Rotel by Martel 100AMP

[sretchko]

Anybody know anything about this little Amp? Service manual or schematics? found nothing in the usual places. It looks similar to a Rotel RA-310 but very different inside.

I have acquired a ROTEL by Martel model 100AMP integrated amplifier that was made 1967 - 1969.
It appears to be all original components. After replacing a single blown fuse I am surprised as it does pass a sinewave signal on both channels, but all of the electrolytics are way out of spec (didn't Leave it on for long) and I plan to replace the e-caps and put it back in service as my bench test amp when servicing preamps, tuners, etc. since it is a small class a/b with a linear power supply that will fit nicely on a shelf above my bench, it measures 12”x8”x4”.
Oh yea, did I mention it has T-03 germanium transistors!

Any information you may have would be greatly appreciated.
Thank you for your help,
Steve

 

[wparks]

I could not find anything. Curious what the output transistor numbers are? Are you sure they are germanium? You almost always see interstage transformers used as a phase inverter driving germanium output devices- are they perhaps hidden underneath? These look like direct drive?

Also- to reach 100W output power with germanium you typically see four devices per channel, usually stacked. That kind of power would almost certainly be Si.

 

[sretchko]

It has four 2SB407 output transistors. the datasheet comes up as germanium. When I tested them they have 0.2v turn on voltage. It also has four driver transistors that I can't see the numbers on due to the little heatsinks glued on them but they appear to be two npn and two pnp, two have a black line on top and two with red line on top. It also has a 3 pin SB3 bridge rectifier. I've never seen a 3 pin rectifier before.

Just has the main transformer, no other transformers.

The model number 100AMP is confusing. It is rated at 15wpc.




bridge rectifier:


four driver transistors:

 

[wparks]

Ok - that makes sense. AI "hallucinated" that this was a 100wpc channel amplifier and that did not make sense. 15W would be about right, given the 30V limit of the transistor.

2SB407 does list from all transistors to be a Ge PNP, but then they point to a (Chinese) datasheet for a 2SB407 that is actually a Silicon PNP. Just to be certain, I looked it up in my handy 1981 RCA SK replacement guide, and the 2SB407 maps to an SK3009, which is basically the same type of TO-3 Germanium PNP power transistor that is used in all of this class of germanium AB amps, from Magnavox "Astrosonic" consoles on up. My little Magnavox A531 amp is a good example, and I have a post there on transistor characterization using a simple circuit should you need. Fortunately it sounds like it is quite functional.

Sure hope you can find a schematic for it- that would be academically interesting. Should be an easy restore without it though, mostly just electrolytic cap replacements. Will be interesting to watch!