K

[spritzer]

This amp (plus the matching tuner and a Sony reel to teel) has been sitting at my parents house for as long as I can remember, completely untouched. I was finally curious enough to open it up recently and it is really something special in a vintage kind of way, a germanium SS amp with output transformers. I'm not sure how old this is but late 60's would be a good guess. Now it is about the same size as the larger Stax amps so no powerhouse but I'm a bit interested in hearing it. So here is what it looks like on the inside:

Here is the back panel, DIN connectors everywhere plus the output transistors:

It is a pretty neat layout and the assembly is brilliant for something of this vintage. The PCB's are well made and some thought has gone into the layout. This thing even has a bridge rectifier and the speaker outputs are PCB mount. The whole chassis acts as a ground plane even if it isn't earthed.

Now the issue here is whether I should risk powering it up as it is or just start to replace parts right away. The caps could be drained but still operational or they could be utterly toast and take out the rest of the circuitry when they fail. Same goes for the bridge rectifier, I've seen my fair share of diodes from that era in the Stax boxes and they don't age well.

[kevin gilmore]

I would be real careful with that. Blow a germainum transistor and its game over.

Chances of finding any replacements is pretty much zero.

"Oliver Tex Germanium" charges stupid amounts of money for germanium output transistors.

[mypasswordis]

Replacing seems like a good idea... make sure you replace the germanium diodes with the same or similar as you probably want similar voltage drop and exponential characteristics. I've heard you can reform electrolytics but not a good idea to do so in the circuit as something may blow... probably would just replace with new ones just to be safe.

[Torpedo]

Variac?

[spritzer]

Turns out the local store has a large stock of axial caps so I'll drop by later to see what they have for me. Most of these are standard values except for the three 2500uf caps. 2200uf is close enough or I can just go to 3300uf for the main PSU cap.

I managed to cut and desolder enough to free the small PSU board and the bridge rectifier looks to be a Silicon part:

I can't find a datasheet but it has all the markings on it plus the same on the bottom of the PCB. The red part behind it is a very old Wima MKS cap.

Variac?

Sadly that's one of the toys I lack. I could fire it up at 100V with a transformer, then go to 120V and then full power but just replacing the lot appears to be the way to go.

[luvdunhill]

If you don't have a variac, rig up something with a lightbulb. I would definitely not power it up directly connected to mains...

[spritzer]

The local store really came through on this one with a nice stash of Philips/BC caps so I just scrapped all of the electrolytics. Here is the end result:

There are also some caps which I had at hand, including the rather nice Nichicon ES non-polars. I also changed out the rectifier for a RS402 unit and the Wima filter cap next to it:

Here is that Wima cap next to the modern version. I haven't fired up the amp yet (I'm going to do it with fresh eye's) but I must say it has been a pleasure to work on this amp and the person who did the PCB layout really knew what he was doing. There were often multiple holes for each cap and the true dual mono layout makes it much harder to mess up something. Here is the underside:

Now the next step is to swap out the crazy DIN headphone socket. This amp will most likely never be used as a speaker amp but it could be ok with headphones. While I can live with my nice, all metal DIN input plugs the same doesn't apply to the very fiddly headphone connector. Time to retrace this pile of wires:

[spritzer]

The headphone socket turned out to be quite a complex beast so I just decided to leave it be. There is a switch attached to it which engages resistors to lower the speaker output once the headphones are connected. That is a good feature to have so I'll just have to live with an adapter.

KG also helped me figure out the 4 remaining caps which had rather odd markings so now they are gone as well:

Now I had no excuse not to power it up so after hastily constructing a 5-pin domino DIN headphone adapter and a 5pin DIN to RCA input cable I flipped the switch...

...and it works perfectly. I still haven't tried it with speakers but it works pretty well as a headphone amp.

[Voltron]

That's very cool Birgir! Are you going to try powering a Stax trafo box from it?

Btw, what the heck is the "scratch" dial or button?

[spritzer]

That's very cool Birgir! Are you going to try powering a Stax trafo box from it?

Yup, it will be a good flea power test for the Lundahl's...

Btw, what the heck is the "scratch" dial or button?

Old school vinyl crap, basically a massive HF roll off to take the edge off recordings.

[bhjazz]

Cool find, and nice repair job. Neat to see photos of it in front of a futuristic computer mouse

[spritzer]

Here is what the finished article looks like with all the custom cables I made for it:

I also spent some time with it driving my Tannoy R2's. No excessive volume issues like with so many vintage amps but it did turn the very mild mannered Tannoys into slow and bloated bass monsters. This is with all the EQ dials in the center but then I remebered reading the Goodmans Maxiamp article in the latest HFN. There they confirmed with measurments that the center position for the bass control was indeed some +3db from neutral. So given that the EQ design could be common and similar the vintage of these amps I tried to dial it a bit back and indeed that worked like a charm. My first hours with the amp were without the front dials in place so I had no idea where the controls were, I just adjusted by ear. The bass isn't still quite linear but that can be said about the rest of the spectrum as well. The top end is rolled off and the midrange a bit larger than life but it makes for a nice whole.

The total cost of the restoration was about 100-130$ and it isn't bad considering I bought most of the parts locally and the rest was my usual TOTL crap that costs way too much. I will venture a guess that the amp sounds better than new...