchanged with a set of resistors of a different value based on the desired crossover frequency.

I was able to order extra 16 pin pads from Digi-Key. So when the time comes, I can calculate what is needed for my combination of speakers. Really cool

[Cybershaman]

Hello! As luck wood have it, I'm dealing with the same lamp issue on one of two Richter Scale 3s I own! That burnt resistor is blue, grey, black, gold, so 68 ohms.

Here's something interesting: my other series 3 is wired differently! A look at the board shows that one was made in the late 80s and the other early 90s. The 90s version has that resistor missing and the lamp wire that goes to the resistor on the 80s board is instead wired between the diodes nearby! The wiring from the transformer on the 90s version is also slightly different.

I must apologize because I am doing this all from memory and using a tablet. (Ugh. What a pain.) I am disabled and sitting in bed right now, otherwise I would hop over to the units for more specifics.

I am fascinated by the various Richter Scale models. In fact, I also own a Series 1 and  2. I guess you could say I'm a collector. I also recently bought my second Series 3 unit JUST because it still had its microphone with it! (Don't ask me how much I paid for it. I essentially bought it JUST for its microphone! ) I noticed that both series 3s had slightly different markings on the front. Both of their bulbs are burned out and when I started poking around inside it was then that I noticed that the black wires for their lamps were wired slightly different. I've seen that there are manuals online for a Richter Scale Series 3 version 2 and it always puzzled me. I guess now we know why!

I'm thinking I would like to replace the lamps with LEDs. Again, sorry if I missed specific details in my post. If you would like, I might post some pictures of the differences. I think it would be useful considering these units are getting older and more people might be looking for information. Especially considering how the meters on the series 3 is virtually useless without the lamp working behind the meter. Also, since the ones you see for sale rarely, if ever, have their microphones still with them, it would be great if someone could figure out if one could either make one or build one from scratch to use. I have a Velodyne SMS-1 unit that connects to my TV and shows the frequency response below 200Hz, but I am excited to try out the microphone with my new Richter Scale, just as soon as I fix the darn lamp!

Anyway, that's all I can stand to "type" on this stupid tablet (an Amazon Fire 10, no less!), so that's it for now. I would love to banter about these unique pieces of audiophile history. But I'd rather do it when I'm on my PC. Take care and I look forward to hearing more about your work on your unit!

Oh! You do know how to build new crossover modules for your unit, right? Short answer is to divide 7200 by the frequency you want to cross over and that is the KILOohms of the resistors you use. You can build your own or try to find the appropriate isolated non-bussed resistor network modules of the appropriate ohm values. They are few dollars these days, I think. I just like making my own, so I didn't look too hard for the right kind. Mouser.com should have them, though. And here's a link to the Audiocontrol manual on making your own: https://www.audiocontrol.com/car-audio/accessories/modules/ It looks like Audiocontrol still sells them, but, boy, are they pricey. But, hey, if that's easier than trying to find the right kind elsewhere I totally understand. I guess they still use them in some of their products. They use the same setup in their old Phase Coupled Activities. I actually have two of those, too! A Series 1 and a 3! Fun little things to mess with. Although, not as useful these days. They DO have separate crossover sections, though.

ANYWAY, I REALLY have to go now! Take care and hopefully talk to you again soon!

(I blame all spelling and grammatical errors on this stupid tablet! )

[Cybershaman]

Ha! I see you already know about the resistors! :p

[Cybershaman]

I also note that your unit has blue wires to the lamp. Both of my units use black wires! Check your circuit board for a manufacture/revision date. I'm curious if there are even more versions of the Series 3!

[Smoky]

Thank you Cybershaman. I was able to get access to the lamp area by taking off the front panel.

It doesn't look right. The darkened lamp just flops around on two wires and the solder pads are partially lifted on the backside, plus, there's definitely some heat damage. I can also see a small piece of fine wire laying in the PC board near the resistors

A real bodge-job in my opinion

It surely is 27.5 AC volts going to this incandescent lamp. Wouldn't using an LED would create unwanted noise?

