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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 2nd Jun 2017, 6:23 pm   #1
richardrx4
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Default Philips N4418 stuck in RECord

Ive had my N4418 since around 1974, but it went out of use some years ago, altho I was too attached to it to sell. I recently unearthed it, and have now replaced all the drive belts, and am about to replace the 5 "switch" lamps with led's.
Anyway, problem is when I switched it on for the first time, all seemed to work OK BUT the REC button alone was lit. Nothing could extinguish it (STOP, switch to Amp mode etc) so I switched it off. Following day I tried again and now all seemed well, all function buttons worked as designed, including REC with PAUSE or PLAY etc. So I played an old tape and again (a bit to my surprise) it played wonderfully well, just as I remembered years ago (an Eagles recording...). Then about 5-6mins in, the REC button illuminated all by itself, which together with the already lit PLAY button, put the recorder into record mode. This erased a few seconds of my prized tape before I could hit STOP. The machine did stop, but the REC button remains lit. On switching off the machine, I hear what I guess is the record relay clicking off. Switching the machine on again shows the REC button lit by itself again.
OK so whats the problem??
I have a Service manual parts 1 and 2, and alto I'm NOT an electronics expert, looking at the circuit diagrams and the accompanying prose, I'm guessing there is a problem with the REC flip-flop (FF REC).
So my question is can anyone hazzard a guess as to what or where the problem might be. I'll be pretty much limited to removing and visually inspecting the various mini circuit boards, and have a simple multi-meter to check basics, but no oscilliscope nor the knowledge to use one. Perhaps there is a known weak component or contact I could look for.
Grateful for any help. The machine is otherwise wonderful, in pristine condition, and I'd like to recapture my youth (!) in listening to some old recordings, and the nostalga of looking at a reel-to-reel again. Music streaming from Amazon just isnt the same!!!
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Old 4th Jun 2017, 3:20 pm   #2
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Default Re: Philips N4418 stuck in RECord

OK, an update, although I could still do with some help from someone who knows more about the electronics of this machine than I do...
Convinced purely by looking at the schematics in the Service Manual, that the record flip-flop (U202) was the culprit, and since the other flip-flop (U201) looked identical, I swapped them around. Kinda success !!.
The REC is no longer lit, and everything else seems to work fine, except that the PAUSE key now stops the machine rather than just put it into pause - I think I can live with that.
So I think I've proved that the fault was with flip-flop unit U202, although I dont know what exactly. Would be nice to have some idea so that I could contemplate a repair - or perhaps someone out there has a non-working N4418 that they would be willing to donate a flip-flop circuit board - long shot I guess
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Old 4th Jun 2017, 7:02 pm   #3
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Default Re: Philips N4418 stuck in RECord

Have you a part number for the IC in question?

Simple logic gates are still useful, and hence many of them are still obtainable .....
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Old 4th Jun 2017, 7:44 pm   #4
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Default Re: Philips N4418 stuck in RECord

The service manual for the N4418 on Elektrotanya doesn't imclude the circuits of the plugin boards, but they look to be much the same as those in the N4510. In fact the N4418 looks to be very similar to what you would get if you merged an N4510 and a power amplifier.

Anyway, the flip-flop module is not an IC, it's a little PCB containing 3 bistable circuits, each of which consists of a BC147 (or BC547) cross-coupled to an AC128 (trust Philips to use an NPN and a PNP transistor in such a circuit!). The other components on the PCB are resistors and are unlikely to be the problem, but one of the transistors could well have failed. Even if you test all 6 it won't take long.
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Old 4th Jun 2017, 8:18 pm   #5
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Default Re: Philips N4418 stuck in RECord

I'd test the AC128 first to determine if two or all three of the electrodes are short circuited to the case, i.e. the 'tin whiskers' problem.
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Old 4th Jun 2017, 8:35 pm   #6
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Default Re: Philips N4418 stuck in RECord

So would I, actually. But both the AC128 (tin whiskers) and BC147 (if it's the early version, not the one with BC547s) (lockfit) are not noted for reliability now.
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Old 4th Jun 2017, 9:53 pm   #7
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Default Re: Philips N4418 stuck in RECord

Wow. That circuit is very ..... Philips. (Working from the Elektrotanya N4510 Service Manual):

The RECORD switch ordinarily does nothing unless PLAY or PAUSE is pressed, which grounds one side of the switch via D202 or D203 respectively. So it is necessary to press PLAY + REC or PAUSE + REC together to start recording. This allows current to flow into the W input (which is normally sitting close to the negative supply). This turns on the NPN transistor, which then turns on the PNP transistor; and the two transistors continue to hold each other in deadly embrace even after the switch has been released and the W input has been allowed to float.

When the W input is pulled down close to the negative rail via D228 or D231, the NPN transistor turns off and then the PNP transistor's base floats high, turning it off. And off they both stay until one or the other turns the other one back on .....

