Basic amplifier troubleshooting - Page 1
Forum section: electronics / repair

author: colic
author_rank: Newbie
timestamp: December 30, 2019, 12:05:40 am
content: I have a Rotel RA980BX with low output from both channels, ie they are quiet and the amp can easily be listened to on full volume which was not the usual case. It has a pre-amp section and an amp section and is rated at 100W into 8Ohms. It has worked fine in the past with the exact same setup (speakers cables, music source etc).A quick check on a scope shows the test input of 2V p2p also appears on the output (ie after the main amp) of both channels. The volume control alters the pre-amp output from zero to 2V p2p, and so I conclude the pre-amp is working, or at least it is transmitting the waveform to the main amp. The main amp, however, appears to do nothing with the waveform and it arrives at the speakers the same as it came out of the pre-amp.Looking at the attached diagram, a 2V p2p input at points 3 and 4 on the lower left of the diagram also appears unchanged at OUT1 and OUT2.I concentrated on one channel and checked the DC voltages as ticked on the top left of the diagram. After that, on the scope I see the 2V waveform on the base of the related output transistors, Q637, ( and so Q633 and Q629). However the same 2V waveform appears on the emitter, and the main amp main outputs (OUT1 and OUT2). The output transistors (Q637 etc) have -55V on the collector as expected.The problem in further diagnosis is Im not completely sure how the amplifier is designed. With the DC voltages appearing correct in the part of the circuit I tested, and the pre-amp working, Im unsure now what to test further to find the fault.Any further help or guidance gratefully received.

author: SpecialK
author_rank: Regular Contributor
timestamp: December 30, 2019, 01:09:19 am
content: Maybe try replacing the pre-out main-in jumper with an RCA cable? See if that makes a difference.

author: oPossum
author_rank: Super Contributor
timestamp: December 30, 2019, 05:39:55 am
content: Make sure the mode switch is not set to mute.

author: andy3055
author_rank: Super Contributor
timestamp: December 30, 2019, 06:28:54 am
content: It has to be something common to both channels. Check the relay RY601. If it is in the off position, both R/L pre-amp outputs will be grounded. If there is power to the coil and it is not energizing, the relay could be bad. If the relay is not cutting in but the coil is good, follow the relay coil supply Q643/644 and associated components. Check F601/602. If they are blown, you have some issues in the power amp area. Check the power transistors.

author: colic
author_rank: Newbie
timestamp: December 30, 2019, 10:49:17 pm
content: Thanks for the replies. Let me go through them see if I can get this amp fixed!Firstly I didnt include some of the things Id tried before getting here, but had already discounted in light the measurements on the scope as follows:QuoteAs per my post Im getting 2V p2p output from the pre-amp, into the main amp and through to the main output so both the jumpers or cables have to be working for that to be the case. As i happens I've used two sets of RCA cables to be sure before I put the scope on, so its definitely not the RCA cables/jumpers.QuoteAgain the scope shows pre-amp signal gettingall the way through the amp. But before putting the scope on Id checked and cleaned al switches and pots. When I turn the switch to mute there is no output at all from the amp, as expected.QuoteAgreed :-)QuoteIt cant be the relay as Im getting the same 2V pre-amp signal out of the pre-amp and all the way to the output? The relay has to be allowing signal out from the pre-amp and into the main amp for the same signal to appear at the main outputs into the speakers. If it wasnt energising or the contacts were bad Id see no signal or an attenuated signal getting into the main amp, surely?QuoteI noted I see +/-56V on the power rails, but I also see this at the PSU outputs. Again before putting the scope on Id pulled both fuses and cleaned them and the holders. I have also checked the voltages going into the pre-amp (+/18V) which are good. So for me to see +/-56V and +/-18V the fuses have to be good (and they are, I tested them!).QuoteThis is where I suspect I may have a problem. My understanding is there are 3 of them on each channel. As I mentioned I see signal on the base and emitter of each, but its only the pre-amp signal and isnt amplified at all.But how do I determine if they are good or not? What's the best way of checking these, preferably without having to pull all 3 out as they're heatsink mounted along with 6 others?

author: SpecialK
author_rank: Regular Contributor
timestamp: December 31, 2019, 12:43:18 am
content: There are four fuses on the schematic you posted. Are you sure you tested F601 & F602? Is it possible you only tested F901 & F902? Maybe they are concealed somewhere?