Here's what I have in 28v lamps, a 6838 (.024mA) and a CM7009 (~.020mA). I did buy a small box of 7839 (.024mA) lamps but they aren't here yet. The candle-power is ~ .15 MSCP for all three. It doesn't require much light to bring out the meter face.

I definitely want it to be cooler-running from here on.

[Cybershaman]

Where do you get your lamps from? I would like to get both of my RSs' lamps fixed and then possibly sell my extra one.

I'll be honest, my electrical knowledge/experience is VERY limited. Between difficult times in relation to my disability/medical issues, I dabble with little "mad scientist" projects/ideas I have and learn as I go. The thought that LEDs gave off significant RF interference never occurred to me! A while ago, I swapped out some incandescents on the meters of two Carver TFM-15cb amps I have and have seen others do the same and I haven't had any problems I know of. Well, that's not entirely true. After a while, I noticed that my meters seem to be reading a little something when there is nothing playing through them. I don't hear anything other than your usual tiny bit of hiss coming out of my speakers. I assumed I was just picking something up from a cable that was too close to a power cord or some other source. Maybe it's the LEDs interfering? Can they do that? I've never heard complaints/warnings from others who did that mod. Anyway, I don't want to get TOO off topic.

I took some pictures of my RS series 3 units and thought you (and anyone else who stumbles on this thread in the future) might find them interesting. I was very mistaken about the years! Both boards were from 1989! Oops! They are actually just two revisions; B and C. Anyway, I just thought I would just throw them up for future reference.

I will have to post them later. Having a rough day today physically. Take care and talk to you soon! Oh, and I would love to hear more about your setup and how you are planning on integrating your Richter into your system! BTW, do you have an original microphone? Or are you using another method or just going by ear? Later!

[Smoky]

The first lamp I'm going to try is the 6838. It's rated at .024mA. I installed a 270 Ohm resistor instead of the original 68 Ohm which, I believe, will limit the current of the circuit to ~100mA. You can see the lamp is about 1/3 the physical size of the old one too. I haven't wired it in yet.

I went ahead and replaced the original Paccom capacitors in the rectifier circuit with new matched 105c Nichicons. I also replaced the original 1N4003 diodes with new UF4004 types.

I found two other 220uf capacitors out of tolerance but I don't have any on hand. Looking around the board I can see that the IC's are dated around 1986.

I have the original microphone and User's Manual too.

I plan to go through the unit with a fine-tooth comb!

[Cybershaman]

Ho HO! You have definitely piqued my interest, sir! I will be following this project with rapt attention! In fact, I will be following your lead, especially since, like I mentioned, I am pretty inexperienced as far as electronics are concerned. And, hey, if I mess something up, I have a backup unit! A couple of questions/comments...

- Can you enlighten me on your decisions concerning the rectifier circuit? Were the components just old? And in what way do the higher quality components help with the function of the unit itself? Just more stable power? (Always a good thing! )
- Where were the other two caps that were out of tolerance? Did you go around the board with your meter testing every cap?
- Were you just noting the date of the ICs to reflect the manufacture date? Or is there something significant about that?
- Amazing that you got the mic AND the original manual!

OK. I'm having a particularly bad day pain-wise, but I managed to get some pics of my two units and am throwing them up simply for reference. Looking forward to hearing your results with the lamps and with any other mods/improvements/repairs you do!

Oh! By the way, I was looking over my units again and I noticed where I got that 1993 date for the Rev. C unit! I'll include it in the pics below. Note: The text underneath "Revision D" just says "CIRCUIT SIDE". I couldn't get it to show up when I took the pic from a higher angle. I'm assuming that just means "Put this side facing all the circuit thingies, dumby. Maybe you could help me decipher what is going on there and what information it is conveying. I sometimes have a hard time mentally when my pain gets bad.

