Help with harman kardon a50k
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The heaters in those three tubes, are run from the cathode current of the output tubes (their heaters are connected in series with the combined cathodes of the output tubes, through the balance pots).
Are there known good tubes in V1, V2, and V3? Also, are the output tubes all in the amp, and are they known good?
If so, the next thing you need to check, is the output balance pots. If they're broken (no continuity between all three leads), you won't get current from the output tubes, to that circuit. Also, check the wiring between the output tube cathodes and those balance pots, and those pots and the heaters of V1, V2 and V3.
If not, you need to make sure you have, or get, good tubes. The three input tubes in V1/V2/V3 and the four output tubes all are dependent on each other being good, in order to work...
Regards,
Gordon.
Are there known good tubes in V1, V2, and V3? Also, are the output tubes all in the amp, and are they known good?
If so, the next thing you need to check, is the output balance pots. If they're broken (no continuity between all three leads), you won't get current from the output tubes, to that circuit. Also, check the wiring between the output tube cathodes and those balance pots, and those pots and the heaters of V1, V2 and V3.
If not, you need to make sure you have, or get, good tubes. The three input tubes in V1/V2/V3 and the four output tubes all are dependent on each other being good, in order to work...
Regards,
Gordon.
It would be the potentiometers that adjust the balance of the pair of output tubes for each channel.
You can use an ohm meter to check continuity of each leg of the balance pots to the center leg. You should be reading small-ish ohms, like less than 100 ohms, between each outside leg and the center leg on each channel's balance pot.
Be careful probing around inside as there could be lethal voltages on the power supply filter caps.
You can use an ohm meter to check continuity of each leg of the balance pots to the center leg. You should be reading small-ish ohms, like less than 100 ohms, between each outside leg and the center leg on each channel's balance pot.
Be careful probing around inside as there could be lethal voltages on the power supply filter caps.
In addition to the great info you've been given, you should also check that the basic B+ supply is operating as well. If there is no B+ available to the output tubes, then they can't conduct any current to light the heaters in these tubes to begin with.
Also, most units that use this scheme for powering heaters do not provide the normal full heater voltage across the heaters of the tubes so powered. Therefore, they tend to have a very dim glow even when operating normally. If you are checking their operation visually, be sure you are doing so in a dimly lit room.
Dave
Also, most units that use this scheme for powering heaters do not provide the normal full heater voltage across the heaters of the tubes so powered. Therefore, they tend to have a very dim glow even when operating normally. If you are checking their operation visually, be sure you are doing so in a dimly lit room.
Dave
Here's what I would do as next steps:
1. Continuity check the output transformer primary windings:
Power off. Remove output tubes (remember which tube went to which socket so that you can put them back the same way to keep things consistent while debugging). Check continuity by placing ohm meter probes between pin 3 of an output tube socket and it's associated center tap on the output transformer. The center tap on the output transformer is the red wire. Follow it back to it's solder joint and measure from there to pin 3 of one of its associated output tubes. It should measure right around 120 ohms. Do the same for the other output tube. Then do the same for the other channel with the other channel's output transformer.
2. Measure B+ voltage. Put the tubes back in the same sockets they came out of. Dummy up the speaker load with some resistors (anywhere from 8 ohms to 100 ohms 2 watt resistors will be fine for now). Turn the unit on, after warmup, place the red volt meter probe on pin 3 of an output tube (carefully as to not touch any other pins) and the black volt meter probe on ground. You should measure about 365 to 370V DC. Do this for each output tube.
3. Measure filament voltages
Measure voltage between pin 4 of V2 and ground. On a correctly functioning circuit you should be getting between 28 and 30V DC.
Also measure voltage from the center legs of each balance pot to ground. Electrically this is the same place as pin 4 of V2. Should be getting the same 28 to 30V DC.
Keep track of all measured voltages and resistances.
Note: On older amps like this that have not been refurbished, you should keep an eagle eye on the amp while its on. If you see smoke or smell anything burning, turn it off immediately. Realistically you should use a variac or dim bulb tester first, but since you've already had the amp on (I presume to know that those tubes aren't lighting) then it's probably okay to proceed, as long as you watch it while it's on.
1. Continuity check the output transformer primary windings:
Power off. Remove output tubes (remember which tube went to which socket so that you can put them back the same way to keep things consistent while debugging). Check continuity by placing ohm meter probes between pin 3 of an output tube socket and it's associated center tap on the output transformer. The center tap on the output transformer is the red wire. Follow it back to it's solder joint and measure from there to pin 3 of one of its associated output tubes. It should measure right around 120 ohms. Do the same for the other output tube. Then do the same for the other channel with the other channel's output transformer.
