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1977 Ferguson 3734 ColourStar: Thorn 9600 Chassis

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Katie Bush
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Just a quick word on lock-fit transistors....

I was talking to a chap last Friday, who said "Ah yes, those things - can suffer from crystal dendrites within the plastic package, between the legs/lead outs". It's not a common occurrence, but has caught him out once or twice where the equipment they were installed in were kept in damp/humid conditions (he's ex British Coal mines electronics engineer). It seems to happen if the legs are mechanically stressed during installation. Apparently, the flat face of a lock-fit can shear off completely to reveal the inner workings! They split at the mould flashing (the raised edge that can be seen around the face of the transistor).

Just a wild stab in the dark, but thinking, hasn't this set seen some damp?

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Posted : 13/06/2018 11:43 pm
crustytv
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Posted by: Jayceebee

Ooo, now that is a clue. I mentioned earlier about the small diodes connected to the zeners on the trip rail, are you absolutely sure about them, particularly W523? Might be worth replacing it to be absolutely certain. 

Could you check the voltage at the junction of W520,521 first please?

I'll recheck the zeners and report that voltage later today  ? 

 

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Topic starter Posted : 14/06/2018 6:58 am
crustytv
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Well I thought I had checked the zeners W520 & W521 before but I'm now I'm sure I didn't, I would have remembered the odd zener package for W520 which looks like a transistor. I do remember checking a couple of zeners out as I built my test rig for the purpose, but clearly not these two. 

w520

I removed W521 (BZX79-C5V6) and put it on my little zener test rig, it clamped perfectly at 5.6V. I then removed W520 (T9019W-01V4) this T0-92 package zener and here lies the problem. Its supposed to clamp at 1.4V and clearly as you can see below I stopped when I got beyond 22V.

I cannot find any info about the T9019W-01V4 zener, is it a special type? Why that package?

W520 1
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Topic starter Posted : 14/06/2018 9:24 am
Jayceebee
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Any device with a T as the first letter eg T9013V is a specially selected device for Thorn, there are several zeners that use that package such as tuning 33V stabilizers (which is an ic actually) so it's not unusual. At 1.4V it would have to very accurate and I'm not sure what to suggest as a replacement.

Let's hope you've  found the cause and it might be a good idea to fully test all the diodes in the trip circuit feed especially from the chopper emitter.

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Posted : 14/06/2018 1:19 pm
Nuvistor
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May be able to use 2 silicon diodes in series to give 1.2v.

 

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Posted : 14/06/2018 1:37 pm
crustytv
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Posted by: Jayceebee

there are several zeners that use that package such as tuning 33V stabilizers (which is an ic actually) so it's not unusual.

I guess I should have said unusual to me, remember I've not see as many sets pass my bench as you/those in the active trade.

What I should also have clarified is why that package? The set is full of normal (to me) diodes of varying values in what I consider the standard zener package, so why did they not chose a similar off the shelf packaged 1.4 zener for W520. I thought a zener is a zener, they come in various values and wattages so what is special about a T9019W.

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Topic starter Posted : 14/06/2018 2:26 pm
crustytv
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Posted by: Nuvistor

May be able to use 2 silicon diodes in series to give 1.2v.

 

Can you expand a little on that advice with an example?

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Topic starter Posted : 14/06/2018 2:27 pm
Sir Jules Ghost
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Some well intentioned comments -

1) "T9019W" is the device number and "01V4-765" is the Thorn part number. It is NOT a 1.4V Zener!

2) W520 forms part of the trip trigger feeds from the 34V supply, via D521 (5V6 Zener), and the Collector of VT803 (running nominally at approx 28V). Therefore, "if" W520 were to be a 1.4V device, if you then add that to the 5V6 (D521) you get a total of 7V being the conduction point of D520+D521. So, the trigger for over-voltage of the 34V line would be 7V! Obviously this cannot be the case, even leaving aside the 28V sitting on the Collector of VT803 that would also immediately trigger the trip if should W520 be a 1.4V device.

