Philips N1500: Chassis, Player No.2

I’ve been faced with similar dilemmas before, then I end up getting both of whatever it is working! 

Regards 

Lloyd 

This VCR over the past week or so has coughed up fur-ball after fur-ball and has kept me very occupied.

First the tape transport motor failed, totally seized up good and solid, thankfully I had procured another sacrificial deck so pulled its motor, that got over that hurdle, then a few other faults surfaced.

The tape transport safety circuit ceased functioning, this is supposed to switch the VCR off after 45 secs. No E-E and no video output also manifested itself. The VCR powers on OK, the drum lacing loads the head into position and the Play, FF, REW and Rec functions as expected along with the Off which de-laces the drum/head.

Investigations started with the first problem namely the protection circuit, as mention above the purpose of this is to switch the VCR off if no tape transport button is pressed within 45 sec.  The reason for this is the tape is statically laced into position and the heads come into contact with the tape. If no transport action is initiated then due to the heads being in contact with the tape and there being no tape movement there would be a risk of tape wear in one place hence the safety circuit.

The delay is obtained via TS113 on Panel 10 and the circuit is supposed to function as follows. Shortly after the "ON" button is depressed, a 12V voltage is present. The voltage across C115 and thus on the gate of TS113 is then 0V. The source of TS113 is biased to approx 10V by a voltage divider R129 + R130 so that TS113 is cut off. C115 is charged via R125 and R126. After approx 45 secs the voltage on C115 has increased so far that TS113 becomes conductive.The resulting voltage drop across R131 drives TS114 into conduction so that a positive direct voltage is fed to the bases of TS116 and TS112 via R133 and R132 and the VCR is switched off.

The first thing I checked was the 12V supply which should be present at R125 delay circuit, it was not present. Working backwards to where the stabilised 12V rail is formed I made some voltage checks and soon startred discovering the lower voltages for TS104,03,02 & 01 were way off. This took me to check the output of the four diode bridge rectifier.

As can be seen below output is just a mere 726mV and not DC!!!. Input to the bridge should be 17.5V, I have 19-20V. I suspected the bridge but in circuit checks which I admit were tricky to say the least, seemed to suggest all is well with them when clearly voltage output suggest not.

Access is not ideal but not impossible but before I go delving into this difficult area anyone have any tips for checking a bridge in circuit other than the usual diode tests, so as to narrow it down to a culprit? If not I guess I'm going to have to bite the bullet and remove D101,102,103 & 104 and all the rats nest below them.

First thought is the bridge is shown as a BY164, not the most reliable bridge rectifier, second thought is the frequency of the 700mv, it is 50hz which suggests one leg of the bridge is O/C, perhaps one O/C the other nearly so passing very little voltage.

Edit.

Your circuit show a 3.15A fuse not in the original diagram, presume this was a mod, the BY164 were prone to going S/C, I thought the bridge was rated around 1.4A, still a 3.15A fuse is better than none.

Sorry Frank you've made an assumption that the legend below the cct was referencing the bridge rectifier, it is not a BY164, the legend is for another rectifier (rectifier panel 15 for another rail). The bridge rectifier on the 12V rail is made up of 4 x BY127 arranged as a bridge. With regards to the fuse, the circuit snippet without it was used by mistake and correct one below shows fuse.

I guess I'm just going to have to remove all the diodes and all the wiring to find the culprit. PITA.

Ok about separate diodes, is that 50hz measurement correct, if so, it still points to the bridge circuit, whether faulty diodes or dry joints etc?

Posted by: Nuvistor

is that 50hz measurement correct?

No its not, I made a boo boo on one of the sample points for the DC out of the rectifier. ?

I'm now correctly on sample point 119 & 120, I do have DC output but as previously indicated, only 726mV.

AC in

DC out

If you have the correct AC going in to the bridge, virtually nothing coming out, nothing getting hot or fuse blowing etc then you are back to checking the diodes and soldering etc.

Looks very tight with little room to work in. That type of work wasn’t a problem for me years ago, couldn’t do it now.

Yesterday I was asked via pm when I was going to post some more repair work, in truth my response was "I won't be, why bother posting as nobody else is". Admittedly a churlish response (apologies) but it was borne out of frustration after having posted multiple projects since January with very few if any members doing likewise.  It doesn't mean I'm not repairing, it just seemed little point posting about it on the forum. Anyway for those that might be interested here is a post update and time will tell if I do any more. 

After having a break from total defeat on Laserdisc players, I thought I'd go back to the Philips N1500 VCR restoration that had been put to one side. To recap on where I was at with it and faults upon its arrival:

1. Drum lacing failed
2. Transport head safety circuit not kicking in after 45 seconds
3. No E-E
4. No Video output

Item 1 was addressed by installing a brand new drum lacing cord, its now lacing correctly.

Next item two, this is where I left off, its worth going over again exactly how the head protection delay circuit is supposed to operate.

The delay is obtained via TS113 on Panel 10 and the circuit is supposed to function as follows. Shortly after the "ON" button is depressed, a 12V voltage is present. The voltage across C115 and thus on the gate of TS113 is then 0V. The source of TS113 is biased to approx 10V by a voltage divider R129 + R130 so that TS113 is cut off. C115 is charged via R125 and R126. After approx 45 secs the voltage on C115 has increased so far that TS113 becomes conductive.The resulting voltage drop across R131 drives TS114 into conduction so that a positive direct voltage is fed to the bases of TS116 and TS112 via R133 and R132 and the VCR is switched off.

I had checked the 12V supply which should be present and the R125 delay circuit, it was missing. Working back to where the stabilised 12V rail is formed voltage checks back through TS104,03,03, & 01 revealed they were way off.

