Reapiring a Denon DRA-335R - Help needed :) - Page 1
Forum section: electronics / projects

author: toli
author_rank: Frequent Contributor
timestamp: July 16, 2010, 07:27:37 pm
content: Hi Guys,I've posted it on HF's DIY section earlier today, and than remembered that EEVblog is all about electronics, so it might be a good idea to post here as well.I've picked this Denon amp today, and I'm trying to get it up and runing. I've tested it and all of the voltages in the PS look good. I've entered a 1KHz ssine wave into the amp, and with my Rigol scope was able to follow iit all the way to the output of the power BJT's. However, the left channel is not even close to what it should be, and the protection circuit stays on and disconnects the output of the amp. I was only able to find the schematics of the DRA-435 which is almost the same. It can be downloaded here:http://www.eserviceinfo.com/download.php?fileid=31997I've tested the DCV in a few places (with no input to the amplifier), and have drawn it on an image:The left channel is the problematic, the right is working well. Other than that, the preamp section, and the tuner are working just fine.Any idea what I should check first, or what the cause to the problem might be?I can always start taking out every transistor and making sure it works as it should, but it's a lot of work to remove them on this old amp.

author: jahonen
author_rank: Super Contributor
timestamp: July 16, 2010, 07:41:42 pm
content: Since it seems that output is saturated to negative extreme, I'd suspect output transistor TR321 or driver transistor TR317 is shorted. Check those first.Regards,Janne

author: DJPhil
author_rank: Frequent Contributor
timestamp: July 16, 2010, 08:20:56 pm
content: I'm still a noob at this, but wouldn't TR303 and/or TR305 require base current to drive 20V negative?What's the voltage at TR303's base?Am I even close?

author: saturation
author_rank: Super Contributor
timestamp: July 16, 2010, 11:02:36 pm
content: Since the two channels are identical designs, any severe asymmetry suggests parts near the asymmetry are at fault. If the right side is good, then it points to TR: 303, 305 on the left channel and so on. I think you can see were those voltages differ quite a bit. While the transistors are the most likely to fail, you need to check the status of capacitors and resistors that couple the fault transistors to the input.Ah, nostalgia. In them olden days, a simple way to troubleshoot electronics designs was to add test point voltages at key junctions. It could either be referenced to ground or for transistors, at CE or BE or BC junction voltages. It was one reason very accurate DMM were desirable with high input impedances so the measured voltage was close to a NIST reference as possible and did not affect the measured circuit appreciably, and thus show voltage as close to those written on the schematic.

author: toli
author_rank: Frequent Contributor
timestamp: July 17, 2010, 07:15:26 am
content: So here's an update. I've disconnected all 4 power BJT's from the board (TR321,322,323,324) and checked them.Using the diode test, all 4 pass the standard tests (one diode drop Vbe and Vbc), and out of range for Vcb, Veb, Vce, and Vec. Obviously for PNP the voltages are one diode drop for Vcb and Veb, and the rest are out of range.My DMM also has hfe tester so I've used it as another test. TR321 has hfe of 144, TR323 is 56, TR324 is 35 and TR322 is 48. I know that transistor of the same type can have quite different hfe, but TR321 seems to be quite far from the others.Any idea where to head next?

author: saturation
author_rank: Super Contributor
timestamp: July 17, 2010, 10:49:48 am
content: According to the voltages you posted those transistors are not likely the problematic ones.If you measured those voltages true, the problematic ones are at least 303, 305, and 315.You need to troubleshoot from the input side first, and work towards the output, because if the first stage driving amps are defective, they will bias the next stage, and so on, so the measured voltages may be also erroneous. Fix the first stage first, and remeasure the voltages as you did, comparing left and right channels. If all is well, then test with music.Quote from: toli on July 17, 2010, 07:15:26 am

author: toli
author_rank: Frequent Contributor
timestamp: July 17, 2010, 10:56:17 am
content: no problem, I will check TR303, 305, and 315. Than report back.

author: toli
author_rank: Frequent Contributor
timestamp: July 17, 2010, 12:44:03 pm
content: As promised. I've removed and did the diode test on TR303, TR305, TR317, TR315. All four pass the test with flying colors. No reading where the should be none, and a diode drop (0.6-0.7 depending on the transistor) on Vbe and Vbc (the opposite polarity for PNP of course).

author: saturation
author_rank: Super Contributor
timestamp: July 17, 2010, 01:59:22 pm
content: That's great, next check the caps and decoupling caps around those TR. Its unlikely to be the resistors but you could check them if the caps return OK.Quote from: toli on July 17, 2010, 10:56:17 amQuote from: toli on July 17, 2010, 12:44:03 pm

author: toli
author_rank: Frequent Contributor
timestamp: July 17, 2010, 02:06:10 pm
content: What exactly should I be checking? I guess I should make sure they aren't shorted, and I can do this using a simple continuity test, but should I check anything else? Isn't it best to make sure they don't break under voltage? I'm not sure how to do that.

author: saturation
author_rank: Super Contributor
timestamp: July 17, 2010, 02:17:34 pm
content: Yes, shorted, open or off spec. I see you have a post on another forum with a pic of the open casing. Are many of those caps electrolytics? If they are, they are likely to simply die from aging.Quote from: toli on July 17, 2010, 02:06:10 pmQuote from: toli on July 17, 2010, 02:06:10 pm

author: jahonen
author_rank: Super Contributor
timestamp: July 17, 2010, 02:18:21 pm
content: Another possibility if none of the transistors are faulty is that the feedback loop is broken somewhere. Amplifier needs DC feedback to stabilize the operating point. The feedback loop seems also to have the tone controls and bass EQ switch, it is quite complicated. Voltage at TR305's base is the critical voltage, it should be 60 mV.Regards,Janne

author: toli
author_rank: Frequent Contributor
timestamp: July 17, 2010, 03:09:07 pm
content: I've tested every single electrolytic cap I could find in this side of the amp, and I've tested TR319. All of them are ok.

