Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Rotel RA320. No 0mv bias in one channel?
Rotel RA320. No 0mv bias in one channel?
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 6th November 2017, 09:26 PM   #11
brittney is offline brittney  Ireland
diyAudio Member
 
Join Date: Oct 2008
Yip, I did initially suspect the diodes as other threads on the internet about old Rotels mentioned these as being problematic. I will remeasure the voltage drop across them again tomorrow but I do recall both being similar, around 2.4V maybe. The are actually mounted (glued) on the heatsink between the power transistors.

However, comparing to the image you attached and others online both SV04s look a bit fried on my Rotel board....
They are just about visible on this heatsink in the picture below.
Click the image to open in full size.
  Reply With Quote
Old 7th November 2017, 09:00 AM   #12
brittney is offline brittney  Ireland
diyAudio Member
 
Join Date: Oct 2008
I measured across both sv04 diodes this morning. Both read very close to 2.5v. I'm still a bit suspicious due to the fact they look a bit scorched
  Reply With Quote
Old 7th November 2017, 09:16 AM   #13
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Rotel RA320. No 0mv bias in one channel?
Have any of the transistors (drivers and outputs) been replaced for modern versions ?

The four diode array gives around 2.5 volts bias voltage and that is marginal for turning on four B-E junctions.

Non original semiconductors could give rise to this.
  Reply With Quote
Old 8th November 2017, 09:41 PM   #14
brittney is offline brittney  Ireland
diyAudio Member
 
Join Date: Oct 2008
Quote:
Originally Posted by Mooly View Post
Have any of the transistors (drivers and outputs) been replaced for modern versions ?

The four diode array gives around 2.5 volts bias voltage and that is marginal for turning on four B-E junctions.

Non original semiconductors could give rise to this.
Hi,

Checked the transistors - all are original, at least the are the same as whats stated in the circuit diagram. The diodes were glued to the heatsink but have come unstuck. Would this cause incorrect voltage across the diode?

As the SV04 diodes are no longer available I will try with 4 signal diodes in series in its place to see if boosting the bias voltage helps
  Reply With Quote
Old 9th November 2017, 05:42 AM   #15
Ian Finch is offline Ian Finch  Australia
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour, NSW
As you already have 2.5V across the SV04s, its unlikely they are faulty or need replacing but if the glue has failed, then bias won't track with temperature as it should - which is not good but probably not like the problem of no bias at all. Though bias would be higher on average, it can't easily skyrocket into thermal runaway as some Vbe multiplier circuits can.

As a suggestion, measure the voltages of Vbias (the actual forward bias voltage applied to the output stage) by measuring across the bases of Q607 & 609. Compare this voltage with the measurement across Q608 & 610 bases. The expected voltages at the output transistor bases are marked on the schematic and I guess this where the disparity shows up.

p.s. I believe there are hard setting thermal compounds out there that should be good for re-fixing the diodes. A freshly roughened aluminium surface with a little clamping force until firm is best - the compound will be a tad pricey though.
__________________
If only I had listened....

Last edited by Ian Finch; 9th November 2017 at 05:58 AM.
  Reply With Quote
Old 9th November 2017, 07:26 AM   #16
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Rotel RA320. No 0mv bias in one channel?
Something else to consider, a purely low bias condition would not give rise to the fault of badly distorted sound at higher levels. Something else is going on here. There are only four transistors and a handful of resistors in the output stage.
  Reply With Quote
Old 10th November 2017, 04:55 PM   #17
brittney is offline brittney  Ireland
diyAudio Member
 
Join Date: Oct 2008
Quote:
Originally Posted by Ian Finch View Post
As you already have 2.5V across the SV04s, its unlikely they are faulty or need replacing but if the glue has failed, then bias won't track with temperature as it should - which is not good but probably not like the problem of no bias at all. Though bias would be higher on average, it can't easily skyrocket into thermal runaway as some Vbe multiplier circuits can.

As a suggestion, measure the voltages of Vbias (the actual forward bias voltage applied to the output stage) by measuring across the bases of Q607 & 609. Compare this voltage with the measurement across Q608 & 610 bases. The expected voltages at the output transistor bases are marked on the schematic and I guess this where the disparity shows up.

p.s. I believe there are hard setting thermal compounds out there that should be good for re-fixing the diodes. A freshly roughened aluminium surface with a little clamping force until firm is best - the compound will be a tad pricey though.
Cheers, Just measured voltage for Base of q608 to q610 and got a value of 1.96v. Similarly measurement on the left side was 1.84v. Is this difference significant?

The voltages marked on the circuit diagram, what are these in relation to. For example the the 0.8v at Q607. Is this across the transistor, Base to emitter?
  Reply With Quote
Old 10th November 2017, 05:03 PM   #18
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Rotel RA320. No 0mv bias in one channel?
Always measure the B-E voltage by reading across the junction. A silicon junction such as these will begin to conduct at around 550 to 600mv and will clamp (will not go higher than) at around 800mv.

With your bias preset turned to give maximum voltage (the 2.4 volts) between the bases of the two drivers, measure the B-E voltage of all four devices and report back with the results.
  Reply With Quote
Old 10th November 2017, 05:11 PM   #19
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Rotel RA320. No 0mv bias in one channel?
Like this. Voltages quoted in service manuals can often be wide of the mark and are subject to interpretation. When dealing with problems such as this the important quantity is the voltage across the junction.

Measuring from ground to a given point in the circuit is how the voltages in the manual are given but these will alter significantly with temperature and from one amplifier to another. The actual supply voltage will also reflect in the overall reading. Also a DC offset would skew the result.
Attached Images
File Type: jpg Bias.JPG (60.1 KB, 28 views)
  Reply With Quote

Reply


Rotel RA320. No 0mv bias in one channel?Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
rotel bias mark adams Solid State 29 19th September 2013 01:09 PM
Rotel 840BX Bias Adjustment Texun1 Solid State 12 21st March 2012 06:06 PM
Rotel RB 980 BX bias question recorder Solid State 6 14th February 2012 09:39 PM
Help! Rotel rb1080 bias setting birdyman Solid State 10 21st June 2010 09:55 PM


New To Site? Need Help?

All times are GMT. The time now is 10:06 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki