KM-60 Tuner Advice Please

[Jeff W.]

Hi All, Recently I was fortunate enough to find and purchase this tuner along with a KX-90 integrated amplifier. I am in the process of doing some restoration/preventive maintenance for both, but need some help due to my lack of experience with tube equipment in general. By the way, this is my first venture into the Fisher camp and I'm looking forward to listening to these 50 year old gems.

I've read up on the threads about this tuner and see that it is generally recommended that selenium rectifier be replaced with a silicon one. I saw that one from Radio Shack was recommended: #276-1181 rated at 8A/400V. I have one from Parts Express (050-030) rated at 6A/400v. Will this be sufficient and have a safe margin or should I get the one from R.S.?

When I do install the new one I see that it is suggested by most to use a resistor or diodes to drop the voltage down so that it will be at the level equivalent to what would have come from the selenium one. I see on the schematic that there should be 192V coming into the 4 x 40uf can capacitor after rectification. In a thread on Audio Asylum someone reported that they had 224V here after installing the silicon rectifier then used diodes to drop the voltage. Others mention using a 5w resistor (but that it gets fairly hot). What is the best course of action here? If the voltage is 224 what type(value) diode(s) or resistor would be a good starting point to to try to end up with 192v on the other side? I couldn't find any information with a recommendation as to a specific value resistor or type of diode(s). I hope to hear from those who've been down this road and hopefully it will help others in the future to have this more specific information. Thanks

 

[dcgillespie]

Jeff -- Welcome to Fisher gear and the AK Fisher Forum!

The selenium bridge rectifier should definitely be replace with a silicon one -- your 6A unit is more than enough for the job.

To properly compensate for the greater voltage drop produced by even a good selenium bridge, you will want to add a 100 Ohm 5 watt resistor between the output of the new silicon bridge, and the first filter cap connection. There should be no filter cap attached directly to the output of the new silicon unit. That way, the silicon bridge acts as virtually a zero drop device, while the resistor then mimics the resistance of the original selenium unit.

Good luck with your KM-60! It's a wonderful tuner, capable of superb performance.

Dave

 

[cc43fan]

Jeff,
Welcome to the Fisher family! I'm sure you'll have a great time :thmbsp:

 

[IMTubeHead]

Welcome

Nice to have you here! I remember the good feeling I got when I accidentally found this group. You will very soon discover why the Fisher group is the VERY BEST group that Audiokarma has! Everyone here is so very helpful, and I've learned SO MUCH here that I can't even begin to describe! My very first Fisher was a tuner as well...a FM-100-C. There was no turning back for me after that! Good luck with your KM-60.

John

 

[Jeff W.]

Thanks to all for the warm welcome and well wishes. I've got her recapped, including a new 40uf x 4 can and will pick up the 100 ohm resistor, as Dave has kindly recommended, later today. I'm sure I'm going to have a few more questions. Thanks for the willingness to help. :yes:

 

[Jeff W.]

Good news BAD news

Good: I installed the 100 ohm resistor between the silicon rectifier positive lead and the 40uf can (photo). I utilized the two existing holes in the chassis where the selenium bridge was mounted. One to mount the new silicon bridge and the other to mount a terminal strip facilitating the installation of the resistor. A 6-32 screw fits these holes perfectly along with the bridge and terminal strip. Now I was ready to fire her up.

Bad: I hooked up the tuner to my dim bulb tester and gave her a try. It lit up the 15 watt bulb brightly . I turned it off immediately.

Of course, I'm going over my work now. the most complicated thing I did was replace the 4x 40uf can cap and was very careful about it, but I'm always capable of a mistake as I've often found :sigh:. Otherwise I was replacing capacitors one at a time. I will be reviewing the schematic against the tuner.

Back Story: I purchased this tuner along with the KX-90 amp from an ad on craigslist. I saw them and called at lunch arranging to pick them up after work knowing at the asking price they'd soon be gone otherwise. They were both VERY clean with wood cases. The tuner has all of its original "Fisher" tubes.

