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samsp99 wrote:

I don’t see any screw holes and I don’t want to go randomly prying.

I assume that you didn’t have any luck finding a service manual.

If you do have to resort to “randomly prying”, having the proper tools can make a difference. Pry tools like they sell at iFixIt can help getting stubborn devices open with minimal damage. The plastic pry bars are softer than many of the plastic cases.

When it comes to opening cases, everything is fair game. I’ve used everything from credit cards, to guitar picks, to painter’s pallet knives to get into devices.

When I don’t have any clues on the proper way to get something apart, I start with the spot that will be least noticeable if the device is damaged by opening. Some manufacturers are kind enough to design the initial pry point in a discrete location to help out the people servicing the device. YMMV.

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Guy wrote:

I do have internet connection (obviously!).

Do you also have a firewall?

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Very close inspection of the connector pins is recommended. The attached picture is of the board side of a connector. The solder holding the connector to the board was actually fine, but the plating came loose from the pins. You can see some corrosion on the raw metal of the pins (second pin from the left was worst). Just reflowing the solder on this connector probably would not have resulted in a long term fix.

Here is what the solder joint looked like…

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Craig:  I’m pretty happy with the TinySA. It has already provided me sufficient amusement for how much I spent on it. It seems like most of the people using it are HAM radio enthusiasts. But there also seems to be many people trying to adapt it to other, more imaginative, uses. The author of the TinySA software seems to embrace the “alternative usage” and tries to accommodate with patches and Easter eggs. I’m still not 100% sure of the TinySA’s suitability for what I’m trying to use it for. I keep seeing the advice on various websites to buy the “proper equipment” for the job, but it is still not clear to me what the “proper equipment” actually is, or if it can be obtained for a price that I’m willing to pay.

I keep having the the thought that “this would be so much easier if I knew what I was doing” ;-). I’ve found there to be a simultaneous “information vacuum” and preponderance of information relative to this subject matter during my web searches. I’ve been struggling to find information relevant to the tuner design that my particular Beomasters have. I guess the best I can hope for is to plod-on with my experimenting and try to stumble upon a method that gives acceptable results.

On a more positive note, I did make progress on interpreting the “markers” that are on the sweep traces. It looks like the TinySA injects a marker at the ~320kHz and ~640kHz points during the sweep. I set my total sweep to 960kHz so the markers are even. I’m not sure why the TinySA uses these particular values or what the utility of the markers are, but at least it is clear when they are happening.

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The best and most reliable option is to replace the connectors. However, you will likely be fine with just protecting the newly cleaned connector so it doesn’t re-oxidize quickly.

A light coating of Deoxit is most likely all you need. Any light oil would also work in a pinch.

Another option is to “tin” the connectors with a very light coating of solder. If the pins are brass, this is quite easy to do.

I would start with one of the above options, then if the issue returns, go though the trouble of replacing the connectors.

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I’ve made some progress with my quest to properly set-up FM tuner boards. I decided to experiment on the the tuner from my most beat up BM6000 (~5.5 of 1o condition). This receiver seems to be becoming my “parts machine” (or garage machine) as the best parts slowly migrate to my nicer machines. If I am successful with this experiment, I’ll use what I learn in the restoration of the nicer BM6000’s and BM8000’s.

I’ve worked through replacing all of the capacitors and most of the trim pots. I replaced only the trim pots related to a specific adjustment in the service manual at a time. I noted the settings of the old trim pots and checked/documented the “before” measurements of the circuit. For some of the measurements, I tried tweaking the old trim pots to learn the effects before replacing them. I skipped the steps that involved adjusting the inductors.

In general, I was able to make measurements and adjustments that “seem” like they are correct and consistent with what I’ve researched online. However, I don’t have the experience to know for sure if they are actually right.

Tuner/IF Adjustment:  Using the TinySA and an oscilloscope, this is what I got for a IF curve.

Here is the “before” capture with the OLD trim pots at the LOW end of the frequency band, 85.5 MHz. I don’t have an “after” capture for this since this measurement is adjusted only with inductors.

Here is the “before” capture with the OLD trim pots at the HIGH end of the frequency band, 108.0 MHz.

Here is the “after” capture with the NEW trim pots at the HIGH end of the frequency band, 108.0 MHz.

I was able to see the amplitude of the IF curve change as I moved each trim pot and adjusting to a point where it peaked seems to make sense. I wasn’t able to change the symmetry of the curve significantly. I did note that the “centering” of the IF curve was sensitive to the tuning knob position. I tried to position the knob half-way between where the display changed between 0.1 MHz below and 0.1 MHz above the intended frequency. I don’t know it was the right thing to do.

