Philips N2400 cassette recorder, muffled sound

[agent_power]

Hi all

I've got a Philips N2400/16 tape recorder here from the 70s (including customary nicotine glaze).

After cleaning out the gooey belts (which got everywhere and will now forever stain my workbench) and lubed up the tape mechanism the mechanical side of things appears to be working very well.

Fortunately, there doesn't seem to be any technical problems with the amplification circuits except that the sound is very muffled with known good tapes.

There are a lot of complicated adjustments (see service manual) which I don't fully understand (step 6 refers to a reference cassette which I don't have). This video mentions a worn head:
Where would I get such a head? Are they interchangeable? I haven't changed a single capacitor either, but there's no hum anyway. Any help is appreciated on how to diagnose this fault without shotgun replacing parts blindly

[themadhippy]

stating the bleeding obvious,but you have cleaned the tape heads?

[Haenk]

stating the obious (2) - maybe the heads are not correctly aligned
and (3) it's not impossible the amplification section is not working as it should (dry caps)

A worn head would be recognizable immediatly (the "stripes" on the head will be deformed or visibly worn out) but that is rarely the case other than due to extremely heavy use.

[wasedadoc]

.. it's not impossible the amplification section is not working as it should (dry caps)

Dry capacitors in an audio chain invariably give impaired low frequency response, ie tinny sound, not muffled.

The unit does have a treble control.  Does it have any effect?

This is a stereo machine.  If only one channel is muffled then most likely an amplifier fault.  If both channels muffled, most likely the head azimuth is misadjusted or head is well worn. Azimuth error only affects tapes that have been recorded on a machine with correct azimuth.  Do a test recording on this machine and if it plays back non-muffled on this machine, almost certainly azimuth has been twiddled by someone.

[agent_power]

Hi all

Thanks for your replies

stating the bleeding obvious,but you have cleaned the tape heads?

Yes, I cleaned them again and more crud came out. Maybe one of my tapes is shedding. I let it run all day with various tapes.

stating the obious (2) - maybe the heads are not correctly aligned
and (3) it's not impossible the amplification section is not working as it should (dry caps)

A worn head would be recognizable immediatly (the "stripes" on the head will be deformed or visibly worn out) but that is rarely the case other than due to extremely heavy use.

I'm suspecting (2), I wiggled around and it could be that the cassette holder is deformed. I think it sounds better with the chassis out, but I would like to verify this.

.. it's not impossible the amplification section is not working as it should (dry caps)

Dry capacitors in an audio chain invariably give impaired low frequency response, ie tinny sound, not muffled.

The unit does have a treble control.  Does it have any effect?

This is a stereo machine.  If only one channel is muffled then most likely an amplifier fault.  If both channels muffled, most likely the head azimuth is misadjusted or head is well worn. Azimuth error only affects tapes that have been recorded on a machine with correct azimuth.  Do a test recording on this machine and if it plays back non-muffled on this machine, almost certainly azimuth has been twiddled by someone.

It has treble and bass, they both work (I did give the pots a cleaning now). It sounds similar on both channels.

I'll go look for DIN adapters to feed some sound in.

Any tips on doing the azimuth adjustments? I have one of those mp3 adapters, is that ok to feed in a test signal? What else do I need to check the output for correct alignment?

Is it possible the heads are magnetised and need degaussing?

[wasedadoc]

If you have a head demagnetiser, by all means use it.  Done properly it cannot do any harm.

You cannot adjust azimuth by recording and playing back on the same machine.  Without an azimuth adjustment tape the best you can do is to play a commercially prerecorded cassette and adjust for maximum treble while listening to it.

[tunk]

You cannot adjust azimuth by recording and playing back on the same machine.  Without an azimuth adjustment tape the best you can do is to play a commercially prerecorded cassette and adjust for maximum treble while listening to it.

But you couldn't you record some music and then play it back,
and if the sound is ok, then you know it's the head adjustment?

[wasedadoc]

You cannot adjust azimuth by recording and playing back on the same machine.  Without an azimuth adjustment tape the best you can do is to play a commercially prerecorded cassette and adjust for maximum treble while listening to it.

But you couldn't you record some music and then play it back,
and if the sound is ok, then you know it's the head adjustment?

Apparently you have not read what I wrote in my previous post.

[themadhippy]

But you couldn't you record some music and then play it back,
and if the sound is ok, then you know it's the head adjustment?

that'll only work for tapes  recorded on that machine,and only then if the record and play head are the same unit.Anything recorded on another machine could be out of line

[wasedadoc]

But you couldn't you record some music and then play it back,
and if the sound is ok, then you know it's the head adjustment?

that'll only work for tapes  recorded on that machine,and only then if the record and play head are the same unit.Anything recorded on another machine could be out of line

That machine has a single head which is used in both record and replay modes.  As such it is impossible to check azimuth by playing a recording made on it.

[agent_power]

Sorry, more questions as I don't have a clue about tape recorders of this vintage:

1. does the tape formulation matter?
2. does it matter if the tape has Dolby encoding on it?

I tried to make a recording and it still sounded a bit muffled, even on the monitor before recording on it. The 'best' quality I had with the mp3 adapter.

How do I check the frequency response of all the different parts (input, tape head, amplifier)? I have an analog and digital scope Rigol DS1054Z, but no dedicated spectrum analyzer

[wasedadoc]

The tape formulation does matter but this is a machine from the era when there was only one type.  Ferric oxide, the same ones that are just about the only ones that are available today.

Playing back a Dolby encoded tape on this machine will sound too bright, not muffled.

Neither the record nor playback frequency response is designed to be flat. Also it is very unlikely that there is a frequency response fault in the electronics of both channels.

[FIXITNOW2003]

 check the AD161 and 162 Germanium transistors know to go leak and effect sound

[wasedadoc]

check the AD161 and 162 Germanium transistors know to go leak and effect sound

Connect an amplifier to the LINE level outputs from the DIN sockets.  If muffled there, the germanium transistors in the N2400's power amps are not the cause.

[agent_power]

I connected some BT-speakers via AUX input to the LINE out and found a commercial tape with speech on it which I used to screwdriver the azimuth to sound most natural to me.

When connected via the recorder's amp, it still sounds muffled but only on lower volumes. When I turn up the volume the highs start to appear.

I can however still adjust the sound via the bass and treble controls on lower volumes to make it sound okayish.

Now I'm not sure if my speakers are incorrect for this (CANTON LE500, should be ok re. impedance) or it's just how this amp works. I don't think this was meant to be a HiFi device by looking at the internals.

Unless you have some hot tip I think I'll leave it like this

[agent_power]

https://streamable.com/1i01d0

I think it sounds decent for not having a single cap replaced. It definitely needed the cleaning and azimuth adjustments though.... and just as I'm about to finish writing this the pinch roller is starting to slip.