The seller said he bought them at an estate sale and had not tested them there or at home. I knew better than to plug them in and "test" them, so based on the condition and price I went ahead and bought them. I ordered caps and started replacing them without ever turning the tuner on. I've cleaned all of the tube sockets, tube pins, pots and switches. The person who assembled this kit did a very nice job. Things are laid out neatly and the solder joints look good.

My understanding is that the likely problem would be a short somewhere in the power supply. Other than going over the tuner with the schematic to check my work and the rest of the tuner, are there some things that I should check that have a high likelihood of being the cause? :scratch2:

 

[cc43fan]

Jeff,
I'm sure you've already thought of these, but...is the fuse good? Also, if you want to check the work in the photo, one way would be setting your DVM to ohms, attach one lead to the chassis, and look for continuity at the "+" lead of your rectifier. I can't see the hidden side of the terminal strip (I rarely see pics on my laptop screen as well as I'd like to), but I like to ask anyway...the terminal strip mounting screw isn't shorting that strip, correct?

Good: I installed the 100 ohm resistor between the silicon rectifier positive lead and the 40uf can (photo). I utilized the two existing holes in the chassis where the selenium bridge was mounted. One to mount the new silicon bridge and the other to mount a terminal strip facilitating the installation of the resistor. A 6-32 screw fits these holes perfectly along with the bridge and terminal strip. Now I was ready to fire her up.

Bad: I hooked up the tuner to my dim bulb tester and gave her a try. It lit up the 15 watt bulb brightly . I turned it off immediately.

Of course, I'm going over my work now. the most complicated thing I did was replace the 4x 40uf can cap and was very careful about it, but I'm always capable of a mistake as I've often found :sigh:. Otherwise I was replacing capacitors one at a time. I will be reviewing the schematic against the tuner.

Back Story: I purchased this tuner along with the KX-90 amp from an ad on craigslist. I saw them and called at lunch arranging to pick them up after work knowing at the asking price they'd soon be gone otherwise. They were both VERY clean with wood cases. The tuner has all of its original "Fisher" tubes.

The seller said he bought them at an estate sale and had not tested them there or at home. I knew better than to plug them in and "test" them, so based on the condition and price I went ahead and bought them. I ordered caps and started replacing them without ever turning the tuner on. I've cleaned all of the tube sockets, tube pins, pots and switches. The person who assembled this kit did a very nice job. Things are laid out neatly and the solder joints look good.

My understanding is that the likely problem would be a short somewhere in the power supply. Other than going over the tuner with the schematic to check my work and the rest of the tuner, are there some things that I should check that have a high likelihood of being the cause? :scratch2:

 

[dcgillespie]

Jeff -- Without knowing for a fact that the tuner worked before starting work on it, it does introduce a number of variables, ranging from the work you just performed, to did it ever work correctly......

Your work installing the new bridge and resistor look good. Therefore, unless the bridge is bad for some reason or there is a hidden short not visible in the pic, I don't think that is the area of concern. However, you can always test at least if the HV circuit is the issue, by simply lifting (and isolating) one of the AC leads to the bridge, and then trying your dim bulb test again.

I imagine what you'll find however, is that you are simply using too small of a bulb in you dim bulb tester. Tuners -- good ones that is -- pull a deceptively large amount of current. In the KM-60, which has fewer tubes that other Fisher tuners, the heaters alone pull nearly 3.5 amps -- and that's when they are fully warmed. From a cold start, they will pull considerably more. When power transformer efficiencies are considered, that circuit alone -- after the turn on surge -- draws almost 27 watts from the AC line. On the B+ side, the tubes and ripple current amount to another 15-16 watts of draw from the line, so you see that the KM-60 -- when operating properly -- and after the turn on surge -- is trying to draw nearly 3X the rating of the bulb in your tester. When the surge is considered, the draw is considerably more. Therefore, while there may be other problems, you won't know until you first "calibrate" your dim bulb tester more accurately for the device you are testing. In this case, calibrate means conducting the test using a 100 watt bulb rather than a 15 watt bulb........

