Olsen model AM 256 stereo amplifier

[electra225]

I use this little stereo amp on the bench when I need one. I don't remember the last time I used it, but it has been at least two years. I wanted to bring it up to power slowly, so I connected it to my iso/variac/dim bulb contraption. I set the voltage at about 60 volts, and put the Kill-A-Watt in series with the amp and power supply. As soon as I flipped the power switch on my power supply, the 5 amp circuit breaker tripped. I fired up my trusty old VTVM, and found a dead short at the filter caps. The four-section filter cap is a Rubycon with two 40uf @300 volt caps and two 10uf @ 300 volt caps. All four show shorted. Next step is to disconnect the circuit at the rectifiers, check the rectifiers, and also check to make sure the PT has not been damaged. Then I will lift all four legs of B+ at the filter caps and check each of them for potential shorts. Finding all is well, I will replace the filter caps. There is precious little room under the chassis, but the plan right now is to restuff the can with the two 40uf caps with 47uf @ 450 volt caps. The two 10uf caps I believe can be mounted under the chassis on a terminal strip. Tight, but doable. The 6AQ5 SE amp is not a barn burner, but it worked real nice for a test rig. One of my next projects is to start working on changers, so I wanted all my test equipment up to snuff.

[TC Chris]

The 6AQ5 SE amp is not a barn burner, but it worked real nice for a test rig. One of my next projects is to start working on changers, so I wanted all my test equipment up to snuff.

Olson was not Mac-level stuff to be sure, but I like the thoughtful way they recessed the output tubes. I assumed it was for ventilation, the neglected issue for so much tube equipment, but then it occurred to me that maybe it was just for vertical clearance purposes, with venting an unintended benefit.

Chris Campbell

[electra225]

This thing works mono or stereo, so it was handy for the bench. The audio is so-so. All I want it for is to make noise when appropriate, and it fills that bill nicely.

[Firedome]

IIRC these Olsen amps were made in Japan, not that that's a bad thing. I had a very early Kenwood tube receiver that was made in the early-to-mid '60s and it was excellent, had 4 x 6BM8 PP output I believe.

[electra225]

I'm sure this one is made in Japan. IT seems to be well-built with a beefy power transformer, and wimpy output transformers. It is handy for a test amp. I hope I can resurrect it.

[electra225]

I have a high current draw situation still in this amp. At about 60 volts AC input, dim bulb and KAW in circuit, it pulls 25 watts with no tubes populated in the chassis. I have cleared the short by replacing the filter caps. Something is still drawing lots of juice someplace. I am going to have to see if I can locate a drawing for this amp before I go much farther. I can guess, but that won't accomplish much. I have been hand-drawing a rough schematic, but I need a real one. IT kinda acts like an output transformer is shorted, but surely both of them aren't. The 200 ohm power resistor gets pretty warm. I haven't seen any smoke, but I still have the dim bulb in the circuit along with the KAW. It pops a 1 1/2 amp fuse if I increase input voltage much above the 60 volts, without the dim bulb in circuit. It still acts like all four filter caps are shorted to common negative.

[electra225]

If you look in the first picture I posted, in the extreme bottom right hand corner, is the rectifier bridge. There is a 40 ohm resistor that points straight up in that shot (black resistor). In order to isolate a solid state rectifier from the circuit, I had to disconnect just past that 40 ohm resistor, at the first filter cap. The heavy draw was still there. I unbolted that rectifier bridge assembly from the chassis, then disconnected the PT from that rectifier bridge by unsoldering the wiring. The draw went away. The PT draws 1.68 watts at 120 volts line voltage, with no noise and no extra heat. There is 200 volts AC on each half of the PT secondary. I'm not sure that's enough, the tube manual says 155 volts on the plates of the 6AQ5's, for what that's worth. I know better than to work on something without a diagram, but here I am. The rectifier bridge consists of two diodes paralleled by two .01 at 600 volt caps. My question: Can rectifier diode(s) short? And, if so, what is the effect? This thing suffered a catastrophic failure at some point. There is no fuse in the PT. I reckon I just keep throwing parts at this thing until I clear the heavy draw with everything connected. I need to bone up on which end of a rectifier diode is which, but I think I have that written down somewhere. I'll replace both rectifier diodes and both the capacitors, then see what I have. There is a 200 ohm resistor on the other side of the filter caps that was getting warm, but that might be its nature. Also, when I was checking that draw, I had 4.5 volts on the filaments. After I cleared the draw, I had only .6 volts on the filaments. I have no idea about that. This one has a problem I've never ran across before.

[TC Chris]

How about those parallel caps (snubbers of some sort?). They're not shorted or leaking badly?