One other little note: you'll notice that one unit came with a self contained resistor network DIP for the crossover and the other has a handmade one (80.6 ohm which I believe comes out to almost 90Hz). I wonder if they made some by hand at the factory? I just can't see someone (well, at least after the year 2000, maybe?) wanting to use a 90Hz crossover. But, hey, who knows?

EDIT: The last picture showing the front panel of the two units, the unit with the Rev. B board is on the bottom and the Rev. C is on top. Personally, I like the Rev. B version!

[Cybershaman]

PS: I am curious what you think of the different wiring configuration for the lamp in the Rev. C board. Looks like they just did away with that resistor altogether! Maybe they were having repairs coming in with them blowing out? I would love to hear what you think!

Well, I have to take it easy. Having a rough time. Ordered some components so that I can start soldering when I get better. Am I a nerd if I really enjoy purchasing semiconductors? heh, heh.

[Smoky]

I found out that the Richter Scale III that I own is the first version.

Here's one comparison between your boards and mine. Notice how AudioControl added a trim-pot in the microphone circuit. I was told it was added to allow adjustment to the microphone's sensitivity.

If possible, let me know if you can read the value on that trim pot and of the resistor that is swung over to it. Plus, let me know what part number is stamped on the four diodes to its immediate left.

[Smoky]

The following issue most probably pertains to your unit. I won't know for sure if it is also an issue in mine until it's back together and tested.

The first picture is of the area of the board relating to the infrasonic filter.

When the equalizer is enabled, an anomaly appears on the waveform. It is supposedly caused by the incorrect selection of the stock capacitor and resistor values. The issue shows itself on a graph in picture two.

In picture three, and with the modified capacitor and resistor values installed, the graph shows that the issue is almost completely resolved.

Let me make it clear too, I'm being helped by a very generous individual on AudioKarma.org

[james_s]

I've always thought lighted analog meters look really classy.

Wow the back of that one is cooked! I'd be tempted to replace the lamp with an LED, feeding it with DC so it doesn't flicker, that will prevent further deterioration of the plastic meter scale.

[Smoky]

These little 16 pin plug-ins came in today too and gold plated on top of that. I even bought a cover so to hide my special resistor "recipe"

As for the lamps, I'll stick with the incandescents. The smallest one of them all arrived today. It's the CM7009 and runs at .020mA. The largest one is a 2185 that looks really similar to the original. The one with a base is a CM7839. All of the lamps are 28 volt types.

[Smoky]

This area of the board is of the equalizer/frequency filter circuit. Unfortunately, AudioControl will not release any of their schematics. I back-lit the board to be able to see the traces better.

Looking close at it, I realized that these are all coupling capacitors, both film and electrolytics. Starting from the left and going right, it is easy to see where the channels separate and then repeat themselves even though the capacitors appear random.

I did some research on these Paccom capacitors and found that several of the black "EV" types have failed. But the capacitors in blue stamped "EVL" are low-leakage types that still measure good, and not only good, they measure better than brand new Nichicon UKL's!

The only capacitors that beat the "EVL" caps were PP film caps that I have in one equivalent value and voltage.

The green capacitors are also Paccom brand but they are the Polyester type of film capacitors. Not the greatest for audio in today's World.

I'm debating on replacing them with PP and PPS caps, and that goes for the low value electrolytics too.

[Smoky]

To me, this is good

The guy on AudioKarma.org just described as to how he figured out the "infrasonic" filter issue inside of the Richter Scale Series Three.

After studying the Eq, infrasonic, and rumble filters for a while he discovered that AudioControl had placed two hi-pass filters in the signal processing circuitry. If the EQ section was switched in, it would put two hi-pass filters back to back prior to the infrasonic filter. When the infrasonic filter was switched out, it was still being powered and its input was still connected to a common capacitor that caused feedback/interaction. And he went on...

He discovered that the infrasonic filter topology in the Richter Scale was exactly the same as the infrasonic filter found inside of an APT preamplifier!