BTW, as confirmation of the quick calculations above, I just checked Fisher's specification for power consumption for the KM-60........ It is listed as 43 watts, for very close agreement with the figures offered.

Good luck with it!

Dave

 

[larryderouin]

I agree with Dave, that your choice of lamp for the DBT is too small. I generally use either a 60W or 75W for tuners, and small (under 30W) receivers. Ideally you should have a BRIGHT INITIAL FLASH for a second, then the lamp will dim down to notice it. It should be approx 1/2 to 1/4 the initial brightness.

As for sizing the lamp, I normally take the Wattage rating on the back of the unit (which is the max. current draw of the correctly running unit) and multiply by 1.5.
In this case a 60 to 75W lamp range will give you the correct indications with slight variations in brightness, lamp to lamp.

 

[Jeff W.]

Boy, you guys are Johnny-on-the-spot! Three helpful replies in less than 12 hours. Thank you. This is encouraging.

I had checked the fuse (for correct rating as well as being "good"), both before and after I powered it up. I did check to see if it was blown before I bought it. The terminal strip mounting lug is not connected to either terminal. I will do the continuity test.

I hadn't used my dim bulb tester in quite some time and went to the web for a bit of a refresher. The site where I was reading advised to start off with a 15w bulb and work your way up in wattage. Since the 15w one lit I thought the worst. I try to never to be in a hurry to ruin something.

Thanks for shedding some light :tongue: on how to use the DBT more scientifically. I hope to be able to report that with the correct bulb everything tests OK. I'll still be going over my work first tonight, but hope to feel confident to give it another try with a higher watt bulb by the end of the evening...

 

[Jeff W.]

Getting better all the time

The higher watt bulb was the answer, thankfully. It lit fairly bright initially and then dropped down just as Larry had described. I brought her up slowly on the Variac afterward and all tubes showed signs of life. No smoke, no funny smells and a cool transformer. Both meters responded when I moved up and down the dial, but the signal meter never moved up over "2". I was in the basement with just a dipole antenna. I'm hoping it will move further up the scale later.

I'm surmising that the next step would be to check voltages at the various test points. I'll have to check my cheap multimeter's manual and see if it can withstand the voltages. I did notice that "V2" (6AQ8) did get fairly hot compared to the other tubes. The shield got too hot to keep you hand on it. I remain open to guidance and am grateful for what has been given. I'm feeling a whole lot better tonight. A few pictures attached of the "patient'

 

[dcgillespie]

Congrats on a successful start up! The 6AQ8 will in fact get quite hot, as will the 6DJ8, and the first two 6AU6s in the signal chain. These four tubes are the hardest working tubes in the tuner, and together likely draw about 70% of the total B+ current drawn by the tuner.

The owners/instruction manual for the KM-60 offers good alignment instructions for the IF transformers in the unit -- without the use of specialized equipment -- if you need to touch these up. However, stay away from the "front end" (the sub-chassis where the 6DJ8 and 6AQ8 are located) and the MPX unit (the sub-chassis in the middle of the tuner chassis). Problems with these units are typically only indicated if the tuning pointer is way off, or the stereo separation is poor.

That's a great looking KM-60, and your work to restore it looks great too!

Dave

 

[cc43fan]

Jeff,
Dave and Larry are the best at troubleshooting...you're in good hands. Great looking tuner you're got there. I'm sure you'll enjoy it with the Fisher amp you picked up. I'm looking forward to listening to the pair this fall :yes:

 

[Jeff W.]

Thanks for the kind words an congratulations. I know I'm feeling pretty good about it :yes:. When you fire it up and it works that is BIG FUN!

I hooked her up to the Variac this morning and set the output to 117V. The schematic shows a range of 105-120V. I remembered a good amount of equipment I've seen from this era would specify 117v. I'm also leaning toward the higher value because I typically have from 120-125V at the outlets in my home.

