Lost bias on Marantz 1060 right channel
I was fixing a Marantz 1060 for someone. Turned out to be a poor contact in the Mono switches. A bit of DeoxIT solved the problem.
I decided to adjust the bias as I found the unit sounded a bit strange. Found the service manual, and lowered the voltage across R747, R748 from 11mV to 7.5mV.
I let the amp running a while, and then, a little pop came from the right channel. Turns out one of my alligator probes moved a bit and shorted two contacts together. I don't know which ones though, as I removed the leads as soon as I found out...
Now, the right channel has 0mV bias... There's just distorted sound coming out. I used an RCA cable between the pre out and main in to be sure the preamp is OK, no problem there.
I checked the voltages and resistance around the bias pots and compared between the two channels. The varistor and diode around the bias pot seem to be OK, I read the same dropout, etc.
In the measurements I've done, I found:
- The right channel's overall voltage is a few volts lower than the left channel.
- 0mV at R747.
- 20V at the negative side of the right channel 3000µF cap.
- 0.01V at the negative side of the left channel 3000µF cap.
- Voltage is about the same at the positive side of the 3000µF caps. (30-something volts)
I haven't checked for shorted transistors. All I can tell is that I don't have my DMM with a sound diode checker, and my oscilloscope with me.
Any ideas? All the components in the service manual's troubleshooting area seem OK.
Oh, one more thing, when I put the voltmeter's positive lead on the H001 collector and negative on emmiter, the resistance is much higher than doing the same reading on H004.
Looking at the schematic, could the 2SC959/H709 be shot?
I just tested H001 compared to the others, it's working fine. The power transistors are still OK.
I pulled out the power amp board, pretty much every readings I've done on both channels are the same.
I might have found the problem. Diode H715 (1N60) looks a bit smokey, like a blown fuse, however reads fine on the multimeter (note that I do NOT have my multimeter with diode and hFE tester).
My local store has some 1N60 and quite a few other germanium devices, so I'm OK. I love these diodes, you can see when they fail :D
If it still doesn't work, I think I'll have a look at H005 and H713, but they seem fine. Fortunately (or maybe not) my local store has ECG replacements for the SV-3A, it's not the case for the S3016 R, for which I found a spec chart, by the way.
So far, reading this topic helped a bit understanding the circuit.
OK, now I have my diode tester and oscilloscope with me. It looks like the 2SC959 is probably bust, but that the 1N60s are OK. I'm about to order two sets of MJE340/350, and MJ15003 output drivers, just in case.
Some specs for the MJEs are lower than the C959/A606 pair. Are they going to be suitable anyways?
Those or BD139-140 (my favourite) should do.
I suggest removing all the semiconductors from the board and check them: also check for open resistors, and when you replace transistors, make sure they're not fake, as happened to me :D
I ordered extra transistors (3 driver sets and 5 output transistors) to first test the amp without checking every components, and if it breaks again, then test everything.
But, I should probably test the transistors individually. I've got a question though, can I test the transistors in-circuit? I found the dead C959 by comparing many readings between the transistor pins on both channels with the diode checker. (They all measured the same, except for the dead one)
Nothing overheated, all the diodes measure fine :bigeyes:, the bias pot still works, even all the output transistors check OK, but one makes a broken glass sound when I shake it... (The die can't be disconnected... It measures fine.)
Maybe I could temporarily try with BD139/140, my local store has those. (But I don't want to put back the possibly broken output transistor in service...)
I'll start to check resistors in-circuit, if I see highly different values between both channels, I'll start looking for drifting resistors.
Received the transistors!
Hey! I can't attach pictures...
OK, so I replaced the 2SC959/A606 drivers with MJE340/350 on the right channel, and the one of the 2SC897 with an MJ15003.
I don't understand those who are saying changing devices would make the amp lose it's tone... (Like on AudioKarma, but it can't be considered a valid comment, as AudioKarma remains AudioKarma.)
Question 1. The 2SC897 I replaced makes some sound when I shake it, as if the die wasn't sticking to the case, but at the same time, the measurements are fine. Should I put it back, or switch all the transistors to MJE15003?
Question 2. I lost my desoldering pump (...), so desoldering the drivers on the other channel would remain a tedious job as it was for the channel I fixed. Do I really need to switch the original drivers for the MJEs?
Question 3. There are currently no heatsinks on the MJEs, I'm not driving them too hard. Do I need to find TO-225 heatsinks? (Is there anything compatible?)
Transistor making a noise? Like something loose inside the case? I sure wouldn't use it. You should replace all the ouputs with the new On-Semi's.
There are TO-126/TO-225 heatsinks, but I doubt you need them on a 30WPC amp.
Heatsinks are suddenly much easier to find :D (They're all made for TO-126)
I tried the amp louder, the drivers and output transistors don't touch warm at all, however there's a funny feeling when you touch the individual output transistor small heatsinks (kidding;)). It looks like the whole amp wouldn't need any cooling at all.
I'll try to find heatsinks at my local store, and if they don't have them, I'll order $0.05 ones with my next Mouser order sometime.
Now, where to find a 51mm dia., 68mm L, >=80v, >=4700µF capacitor...
BTW, are there any risks putting higher value caps for the DC-coupling electrolytics?
|All times are GMT. The time now is 07:50 AM.|
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright ©1999-2015 diyAudio