2. Measure B+ voltage. Put the tubes back in the same sockets they came out of. Dummy up the speaker load with some resistors (anywhere from 8 ohms to 100 ohms 2 watt resistors will be fine for now). Turn the unit on, after warmup, place the red volt meter probe on pin 3 of an output tube (carefully as to not touch any other pins) and the black volt meter probe on ground. You should measure about 365 to 370V DC. Do this for each output tube.
3. Measure filament voltages
Measure voltage between pin 4 of V2 and ground. On a correctly functioning circuit you should be getting between 28 and 30V DC.
Also measure voltage from the center legs of each balance pot to ground. Electrically this is the same place as pin 4 of V2. Should be getting the same 28 to 30V DC.
Keep track of all measured voltages and resistances.
Note: On older amps like this that have not been refurbished, you should keep an eagle eye on the amp while its on. If you see smoke or smell anything burning, turn it off immediately. Realistically you should use a variac or dim bulb tester first, but since you've already had the amp on (I presume to know that those tubes aren't lighting) then it's probably okay to proceed, as long as you watch it while it's on.
IMHO, to amplify what kward just said- I'd NEVER run an A50K, UNTIL it has been recapped, period.
I've just gone through trying to resurrect one, which had a burnt out output transformer- caused, most likely, by cap failure to one of the output tubes. This made the tube go "full on" all the time (i.e, red-plating, pulling much too much current)... which in turn, overloaded the output transformer, making it burn out internally and short.
These output transformers are EXTREMELY hard to find. Do everything you can, to NOT EVER blow one out...
Regards,
Gordon.
I've just gone through trying to resurrect one, which had a burnt out output transformer- caused, most likely, by cap failure to one of the output tubes. This made the tube go "full on" all the time (i.e, red-plating, pulling much too much current)... which in turn, overloaded the output transformer, making it burn out internally and short.
These output transformers are EXTREMELY hard to find. Do everything you can, to NOT EVER blow one out...
Regards,
Gordon.
OK.. the continuity tests on the output transformers are all around 120-30.. my cheap radioshack multimeter fluctuates a bit and makes an exact reading difficult.
For measuring the voltage at pin 3.. What is the safest way to get the red lead on pin 3? Do I have the amp on its side? use alligator clips? go from the top of the chassis? Trying not to get electrocuted here..
For measuring the voltage at pin 3.. What is the safest way to get the red lead on pin 3? Do I have the amp on its side? use alligator clips? go from the top of the chassis? Trying not to get electrocuted here..
The amp should be upside down on good supports, with plenty of lighting on your work area. Place the volt meter at a location and angle that you can see the readout without having to move your body. Ideally you will only need to move your eyes to see the reading without turning your head. Any body motion may cause the probe to slip off of the point you're trying to read. Turn the volt meter dial to the proper setting.
Alligator clip the black volt meter probe to circuit ground. Turn on the amp. Then carefully touch the red probe to pin 3 of an output tube with enough force to make good contact (but not too much force). Usually I stick the pointy end of the volt meter probe down near the base on the inside of the socket where the pin comes out of the phenolic material and push outward toward the outer rim of the socket. Be careful not to let the probe slip off of the pin and touch the chassis or another pin.
Use only one hand for this. If you're new at it, put the other hand in your pocket so you're not tempted to use it while the amp is on. Wear rubber soled shoes (like sneakers). Put the probe down while you write down the voltage, then pick it up again for the next measurement. Don't be rushed. Don't have any background distractions such as music, children, or television. Use a clean and uncluttered work surface. After a few attempts you'll get good at it so you can do it with confidence.
Try all of the above with the amp off first so you know where your hand and arm motions will be and how much force to apply and how "slippery" the tube pin might be.
Well...I think that covers everything I can think of. This may sound like overkill, but each step is there for a reason. One time I got shocked from my power supply. My hand veered a little too far accidentally and touched the positive pin of the first power supply filter cap. Luckily I had shoes on (on my concrete basement floor) and only one hand in the amp. I had a big red welt on my hand for about a week, but I remained alive. That was the ONLY time I ever made that mistake as it scared the bee-jeebies out of me enough for me to remember from then on.
Alligator clip the black volt meter probe to circuit ground. Turn on the amp. Then carefully touch the red probe to pin 3 of an output tube with enough force to make good contact (but not too much force). Usually I stick the pointy end of the volt meter probe down near the base on the inside of the socket where the pin comes out of the phenolic material and push outward toward the outer rim of the socket. Be careful not to let the probe slip off of the pin and touch the chassis or another pin.
Use only one hand for this. If you're new at it, put the other hand in your pocket so you're not tempted to use it while the amp is on. Wear rubber soled shoes (like sneakers). Put the probe down while you write down the voltage, then pick it up again for the next measurement. Don't be rushed. Don't have any background distractions such as music, children, or television. Use a clean and uncluttered work surface. After a few attempts you'll get good at it so you can do it with confidence.
