Tempo #158 schematic?

[glebert]

I was given this little Japanese solid state amp with the proviso that it was "really sketchy." It absolutely was really sketchy. The funny thing was that it worked when I first got it and had it on my variac and LBL, being careful not to touch anything else because there was no grounding. Actually sounded pretty good. Installed a grounded cord and inline fuse and all the sudden no worky. Power transformer measuring completely open primary.

I got it working again wiring in a center tapped 12VAC transformer I had sitting around but I have no idea what the secondary voltage really should be. The only marking on the first transformer was the number 847 inked on the paper around the windings. One main filter cap 1000uF 35WV so presumably there is one voltage rail (totem pole output) that is less than 35V but other than that I got nothing. I haven't even measured my rail voltage yet, but output is distorted.

Like this one. Thanks in advance.

edit: measured that I am getting about 15.5V on the main rail and about 6.5V on the midpoint of the outputs. I believe this was sold as a 10 watt amp (for whatever that is worth). Seems like to get 10 watts out with an 8 ohm load you need about a 25V rail ( ((25/2)/1.414)^2/8)

[J M Fahey]

Are you measuring straight at the transformer wires or terminals?
You replacing cord opens opportunities for miswiring between outlet and primary.

Show a closeup of that, also power transistors and filter caps all with readable values.

We are wizards but only that much

Yes, it might need some 25-30 Vdc as you calculated, so some 18-21VAC secondary.
24Vac sounds slightly over the top.

No need for CT since itīs not a split supply.

Whatever rail voltage you measure now is wrong, because you have the wrong PT

[glebert]

Yes, I measured PT primary directly at the transformer. I also think the transformer it came with wasn't the original, based on the way it was mounted and the hacked wiring.

Output devices are 2sb473, so pretty low voltage (Ucb=32V is the only rating I can find) and power diss.

Regarding CT, since it had a CT secondary PT and two diode rectifier that is what I kept for now, but am not married to it. I'll try to get some pics later.

Looking at other schematics of similar amps almost none of them say what the rail voltage is anyway.

[glebert]

Pics.

Main filter cap


Output capacitor and main rectifier diodes.



Output transistors (both same)


Top view



Bottom view (power supply at top of pic)

[g1]

Apparently they were also sold as Univox. This 150RX schematic may be similar.

[glebert]

Apparently they were also sold as Univox. This 150RX schematic may be similar.

I've been using the Univox U65rn schematic which also has some similarities to the Tempo, but also some differences for sure. For example the Tempo doesn't have the resistor chain which sets the output transistor bias points and the midpoint. This is a bit strange since it does seem to have a interstage transformer to do phase inversion.

The 150rx schem is nice because it does give the rail voltage (a volt higher than the cap rating!). Thanks!

[dmeek]

Don't know the model number of this schematic but it looks close. I see only 2 rectifier diodes in your photos. Are there 2 more not visible? This amp should be running on a negative power supply. The rectifier bridge in the schematic is reversed.
If it uses 2 diodes and a grounded center tap I would use a 45vct transformer. With a 4 diode bridge use 20v-22v.

[The Dude]

The fact that it worked initially, and then didn't makes me wonder if there's not an intermittent connection in the transformer itself. I'd pop the end bells and see if maybe there's a broken lead or a poor connection on one of the lead wires.

[glebert]

The fact that it worked initially, and then didn't makes me wonder if there's not an intermittent connection in the transformer itself. I'd pop the end bells and see if maybe there's a broken lead or a poor connection on one of the lead wires.

OK, the mystery of the first transformer is resolved. I had forgotten that there was a little extra piece of wired attached to some leads on the primary side but the wire went nowhere and was just taped to the insulation of a different lead. Turns out that piece of wire was actually connecting two windings of the primary side, so when I took that bit of wire off the primary went open. I had tried for continuity to those leads but forgot they were covered in clear enamel so it only gets connection if you probe at the very cut end. Mistakes=learning...

So I could theoretically revive that transformer, but I had already rewired the PS to use the 22V CT I had tried earlier with a full bridge rectifier and not using the CT. Thanks to dmeek for the tip about this being negative voltage, I would have blown it up for sure without that. Now I'm getting -32V main rail and -16V midpoint. Output is cleaner but really quiet. Main signal barely louder than the hum I'm getting. I will take a closer look and compare to dmeeks schematic and see what I can find. Probably not worth spending much time on this guy, but now I'm invested.