3) Therefore, we can take a guess at the conduction (Zener) point of W520 by looking at the circuit voltages that it is involved in monitoring. Let's say that we want our over-voltage trip for the 34 volt line to trigger before it reached a figure of 39V. We have D521 which already puts the start figure of the trip trigger point at, you guessed it, 5.6V, so if we also included a 33V Zener in series with that our trip would operate at 38.6V (or thereabouts in the real world). Use a 30V Zener as W520 and it would trigger the trip at 35.6V. So we can deduce that W520 should be in the region of 30 to 35V from the way the circuit is designed.

4) Now that you know this, you may want to test w520 at a higher voltage than the 22V you stopped at due to the part number confusion. I suspect there is a good chance it will test OK.

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Posted : 14/06/2018 2:37 pm
Nuvistor
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I have not looked the circuit of the 9600 so treat with caution.

The forward voltage drop of a silicon diode is approximately 0.6v so two in series would give a 1.2 volt drop,  very close to  the 1.4 v Zener. Like the Zener diode, suitable power and current rating must be used.

The technique was used in various types of circuit, one, not related to the Thorn set, was used to increase the output voltage of a 3 terminal 5 volt regulator such as a 7805. A diode placed in the common ground connection and forward biased would increase the regulated output by 0.6v , two series increase the output by 1.2 volts. Zener diodes were also used if a voltage was required that fell between the standard range of the 78xx series.

 

Hope that makes some sense.

edit, from Sir Jules post it seems my comment about silicon diodes does not help, however it may help at another time and well worth knowing.

 

 

 

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Posted : 14/06/2018 3:05 pm
crustytv
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Thanks Frank, makes sense now  ? 

 

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Topic starter Posted : 14/06/2018 3:10 pm
crustytv
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Well in light of the recent invaluable input from beyond the grave  ? it seems I misread the zener spec from the manual.

I must admit I did add the 5.6V and the 1.4V together this morning and thought what on earth is the 7V for! I didn't say anything as well, I thought I was looking stupid enough already. Well that was a bad move as its even worse now  ? 

I cant test beyond 30V as my bench PSU is at its max.. No doubt as Sir Jules points out it will be fine and I'm back to square one. I think I'm going to put the back on this one and admit defeat, each time I've tried to repair 9k's its been a total nightmare, I didn't enjoy the last one and I'm certainly not enjoying this one. At 23 pages I think enough is enough, let other threads have some air time.

Edit:

I put everything back in circuit VT803, VT814, W520, W521, W523 and C830. I thought whilst I'm at it I would test W528 the 27V zener on the test rig, this seemed a little high (my bench PSU tops out at 28V so can't push the zener further). I replaced it powered the set up and I had 26kV!!! However..... it was hunting, by that I mean going low (22kV) then high (25kV). The raster was reduced width and raggedy on both sides. The hunting continued, whilst this was going on I checked TSA/9 and peak fly-back was 647V the 80V rail was up to 73V. Then the 80V rail collapsed back to the 67V, EHT settled back at 20kV and I'm back to where I was.  At least I now know the part of the circuit that is causing the troubles, a matter of working out why the hunting and if/why it causes the demise of W528 or not. Reading up, W528 purpose is to put a +ve voltage onto the trip line if the 24V rail goes high but I won't bore folk any  longer with this painful 9k6 exploration as I'm sure you've all had enough.

trip 2
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Topic starter Posted : 14/06/2018 3:17 pm
PYE625
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Getting it down to the area of the circuit is half way there  ? 

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Posted : 14/06/2018 5:20 pm
Jayceebee
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Unfortunately I feel a bit responsible for you misreading W520 as a 1.4v zener, in my defence I was reading the posts on my phone whilst I was supposed to be working as this just has me totally gripped, unfortunately because of this the pictures are of low quality and cannot be resized so I wasn't able workout exactly where in the circuit it was.

Going back to the packaging of the device, that I can't answer, maybe something to do with auto insertion requirements in that area of the board or maybe it consists of multiple elements akin to an IC as in the 33V stabilizers for varicap tuning. Whatever it is as I said earlier Txxxx means a specially selected device.

A couple of things I will say is that with that last episode you seem to probing be in the right area and shows how much the the circuit is able to put up with. In a lot of chassis I've worked on the dead count of chopper transistors with even the slightest glitch in the conditions could have been quite significant by now.  Chris if you ever attempt an FV31R or FV32L VCR you won't see me for dust ? .

