Checking a Sonab R4000

[kristerj19]

Hi Guys,

I just started a checking a Sonab R4000 (quite an unusual amplifier) and it has really nice "heavy metal" knobs.


All three inputs works nicely and the receiver plays OK. After replacing the bulbs and I measured the DC offset at no load and zero volume. The Dc offset was around 200 mV on each channel, which I believe is a bit high?



So I unsoldered transistors V701 & V703 plus V702 & V704. These seemed OK, but I'm a bit concerned about the matching:

V701: hfe = 244, Vbe = 0,762 V
V703: hfe = 224, Vbe = 0,751 V

V702: hfe = 248, Vbe = 0,757 V
V704: hfe = 212, Vbe = 0,761 V

Would you consider these transistors to matched?

Also, some clever guy has thoroughly painted most the components, which make reading the actual transistors difficult. At least on one I could confirm that it was a BC207B.



Does anyone have any ideas for replacement for BC207B?

 

[cademan]

It's fine as is but if you are a novice, you're just going to introduce new problems where there were none before.

 

[kristerj19]

It's fine as is but if you are a novice, you're just going to introduce new problems where there were none before.

I'm clearly a novice, so if you think it is more likely that I will introduce more problems, I might just leave it as it is and only replace the capacitors.

 

[ivandezande]

Hi Guys,

I just started a checking a Sonab R4000 (quite an unusual amplifier) and it has really nice "heavy metal" knobs.

View attachment 1588638
All three inputs works nicely and the receiver plays OK. After replacing the bulbs and I measured the DC offset at no load and zero volume. The Dc offset was around 200 mV on each channel, which I believe is a bit high?

View attachment 1588641

So I unsoldered transistors V701 & V703 plus V702 & V704. These seemed OK, but I'm a bit concerned about the matching:

V701: hfe = 244, Vbe = 0,762 V
V703: hfe = 224, Vbe = 0,751 V

V702: hfe = 248, Vbe = 0,757 V
V704: hfe = 212, Vbe = 0,761 V

Would you consider these transistors to matched?

Also, some clever guy has thoroughly painted most the components, which make reading the actual transistors difficult. At least on one I could confirm that it was a BC207B.

View attachment 1588653

Does anyone have any ideas for replacement for BC207B?

Offset at 200mV is not fine at all, you are correct. Try matched KSC1845 for replacements, ignore the other unhelpful comment.

 

[kristerj19]

Offset at 200mV is not fine at all, you are correct. Try matched KSC1845 for replacements, ignore the other unhelpful comment.

Thanks, if there is no improvement with the KSC1845, I can always revert to the original transistors without too much trouble. I'll will be posting progress...

 

[cademan]

Its not sending DC to the speakers, and its not going into protection, and your speakers are not smoking and you yourself said it sounds fine so...……...don't listen to ivandick. Ignore his unhelpful comment!

Been doing this for far too long but if you want a doorstop, go ahead!

200 mV= 200/1000 V= 0.2V

 

[century tek]

With the schematic you have showing, it looks like there is no adjustment for DC offset but I could be wrong.

But for now, it's fine!

 

[kristerj19]

Time for a little update. Mostly out of curiosity and learning, I decided to replace the pair of BC207 on left channel with a pair of KSC1845, to see if it would make a difference. Ordered 10 KSC1845, and all of these where actually very close in both hfe and Vbe, so it was easy to find a matching pair. After replacement the DC offset on the left channel went from around 200 mV to around 95 mV, so clearly an improvement. So I did the same on the right channel and this also went down to around 100 mV. However, I'm not sure it will make any difference when listing to the amplifier, but at least I feel better. I have tested it with some old car speakers, just to check that there is no serious issues.

Next up will be replacement of electrolytic caps, but I will not work on the tuner board as I do not have the proper test equipment for this.