Weltron 2007 UFO (vintage) - setting bias and crossover

[dkneyle]

Currently restoring a Weltron 2007. Have replaced all the small signal and driver transistors in the the power amp stage. In theory I need to reset the bias and crossover. The service manual provides no guidance. Anyone got any thoughts about how to do this?

[These were amazing looking all-in-one systems from the 70s.]

[redwire]

Re: Weltron 2007 UFO (vintage) - setting bias and crossover

Hey man that's so groovy. Some movie had it and the miniskirts were dancing...

There's two trimpots per channel, one for amplifier DC offset and another for bias current.
Because the amp has an output coupling capacitor 1,000uF it doesn't matter if the loudspeaker is connected or not for setup.

Measure the top 2SC1060 collector (tab) voltage to see what VCC is, then adjust 20k trimpot at the small 2SC458 (offset) to try to get around 1/2VCC at the output node before the big cap. i.e. bottom 2SC1060 collector (tab) voltage target for around half of Vcc. It is not critical and will move around with mains.

For bias current, it is measured across either 0.5R emitter resistors and guessing around 20mA for maybe 0.15W idling per transistor. So target around 10mV, 5-25mV range I think would be fine.
The three 1S2076 diodes are supposed to track heatsink temperature, some amplifier designs do- others do not.
You have to set bias current and wait and see if the amp runs too hot (too high bias), or bias current keeps climbing up which is not good for thermal runaway. The 1k trimpot connecting to the diodes sets bias current.

The two trimpots likely interact a little so you have to go back and forth a couple time.
As long as the bias current is not too low or zero, that sounds terrible crossover distortion not cool.

[dkneyle]

Re: Weltron 2007 UFO (vintage) - setting bias and crossover

Thanks Redwire for the comprehensive "How-To' guide. For some reason they don't include it in their Service Manual.

Yes, it's very much 1970s sexy looking machine. Owner claimed it had strange distortion in one channel, but as is so often the case, when I got it into the shop I couldn't fault it. He requested a re-cap and whatever else. The experts on here said it was a cheap and simple circuit and to leave well enough alone. But owner was insistent he wanted it future-proofed, so I did a complete re-cap of the power amp, pre-amp and phone amp, and power supply. Up the value of the main supply cap to 3300uF. I then replaced all the small signal transistors with modern equivalents. Some of the old PNP driver transistors were showing gain of only about 5!

I do actually believe I can hear an audible difference. Originally I noticed a distinct 50Hz mains hum when I pushed the amp hard. That at least seems to have gone.

I'll let you know how I go after the set up adjustments.

[dkneyle]

Re: Weltron 2007 UFO (vintage) - setting bias and crossover

Got DC offset adjusted without too much drama. Vcc is 42 volts, and was able to adjust the offset to 21 volts as per Redwire's instructions. (It was about 18 volts on both channels prior to adjustment).
Bias I couldn't get anywhere near Redwire's calculated numbers. At maximum adjustment I was getting just a fraction over 4mA. Mid point on pot was about 2mA and that was approximately where the pots had been set. So I decided to adjust bias for 2mA because that intuitively seemed about right.