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Hi,
Just finished a big job refurbish a Beomaster 4400, idle ciurrent is 11,7 mV (meassured on the Emitter resistors) for both channels and stable after warm up, however i meassure a DC unbalance at left channel 43mV and right channel 21 mV ??
Do i have to live with this? or can i do something to get it closer to zero mV?
I have renewed all capasitors , also the filter caps for the +- 35v supply, no semicunductors renewed nor checked, however the Beomaster is working very well, good sound, output meassured to 85-90w into 4 ohm before clipping both channels driven at the same time.
Also i have meassured the difference in the output into 4 ohms with 1kHz sine on tape2 input equal signal level on both input left and right, but the output is not equal when turning the volume up the output is not always equal, it is different depending on the volume setting.... it is not possible to adjust this correctly on the adjacent pots in the preamp. there is always some volume levels with a small difference...
1 DC level on output is it OK with 43mV and 21mV? what to do
2 Difference in output with different volume settings?
Can someone give me a hint
Kind regards
Claus
1.43mV is not a lot and certainly not something that will ruin anything in your speaker, but it's more than most adjustable amplifiers are spec'ed for.You could try to replace the two transistors in the differential amplifier stage.They can cause DC offsets if not reasonably matched. Fitting two from the same production batch will usually be fine.
2.Could be as simple as a dirty (or bad) volume potentiometer.
Martin
Hi Martin,
Thanks for your reply, i will purchase some BC547C and select some pairs with hfe as close as possible.
as for the volume pot, it is carefully cleaned with kontakt 60, but i think it suffers from aging, when moving sideways it feels very loose, compared to Bass Treble and balance pots which feels more firm
My 4400 is one of the first, with the full plastic potmeters, any idear where i can find those? i searched a lot but maybee its a dead end?
One more thing, i found bigger filter caps 15000 uf which fits in the 4400, would it be an advantage to replace to those? or is the difference not noticeable
/Claus
PS må man skrive på dansk her? eller skal det så være nordisk forum?