I would like to check the Bias on my 840BX. The service manual states to test with the + lead on TP3 & - lead on TP1 and adjust VR601 to 4.0mv.
Then repeat on TP4 &TP2. I have the wiring schematic in the manual but there is no TP1 thru TP4 that I can find. Any suggestions? I can forward the manual to anyone that can help me out.
Thanks,
Bill/Texun1
Then repeat on TP4 &TP2. I have the wiring schematic in the manual but there is no TP1 thru TP4 that I can find. Any suggestions? I can forward the manual to anyone that can help me out.
Thanks,
Bill/Texun1
Thanks for the response gents. I did find the test points and my readings are 4.1 & 4.3. Sakis you said this is low. Is there a higher setting that you would recommend.
Regards,
Bill
Low? Odd. I was just wondering if it higher bias and that was one of the keys to why it sounds so darn good when the basic circuit and parts are mid-line at best. 125mA is more typical but I have no idea if the heat sink can take it. I have a pair of RA-951's too. Same listenable sound but with a few modern touches like a protection circuit. I am keeping my paws out of my RA-840II as it just sounds so darn nice.
125ma will be far to high for this amplifier ( assuming that we are talking 125ma per device) i don't think that the heatsink will ever make it
still calculation of 18ma to my opinion is low still this is the recommendation of the manufacturer ..... i presume that this much is only required to reduce crossover distortion and not to bias the very first watts of the amp in class A
Kind regards
sakis
still calculation of 18ma to my opinion is low still this is the recommendation of the manufacturer ..... i presume that this much is only required to reduce crossover distortion and not to bias the very first watts of the amp in class A
Kind regards
sakis
This all started when I wanted to just check a few things on this old amp to see if she was still in basic good health. I started with the DC offset at the speaker terminals. Ideally it should be very close to 0.000Vdc. Well one channel was -57 and the other was 1.4. Most amps have a pot that can be adjusted to help bring these values a little closer. As far as I can tell this is not an option on the Rotel. It seems to me the only option would be a set of new matched transistors.
Next I wanted to check the bias while I had it opened up. That was the reason for the post. Once I found the test points everything was pretty much on spec. I agree this old girl sounds great for her age. I did bump the Bias to 5.0 and will keep an eye on the temps.
Next I wanted to check the bias while I had it opened up. That was the reason for the post. Once I found the test points everything was pretty much on spec. I agree this old girl sounds great for her age. I did bump the Bias to 5.0 and will keep an eye on the temps.
Last edited:
if this is the case then the bias voltage is very much lower than for an EF output stage.
From what I think I understand, it is not the class A region, or even crossover distortion, that we run higher bias unless you are going for several watts, but it has more to do the the linearity of the devices themselves.
Agree ... no probs with that i just stated my opinion
the OP has pretty good understanding of the troubles related to temperature if bias is increased and generally stability issues ... but also owns the amp for quite some time so he may understand if it sounds better or worst with a click of bias more .
Another thing i wanted to add is that offset and other problems might originate from capacitor related issues ...sampling a couple of them for evaluation will be an easy thing to do ( to my world replacing them all will be the best to do ) since amplifier is old enough for a thing like that ...
As a million times said before ...given the opportunity you always have the option to put something better bigger in the same space
kind regards
sakis
the OP has pretty good understanding of the troubles related to temperature if bias is increased and generally stability issues ... but also owns the amp for quite some time so he may understand if it sounds better or worst with a click of bias more .
Another thing i wanted to add is that offset and other problems might originate from capacitor related issues ...sampling a couple of them for evaluation will be an easy thing to do ( to my world replacing them all will be the best to do ) since amplifier is old enough for a thing like that ...
As a million times said before ...given the opportunity you always have the option to put something better bigger in the same space
kind regards
sakis
Yea, that's the problem with the "good" amps. They are all old enough to need their electrolytics changed. About 7 to 10 years max unless abused. The 840 sure is worth it though. I prefer it to my Creek, and compared to the NAD, well, lets just say I prefer the Rotel. Newer caps give the opportunity for lower ESR, which is no bad thing.
Strangely, my manual shows a reading of 2.5mV between TP1 & TP3 (TP2 &TP4) after the suggested warm up of 50W into 8Ω, but adjusting to this figure showed high levels of crossover spikes on the distortion residual on my THD meter. At 10kHz and 50W into 8 Ω, I found that when I adjusted VR601 (VR602) so that the the crossover spikes were minimal, the bias readings were around 3.5mV.
Brian.
Brian.
Brian,
There seems to be several versions of the amp that was produced. It does not surprise me at all that your manual has different values stated in the specs. I have been thinking about giving mine a new cap job due to the age of the unit.
However, my last recap was on my Optonica SM-4646. Although it did make it a better unit overall it seemed to change the sonics and that was a change that I am not sure I care for. No one else may notice the difference but this was my favorite amp that I have listened to for many years. I just don't want to have the same regrets with the 840.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.