author: andy3055
author_rank: Super Contributor
timestamp: December 31, 2019, 02:56:58 am
content: Fuses 601/602 are the speaker protection switches on the output. They are at the top right on page 13 of the service manual. If they are blown, you can be sure the power transistors are blown as well. You will still hear some sound that is coming thru R 695 and 696 which are going back to points before those fuses-downstream of R643/644-feedback. So, even if the relay is not closing, you will get some sound-like chipmunks.First, check if there is any DC at the points marked OUT1 and OUT2. That will tell you if there is any damage to any of the output transistors. You have 6 output transistors on each channel. They are in a parallel push-pull configuration. 3x 2SD1047 (NPN) and 3x 2SB817 (PNP). This is how the +/- 55 volts comes to 0 volts on the speaker lines. Hence the need for the fuses so that you don't blow the speakers.You can do an in-circuit check of the outputs with a DMM on Ohm scale while power is off. If you compare the identical transistors (Q635/636, 631/632, etc.) from the 2 channels you should see the difference immediately. Ultimately, you have no option but to lift the legs for a proper check. If even one of them are bad, I would replace all 12 of them. You have to check the drivers and the pre-drivers also. There is no shortcut in these direct coupled stages as they blow the whole chain when one goes bad.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 31, 2019, 03:12:34 am
content: Cant recall a brand ive repaired years ago,Be sure your center tap x-former ground is good all the way around ... i tought i had bad supply voltages in the circuit, but it was ground related ... the preamp section was "floating"

author: floobydust
author_rank: Super Contributor
timestamp: December 31, 2019, 05:41:35 am
content: OP your numbers aren't making sense.The power amp section has a voltage gain of about 28 so a 2Vpp input giving 55.7Vpp output, or about 48W. You are instead measuring a voltage gain of 1? That is strange.Ensure the non-polar feedback divider capacitors are not open-circuit C607 and C608. That's the only thing that could cause this beyond making the wrong measurements. The only circuit portions common to both channels are for input muting and protection. There's nothing to make both channels quiet.Interesting that Rotel is taking partial negative feedback after the fuses (via R695, R696).edit: if C607, C608 were going low value, you'd also lose bass, it would sound tinny, as another symptom.

author: mzacharias
author_rank: Frequent Contributor
timestamp: January 01, 2020, 04:01:00 pm
content: If the relay is activated the input impedance is dropped to 1 kOhm. So even if you have 2V going into the amp, that level will be reduced dramatically before the amp section even has a chance to do it's job.Check the input resistance at the input jacks while the amp is running. The input signal cables are best left unconnected to your generator, but if plugged into the amp and open at the free end, make a convenient place to test the input resistance.1 kOhm suggests a problem with that relay circuit, or possibly an amp problem triggering this "protect" circuit but that seems unlikely since you would have heard BIG pops/hum through the speaker(s).

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 02, 2020, 01:09:40 am
content: +1With the Canadian's !Figures aren't making sense, even less considering same problem on both channels.Check ground continuity through transformer, PS filter caps, speaker connection.Beware of corrosive glue that may cause and hide the problem.Quote from: floobydust on December 31, 2019, 05:41:35 am...most negative feedback, further improving the damping factor?

author: oPossum
author_rank: Super Contributor
timestamp: January 02, 2020, 02:53:44 am
content: Quote from: floobydust on December 31, 2019, 05:41:35 amTaking it after the fuses compensates for the resistance of the fuses. Think about what would happen if the fuses blew. An additional feedback path before the fuses will keep the amp stable when the fuses blow.

author: floobydust
author_rank: Super Contributor
timestamp: January 02, 2020, 03:37:15 am
content: Taking partial feedback after the fuses seems weird, like Rotel wanted to do remote sense at the output jacks. When a fuse blows, the amp's voltage gain goes up 3XLoudspeaker fuses do add distortion at low frequencies, the element heats up and cools with a LF sine wave.The muting relay hard shorts the amp's input. I'm not seeing much that can make things quiet yet no distortion.