Checking the AC input of the full wave bridge confirmed via the scope I have 18.4VAC going in but only 2.5VDC coming out. By my reckoning you calculate the full-wave bridge VDC out as VAC * 1.41 which gives me an expected output figure of 25.944VDC. Allowing and deducting the voltage drop across the four diodes, (0.7V x 4) it should be giving me an actual VDC output from the bridge of 23.144VDC.

The Philips circuit actually states a 17.5VAC into the bridge  so applying the same math 17.5 * 1.41 = 24.675 - the 4 diode drop should give the DC output of 21.875vdc (near as damn it to their stated 21V). My input is 1V higher and my output is a lowly 2.5VDC a whopping discrepancy as can be seen above in the scope trace of the bridge rectifiers input and output

Testing of Bridge rectifier diodes (Module 10)

In circuit tests of the diodes shows roughly .500v drop across all diodes.

I suspect the bridge, however in circuit testing is leaving me more confused than anything. Therefore nothing for it but to unsolder the rats nest of wires to module 10 so as to be able to gain access to remove the 4 BY127 diodes and test them. What's the betting they will be innocent, though I fail to see what else could be the cause other than a failing bridge. Happy to stand corrected if I'm missing something blindingly obvious to others looking in.

Those test would seem to suggest the diodes are OK and something else is afoot but replacement is the only sure way to rule them out. Could you possibly do the checks below before changing them.

Posted by: Jayceebee

Those test would seem to suggest the diodes are OK 

Hi John,

That's why I was confused as I also thought they appeared OK. I was mainly unsure about the reverse, changing readings charging up through caps.

Posted by: Jayceebee

 something else is afoot but replacement is the only sure way to rule them out. Could you possibly do the checks below before changing them.

I always seem to pick up kit that has obscure and confusing faults. Test results to your queries below.

** Ignore this post, see follow up post below **

If the resistance between the junction of D103/4 is varying between ground but not between point 120 then something very odd is going on. Check how 120 is grounded, maybe a loose eyelet or something?

Sorry John scrap my ramble above re: cct point 120 it was late.

So to clarify the four tests you sought:-

1. Resistance of TX winding between the cct trace points 105 - 106 is .48R
2. Resistance reading of Junction of d103/d104 cct trace point 119 to chassis, is varying  from 1M to 500R and all in between.
3. Continuity of cct trace point 119 to TS1 collector is confirmed as good.
4. Continuity of cct trace point 120 to chassis GND is confirmed as good.

BY127 are normally reliable but they CAN break down on load....I've had an obscure fault like that....

Hi Rich,

I'd normally agree if this were in a high load part of the circuit but all they are seeing is 17.5V input and a 21V output across diodes rated at 1250V and a forward current (average) of 1A .

They are so overrated for the function application and all the in circuit tests scream OK. I know I've got to bite the bullet and remove that nest of wiring ( a lot of work) but I'm almost certain it will be no further on afterwards. Just wish I could come up with a theory to pursue as to what would or could cause a bridge to behave in such a manner of 18.4VAC in, 2.5VDC out.

Hi Chris, I can't see properly from here but why the need to remove the board? I would just remove the four or five leads from the vicinity of the diodes and remove and replace. As none of the diodes are short could you not just tack four replacements on the print side as a test. If it restores the correct voltage then remove the new ones one by one until the low voltage reappears?

As suggested by John I tacked in 4 new 1N4007 diodes on the print side. The stabilised supply is now reaching the delay circuit which in turn means the protection circuit is now also working, it kicks in as it should at 45 seconds unlacing the head-drum from operation and powering down.

I monitored the AC in and DC out of the bridge on the scope. The AC into the bridge appears 3V reduced from the previous voltage but the DC output is almost spot on now. Equally the waveforms look horrid, I can only surmise the AC reduction and the crap waveform is due to the paralleling up of diodes.

This seems to fall smack into the Sherlock Holmes quote "When all else has been eliminated, whatever remains however improbable, must be the truth". So despite the bridge testing in circuit as OK, the bridge was behaving as faulty and it indeed was.

Now to tidy up the bridge, fault 1 & 2 resolved now onto faults 3&4.

One thing I used to do if parts were difficult to replace or the print side was difficult to access was to cut the old component in half and clear the lead-outs and then as neatly as possible (and as quickly as possible) solder the new component to the original leads. It isn't the best option but with care (and the fact that it is for your own use) it should be satisfactory.

Upon removing the old BY127's I found that indeed D102 and D104 had failed.

The pads were cleaned and the new 1n4007's installed.

I then powered up and scoped the bridge input and output. Again the AC is still lower (by 1.8V) at 15.7V than specified on the cct and what I was getting before changing or adding diodes (18.4V). The DC is much higher now at 23V but the AC waveform is still horrid not a good sinewave at all but significantly clipped.

This is how the AC waveform looked right at the very start when I suspected a problem with the bridge but hadn't done anything to it, as you can see the AC looks cleaner. No idea why this should not be OK now it almost looks like its full-wave zener clipped but its not.

Oh and the safety circuit is operating at 20 sec now not 40sec. Not a real problem as its only there if no transport keys are pressed to save the heads, so 20 secs makes no difference really.

Don't concern yourself about the AC input as it's unreglated, take the machine to three different locations and you will get three different results. As long as TS1 can output a constant 12v with minimal ripple you've no need to worry, anyway onwards and upwards.

I have to say I was going to have a little bet with you that after removing the tacked on diodes you would still have the correct output, it's happened with me several times before, maybe some sort of short term self healing due to heat from the iron? By the way I've never seen a Philips/Mullard BY127 that colour before, BY126 yes, also seen BYX--- in that style with a red body.