Reel to reel player hum on playback

[glebert]

Acquired a "vintage" reel to reel (Sears solid state, made in Japan) and a bunch of tapes, all untested. I actually have someone interested in buying them if the machine works. Hacked a power cord to make it fit, unit is kinda working, but there is something odd. When playing back the stuff recorded on the tapes it sounds ok at low volumes (maybe a bit muffled) and no hum. When I turn up the volume past a certain point I get a ton of hum that swamps out the recording. This is present on the headphone and line outs. I find this a bit odd because if it is something like a capacitor I would expect the hum to either be there all the time and constant, or it should vary evenly with the level of the playback signal. Is there any simple explanation for something like this (e.g. magnetized heads). I don't have a schematic for this, nor do I think I would spend much time fixing it.

Thanks,
Greg

[Enzo]

It is not the heads.

I suspect the unit is breaking into RF oscillation at advanced volumes.

[glebert]

It is not the heads.

I suspect the unit is breaking into RF oscillation at advanced volumes.

Yeah, that would fit the bill. I wish it was at "advanced volume" though and not "normal volume." Oh well, don't really have anything into it anyway.

[Enzo]

Oh well, advanced is just a fancy word to indicate turned up to some point.

[glebert]

I may have to get this thing fixed, I am starting to really enjoy the 70's country band that in on these tapes. Can't tell if all the songs are covers or not, some of them are either originals or so obscure that Google can't find any match to the lyrics. Pretty good performances. Makes me want to cover them, whoever they are.

[glebert]

I figured might as well try cleaning the pots since I have heard of amps oscillating when having an open on their input. In good news it seems to have helped the oscillation, but now it seems like the playback slows down after a few seconds. I was pretty careful not to get cleaner on to anything else, so I will need to check it out a bit further. Capstan isn't direct drive on this one, it is on a flywheel that is driven by a bogey wheel that frictions onto the main motor shaft, which has three different diameters for the different speeds. Seems like not enough friction between the bogey and the main shaft.

[glebert]

So much for not spending much time on this reel machine. The original oscillation problem is fixed, as are the slipping friction drive wheels and also the stuck VU meters and dirty heads.

The only problem I am having now is that anything I record sounds muffled, like a blanket thrown over a speaker. Playback seems decent, but I don't have known very high quality recorded tapes. The old stuff I do have sounds much better than anything I record to it now. Would my next step be to look at the head biasing? BTW, this is a two head machine, so supposedly the heads should self degauss when bias is applied to them while in use.