When I did these adjustments, I did not pay much attention to the “blip” near the center of the IF curve. I originally just thought it was an anomaly (random noise) in my setup. This may actually be a feature of the TinySA and be a “marker” at the center of the sweep. Unfortunately, the documentation for the TinySA is sparse and the firmware is still somewhat fluid, so some educated guessing is needed. I have some more research to do here.

If the “blip” is a center marker, would it make sense to center the marker with the tuner knob, then try to adjust for symmetry with the trim pots?

Another area that I have questions relates to an earlier question about the “mV EMF” level. I noted that some of the settings seems to be pretty robust to the input signal level. Other settings, like the “AGC” and “Opening of the Stereo Decoder” were sensitive to the input signal level. I think that the interpretation of the meaning of Vac is what is messing me up, or at least is adding to the error. There is a 3 dBm difference between Vrms and Vpeak, and 9 dBm between Vrms and Vp2p. Any comments on how this should be interpreted would be appreciated.

How I handled this was to note the signal level needed to get the correct voltage measurement with the old trim pot, then initially set the new trim pot to match. Afterward, I then played around with resetting the voltages based on a Vrms assumption of the signal level. I am not very confident that this is giving me a 100% correct setting.

In general, the FM tuner successfully receives stations and sounds similar to what it did before I installed the new parts and readjusted. Some aspects of the sound quality actually seem slightly improved. However, I still don’t think that it is exactly right. On some stations, on particular songs, there seems to be a slight fuzziness or distortion. I’ve also noticed this on one of my other tuners (that I haven’t touched). Any ideas on what I could be experiencing?

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As a side note, I tried experimenting with using the “3C” method from the original post. For some of the measurements, it provided a results that were pretty close to the final setting. The FM would “work” but could easily be improved with a proper adjustment.

Here is my 108 MHz IF curve with setting R1-R4 with the “3C” method.

I think that I actually just got lucky with this result. I tried to see if the procedure was repeatable and the second time was an epic fail.

 

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Good idea to look at the chip spec sheet for clues.

I did some more reading and EMF likely refers to electromotive force. I think that last time I heard that term used was a very, very long time ago in college. It isn’t clear to me why it is being stated in that way. My best guess is that they are trying to indicate making a measurement on a signal generator with a low impedance built-in load with a high impedance measuring device (i.e. without a 50 ohm load at the scope)?

The spec sheets may have provided a clue to another question that I had. Which aerial input to connect the signal generator, the 300 ohm or 75 ohm? Seems like the 75 ohm is the proper one(?).

I’ve been playing with the TinySA to get a feel for how it performs. So far, the settings on the TinySA match what I measure on my scope. I still need to work out ways of measuring what I need with the equipment that I have. For example, I don’t have anything that can measure a 10 uV signal in the MHz frequency range. I think the many of the measurements that I need to make will have to be done indirectly.

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The TinySA seems to be a pretty capable device, especially considering the price. It has its quirks and limitations, but I’m hoping that I can work around them.

I am having trouble understanding exactly what setup is be specified in the service manual. My (possibly incorrect) interpretation is that they are calling for a settings on a specific piece of unnamed equipment. I think I understand what they are trying to accomplish, but might be getting hung up on the nomenclature. For example, in the service manual…

The 94 MHz and 1 mV part is clear. Is the voltage amplitude, peak-to-peak or RMS? Does “EMF +-75kHz” refer to a injecting a wide band FM modulation onto the signal or is it just a tolerance? The term EMF is confusing since it means electromagnetic field to me. But that interpretation would have different units.

I’ve been searching the web for information on setting up FM tuners, but most of what I’ve found doesn’t seem to be applicable. Please let me know on any good resources that you are aware of.

It is clear to me that I have a lot to learn and that the learning curve is steep.

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I decided to take the “restomod” path with my Pentas since they were a bit rough when I bought them used. The yellow covers were cracked and glued back together several times and looked pretty bad. The grill fabric was also in need of replacement. The bases had their share of scuffs and scratches. I ended up repainting all of the gray and mirrored parts. I went through two iterations with the display covers.

The first iteration was fill in the cracks and paint the covers. Of course, I lost the use of the displays. This is a feature that I really didn’t “need” and I haven’t missed having it.

The second iteration was to make a 3D printed part that matched the grooved trim piece. This actually matches much better than the picture makes it look (flash problem?).

