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1974 22" Ultra 6714: Thorn 3500 chassis

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Red_to_Black
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To get 6.2V at the wiper of R504, assuming about 18V at the top of R502 (1.2K), your total combined resistance including the pot in series with the paralleled R524 and X501 must equal around 600 ohms, so unless you have a cracked track or the bottom end is floating /hi resistance in some way you should be able to achieve about 6 V at the wiper.

What is the resistance of X501 ? or even X501 and R524 in parallel ?

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Posted : 13/09/2016 11:28 pm
Cathovisor
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Okay.

Looking at the diagram, it infers that the voltage at the junction of R501/R502 is 18V. That means that if X501 were o/c, the current through the chain of R502, R504, R524 and R505 is 8mA: because it's (18/2250).

So, assuming no current out of the wiper (there will be, but for now we'll ignore it) that means that .008 x 800 = 6.4V at the bottom of the pot R504. More than you need.

But R524 is shunted by a thermistor so the voltage at the bottom of the track of R524 will be lower (let's say the value of X501 = R524) which would make the resistance at the bottom of the track of R504 (330 + 235) = 565 Ohms, and with 8mA flowing that puts 4.5V at the bottom. In reality, there'll be slightly less current here because there'll be current flowing out of the wiper into the reactance transistor.

R502's not been checked yet...

 

(Edit - I see R2B's said much the same as me!)

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Posted : 13/09/2016 11:37 pm
crustytv
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X501 ( VA1039) tested at 490R and is OK as the data sheet states 500R a 25C and I tested it against tow in stock whic read the same.

R502 is 1.3K instead of 1.2K but within the 10% tol

R524 is 543R instead of 470R which is a little 26R out of tol

ccct5.jpg

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Topic starter Posted : 14/09/2016 12:33 am
Jayceebee
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Just back home from Ireland and it looks like I may have led you on a merry go round, I thought that in post 20 you had achieved 6.2v at the wiper. I now see that this is what you were aiming for. I'd also presumed C508 had been replaced but it's not marked in post 43.

John.

Edit. post crossed with Chris, Is R505 within tolerance?

Re the 180v line pulse, what is the resistance from the top of R506 to chassis? 

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Posted : 14/09/2016 12:34 am
crustytv
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Jayceebee said

Re the 180v line pulse, what is the resistance from the top of R506 to chassis?   

Using post (R) the LTB earth point as the ground reference R506 measures 4.7K one end and .3R the other. I'm a little confused as to what you mean by top end.

ltb1.jpg

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Topic starter Posted : 14/09/2016 1:06 am
crustytv
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Jayceebee said
I'd also presumed C508 had been replaced but it's not marked in post 43.

Oh Bum!

I'd completely over looked that blighter, fixated on the resistors over in the discriminator and missed the flamin obvious one bash_gifThis is what has happened to me time and time again.... blinkers.

So C508 the electrolytic .......connected to R504........

Out of value!!!!

High ESR!!!!! 

Leakage!!!!! 

well that's really not going to help is it!!!waa_gifembarassed Can I erase all the last posts where I'm chasing my tail. Never mind I always said my threads are warts-N-all.

Well its late now I will slope off to bed to return tomorrow morning with a new C508 and I suspect the ability to set up the Hoz osc. non_gif

c508.jpg

ccct6-1.jpg

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Topic starter Posted : 14/09/2016 1:42 am
Jayceebee
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chris said , I'm a little confused as to what you mean by top end.

The upper most end as shown on the circuit diagram which is connected to pin B of the EHT transformer. Measuring here to chassis shows if the winding is OK and 0.2 ohms shows you have continuity.

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Posted : 14/09/2016 8:40 pm
Red_to_Black
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So has replacing the cap cured it then ?

 

Edit: Yay able to post again Chris smile , cheersthumb_gif

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Posted : 14/09/2016 10:01 pm
crustytv
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Red_to_Black said
So has replacing the cap cured it then ?