author: saturation
author_rank: Super Contributor
timestamp: July 17, 2010, 04:24:58 pm
content: Sorry to hear, at this point, it looks like the fault is outside the amp section as Janne suggests, the only TR left in the left channel input is TR 301, with a test Vdc of 0V.Quote from: toli on July 17, 2010, 03:09:07 pm

author: toli
author_rank: Frequent Contributor
timestamp: July 17, 2010, 04:43:56 pm
content: I'll try and test it later today or tomorrow.

author: toli
author_rank: Frequent Contributor
timestamp: July 18, 2010, 07:19:07 am
content: Ive taken out and tester TR307,309,311,313. All of them are fine. I've also tested TR301 and TR302 (for comparison) and both seems to be identical in terms of voltage drops so I'm assuming TR301 is good as well.I've tested the voltages. At the base of TR301 and 302 its -16V (they are connected). At the emitter of the TR302 there is 0V, and at the emitter of TR301 there's -4.7V. Any suggestions as to what I should check next?Thank you.

author: saturation
author_rank: Super Contributor
timestamp: July 18, 2010, 12:35:27 pm
content: Wow, you've pretty much disassembled the entire left channel. These TR 302 and 301 are connected to something off the schematic, where does it go?Again, these are symmetrical, so something is causing a short in this area and it seems at the very input of the L channel. Check the status of R301, especially C373.Quote from: toli on July 18, 2010, 07:19:07 am

author: toli
author_rank: Frequent Contributor
timestamp: July 18, 2010, 01:38:27 pm
content: I won't be able to check the amp until the weekend as I have exams this week (second year EE student). I will keep you posted this weekend once I've check R301 and C373.BTW, I've posted the full schematics in the first post so you can see where everything goes.

author: saturation
author_rank: Super Contributor
timestamp: July 18, 2010, 03:13:15 pm
content: Good luck with exams, yes I see it now, the URL link has the full schematic.Quote from: toli on July 18, 2010, 01:38:27 pm

author: toli
author_rank: Frequent Contributor
timestamp: July 22, 2010, 02:29:21 pm
content: Hope you've not given up yetR301 is 1K as it should be. C373 (and C374) are 0.9uF so within specs. R297 is 471K so its fine as well.

author: saturation
author_rank: Super Contributor
timestamp: July 23, 2010, 08:49:35 pm
content: Getting tough!As you've removed the L channel, can you power up the unit and the right channel test points are OK?I also presume the protection circuits no longer disconnect power to the main amps?Tracing the amp leads to IC 201, are these miniswitches ok and not shorted? Also the output path from the left channel, no switches are shorted to ground? IU 2345 and the speaker terminals ok? IC 251 the volume control?Quote from: toli on July 22, 2010, 02:29:21 pmQuote from: toli on July 16, 2010, 07:27:37 pm

author: toli
author_rank: Frequent Contributor
timestamp: July 24, 2010, 05:58:43 am
content: Hi,The right channel still seems to be just fine. The protection circuit remains on and disconnects the output from the transistors.However, something strange just happened. Since I haven't touched the amp for a few days I wanted to test some point once more to make sure all is still the same, I've connected the DMM and started the amp and it actually clicked (connected the output) for a second or so, then disconnected and repeated itself a couple of times. I've turned the amp off and than back on and now it remains disconnected. Very strange and I don't really know why it happened.IC201 looks ok. No shorts on either side. I've tested with the scope using a 50Hz (from a PC not from the mains) signal. The left (pin 12) is 2.62Vp-p and 26mVavg. The right (pin 17) is 2.78Vp-p and 21mVavg. So I guess this one is fine right?As for IU2345, the problem is I'm not really sure how to check that as I've never came across something similar. I guess if I'll remove the entire volume control unit (it sits on the board using a socket and can be removed), and will just short it as if its at full volume, I should see if its the cause for the problem, right?I'm unfamiliar with IC 251 but I guess if its a motor driver its nothing more than a buffer to supply the motor with the needed current, or am I wrong?BTW, here's a photo of the volume control. As you can see it can be removed from the main board:Edit:Something seems to be very strange. Looking at the schematics, since I've checked IC201 and the output is ok, and since I know there's a problem with the voltage at the emitter of TR301. It seems logical that the problem will be somewhere in between. However, since the motor+IC251 aren't connected to the signal, but only control the volume pot, they shouldn't cause any problem. So is there something wrong with my logic/understanding of the circuit or is there something else that I'm missing?

author: toli
author_rank: Frequent Contributor
timestamp: July 28, 2010, 08:19:02 am
content: After testing so much more things, it finally turns on and play's. The problem was a bad trace it seems. There are a few caps glued at the bottom of the board. After noticing that a trace doesn't give me a good continuity reading, I've remove the glue the covered it, and indeed the trace was damaged. I've placed a piece of wire instead and now it worksNow to my next challenge, adding a remote control. I'll open a new thread for that in a few minutes.

author: saturation
author_rank: Super Contributor
timestamp: July 28, 2010, 10:27:46 am
content: Glad it worked out.