I also experimented with designs with a painted cover with a cutout for the original display. I also tried fitting a replacement color TFT display in place of the original display. In the end, we (by “we”, I mean my wife) decided that the speakers fit our decor better without the displays.

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The TinySA can generate signals in the FM frequency band with various modulations, as well as sweeps. I can compute FFT’s with my scope which should give an idea of the distortion?

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I have looked around for the equipment needed for working on the FM tuners. I originally started looking at used, professional grade equipment. I found myself gravitating towards the equipment that we had in our electronics lab at work in the 1980-1990’s time-frame. Though this equipment has dropped significantly in price since new, most of it is still out the price range that I would want to spend for this project. Additionally, I have the usual concerns about the operating state of such old equipment.

While researching FM tuner repair and alignment, I came across a device that might fit my needs. It is called a “TinySA” (https://www.tinysa.org). This appears to be able to generate the signals that I would need. Does anyone have experience with this piece of equipment? If so, would a TinySA and a 200 MHz scope be sufficient to do the basic adjustments?

chartz: I was reading some old posts in the Beoworld archives. Did you ever get your BM8000 to tune to the higher frequency stations?

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Good catch on the power cord. Maybe there is a clue in the seller’s name (i.e. ishams*123)? 😉

However, looking at the rest of the B&O stuff that he has for sale, it is probably a legitimate oversight in the pictures. Why scam people on a $50 item when you are selling a bunch of more expensive stuff?

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My first thought was similar to Sia43’s suggestion. Of course the labeling doesn’t make sense, unless, it is trying to indicate which port on the OTHER equipment to hook it up to. For example, SUBWOOFER IN means to hook it up to the input port of the subwoofer.

Otherwise, maybe it is a remixer for someone that has a high-passed center channel and low-passed subwoofer channel and wants to run left and right full range speakers?

I also noticed the lack of an IR connection. Could it be on the side of the box and not pictured?

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Looks great! That is very beautiful veneer.

How are you going to handle the veneer overlap on the corners?

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Isn’t most of the “alignment” done with the variable inductors?

My assumption is that these don’t “go bad” and are (very much) to be left alone. Is that true?

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Note: It is probably obvious that I don’t have ANY experience with messing with the tuner sections. I do have general, theoretical, engineering knowledge and can follow along with most topics.

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Thanks for letting us know that you got the amp working.

I’m glad you didn’t have to resort to the Beovox option. IMHO, the BeovoxPentas just don’t “look right” once you are used to the BeolabPentas.

Finding a good condition, intact display cover will be a challenge. I had the same problem and ended up with an alternative solution.

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Martin,

Thank you for your sage advice.

Your reply is pretty much what I suspected. I had to ask in case the settings of the trim pots were far less critical than what I expected. With old equipment it is sometimes hard to judge which is the lesser of two evils, old worn out parts versus new out of spec parts.

My game plan is to stick with “1” if there are not issues, then maybe try “2”. I’ll research what is needed for “5”. My bench equipment most likely falls into two categories, too new (modern digital with all of the digital issues) or too old (like vacuum tube old, my wife’s grandfather’s test equipment). My gut feel is this is a job for quality analog equipment(?)

I’m not too sure if “6” is really an option. One of the reasons that mothballed the stereo equipment years ago is that all of the really good electronics technicians that I knew either retired or otherwise closed down shop. Besides, option “5” sounds like more fun.

The strange thing is that I really have no interest to listen to FM (same goes for cassette tapes). I just like the idea of having nice, vintage stereo equipment that is fully functional.

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chartz:  I haven’t had the need to veneer any of my B&O equipment, but I have used veneer on other things. I second the recommendation for contact cement to attach the veneer to plastic or metal parts. Another option is a heat activated veneer glue. I haven’t used this myself, but know people who swear by it. I assume that you are using a raw veneer and not one with a pre-applied adhesive.

How do you plan to trim the veneer? If you plan to use a razor knife be sure to pay attention to the direction of the grain so you don’t inadvertently split the veneer.

Looking forward to seeing pictures of your final results.

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I hope that fixes your problem. Best of luck and be sure let us know how it turns out.

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The deck certainly would not work if the transformer was only putting out 0.018Vac. It wouldn’t even power on to the point where you would see any significant activity on the deck. Since you said that there were DC voltages on the output of the power supply and other signs of life, I would question your readings.

My only guess right now is that you aren’t getting accurate readings from your meter. Have you tried another meter or checked the meter with a known working source?  I would try something like a low voltage AC output wall wart if I wasn’t sure that my meter wasn’t 100% OK.

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