As he explains it, AudioControl used large value caps with small value resistors, while APT used small value caps with large value resistors in its similar circuit. He, literally, plugged in APT's values and the issue disappeared!

Would this be described as intellectual property infringement

So with the modified components installed and both the Eq and infrasonic filter in/on, the roll off is at 0.4dB down at 20Hz, as seen in the graph below. Btw, the graph shows both of the filters on and also off.

The second picture is of the capacitors and resistors that need to be changed-out.

Here are the new values:

Replace 1uF with 0.1uF (100nF) 5% film caps
Replace 4k7 with 75k 1% MF
Replace 1k2 with 27k 1% MF
Replace 68K with 640k 1% MF

**The last picture was drawn by HalfApt describing the circuit with the Equalizer switched in.

Many thanks to HalfApt of AudioKarma.org 

[Cybershaman]

In bed and on phone. About to hit the lights. Will dig in to what you wrote tomorrow,  Smokey.

I just wanted to thank you for sharing SO MUCH information. I feel like I've hit pay dirt, here. I've had a nearly lifelong love of the Richter (as well as another AudioControl toy, the Pase Coupled Activator, of which I have a Series 1 and 3) and am looking forward to giving attention to my units. As I mentioned, I have a lot of physical/medical challenges and things have been rough these last few years. I haven't felt this good in a long long time. Thanks, again. I'll check that pot value/number tomorrow. 

[Cybershaman]

(Post 1 of 3)

OK! I have some interesting things to report! I'll try to be brief and to the point.

First up, the trim pot you are interested in is marked as a 1K 524M on the Rev. B board unit and a 1K 104M on the Rev. C board unit.

The Resistor attached to the pot in both revisions is 270 Ohms 5% (Red - Violet - Brown - Gold). (I'm including the colors with resistors so you can verify in case I make a mistake. )

The black diodes on the Rev C board are marked 818. I tried measuring them, but I'm not sure if I'm getting the right results. My multimeter might be dying and I've ordered new batteries. Did some searching and I came up with a 1N5818. Does that sound right?

I'm including pictures of both boards. I also noted that it looks like something funny is going on with the the Rev. C board. Looks like they took out a nearby resistor. (EDIT: Nope. A resistor was hiding under the pot on the Rev. B board! ) I tried to illuminate the board traces on the Rev. B board for comparison.

Hope this helps! Let me know if there's anything else that I can do. As always, I would love to hear your thoughts/plans!

EDIT: I find it interesting that they used clear diodes on the Rev. B board. Are those "signal diodes"?

[Cybershaman]

(Post 2 of 3)

Each board has some different jumper wires in the rear. Each unit uses a different style of RCA connectors, so this might not be anything special. I'm just posting it for posterity.

[Cybershaman]

(Post 3 of 3)

Posting this last because I didn't want to end on something mundane.

On the Rev. C board, they put a 26 kOhms 5% (Red - Blue - Orange - Gold) resistor on the pot that changes the frequency of the warble tone!

Take care!

[Smoky]

Thank you for the pictures Cybershaman!

I asked about the diodes because mine are clear glass and I thought that they were Zeners having one green band. When you mentioned 1N5158, I turned to the internet for an answer. The diodes turn out to be a PNPN 4-layer Shockley diodes. This is all a guessing game for me when schematics aren't available. Anywho, I found a picture of an early Motorola 1N5158 part in a glass version, so they appear to be the same part.

About that added resistor on the Warble-tone pot. I would guess that the resistor was a patch needed to refine the range of the potentiometer so that when the knob was turned its pointer would align correctly to the printing on the faceplate. I'm not sure. Everything will be tested on my end since no resistor is present. As you can see, the pot used in this first version Richter Scale is different than the ones used in later models.