author: colic
author_rank: Newbie
timestamp: January 02, 2020, 04:13:12 am
content: Thanks for the replies. Here's what I found based on them.Quote from: andy3055 on December 31, 2019, 02:56:58 amJust to clarify the syptoms, a signal generator app on my phone generates about .1V p2p at 100Hz.With the amp volume turned up full, this appears as .1v p2p at the pre-out and main in. It appears as about 0.025 at OUT1 and the left output channel. and 0.3V p2p at the OUT2 and the right output terminal.At 1000Hz its pre-out .1, out2 0.1V OUT2 0.6VAt 1000Hz its pre-out 0.05 OUT1 0.05V, OUT2 0.3Neither waveform is distorted or altered in any way except its amplitude.That's a bit more scientific than "music comes from the speakers but isnt very loud although it is clear and not at all distorted." So it appears from that neither output is working, one less so than the other.The above confirms it, but the top right fuses are intact and have continuity from OUT1/2 to the terminals (resistance from the out1 cable, via the fuse, to the terminal is zero) .DC at OUT1 is 0.008V OUT2 is -0.022V. I take this to be effectively zero as expected.Comparing identical pairs was problematical! Base to emitter measurements were consistent being 11M or 8M depending which way round the DMM leads were. But Base to collector and collector to emitter readings would very from 200k to .4M or so, and would steadily climb for some readings. So if the readings look erratic between some pairs, that's why. I took many measurements and concluded its because they are in circuit that I cant get a stable measurement.Hence I cant draw an easy conclusion from these measuements. What should I learn from them, do they tell me anything or just that I cant take meaningful measurements while in circuit!QuoteIm happy to pull the output transistors to measure them if that's whats needed next. Id just like to know these readings suggest the problem is in that area first. Id hate to pull them and find they're all fine and the real problem is something really simple.For example if no input was getting into the main amp and maybe as you say it still reaches the output via the feedback resistors. Although wouldnt it be distorted as you say, and it isnt at all. But I can also see the .6V signal on the base of Q629 for example, which suggests to me the signal reaches the main amp, it just doesnt amplify. Is the gain of the output stage sufficient to turn the .6V signal into 56V peak, or the design maximum, or could it still be an issue further back. At any rate the power transistors do not appear to be amplifying despite the signal on their base (and at the output).Quote from: mzacharias on January 01, 2020, 04:01:00 pmDC resistance on both input channels measures at 23k with the amp powered on or off, regardless of the protection relay being on or off (according to the protection light and audible click from the relay)Quote from: floobydustlink=topic=224052.msg2851226#msg2851226 date=1577770895I agree both channels failing should narrow it down and aside from the switches there really would have to be two points of failure for both channels to fail. I suppose the output stages are the most likely since they run hot and are connected to external speakers and cables.I think my measurements (plus those above) confirm the pre-amp is working and that the main amp is not, while it has power and a path to the output terminals, and that all fuses are OK.I think the other suggestions are addressed by my measurements above. If Ive missed something please let me know, Im open to all suggestions because if I knew what to replace I wouldnt be asking :-)

author: fzabkar
author_rank: Super Contributor
timestamp: January 02, 2020, 04:29:33 am
content: Quote from: floobydust on December 31, 2019, 05:41:35 amCould I ask how you arrived at that figure? I think I'm having a brain fart because I could only manage a gain of 3.7, which must be wrong.Edit:I read the value of R619 (562) as 56 x 10^2 = 5600 ohms. I now see that I should have taken it at face value, namely 562 ohms. Why do designers use these odd values. Grrr!

author: andy3055
author_rank: Super Contributor
timestamp: January 02, 2020, 04:59:14 am
content: Where are you injecting the test signal at? Have you checked the 0.22 Ohm emitter resistors of the power transistors?What position is your speaker remote switch S 601 on?Your checks on the transistors show a lot of deference between the 2 channels. I think you have failed output transistors. There are voltage readings marked on the circuit diagram. Have you checked if they are the same as your amp? Can you check those fuses out of circuit? If they are soldered in, disconnect the speakers and check them on their ends. If the fuses are good, stick your DMM probes on (DC volts) in to the speaker terminals of the amp (with speakers disconnected) and see if you get DC when you switch the amp ON. If the fuses are blown, check the DC voltage against ground on the bases of Q639/Q640 when the amp is on. It should be 0 volts.If the fuses did not blow while the output transistors failed, there is a good chance that your speakers are burnt out. Speaker protection by fuses is a poor design as most often they do not blow and the result is burnt speakers. To check the speakers, hook up a 1.5 volt flashlight cell and see if the speaker cones are moving forward/back. If you hear a little noise but see no movement, time to get new speakers. But you will need to fix the amp first! I am saying these little things as I am not sure what your level of experience is. So, don't take this on the wrong side.

author: colic
author_rank: Newbie
timestamp: January 04, 2020, 10:40:24 pm
content: Thanks Andy3055. Think Ill pull the output transistors and see what I find. Just for completeness, here's the responses to your last suggestions thankyou.Quote from: andy3055 on January 02, 2020, 04:59:14 amQuote from: colic on December 30, 2019, 12:05:40 amSo to expand on that, an audio lead from my phone into the amp phono inputs.Quote from: andy3055 on January 02, 2020, 04:59:14 amYes, they are all good both electrically and visually.Quote from: andy3055 on January 02, 2020, 04:59:14 amThe switches are set Tone off, volume max both channels, mode mono, recording off, listening aux. Changing the tone, mode and recording switches makes no noticable difference.Quote from: andy3055 on January 02, 2020, 04:59:14 amQuote from: colic on December 30, 2019, 12:05:40 amSpecifically, the diagram shows small pencil ticks(checks) for the ones I verified. After that I decided it wasnt telling me much and didnt check the other channel/half of the amp.Quote from: andy3055 on January 02, 2020, 04:59:14 amThey are soldered in, look like wide copper strips. However:Quote from: colic on December 30, 2019, 12:05:40 amQuote from: andy3055 on January 02, 2020, 04:59:14 amQuote from: colic on December 30, 2019, 12:05:40 amQuote from: andy3055 on January 02, 2020, 04:59:14 amQuote from: colic on December 30, 2019, 12:05:40 amTo elaborate - the music is clearly audiable and sounds normal, but quiet. Maybe like I have the volume up 10% of the max. I can just see and certainly feel the bass cones moving and they have continuity. This is what Id expect of only 2v p2p going into them. Of course they still could be damaged, but as you say I have to fix the amp first :-)

