Sync head hum

[EmilFrid]

So I've got an interesting problem. I own a Tascam 85-16 and it's in great shape overall. One problem I've noticed recently, though, is a hum coming from the sync head, not mechanical hum but electronic hum leaking into the sync amp via the sync head. The more channels I arm, the louder the hum. It's barely noticeable on 4 simultaneously recorded tracks, but if i keep arming more channels, the hum increases and it's clear that it increases with every single channel, so it's nothing to do with groups of 4 or 8 for instance. And the hum only starts once I push "Record".
The hum can be heard on all channels of the Sync head, regardless of said channel's safe/ready status.

I've recapped the psu completely, and while it removed some other issues, it did nothing for this. I also recapped the board that houses the master oscillator since I thought this hum might somehow sneak into the bias signal. No luck there either.

The machine is completely quiet otherwise, and all the power going to amps, N/R and transport are clean looking on my scope.

If anyone has encountered the same issue, I'd be very greatful for some guidance.


Emil

 

[Bo Deadly]

Try moving it physically as far as possible from it's current location. Even if it's just away from the wall a few feet, see if that makes any difference in the hum level. Use a screen shot of an FFT in your daw to compare before and after if possible.

 

[EmilFrid]

@Bo Deadly I've done this, but it made no difference. Also no difference using the hum shield. I'm perplexed.

 

[radardoug]

Is it quiet in repro? Do the head shields make a difference in repro or sync? Is it mains frequency or a harmonic?

 

[EmilFrid]

@radardoug yes, it's quiet monitoring from repro head, and head shields make no difference. I think it's 100Hz. Will measure to be sure though, I can't imagine the hum in my head atm.

 

[gyraf]

If the hum only starts once you push "Record", it's in input monitor, not in sync read head? Or is it the other channels than the ones being recorded that hums more with more channels recording?

 

[EmilFrid]

@gyraf that's correct, the hum can be heard even on "Safe"-channels, while recording several other channels simultaneously. You can see it increase across all 16 channels on the vu meters as well. I tried unplugging all tape ins and outs to and from the patch bay and, again, punching "Record" while adding channels, but that didn't make a difference either.

Edit: to be clear; the hum affects both recorded signal  and sync head monitoring.

 

[gyraf]

Somehow you have poor regulation of the power supply line for record amplifiers?

Are your supply voltages correct, and which ones of them gives variation in correlation with number-of-tracks-in-record?

 

[EmilFrid]

@gyraf while recapping, I also checked the various voltages and everything looked good and up to spec.
Yes, maybe I should check if regulation is poor, ie voltage fluctuations in response to tracks going into record. But if I remember correctly, both repro and sync amps are fed from the same rail. One thing changing, though, going into record, is the record relays shifting. But I wonder how that would make safe channels hum.

 

[gyraf]

..loading down supply lines below regulation onset voltage would do that,,

 

[EmilFrid]

@gyraf yes, I'm just confused as that's not happening if I'm monitoring in repro mode while recording on several channels. The hum still get recorded, but the safe channel doesn't hum like in sync mode. And both amps get their juice from the same supply line.

I will check the effect on supply line regulation now, just arrived at the studio. I think it sounds like a good idea. I might have overlooked something in regards to the demands of the different amps.

 

[EmilFrid]

@gyraf update! So I measured the +/-15V supply feeding the amps. Stable regulation and no voltage drop across the output. BUT when I scoped the -15 line while adding recorded channels I could see a sinusoidal type pulse intensifying for each added channel. Could it be a case of a bad regulator? Or well, it's actually two transistors, the potentially bad one being a d361 and a chip, mc1468L. Including a pic of the board in this post.

Edit: scoped again, and now I actually see the pulse on both rails...

 

[Bo Deadly]

Could it be a case of a bad regulator? Or well, it's actually two transistors, the potentially bad one being a d361 and a chip, mc1468L. Including a pic of the board in this post.

Presumably the regulator chip is fine since it's not handling load. It's probably just the transistor(s).

Silicon parts that handle a lot of power can get burned up inside over time and start to become slow to turn on / off or won't turn on / off all the way. So yes, one or both of those transistors could definitely be bad.

Looks like SD361 is NPN TO-220 10W pinout: b c e

A good replacement for that would probably be something like TIP31C. But I would definitely replace the other with the complementary part. Presumably it's a PNP which would be TIP32C. There are lots of parts that would probably work there so maybe you can use something laying around.