Try all of the above with the amp off first so you know where your hand and arm motions will be and how much force to apply and how "slippery" the tube pin might be.
Well...I think that covers everything I can think of. This may sound like overkill, but each step is there for a reason. One time I got shocked from my power supply. My hand veered a little too far accidentally and touched the positive pin of the first power supply filter cap. Luckily I had shoes on (on my concrete basement floor) and only one hand in the amp. I had a big red welt on my hand for about a week, but I remained alive. That was the ONLY time I ever made that mistake as it scared the bee-jeebies out of me enough for me to remember from then on.
OK.. So you, along with everyone else on the internet, has me shaking in my boots every time I get close to this thing while powered on.
Somehow I managed to get that probe into the power tube sockets and they all seem to be reading in that appropriate range..
How likely is this cause by bad capacitors? I think I should take some advice and change all of them before trying anything else.. Some of them that connect to the preamp tubes don't look like they're in the best shape, (siding of the capacitor tubes appear a little torn, but not bulging..
Any recommendations on where to source these from?
Somehow I managed to get that probe into the power tube sockets and they all seem to be reading in that appropriate range..
How likely is this cause by bad capacitors? I think I should take some advice and change all of them before trying anything else.. Some of them that connect to the preamp tubes don't look like they're in the best shape, (siding of the capacitor tubes appear a little torn, but not bulging..
Any recommendations on where to source these from?
Tubefan,
For recapping the power supply in stock form, you need 7 caps total split across 4 cans. Back when I did mine (like 7 years ago), I used four of these http://tubedepot.com/cp-jj-100x2-500v.html. (need the clamps too.) You would also need one additional cap perhaps like this http://tubedepot.com/cp-jj-47-385v.html, and just tuck it under the chassis somewhere safe.
For caps C27 and C28, I bridged the two sides of a 100+100 cap to make a single 200 uF cap per can, then connected them in series as shown in the attached schematic, which forms the voltage doubler.
The remaining caps show 50 uF each, but I doubled that to 100 uF each, using one side each of the remaining 100+100 caps.
Some other AK members may know where to source the exact replica caps (or close enough) so you get a stock appearance as well.
Also, when I recapped mine, I converted it to EL34 output tubes, and in the process I redid the power supply just a bit (combining the 370 and 350 V taps into one tap), so I did not need a 7th cap. In retrospect, I should have kept the power supply stock..maybe one day I will go back and fix that.
For recapping the power supply in stock form, you need 7 caps total split across 4 cans. Back when I did mine (like 7 years ago), I used four of these http://tubedepot.com/cp-jj-100x2-500v.html. (need the clamps too.) You would also need one additional cap perhaps like this http://tubedepot.com/cp-jj-47-385v.html, and just tuck it under the chassis somewhere safe.
For caps C27 and C28, I bridged the two sides of a 100+100 cap to make a single 200 uF cap per can, then connected them in series as shown in the attached schematic, which forms the voltage doubler.
The remaining caps show 50 uF each, but I doubled that to 100 uF each, using one side each of the remaining 100+100 caps.
Some other AK members may know where to source the exact replica caps (or close enough) so you get a stock appearance as well.
Also, when I recapped mine, I converted it to EL34 output tubes, and in the process I redid the power supply just a bit (combining the 370 and 350 V taps into one tap), so I did not need a 7th cap. In retrospect, I should have kept the power supply stock..maybe one day I will go back and fix that.
Last edited:
RevMen
The Reverend Menacer
Is this common?
Rev -- More common than you think. Fisher is quite famous for it, as are some other manufacturers as well. It's an easy way to power the preamp tube heaters with DC power, without having to go to the expense and trouble of providing a separate power supply for that purpose.
Dave
Dave
I aquired an HK A50K myself a few months ago (ebay, paid WAYY TOO MUCH FOR IT) I KNOW it must be refurbed - it is all original, even original tubes. All tubes test very good. Right channel almost never works beyond the faintest tinny sound. I use is exclusively for playing LP's from my Garrard turntable. Left channel goes in and out at whim, but mostly plays well. I have problems with my hands doing fine work (soldering), bnut my brother used to be an electrician, and has offered to do all the rebuilding work on it for me, but I have to get the parts. I have a sam photofact for the amp, and know many of the caps, etc are no longer going to be found using those part numbers. Is there anyone who can point me in the right direction for a good source for the resisters, caps, etc? I have a excel spreadsheet someone made up with the schematic in it, but I have been away from the tube world for quite a long time, and being where I am mail order is probably best. I hope I posted in the right area, I have never really used forums before, but looks like a rich place for good knowledge
Thanks everyone in advance!!
Thanks everyone in advance!!