Either of those transistors being faulty could cause your original "sticking in record" problem, and both AC128 and BC147 have known failure modes. BC547 is a straight replacement for the BC147 (it's the same silicon, just in a more reliable encapsulation). As for the AC128, I'd be tempted to try a BC327 as a first replacement, even despite this being a silicon device with a different turn-on voltage. After all, it's switching hard on or off, not biased critically into the middle of a linear region .....
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Old 5th Jun 2017, 12:00 am   #8
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Default Re: Philips N4418 stuck in RECord

Wow, thanks for all your really helpful replies - first time I've posted on this forum and I'm impressed.
Having looked at the little U202 circuit board, I can't see any tin whiskers (maybe I wouldn't anyway), and nothing looks amiss. There are 3 pairs of AC128/BC147's, and again by looking at the Service manual I've managed to identify what I think are the REC flip-flop pair.
On Julie's advice (thanks) I've ordered a good few BC547/BC327's so will post an update once I've had a go at a repair...
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Old 5th Jun 2017, 5:00 am   #9
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Default Re: Philips N4418 stuck in RECord

You won't see the tin whiskers, they would be inside one of the (metal-cased) AC128 transistors.
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Old 6th Jun 2017, 11:15 am   #10
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Default Re: Philips N4418 stuck in RECord

Julie is correct about silicon transistors working here. I thought she was, but wasn't prepared to agree without evidence.

I have an N4510 on the bench at the moment (this is essentially the same machine as the N4418, but without the power amplfiiers). Most of the tape controls didn't work, etc and after a bit of tracing it appeared to be problems with the flip-flops. I found some of the AC128s were leaky (not shorted, and still tested as transistors, but leaking emitter to collector). I have replaced all 6 (2 boards, 3 flip-flops on each) with 2N3906 transistors (this is the common silicon PNP transistor I normally use for general-purpose jobs). It now works fine, the control buttons start motors and operate solenoids as required.

I would think the BC327 will be OK too (but haven't tried it).

Do make sure you get the pinouts right if you are substituting transistors. They may not be the same
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Old 6th Jun 2017, 12:53 pm   #11
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Default Re: Philips N4418 stuck in RECord

Tony, thanks for the advice. I've Google'd the pinouts of the current and replacement transistors, and noted the layouts, so I shouldn't go wrong!? Will post my success or failure in fixing this.
As I said originally, I'm no expert at electronics although dabbled a bit over 40 years ago! We'll see how my soldering skills hold up...
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Old 6th Jun 2017, 5:26 pm   #12
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Default Re: Philips N4418 stuck in RECord

Those little Philips PCBs are not that easy to solder. The tracks lift fairly easily (although they are simple enough to repair, they are just single-sided). I found it best not to try to suck the solder off and remove the old transistor but rather to heat one of the joint and bend the transistor away from it. Then carefully clean up the board.

On the AC128, the base is the middle lead and the collector the one with the red spot. On the PCB they connections are in a triangle, the base is the one nearest the edge connector fingers. The emitters of all 3 AC128s are connected together, so you can fairly easily trace what goes where if there's any problem.
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Old 6th Jun 2017, 6:43 pm   #13
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Default Re: Philips N4418 stuck in RECord

Tony -- thanks very much for confirming my gut instincts
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Old 6th Jun 2017, 10:52 pm   #14
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Default Re: Philips N4418 stuck in RECord

Tony, thanks for those tips, really helpful. Replacement transistors not here yet, but you've given me a bit more confidence for when they do. I'll most likely replace the guilty pair first to see if that fixes the problem, before contemplating replacing all 12 transistors on the rule of "if the rest aint broke...".
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Old 7th Jun 2017, 5:28 am   #15
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Default Re: Philips N4418 stuck in RECord

I had no problems with the BC547s. Had they been BC147s (in the lockfit package) then I would have been more suspicious of them. I did not change those.

In my case I think 4 of the flip-flops were malfunctioning, and the AC128s tested leaky. I don't normally replace good components, but in this case I thought that with a 2/3rds failure rate (and I assume all were originally fitted at the same time) the other 2 wouldn't last much longer so I changed the lot to avoid future problems. It's up to you what you do though.
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Old 7th Jun 2017, 8:40 pm   #16
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Default Re: Philips N4418 stuck in RECord

It was common back in 'the day'. I used to work for Philips and these (in 1974) were new. Of course back then, all we had to do was order a new flip-flop but when they occasionally went out-of-stock we were forced to try and repair the original. Invariably it was one (or more) of the AC128's. Mind you they were easily available as well.....
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Old 8th Jun 2017, 7:28 pm   #17
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Default Re: Philips N4418 stuck in RECord

Success !!
Decided to replace all 6 AC128's with the suggested BC327's once I realised how easy replacing the first one was, although for now the BC147's stay. All the transport functions now work perfectly.
Also replaced the 5 "switch" bulbs with 5mm 3.2v led's in series with a 270 ohm resistor as advised from an online led calculator, and again works a treat.
So for now at least all is well, although you should never be too confident with 40+ year old tech.
Thanks to all of you who gave invaluable (to me, probably obvious to you) advice, I couldn't have attempted this otherwise .
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