author: SpecialK
author_rank: Regular Contributor
timestamp: January 05, 2020, 03:26:42 am
content: Quote from: colic on January 04, 2020, 10:40:24 pmI see the copper strips to which you refer. I dont believe these are fuses. I believe they are on the underside of the board to which the speaker terminals are attached.Can you take some pictures of your unit?

author: andy3055
author_rank: Super Contributor
timestamp: January 05, 2020, 05:12:43 am
content: This is rather strange! F 601/602 are not listed in the parts list. Looking at the image posted by SpecialK, it looks like they never put a fuse there but put a thick copper strip instead! I also noticed that the schematic has an error on the OUT 2 line. It should go to the R output but the schematic shows it as going to the negative speaker terminal!

author: Haenk
author_rank: Super Contributor
timestamp: January 06, 2020, 07:00:17 pm
content: the "stuff" right next to C607 looks like leakage, but hard to tell from that (unsharp) picture - be sure to check that, and the other channel as well

author: SpecialK
author_rank: Regular Contributor
timestamp: January 07, 2020, 01:38:21 am
content: Quote from: Haenk on January 06, 2020, 07:00:17 pmNah. I wouldn't worry about that, as it's not pictures from his unit. Rather these are pictures I snagged from another online source of the same model.

author: magic
author_rank: Super Contributor
timestamp: January 07, 2020, 08:56:06 am
content: Quote from: floobydust on December 31, 2019, 05:41:35 amIndeed; this is basically one giant opamp and R619 is its gain resistor and C607 couples it to ground. If you can verify that there is undistorted 2Vpp at the inverting input too (and I'm pretty sure there is) then nothing is wrong with the amp proper and the gain network must be screwed. Most likely the elcos dried up.editOr perhaps they lost connection to ground, which could explain why both channels crapped out at the same time. Check if there is continuity to ground wherever it should be on the main board.Quote from: floobydust on December 31, 2019, 05:41:35 amQuite the contrary, it attenuates audio frequency to unity gain so that frequency response is flat all the way down to DC, as the amp always was unity gain at DC

author: floobydust
author_rank: Super Contributor
timestamp: January 07, 2020, 07:56:08 pm
content: I get this for a simplified block diagram. As the value of C607 drops, the HPF corner frequency moves up. 100uF is 2.8Hz but 1uF is 283Hz or tinny sounding. Gain is always unity at DC.Other repairs I've done with a quiet (but not distorted) amplifier channel:-Bad speaker/headphone jack switch.-Due to shorted amp output. There was another daughter board with output inductors on it that shorted due to corrosion. But this Rotel amp is missing that and has no Zobel network, which is odd. I keep looking for that on another pc board. OP could measure ohms to GND at OUT1 (with power off and spkr disconnected).Best not to inject to the phono preamp input, there is very high gain there +35dB and RIAA eq curve can make it confusing because freq. response is not flat. Moving-coil cartridges output around 4mV max. - much less than your phone. Using a Tape or Aux input would not overload anything.

author: magic
author_rank: Super Contributor
timestamp: January 07, 2020, 08:27:15 pm
content: We know that gain is unity over entire audio band, so the corner frequency already is way above 283Hz or even 20kHzThe capacitor may have completely dried up or lost ground connection.At this point I would definitely check this feedback network.

author: colic
author_rank: Newbie
timestamp: February 13, 2020, 02:31:39 am
content: My delayed response doesnt imply apathy, Ive just not had time.My thanks to everyone who chipped in. Particular thanks to Floobydust and Magic for 100% correctly identifying the fault. And thankyou Floobydust for your diagram, I actually understand what's going on now and why these symptoms can occur (ie my amp converted into unit gain).So, on closer inspection of C607 and C608 I could see a small amount of leakage (but there's also slight leakage on the PSU caps and so far they're fine). The leakage had literally just touched both R619 and 620, neither of which gave me a sensible reading in circuit. One of them fell apart on removal, the other gave 2M2 for resistance.I replaced all 4 components and my amp sounds awesome again. You've clearly spent a lot longer looking at and fixing circuits than I have, and Im glad you share that with the less experienced. Thankyou.The only remaining mystery is how both channels failed at apparently the same time. However the amp was being used by my daughter and friends so I suspect in fact this happened over some time, up to the point I happened to run the amp and notice. Im not sure what it says about the youth of today and HiFi, but that's probably a different forum :-)