Philips EL3576 (new) restoration

[borispoodle]

I have recently aquired an immaculate Philips EL3576 tape recorder identical to the model I owned as a child. This one was brand new and never used, sitting unopened in its box for over fifty years. Must be unique!

The decomposed synthetic rubber components were easy to source on ebay from Germany, all bearings relubricated and heads degaused.

The machine runs almost perfectly, certainly no detectable wow and flutter. A very nice mono 4 track 4 speed machine. Its only fault is a mains hum apparent when the volume control is advanced beyond half full. This hum is only present on playback (tape loaded or not) and completely disappears when the stop button is depressed. Likely then not to be a fault in the power amp. All functions of the recorder work just fine. The hum is present playing back tapes made on a Revox and certainly not present on the tape itself.

There are lots of electrolytics present of course which might be responsible. I wonder if any one has any experience of this and can advise? I cannot recall such a problem from my youth though this just might be a characteristic of the machine. I do have the recorder's circuit diagram.

Many thanks.

[Michael Maurice]

Hi and welcome to the forum.

If the hum is only present when the volume control is advanced and in play mode, it means that the power supply and electrolytic are ok. The hum is most likely being induced through the wiring to/from the head or even the control itself.

Do you have access to an oscilloscope?

[borispoodle]

Yes I do have a scope. I have noticed that the wiring from the rec/play head down through the chassis to the amplifier board is simply a twisted pair. touching this on its insulation with a finger amplifies the hum immediately, the twisted pair probably acting as an antenna picking up motor radiation is my guess. The head is earthed to the chassis via a short cable but that leaves the relatively long twisted pair run to pick up radiation. I did by the way check out the power supply smoothing with the scope and found nothing.

Swithing to record mode, where the head is driven rather than acting as a signal source removes the hum completely. Perhaps rewiring with screened two core cable (both channels needed) is the answer securing the screen to the chassis.

[Refugee]

Is there an earth between the negative of the power supply and the chassis. I guess it is a transistor amplifier with negative earth.

[radiotechnician]

You have mentioned the twisted pair. From the schematic, how does the machine select which of the tracks to use?

The twisted pair is a balanced line, and if the machine records and plays back, is working.

From the schematic determine which switch is coupled to the speed change selector. The machine will adjust equalization to optimize frequency response for recording and playback, which differs with speed.

Were one of the contacts failing to close during playback, it might cause a feedback path to open. This could cause the gain to rise, making the existing hum levels much louder.

Do you have the entire manual, with the adjustments for the switch linkages? Also the schematic should show the contact which operate at each speed and function.

I would not modify anything. I repaired the four speed Philips transistorized machines in the 60s, and I suspect I may even have that model in a great pile elsewhere.

[Welsh Anorak]

Of course this could be a 'feature' of the machine! I have a Grundig that does the same and is hardly noticeable when a tape is played. However, if it is too loud, I'd leve the machine idling for a few days which should give and electrolytics chance to fully wake up. These had the excellent Philips silver or blue capacitors and even at this age rarely give trouble.
Maybe wrapping some aluminium sleeving over the head wires (say from a piece of satellite cable) and earthing itcould help.

[borispoodle]

Thank you everyone,
Cannot find a problem with the mechanical switch driven from the speed selector so that seems ok for equalisation selection, also checked the fault is indeed present for all four speeds. Everything else seems correct and being brand new not worn out. The pots were in fact completely silent in operation for instance. Will continue to investigate. Especially improving the screening as suggested thats easy to try.

[Refugee]

I would also check the plug wiring too. They are often badly wired from new as it was made before factory fitted plugs.
A bad earth in the plug would cause static hum that is often "buzzy".

[Michael Maurice]

I think you'll find these are double insulated with two core cable, it no earth.

[borispoodle]

Yes indeed, this recorder has no earth. The rec/play stereo head (4 track) wiring is strange: each head coil is wired back to the preamp using simple screened single core cable with the screen actually used as one of the active conductors. One of these cables per channel so vunerable to induced hum. Philips being just a bit too economical?

[TonyDuell]

A couple of things that I would try :

1) Earth the machine. It must be safe to do this as it was designed to be connected to other equipment that may well be earthed. Run a wire from the metal chassis to the earth pin of the mains plug.

2) Swap the Live (Line) and Neutral wires in the mains plug. Some machines hum more with the mains one way round (I guess capacitive coupling from the mains wiring inside the machine or even between the transformer windings).

[borispoodle]

Thanks Tony, did try that but to no effect, here is a photo of the head block

[cheerfulcharlie]

Have you tried disconnecting the head and just feeding the cables with a tune out the 3.5mm socket from something battery operated such as an iPod (my generic term for any modern cheap music player or phone)..as maybe the head has aged in some way or other? It might sound distorted but you are listening for hum.

[borispoodle]

Thank you cc, the head output will be very low and according to the circuit diagram feeds the same transistor as the mic input when that is used during record or mic input for p.a. amplification. As that path when switched in causes no hum it really looks like cable hum pickup from the recording head when the machine is switched to that.

Best tested in two steps. First simply parallel up some suitable electrolytic power supply smoothing just in case that's the problem. Then try a better head cable.

[ricard]

Originally Posted by borispoodle

Yes indeed, this recorder has no earth. The rec/play stereo head (4 track) wiring is strange: each head coil is wired back to the preamp using simple screened single core cable with the screen actually used as one of the active conductors. One of these cables per channel so vunerable to induced hum. Philips being just a bit too economical?

Well, frankly, one of the two wires from the head would probably be connected to chassis anyway so it might as well be used as a screen.

[cheerfulcharlie]

Well here is an odd thing you can try.

Pick up the tape machine and wander about with it ..see if that affects the hum.

[radiotechnician]

In the Philips Continental 401 the R/P head returns to earth through
a resistor. It is there to provide a measurement point for setting
record bias. Can you post the part of the schematic that shows
where the heads are wired to in your model ?

[borispoodle]

The problem was actually magnetic fields from the running motor. Really a characteristic of this machine. Improving smoothing had no effect, the original capacitors are fine. Improving the head wiring has significantly improved matters. Not much else to do really.

[borispoodle]

For anyone who needs it, the full schematic is available here:
https://www.radiomuseum.org/r/philips_rk65s.html

[cheerfulcharlie]

Glad you found it and that is useful.

Probably not going to be a bother to you, but if you're ambitious you could try making some screening with some 'Mu-Metal' foil... but as a metal sheet for fabricating this could work out expensive unless you can find some scrap cans, etc.