Rotel RA-920AX - DC output at binding posts

[desperaudio]

A colleague brought this little Rotel to me (to have a look at it and try it out). It had been sitting for some time so I took my trusty DMM out and had a look at the DC output at the speaker connections (no point in cooking a pair of speakers - once bitten - twice shy. It seemed a wee touch high (96mV and 106mV).
I went inside and touched up the bias (set to 4.4mV) but that did not seem to affect the readings at the posts.
I don't see any obvious adjustment points. Could someone perhaps tell me if this (rather high to me) reading is normal for the model or do I need to get some fixing underway?

thanks

jac - desperaudio @ large

[Kevin P]

Was this with the terminals open or under load?

Try putting a load across the terminals (say, a 10 ohm 10W resistor) and then see what you get for DC voltage. It may be a lot lower under load.

[desperaudio]

Under load and warmed up

Kevin - I should have mentioned that part. I warmed up the unit for an hour before I took any readings.
I then took a chance with a pair of disposable speakers and hooked up a tuner and let it go for another hour and then took another reading with the speakers functioning and again with the speakers disconnected. No change.
There's a nasty little thump when that unit powers up too. The speakers are just cheap little Sony bookshelf (really cheap) units but that woofer manages to move quite some when I turn on the power!

[Kevin D]

Service manual states bias at 4mv.

http://www.bwgroup-support.com/downloads/techmanuals/rotel/RA-920AX-TM.pdf

Kevin D.

[Glen B]

DC offset measurements should be made with no input and no load connected. DC offset in any amplifier should be less than ± 15mV, with ± 0-5mV being ideal. The thump is the result of the highish DC offset. The 920AX has no DC adjustment trimpot as you've already discovered. The high DC is likely the result of the input differential pairs in both channels drifting out of spec with age. The fix in such a case is to replace the diff pair. Try replacing transistors Q601/Q605, and Q602/Q606 with new closely matched pairs.

[desperaudio]

Thank-you very much - the one thing I couldn't find anywhere! I'll re-set the bias a touch more and see what gives!

[Glen B]

Adjusting the bias is not going to help. You should really focus on the differential pair to begin with. You can get replacement transistors (2SA1016K) from B&D Enterprises. They're cheap enough that you can buy a bunch and beta match pairs.

http://www.bdent.com/index.jsp

[desperaudio]

Thanks for the advice guys. I'll see if I can source the parts locally (bringing stuff in from the USA is kind of a PIA.
Once that happens I'll match up a few (if I can - I'm a bit of a novice) and we'll see if my colleague's baby Rotel can get all fresh and shiny new.

Best regards.

[gunter]

Hey! I have this same problem! how did you get on desperaudio?
I replaced the differential transistors as per the advice but no luck, still the same issue with DC on the binding posts and a nasty thump when I turn the amp off and when I turn it on the drivers suck in and then release - scary stuff
more advice anyone??

[Glen B]

Gunter, is the problem in one or both channels ? Exactly what DC figure(s) did you measure ? As a next step, replace caps C605 and C601. If either or both of those are leaking, they can cause DC offset problems.

[gunter]

Thanks Glen, yes both channels have an issue. from memory it was something like 100mv and 70mv - I will replace those caps(605 &601, 606 & 602) and see what we get.
if those caps are leaking maybe there are other caps that need replaced too? do you think there are any upgrades to other caps or components I should do while I'm in there?

[Glen B]

I think you would benefit from replacing all of the electrolytic caps.

[gunter]

I have replaced those caps and now get a reading of 92mv and 65mv. I have replaced the caps with ones that say TK on them instead of Sk i'm hoping that is related to thermal rating and not anything significant. is there any particular audio grade cap I should be using or does it not matter so much with elect's?

[Glen B]

Yes TK caps have a higher temp rating than SK. Are you saying you replaced all the electrolytic caps including C601, 602, 605 and 606 and you're still getting those levels of DC ? You're checking DC with nothing connected to the amp input, right ? Any good quality elect is fine, Elna, Panasonic, Nichicon, Vishay/Sprague, United Chemi-Con, Mallory, Cornell Dubilier.

[gunter]

Thanks heaps.
at this stage I have only replaced those 4 and measuring with nothing connected. I have to order the rest of the caps - it looks like most of them have a lifetime of 2000 hours before they start degrading and as the amp is from the 90's I assume that they might all be adding to the problem

[Glen B]

In addition to the caps, try replacing the differentials Q601, 602, 605 and 606 (again) with pairs of Zetex ZTX795A PNP transistors. The gain of these are very closely matched and may help bring your DC levels lower. Just note that the center lead of the ZTX795A is the base while the center lead of the stock transistor is the collector, so you will have to bend and re-orient the leads accordingly when installing them in the PC board. The ZTX795As are available from Digi-Key.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ZTX795A-ND

[gunter]

Ill give that a go once I've replaced the caps but I'll have to source it from inside NZ otherwise it will cost me $30 for shipping!
I have ordered the caps and will replace a few at a time and retest to see how it progresses

[Glen B]

If you can't obtain the ZTX795A in NZ, you can also try Toshiba 2SA970BL, but the Zetex is better.