Make sure you have proper heat sink coupling though.

If you have any data on that board it would help confirm.

Note that you might not use the machine until you make the change. If one transistor has a catastrophic failure, it might leave one rail up and the other down which can burn out things.

 

[EmilFrid]

@Bo Deadly thanks for the advice! I'll see what I have in my transistor stash. I hope it's just the transistor(s). One other thing I just noticed, is the rectifier feeding the +/-15 circuit gets really hot. Like "burn your fingertips" kind of hot. I fail to see how it would draw enough current to be that hot, while not showing any kind of voltage drop (measured across the rectifier). Maybe I need to replace the rectifier?

Yes, here's a screenshot of the board. Bear in mind, though, that the schematics for the A-version (my machine) is impossible to find, so I only have the B schematics. They look similar enough, just qlightly different values here and there.
Sorry for the poorly printed schematic btw, this was the only one I could find online.

 

[johannes]

With this new information about the rectifier I would be inclined to suspect that something is drawing too much current.

 

[EmilFrid]

@johannes possibly, but wouldn't the transistors driving the circuit be very hot too, In that case? They're all ok temperature-wise

 

[Boomerang]

So I've got an interesting problem. I own a Tascam 85-16 and it's in great shape overall. One problem I've noticed recently, though, is a hum coming from the sync head, not mechanical hum but electronic hum leaking into the sync amp via the sync head. The more channels I arm, the louder the hum. It's barely noticeable on 4 simultaneously recorded tracks, but if i keep arming more channels, the hum increases and it's clear that it increases with every single channel, so it's nothing to do with groups of 4 or 8 for instance. And the hum only starts once I push "Record".
The hum can be heard on all channels of the Sync head, regardless of said channel's safe/ready status.

I've recapped the psu completely, and while it removed some other issues, it did nothing for this. I also recapped the board that houses the master oscillator since I thought this hum might somehow sneak into the bias signal. No luck there either.

The machine is completely quiet otherwise, and all the power going to amps, N/R and transport are clean looking on my scope.

If anyone has encountered the same issue, I'd be very greatful for some guidance.


Emil

I think I know exactly what this is. I had precisely that problem in a customer's studio some 20 odd years ago. The owner used to erase unwanted tapes for reuse when the studio wasn't booked. Then, one day, he noticed he could only erase 13-15 tracks simultaneously (any selection of channels), otherwise he got hum. I went to investigate, and while he made us a drink, the problem suddenly got worse. I could only erase about 9 tracks without hum. When he came back with the coffees, the problem reverted to 13-15 tracks. Puzzled, I wondered wether the kettle at the far end of the building could have been responsible for the change. Indeed it was!
The problem was one diode in the bridge rectifier had failed. The other diodes were keeping the reservoir caps full enough, but when the load increased (with more tracks in record) or the mains voltage drooped (with the kettle on) ripple appears on the DC power rails.
New bridge >>> problem solved. I had lots of hot brews that day and learned a valuable lesson. If you get a bizarre fault, check the bridge rectifier.

 

[EmilFrid]

@Boomerang wow, interesting! That would explain why only the bridge rectifier gets hot and nothing else, too. And the problem is exactly how you describe it; sometimes it's worse than others. I will turn a the kettle on in my workshop next door and see if it reacts with the tape machine. Then I'll swap the rectifier, got a bunch of them.

I'll report back later tonight! Having a session on Sunday, so I'll work my ass off to get this working by then.

 

[Bo Deadly]

Indeed it sounds like it's the rectifier. Like I said, when silicon parts start to die, they don't turn on / off cleanly. So current is sloshing back and fourth in one or more of the rectifier diodes when it should be fully on / off. So it gets hot.

If the machine is important to you, I would just replace all of the high power silicon parts in the supply including rectifiers and transistors.

You might also consider replacing some of the resistors that are carrying a lot of power. They too can get burned up inside and become physically brittle. If you see ones that are discolored or the PCB around it looks discolored, I would change those resistors as well.

 

[EmilFrid]

@Bo Deadly you're right. Might as well replace power transistors as well. I think I have most of them in the workshop stash, some brand-new, some just lightly used.

The machine is very important to me indeed.

I already checked power resistors throughout the psu, but the all look fine. The internals of this machine are generally very sturdy and well-laid out, built to handle a lot of work. The power transformer is ENORMOUS, and is properly fused. They didn't compromise on safety.

I just arrived, so will dig out a rectifier (now where did I put them...) and report back.