I measured at each terminal of the 4 X 40uf can cap. I'll show the measured value followed by the schematic value below.

C1-A: 203V/192V, C1-B: 182/176, C1-C: 167/160, C1-D: 144/140.

Given these measurements, should I bump up the value of my dropping resistor that I have between the silicon rectifier and C1-A? If so, what should I step up to from the current 100 ohms? I'm guessing there's mathmatical formula. What would be recommended given my readings and my household current that runs 120-125V?

 

[dcgillespie]

Jeff -- The voltages are so close that it's not worth worrying about. Also, understand that the B+ voltages within an FM tuner can vary quite significantly depending on whether a station is tuned in, and how strong the station is. While the schematic for the KM-60 doesn't specifically specify it, it is customary to provide voltages produced with no signal present at the antenna terminals, and no station tuned in.

When those variables are considered, your voltages look just fine!

Dave

 

[larryderouin]

Plus on more than a few of the late 50's / early to mid 60's schematics in the notes, FISHER would specify that voltages could vary as much as +/-15% based on an input of 117V, due to manufacturing tolerance variances in the components.

When you finally plug it in to house current directly, take those voltages again. See if you can do it with the house voltage around 125. The values will increase slightly but probably will end up within 1% of the nominal voltages for the schematic.

In other words, you're fine. Hook it up and get jammin....

 

[Jeff W.]

Thanks guys, I'll cheerfully proceed to the alignment section of the manual.

 

[Jeff W.]

Back to Bad News

Didn't have much of a chance to get back to the tuner over Father's Day weekend, but got back under the hood last night and tonight.

I started with the "Alignment Without Instruments" procedures. I had a dipole antenna hooked up to the "Distant" terminals then turned the unit on and let it warm up for a bit. I moved the tuning dial up and down the scale. The "Stereo Beacon" tube responded to stations all along the dial. The tuning meter responded and moved, but never moved beyond "1" on the scale. Actually it never made it to "1" fully. I was in the basement, so I moved upstairs and got the same results.

Tonight I hooked the tuner up to my roof antenna and everything seemed the same with the signal pointer not even moving to a full "1", so I thought I'd go ahead and try the next step to see if I could get any results.

I used the hex tool in the limiter coil and turned it clockwise and got no noticeable improvement on the meter. I turned it back to where I started and then further counterclockwise and noticed no improvement in the meter reading. Next I went clockwise further than before with no result, then reversed for the same. This is where I have stopped. Any ideas as to why the meter is not responding more than this? It definitely moves between "0" and (almost) "1". Thanks.

 

[dcgillespie]

Jeff -- If you're getting closure or near closure of the Stereo Beacon (indicating a strong stereo pilot signal), but little response from the tuning meter, then I would suspect the tuning meter itself. Over time, they can become "sticky", or, the glue holding the backing plate of the meter in place deteriorates and lets the plate come loose. If it does it can rub against the back side of the needle and cause it to have limited movement. The other thing is to check the cleanliness of the meter switch contacts as well.

The real clue is how the Stereo Beacon is responding. If it is normal as described, then I suspect the meter or its switch. If the Beacon's response is "there" but minimal, then something else is at play.

Let us know!

Dave

 

[Jeff W.]

Thanks Dave. I gave the meter switch more cleaning just to make sure. I inspected the tuning meter closely and it is in excellent looking shape with nothing visible to make me suspect a problem. The Stereo Beacon would close in on stations but never got close to closing completely. About a 1/2" gap always remained even on the strongest stations.

After re-cleaning the meter switch contacts and inspecting the meter I turned the tuner on again. I had left it tuned to one of the stronger stations in town when I last shut it down. After a few seconds the Stereo Beacon lit and almost made it to full closure. The tuning meter moved to "4". It stayed this way for a short period of time, then went back to the previous condition of the beacon not coming close to full closure and the meter not exceeding "1". I turned it off then came back to it later to experience the same sequence. Any ideas?