Dunno not felt myself today so have not been near the workshop at all, probably the shock of being so stoooopid to have missed the cap. doh_gif

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Topic starter Posted : 14/09/2016 10:08 pm
Red_to_Black
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These things happen to all of us from time to time, I would bet a lot of us have done far worse things at one time or other. electro_gif.

Better to just forget it and move on.

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Posted : 14/09/2016 10:24 pm
Cathovisor
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chris said

Red_to_Black said
So has replacing the cap cured it then ?

Dunno not felt myself today so have not been near the workshop at all, probably the shock of being so stoooopid to have missed the cap. doh_gif  

For what it's worth, I'm not convinced that changing that cap will cure your problems, but naturally I'm prepared to be proven wrong!

Here's a tip suggested by a mate of mine: get 4x AA or AAA batteries (I'd prefer HP7 or HP16 myself, but...) in a holder, lift one end of R502 and apply the 6V it supplies between the junction of R502/R504 and ground. Then see if you can tweak the LO down to 15625Hz or less. That'll prove the LO can be driven.

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Posted : 14/09/2016 10:27 pm
Jayceebee
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Cathovisor said

For what it's worth, I'm not convinced that changing that cap will cure your problems

Me too, the ESR is not too bad for a 40 year old capaicitor but we won't know for sure until you try it. Looking back at previous 3000 LTB posts the core of L501 appears to be just a turn or two in from being flush with the top of the former. Here's a thought, is the core of L501 the correct length ie could it have been broken sometime in the past and just a remnant been put back in? As you can tell clutching at straws now.

John.

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Posted : 14/09/2016 10:55 pm
crustytv
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Couldn't be bothered to reinstall the PSU and LTB back into the set at this late hour so did the testing on the jig.

It is now working, R504/L501 adjustment, as you can see period of 64us and Freq of 15.625kHz

Back into the set tomorrow and tackle perform R504 set-up, then tackle the video fault which is likely the thick film.

finally.jpg

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Topic starter Posted : 15/09/2016 12:45 am
Cathovisor
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Okay... so what did you do? duno_gif

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Posted : 15/09/2016 4:39 am
crustytv
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You're pulling my leg? Replaced C508 as discussed in post #46

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Topic starter Posted : 15/09/2016 8:11 am
Cathovisor
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No, not pulling your leg. Genuinely surprised it was that decoupling cap, but I shouldn't be... 

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Posted : 15/09/2016 8:22 am
Focus Diode
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Really admire this hard work. I was surprised they still made 3000 series receivers with 4-push button mechanical tuners seeing our rented Ultra had a 6 button varicap type. This was installed in August 1973.

Can't find any reference to this model anywhere, the control panel layout is similar to the later 9000 series in your brochures with the slider controls below the 6-button bank. There was a U logo just under the top p/b on-off switch.

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Posted : 15/09/2016 11:52 am
crustytv
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I need to find a WTF smiley.

So I install the modules back in the set, go through the R504 set-up routine, can I get lock, well float? No

So changing C508 allowed me to get the frequency on L501/R504 on the jig, where I could not prior to its replacement. I assumed as this was now achievable it was just a matter of setting it up via the book. Short the flywheel, set R504 for 6.2V, adjust L501 for float, remove flywheel short, job done.

Now I'm at a loss duno_gif

wtf.jpg

Scoped the oscillator and the best it can get is 15.93kHz

wtf2.jpg

wtf3.jpg

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Topic starter Posted : 15/09/2016 2:09 pm
crustytv
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Left the set running ( about 1 hour) whilst I swotted the circuit. Happened to glance up at the scope and noticed the frequency of the oscillator output had dropped to 15,625kHz. I Looked at the screen but not floating picture, odd. Unclipped the flywheel short and I had line lock.duno_gif

I'm going to switch off and let the set cool off for an hour, I will the try again. If its OK god knows if its unlocked again then maybe its a thermal thing, but I would expect it to be the other way around.

15625.jpglock.jpg

p.a. C518 flyback tuning cap is still cold to the touch

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Topic starter Posted : 15/09/2016 3:03 pm
malcscott
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Looking good Chris.

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Posted : 15/09/2016 4:07 pm
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