Chris Campbell

[electra225]

The power supply in this little amp is totally toast. I have determined that the PT is bad. I don't have any filament voltage. I do have high voltage, which is rather unusual. Generally, when you toast a PT, you take out the HV winding. Realizing that I'm now in a "I guess I'll learn something" mode, and also realizing that I'm wasting parts on this thing, I have decided to try and figure out what happened. What took out the power supply. I have great high voltage after replacing both rectifiers and both .01 caps across them. One diode was showing a dead short. That is not my only problem. The big 200 ohm green resistor you can see in the first picture I posted gets smoking hot at about 85 volts on the line. The dim bulb glows at full brightness and the KAW shows 112 watts draw, when the nameplate says it draws only 36 watts when all is well. There is about 55 volts on one end of the 200 ohm resistor, nothing on the other end, so it has a tremendous draw on it yet. The output transformers connect to the business end of that resistor. I disconnected both wires to the output transformers. One transformer has a shorted primary, shorted to chassis. My theory is that the output transformer shorted, then somehow took the filament winding out of the PT. I haven't tested the output tubes yet. This failure may have been due to my not paying attention to this thing when it was running. I don't remember the last time I used it, nor what service it may have performed. I would typically start this amp up when I started working and let it run like my VTVM or signal generator all day whether I was using it or not. So it may have had this issue when I wasn't paying attention to it. I shut if off thinking all was well, when I had smoked it. Kind of too bad. I may keep it for parts, but all that may be good is the cord, maybe some tubes and controls. The rest is junk.

[TC Chris]

Well,with the diodes you don't have a 5V filament winding to deal with, so you could buy a new filament transformer and one new OPT and you'd be all set. Or maybe scrounge around in your junk box....

Chris Campbell

[electra225]

My junk box doesn't have what I need. I have an Admiral solid state amp, but its PT won't work. I think I'll replace both output transformers and see if I can find a cheap PT somewhere that will work. Then I'm going to fuse the primary of the PT and I'm going to fuse B+. I have a ton of parts already in this thing from throwing parts at it. And it's kinda cute and it works very well for what I need it for. I don't believe I can replace it for what it will cost to fix. I may put out some feelers for a couple output transformers and see what I can find.

[TC Chris]

Again, can you get away with just a filament transformer and use the HV portion of the old transformer?

Chris Campbell

[electra225]

I'd thought of that. A walwart should power up three tube filaments. There is no room for anything extra. NONE.

I really believe, in the long run, a new PT would be the cheapest and more trouble-free. I don't have a lot of faith the high voltage winding hasn't been compromised. My luck I would go to the trouble to kluge in a filament transformer only to lose high voltage in short order. Every time I try to shortcut what I know to be right, I end up drawing the short straw.

[back-2-mono]

Me thinks that unit was made by Monarch in Japan. I had one back in the day that looked exactly like it. Same cosmetics, single-ended EL84 outputs, same slide switches and knobs.
You might want to BOLO for one on the bay for a donor.
Just sayin'...

[electra225]

I wonder if the components in a donor would be better than what I have. I will keep an eye out. Thanks for the suggestion, Gary. Good to see you on the forum......

[electra225]

They are asking big money for Olson amps on the 'bay. This one is worth fixing, according to that. Wow!

[electra225]

While I'm waiting for parts and figuring out how to get 70 cents worth of diodes here without paying $9 shipping, I am also in "toss" mode. I am clearing out things I don't use, things I can't sell, things that don't work. I am tired of working around this stuff. This little amp is on the "toss" list. I decided to revisit this issue. I don't really think the PT is bad at this point. I don't have any filament voltage, there is a dead short somewhere. Looks to me like if the filament winding in the PT was open, it wouldn't provide the short. I reckon it could be shorted, but to what? This will make an interesting investigation while I wait for parts. I can't help but think I overlooked something.

[electra225]

Well.....

I may have found a problem. A self-inflicted problem. A rookie mistake. I know better. I ALWAYS check and re-check my work, right? I ALWAYS work with a schematic, right? I SHOULD always work with a schematic. Sams doesn't list this amp in their catalog. I'll try Monarch, but I'm not holding out much hope. I really do need the drawing for this amp. Stand by.....

I had a hot wire connected to a ground wire. Fixing that got rid of the short. Now I'm down to having an open output transformer and an open filament winding in the PT. I like the idea of finding room to place a filament transformer somewhere. I think I can do that if I find a small one. The output transformer may be a different kettle of fish. One should really replace both so it will sound the same on both channels. I'll dig around a bit to see if I have something I can use for test...

[electra225]

Killing two birds with one stone here....

AES has output transformers that should work for $20 apiece. Also have a filament transformer that should work and that will fit for $18 and change. This little amp has been all recapped and checked otherwise, so, unless I have committed another hack, it should be fine. Plus, they have the diodes I need for my TV project, so that will work out well.

[danrclem]

While I'm waiting for parts and figuring out how to get 70 cents worth of diodes here without paying $9 shipping, I am also in "toss" mode. I am clearing out things I don't use, things I can't sell, things that don't work. I am tired of working around this stuff. This little amp is on the "toss" list. I decided to revisit this issue. I don't really think the PT is bad at this point. I don't have any filament voltage, there is a dead short somewhere. Looks to me like if the filament winding in the PT was open, it wouldn't provide the short. I reckon it could be shorted, but to what? This will make an interesting investigation while I wait for parts. I can't help but think I overlooked something.

I don't know if this deal is still available, but DigiKey used to have free shipping if you paid by check. I'll try to see if they still do this and if they do I'll let you know.

Edit: It looks like it's still available. It's a good deal but may take a little longer than a conventional order.

https://www.digikey.com/en/terms-and-conditions

Line 6 under, "II. Conditions of Order"

* When a check or money order accompanies your order, DigiKey pays all shipping and insurance via ground service level to all addresses in the U.S. and Canada.