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Harman Kardon A-250 lets get it operational

Kingbleu

Kingbleu

Well-Known Member
Harman Kardon A-250 tube amplifier. This is the version with three 12AU7 and three 12AX7. I ordered some caps from mouser and others from just radio.



C1, C2 120uf 250V - 120uf 450v Miec
C12 .05uf 400V - .05uf 630v Brown dip
C25 .05uf 400v - .05uf 630v Brown dip (C25 is .1uf on the sams photo fact) .05 factory installed.
C09 .1uf 400v - .1uf 630v Brown dip
C22 .1uf 400v - .1uf 630v Brown dip
C17,18,30,31 coupling caps .1uf 400v - .1uf 630v brown dip

There are some differenes from the photo fact and my amp. For example photo fact shows four caps C16,19,29,32 I don't have these caps in my amp. Also photo fact has C6 in a different location than my amp.

The three multicaps are are going to be replaced. C3 and C5 can probbably be dissconnected and I can instal the electrolytics under the chasis. Space is limited and C4 will have to be restuffed. I have never restuffed and I'm not looking forward to it. I watched some videos on how to do it and it looks like a PITA no matter what method is used. If the restuff is a complete failure I will just purchase one new but would rather save $60.

Im thinking after I replace all the caps mentioned it will be safe to power up.
 

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As it sits now, put the tubes in it and power it up as is. Just use a dim bulb tester in series with a 100 watt incandescent bulb. Using an amp meter, you can watch the current draw as DC starts to flow. This way you can see if it works and or produces any music. Then do a recap to become safe and reliable.

Sometimes the can capacitor will get warm as it starts to reform but its safer with the dim bulb.

Todays capacitors will be smaller so you should be able to replace all of them underneath. Those orange electrolytics in the power supply can be replaced by smaller, better capacitors.
 
cademan said:
As it sits now, put the tubes in it and power it up as is. Just use a dim bulb tester in series with a 100 watt incandescent bulb. Using an amp meter, you can watch the current draw as DC starts to flow. This way you can see if it works and or produces any music. Then do a recap to become safe and reliable.

Sometimes the can capacitor will get warm as it starts to reform but its safer with the dim bulb.

Todays capacitors will be smaller so you should be able to replace all of them underneath. Those orange electrolytics in the power supply can be replaced by smaller, better capacitors.
Click to expand...
I powered it up to 115V and it was drawing 33watts on the watt meter. The dim bulb starts coming on at 60V and slowly increases brightness. Its not glowing like the sun more like a dim night light. None of the tube filiments are glowing but one of the output tubes is warm.
I need to figure out this schematic and take some voltage readings.
 
I hooked it up to mains power and got sound. It didnt like the dim bulb starving it. Im getting 18 watts before clipping. The loudness pot is bonkers it does not track evenly and cleaning it did not help much. The balance pot is also wonky. I have to alternate between balance and volume to match both channels evenly. Balance is generaly in the 11:00 position when both channels match.
With my phone plugged in I have volume knobb at 3:00 for a normal listening level. Doesn't sound great lot of distortion in the low end.
 
This is an excellent amplifier with very good transformers and a stout voltage doubler power supply. Well worth the time and trouble to restore. I generally don't have the patience to work with 60-year-old pots and switches. so I convert older integrated amps like this to straight power amps.

Jack
 
not massively in love with the problems all those old pots and switches bring either, but I'm also something of a preservationist. I might take a middle path if I really wanted this as a power amp and fit a set of preamp out / main in jacks. Pop the jumpers and its a power amp, put them back in and its as-designed.
 
That's why I stick to theater and PA amps that have been stored outside for decades... bulletproof once you clean off the half inch of dirt!
 
I had one of those amps, mine was from 1959. A recap was essential before it produced great sound. The biasing resistors were a bit under rated for current and the values ran the tubes too hot. I replaced all the biasing resistors with higher wattage and used values that would run the tubes at safe current levels. I had issues with the dual volume pot. It never tracked properly but I was unable to find a replacement so I lived with it. A friend of mine has it now and runs it all day, every day. If you can restore it you will be rewarded. If not, sell it to me. :idea:
 
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