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Posted : 14/06/2018 8:45 pm
crustytv
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Posted by: Jayceebee

Could you check the voltage at the junction of W520,521 first please?

Sorry John, only just remembered this, the answer is 32.53V

 

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Topic starter Posted : 14/06/2018 9:24 pm
Jayceebee
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That's high and I suspect must be coming from VT803 via W521 and I'm not sure how relevant it is just that Mikes drops to 16V with the tripler disconnected. It's precariously close to causing W520 to conduct if it is in the region of a 33V device as our friendly ghost suggests. With the EHT facsimile voltage removed from the equation the voltage at the junction I would expect to be around 25V with what you have on the supplies at the moment. 

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Posted : 14/06/2018 10:31 pm
Jayceebee
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If I would be allowed to go slightly off topic here the BBC appeared to use a slight variation of this set in their studios. The set is very noticeable with it's concave front and appeared to have 12 or 16 channel touch tuning and a couple of extra slider controls. Can any of our broadcast members shed any light on these, I'm certain they must have had composite/component input also and if so how were they mains isolated, transformer?

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Posted : 14/06/2018 10:44 pm
jjl
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Chris

If W528 is really being killed, the most obvious cause would be excessive current flowing through it causing excessive dissipation. I suppose this would be possible if the W513/4 input to the trip regenerative switch  presented a low resistance to 0V.

If, for example, C608 in the trip circuit was shorted and say 30V was presented to the cathode of W528, then 30mA ( (30 - 27) / 100) would flow through R624 and hence W528, the dissipation in W528 would then be 27 * 0.03 = 0.81W which is way too high for a presumably 400mW zener.

I've no idea if this is really the cause, but it may give you something to go on. In the scenario above, the fault wouldn't need to be present for very long in order to damage the zener i.e. an intermittent fault could cause failure of W528.

 

John

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Posted : 15/06/2018 11:49 am
Jayceebee
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Hi Chris, I don't know if you're ready to revisit this yet but I think I have an idea of what might going on here and maybe we'll get down to the cause or at least solve one more issue.

As Jon mentioned earlier about checking the the fifth harmonic tuning adjustment and it's effect on the voltage at TSA/4 I now believe it may revolve around this. In light of removing the feed to the tripler causing the voltage to drop appreciably on Mikes set we need to fully investigate whats going on around VT803/814.

My theory as what maybe happening is this.

We know the coil is doing it's job as you observed the increase in A1 volts and EHT but didn't see a swing at TSA/4. Increasing the set EHT to the point of the picture being unstable is I suspect the causing the voltage at TSA/4 to increase to the point where W520 reaches it's breakdown point (thanks Sir Jules as I didn't know it's voltage) and cause it to bias on VT607, not enough to fully remove the charge on C608 via W611 causing a trip but just enough to prevent it charging any higher to open up mark/space ratio further. Not fully sure why the apparent instability but one thing at a time.
Measure the voltages at VT803/814 c,b,&e also W523 anode and cathode with the AC input to tripler disconnected and let's see whats going on. With it removed there should be no bias at VT814 base and so turned off, turning on VT803 consequently lowering the voltage at it's emitter which is connected to TSA/4. If you do see volts at VT814 base could you post a picture of the focus circuit on the CRT base in case there is some feed in point there.

I hope this all makes sense and my theory somewhere near correct. In your own time.

John.

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Posted : 17/06/2018 7:20 pm
crustytv
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Hi John,

right royal pain to get at those two, no way to get access from above. Had to position the tv precariously on the edge of the bench, extend the scanning module out to its max, get down on the floor and use a mag glass to find them.

VT803 e=16.7, b=16.1, c=16.5

VT814 e=16.1, b=6.9, c=33

9k6cct 5
9k6cct 6
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Topic starter Posted : 17/06/2018 8:38 pm
PYE625
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Posted by: Chris

 Had to position the tv precariously on the edge of the bench

Erm….I did that once with a Murphy. ?  ?

But the same won't happen to your set now will it Chris !  You are a bit more safety conscious than I used to be. 

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Posted : 17/06/2018 8:44 pm
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