Philips N4504

[Chindit]

Hello,
I've recently dug a Philips N4504 out of the garage that I bought earlier in the year but put to one side. Following a cosmetic clean up, it looks to be in good condition; the drive belts have been replaced at some point in the past although I did encounter the gooey remains of an original belt on the inside of the case. It was simple to clean up but I now fully appreciate how much grief a decomposed belt could cause if it was still in situ around the pulleys!
The heads look very nice too, with no visible wear. After cleaning quite a bit of oxide from the tape transfer components and demagnetizing, i tried it with a good tape recorded on another machine. It works very nicely on playback!
I then tried recording from an FM tuner but found that the meter pointers would hardly move (yes I've checked the recording level setting this time!). On playback, the output level is very low and quiet and also very muffled with very little treble.
I was wondering where I should start looking for the fault - could the poor recording be due to a failure in the bias oscillator circuit perhaps? If so, could anyone advise which components form this part of the circuit please? Thanks for any help, Colin.

[jamesperrett]

A lack of bias would give a thin sounding distorted recording with too much treble. Muffled playback sounds like dirty heads or the tape making poor head contact. It is also worth checking that the tape is spooled the right way round - for some reason I've encountered quite a few tapes recently which have been spooled with the oxide on the outside.

[Chindit]

Hi, and thanks for the reply. So it points towards a head or a head to tape interface problem rather than a bias problem? I will follow this up and I'll also check that the tape was (is) the right way round. I'm pretty sure it though as I used a portion of a tape that I had already previously successfully recorded on.
Anyway, I'll check it out and report back when I've had another look. Best wishes, Colin.

[julie_m]

Originally Posted by jamesperrett

for some reason I've encountered quite a few tapes recently which have been spooled with the oxide on the outside.

There was a persistent myth about this reducing print-through.

[AndiiT]

Originally Posted by Chindit

…... but found that the meter pointers would hardly move (yes I've checked the recording level setting this time!).

Hi, Head cleaning and bias settings aside be aware that the DIN socket used for input and output on the rear of the machine has two sensitivities - in the record mode (as do many Philips machines) the high sensitivity input (2 to 5mV) is on pins 1 and 4 for left and right channels respectively and the lower sensitivity input being on pins 3 and 5 (around 250 to 500mV), once again left and right respectively.

Depending on the level of your source signal your problem my be that the level from that is not high enough for the pins of the socket that you have it connected to.
On playback the output is on Pins 3 and 5 of the DIN socket; The solo socket on the far right of the machine, looking from the rear of the unit, marked MONITOR has an output on pins 3 and 5 on both playback and record. It always struck me as an odd arrangement but that's Philips for you

Regards

Andrew

[ricard]

Originally Posted by AndiiT

[ ... ] be aware that the DIN socket used for input and output on the rear of the machine has two sensitivities - in the record mode (as do many Philips machines) the high sensitivity input (2 to 5mV) is on pins 1 and 4 for left and right channels respectively and the lower sensitivity input being on pins 3 and 5 (around 250 to 500mV), once again left and right respectively.

Isn't this because the radio socket has high value resistors between the record and playback signals, originally so that one could connect a record player (with high-level and high impedance crystal output), whose output is on pins 3 and 5 to the radio socket, and still get a working signal, alternatively you could connect two tape recorders using a straight cable between the 'radio' connectors for copying.

Of course, in the day and age (not to mention market) when the N4504 was being sold I would expect that crystal pick-ups had been entirely replaced by low-level magnetic ones, and the record-playback signal interconnection would be useless for record players (instead requiring a separate RIAA preamp, normally built into an associated hifi amplifier), although it would still be useful for tape copying.

Depending on the level of your source signal your problem my be that the level from that is not high enough for the pins of the socket that you have it connected to.
On playback the output is on Pins 3 and 5 of the DIN socket; The solo socket on the far right of the machine, looking from the rear of the unit, marked MONITOR has an output on pins 3 and 5 on both playback and record. It always struck me as an odd arrangement but that's Philips for you

I wonder if that isn't related to the aforementioned record-playback resistors. If pins 3 and 5 on the radio socket outputted a signal during record the result would be a feedback signal - either as an echo when monitoring via the playback head, or as an unwanted feedback path around the preamplifier when monitoring via the record amplifier. So in order to supply the user with an output which always outputs a signal in both record and play modes, a separate output was devised.

Of course, as noted above, it would make more sense to skip any record-to-playback resistors entirely and have the connectors do what they were supposed to without any extra tricks, but on the other hand having a certain consistency within the brand is also a good thing.

I always noted that older Philips machines, such as the 1960s machines like the EL3547 and EL3533, never had any monitoring via the 'radio' connector. I always thought it was part of the DIN standard that only one signal direction should be present on the 'radio' connector at any time. I think it was the same with Grundig, who also almost exclusively subscribed to the DIN mantra.

