@Bo Deadly @Boomerang so I replaced the rectifier, but unfortunately the problem persists. One good thing is the fact that the new rectifier doesn't get nearly as hot as the old one.
Sync head hum
- Thread starter EmilFrid
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Help Support GroupDIY Audio Forum:
Scoping the positive output of the rectifier give a sawtooth looking wave. Attached pics show the waveform with all channels in record mode and the waveform without any channels in record mode. The pic showing a bigger waveform is snapped during record mode, channel 1-16.
Could this rectifier be crap too? It's almost brand new, I've just been using it for a small project.
I'm really perplexed.
Could this rectifier be crap too? It's almost brand new, I've just been using it for a small project.
I'm really perplexed.
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Bo Deadly
Well-known member
It's difficult to get any knowledge from that old scope. Can you tell the amplitude of that waveform from that?
Otherwise, I see no evidence that the rectifier is bad. It could be that the transistor is not regulating as Gyraf said.
Just replace everything known to wear out or go bad and see where your at after (this is sometimes known as "shotgunning").
Otherwise, I see no evidence that the rectifier is bad. It could be that the transistor is not regulating as Gyraf said.
Just replace everything known to wear out or go bad and see where your at after (this is sometimes known as "shotgunning").
@Bo Deadly you're right. I tried doing a test recording now, and the ratio is a lot better, probably thanks to the new rectifier. Replacing the semiconductors is the next step, as you said it might be some transistor slowly going bad, especially as this unit has been a workhorse since the 80s.
Will post in this thread whenever I get it working flawlessly again, for any future 85-16 owners having this problem.
Will post in this thread whenever I get it working flawlessly again, for any future 85-16 owners having this problem.
radardoug
Well-known member
Do you have a value for that ripple voltage? If it is a significant proportion of what the output voltage should be, it indicates poor filtering caps.
@radardoug Unfortunately, I only had this old scope today and it's really difficult to read amplitude, since I have no idea where the correct setting is on "vertical gain", but it was enough ripple to cause problems in the audio output. All the filter caps are brand new. I did notice, though, that in the B version of the PSU, they had a 15.000uf cap where mine had roughly 7000uf. So I replaced it with a 10.000uf in parallel with a 4700uf one, and this damped the ripple a lot. And with the new rectifier it actually is better overall, despite my initial thoughts. Didn't realize that until I made a test recording. Just listening to hum doesn't say everything.
It is a long time ago, so I am struggling to remember what the differences were between the 85-16 and the 85-16B. My records show I do have manuals for both in my collection (real paper ones, not PDFs). Now whether I can find them.....
There are quite a lot of rails running from multiple secondaries on that transformer. If you get a chance, use the scope to check the DC on each of the many rails for ripple to see which one is affected by arming tracks. Does yours have the noise reduction fitted. I think it was an option. Looking at the "B" circuit, The DBX has a separate secondary and regulator, so that could be worth checking. It also shows low value resistors used to share the load between pairs of transistors, so checking the DC voltages there may give a clue.
There are quite a lot of rails running from multiple secondaries on that transformer. If you get a chance, use the scope to check the DC on each of the many rails for ripple to see which one is affected by arming tracks. Does yours have the noise reduction fitted. I think it was an option. Looking at the "B" circuit, The DBX has a separate secondary and regulator, so that could be worth checking. It also shows low value resistors used to share the load between pairs of transistors, so checking the DC voltages there may give a clue.
The ripple you see in the power supply should exclusively be before the regulation. If there is any significant ripple after, its not regulating right...
@Boomerang Yes, I have the DBX unit, and even with the psu card for the that unit disconnected, the machine hums along a bit. Only the +/- 24v circuit feeding audio cards and some parts of the autolocator and controller is humming. This has by then been stepped down to +/-15v.
After the mods I did, the hum is only objectionable while recording 13 tracks. So it's a big improvement. Just swapping the rectifier made a difference, so that was one of the problems for sure.
The transistors for regulation do not correlate to the schematics I have, but I've managed to trace out which ones they are. A lot of chassis mounted transistors with wires going everywhere, so it wasn't obvious at first. If you ever found the documentation of the non-B model and scanned it I'd be extremely grateful. It's practically impossible to find, and working with the B-schematics is a bit of a headache at times.
@gyraf Yes, the ripple is not only increasing on the rectifier during record, but also appears on the regulated lines. With no channel being recorded, they're as quiet as Jeffrey Dahmer's boyfriends.
I probably should source those transistors and do a swap. They might be tired after all these years. I really can't find the source of this.
After the mods I did, the hum is only objectionable while recording 13 tracks. So it's a big improvement. Just swapping the rectifier made a difference, so that was one of the problems for sure.
The transistors for regulation do not correlate to the schematics I have, but I've managed to trace out which ones they are. A lot of chassis mounted transistors with wires going everywhere, so it wasn't obvious at first. If you ever found the documentation of the non-B model and scanned it I'd be extremely grateful. It's practically impossible to find, and working with the B-schematics is a bit of a headache at times.
@gyraf Yes, the ripple is not only increasing on the rectifier during record, but also appears on the regulated lines. With no channel being recorded, they're as quiet as Jeffrey Dahmer's boyfriends.
I probably should source those transistors and do a swap. They might be tired after all these years. I really can't find the source of this.
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