Now back to the stock capacitors used in the equalizer/frequency filter circuit. Mine are a mix of brands while your units have the same uniform types. I pulled four of mine just to test because "low-leakage" is a requirement in this area. Surprisingly, the bright blue Paccom 1uf 50v caps performed better than two brand new Nichicon UKL capaictors. But for those two small green Capar 1.5uf 50v caps, the leakage on my Sencore Z Meter would not drop to zero while the blue caps dropped to zero in under 5 seconds. This is the area I'm going to try PP and PPS film capacitors.

[Cybershaman]

Hi, Smoky!

Headed for bed right now, but I just read your post. Very interesting! One quick note before I dash, though. Just wanted to make sure you understood that I was only able to read the numbers "818" (and I can tell they are the last 3 digits) on the diodes and that I was coming up with "1N5818" not "1N5158" in my searches. NOT implying you're wrong in ANY way! I just wanted to make sure that you read what I wrote correctly. Truth be told we are treading into areas where I am struggling to catch up in my understanding of electronics. I THINK I'm understanding everything OK, but bear with me if I ask many questions/clarifications!

Anyway, goodnight and I'll be letting what you said sink in. Might be back later with some questions. Peace!

PS: At some point, I might try and put things into a list to help me keep track of where you're at as far as repairs/replacements/mods go. I am definitely going to be following your lead on, likely, both of my series 3 units, so I want to be triple sure that I'm doing things correctly and understand the reasons. Also, I'm sure that there will likely be some other Richter fans that will come along and just devour this post in the future. There's just so very little information on these things that's easy to get to, as I'm sure you are more than aware. OK. Gotta run!

PPS: I have OmniMic (the PC program with microphone) and was thinking that I could maybe test the output of the warble tone generator on each of the units I have to see if there is any difference in the frequency compared to what's indicated on the front panel dial. It could be that the change the resistor makes is very subtle and I might be unable to tell the difference, but do you think it's worth a shot? OK. I REALLY have to go now! Nite!

[Smoky]

Thank you Cybershaman,

I'm guessing that your units are probably working just fine besides the lamps needing to be replaced. I wouldn't get too carried away.

I think the one I own has seen many miles and I'm just looking to take a few of those years off of it 

I have the film caps on order, and when it's all wrapped up, I'm going to clean the sliders and pots with DeoxIT and put it through some bench testing. I'll take some voltage measurements around the board too.

My problem is that I have no way of doing a frequency response test.

I want to see what the "baseline" looks like first before any filter modifications are done.

Plus, some really nice stereo equipment will be connected to this thing and I don't want it adding extra noise or distortion.

The specification in the manual states its Total Harmonic Distortion is 0.005%.


*I also have a thread regarding the Richter Scale Series Three on AudioKarma.org 

https://audiokarma.org/forums/index.php?threads/audiocontrol-equalizer-and-paccom-capacitors.928669/

The replacement of the electrolytic capacitors are done. The UKL opposing pairs of caps are matched well under 1% in tolerance. The green Polyester capacitors are next. HalfAlt on AudioKarma says it's critical that the L&R channel capacitors match. He said even a 5% difference between the left and right channel capacitors will cause a noticeable deviation.

Since the this original version of this Richter Scale did not include a potentiometer in the microphone circuit, I added one.

I had three used 500mW Bourns 1K Ohm pots that have legs that point downward. The original 200 Ohm resistor measured 197.3 Ohms.  Next I took a 100 Ohm resistor and soldered it to one leg of the pot. I then measured the minimum and maximum of the resistor/pot combination. It's range is from 100 Ohms to 1.1K Ohms. I set the pot to 200 Ohms and installed it. What's nice is that it reads correctly in circuit and easily fine-tuned in the future 

As for the lamp, it's installed. I applied small pieces of Thermo-Tec heat barrier tape to the PC board which allowed me to place the lamp further away from the back of the meter. I works great!

And in the last picture, here's the Richter Scale with the new lamp installed and the front panel back in place. I also plugged-in the microphone and, while humming a low note, notice how the meter reflects it. This is going to be interesting to see how well this thing works