[Chindit]

Hello Julie, Andrew and Ricard,
Thanks for your input on this. Having had a bit of a session today I'm slightly further forward.
I gave the heads another good clean and managed to get some more oxide of them (cotton buds soaked in alcohol are better than random scraps of cloth as you can see what you're doing as you clean.
I changed C57 which is a 10uF capacitor in the motor run circuit - I knew this wasn't connected with any known fault, but it was leaking black goo so I thought it had better go!).
I checked my tape was the right side out - which it was! I then checked the leads and input connections and they are all good - the source is a AH762 Tuner-PreAmp. If I connect to a Philips N2511 cassette deck in lieu of the N4504 I get excellent recording results, so the source and signal level are fine.
I then tried recording to the N4504 again, with the machine set for stereo at 7.5 ips. The left hand channel is dead - no meter movement and no sound recorded on this channel at all. The right channel records and sounds normal on playback- the meter will climb up the scale as I increase the level control but strange to say, it doesn't move about much, just more or less sits still!
When I play back a tape made on another reel to reel on the N4504, both channels are present, but again no indication on the LH meter.
That's as far as I've got today. I'm wondering if one of the many contacts on the pcb mounted record switch could be open circuit? Thanks again, any more ideas more than welcome! Best wishes, Colin.

[AndiiT]

Originally Posted by ricard

Isn't this because the radio socket has high value resistors between the record and playback signals, originally so that one could connect a record player (with high-level and high impedance crystal output), whose output is on pins 3 and 5 to the radio socket, and still get a working signal, alternatively you could connect two tape recorders using a straight cable between the 'radio' connectors for copying.

The above is correct, the resistors are switched out on playback, of course, to allow a DIN lead to provide record and playback via one lead with suitable equipment.
Had the machine been a two-head model I suspect that the additional Monitor socket wouldn't have been required however I have seen at least one two - head Bush (possibly Grundig designed though) cassette deck with the same arrangement of sockets as that of the Philips N4504.

Originally Posted by Chindit

….. I'm wondering if one of the many contacts on the pcb mounted record switch could be open circuit? ...

That is a definite possibility or even on the selector switches that select the individual tracks and speed, these machines are over 35 years old now and lots of Philips equipment suffered from tarnished switch contacts even during its "normal service life"

Regards

Andrew

[Chindit]

Hello Andrew,
Having studied the manual a bit further today (sitting on the train travelling to/from Selby for a meeting) I can see more more pointers along the lines that you suggest.
When the track selector is set to 3-2 (I think from memory), the left channel is purposely deactivated. Therefore failure of the left track to record could be due to a fault in the ST, 1-4, 3-2 switch.
The constant level of the right hand recording meter (without up and down movement) must be due to the presence of continuous DC flowing through the meter. If C142 were leaky and passing DC rather than signal then these conditions would arise.
I will investigate again, probably at the weekend if I get chance.
I just hope its not down to a defective head coil; perhaps I'll check this first.
Colin.

[ricard]

FWIW, I have a 4504 with a defective meter, I mention it just for the statistics, perhaps the meters were prone to breakage (or as mentioned above some associated component tends to fail, either causing the meter to fail to indicate anything, or by actually destroying it).

[Chindit]

Hello Ricard, thanks, I'll check the LH meter by swapping it with the right one just in case! There is obviously a fault in the recording circuitry though as I get no audio output on the left channel.
Trying the recorder with a pre-recorded tape last night, I get good audio from both channels; RH meter reads at about half scale but doesn't fluctuate up and down with the sound, LH meter stays down on the stop throughout.
Colin.

[Chindit]

Not had chance to look further at this - spent a few hours on a recalcitrant N4506 last night. Capstan motor wouldn't run after a thorough rebuild of the whole machine. Turned out I'd got a three and four pin plug/ socket transposed (I know, it should be impossible!). Works fine now though, with all the banging and crashing that was present when operating the switches now gone!
Colin.

[Chindit]

Just a quick update on this. I've not had much time to 'mess' (Wife's term) recently due to holidays and the like. However, as Saturday was a wash out, I managed to have a bit of a fiddle in the afternoon.
It turns out ricard was on to something - both meters on the N4504 were sticky to some extent; the left hand one being particularly bad. Once I'd established that they hadn't been damaged by excessive current due to a leakage path, I swapped them with a pair of meters from another identical non-working machine that has been lurking in the garage for a while. Hey presto! when playing back a pre-recorded album tape both meters now register and follow the ups and downs of the music.
However, when I record to a blank tape I am still getting a normal Right channel output on playback but probably only about 5% of this output from the Left channel - and muffled too. The left meter barely moves off the stop.
Interestingly, this low level of left channel output is only present when the balance control is set in the middle (50/50). If I swing it over to exclude the right channel completely, the small amount of left channel output disappears too.
So I still have a recording circuit fault, but I think the metering section is working OK now. Will have to try doing a comparison of voltages present on each channel during recording I guess.
Colin.