[gunter]

I might have to stick to the toshiba. Can't find the Zetex anywhere in NZ and I spent ages trying to source them. Shame really.

have replaced C903-906 (no great change) and waiting for the others (and the Transistors) to arrive

[gunter]

I have now replaced all the elect caps except those in the phono stage but no change in mV. still waiting for the high gain transistors to arrive but I don't hold much hope that they will make a difference to the main issue.

[Glen B]

I can understand your growing frustration. Just wait on the new transistors and see what happens. The Toshibas are recommended because they normally come closely matched in gain, second only to the Zetex. If the differentials you installed were not closely matched in gain, that could account for the levels of DC you're still getting. In the last sentence of post #5 above, I recommended closely matched transistor pairs. Also check for deteriorated resistors as suggested on diyAudio.

[gunter]

Thanks Glen - I gave the resistors a check and found that r610 was down to 1.5k from 6.81k! (annoying that i haven't noticed until now) there were also r902 and r634 that looked like a slightly different colour but checked out OK so I'm not sure if they were different stock or had a hard time or not so will replace anyway (and their counter parts in the other channel). I'm not sure what the "F" means on the parts list but had to order the correct values @ 0.1% tolerance

do you think there has been other damage to components because of this?

[Glen B]

I would check the condition of the two diodes just to be sure, "F" on the parts list means 1% tolerance.

[gunter]

I have performed a quick in circuit test on the diodes and and get leakage on D604 & D602. The D601 & D603 are OK but will replace anyway.
I would like to replace D901 & 902 just to be on the safe side but can't source the RD18E in NZ and I'm not sure what the equivalent would be as I only know enough to be dangerous - what do you think?

[Glen B]

If your VE+ and VE- voltages are on spec, I would leave the two zeners alone. If you really needed to replace them, you would just have to find ones in NZ with matching characteristics.

http://www.datasheetcatalog.org/datasheet/nec/RD190E.pdf

[gunter]

Thanks Glen.
this maybe a stupid question but how close should the voltages be compared to the specs? I have 17v @ the emitter of Q901 (spec is 17.5v) and -17.5v @ the emitter of Q902 (spec is -17.7v) I am getting -28.1v & 28v at the rectifier which has a spec of +/- 28.4v (measured to ground)

[Glen B]

Those are excellent figures. The slight variations should be from parts tolerance variations/drift in parts values, but still no reason for concern IMO. Its probably best to PM me if you have other questions.

[gunter]

Success!! after replacing R610, both channels have the same offset and after replacing Q601/2/5/6 with the higher gain and matched transistors (Toshiba 2SA970BL) I now have 31mV and 32mV at the outputs which I think is acceptable.
I have a very slight hum coming from the transformer and translates to the speakers but I think that this might be solved by better grounding suggested in diyAudio

Thanks heaps Glen for sticking with me, your a champion!

[gunter]

Success!! after replacing R610, both channels have the same offset and after replacing Q601/2/5/6 with the higher gain and matched transistors (Toshiba 2SA970BL) I now have 31mV and 32mV at the outputs which I think is acceptable.
I have a very slight hum coming from the transformer and translates to the speakers but I think that this might be solved by better grounding suggested in diyAudio

Thanks heaps Glen for sticking with me, your a champion!

[Glen B]

That is great news. Those are acceptable offset figures indeed. Happy listening.

[srb]

DC offset in any amplifier should be less than ± 15mV, with ± 0-5mV being ideal.

Those are acceptable [31mV, 32mV] offset figures indeed.

Glen, the first statement seems to conflict with the second. Are you revising the maximum offset to be ± 30mV for any amplifier?


Steve

[Glen B]

Given the context of the situation, there is no conflict. I did not say the maximum offset for any amplifier should be ± 30mV, nor do I believe I gave that impression. DC offset of less than 15mV is best. A DC offset level between 16mV and 50mV though not ideal, is acceptable, especially when it is at the lower end of this range. At those levels, 2nd harmonic distortion while increased, is still inaudible.

Based on the fact that the amp has no means of zeroing DC offset, other than installing as precisely matched a differential transistor pair as possible, ± 30mV is a good compromise. I was only concurring with Gunter's feeling that his current offset readings are acceptable, which they are.