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Old 21st Jun 2019, 6:16 pm   #1
19Seventy7
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Default Amstrad TVR2

Hi,

I have an Amstrad TVR2 which is in the process of repairs. Inside the relay keeps clicking. I did a visual search first, and found a cracked ceramic capacitor, that got replaced. turn the set on, no luck.

I read on eserviceinfo that if the relay clicks to change a "10R 7watt" resistor in series with the bridge rectifier. I've done a search through the service manual and there doesn't appear to be a 10ohm 7 watt resistor listed.

Does anyone know what resistor it means?

Thank you
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Old 21st Jun 2019, 7:30 pm   #2
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Default Re: Amstrad TVR2

The only one I saw had dry joints on the crt base panel rgb transistors but this may be
useful;
https://www.americanradiohistory.com...on-1996-08.pdf
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Old 21st Jun 2019, 7:41 pm   #3
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Default Re: Amstrad TVR2

hi, the 10ohm 7watt resistor is the surge limiter on the ac side of the bridge rectifier
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Old 21st Jun 2019, 8:24 pm   #4
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Default Re: Amstrad TVR2

It will most probably be a white rectangular object, about 1cm by 1cm by 5cm long and made of unglazed porcelain. If in doubt, post pictures.

You will be able to test this resistor in circuit (but obviously NO POWER!) using a multimeter on its lowest resistance range, since its resistance will be much lower than any possible parallel path (if it's any good).
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Old 21st Jun 2019, 9:08 pm   #5
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Default Re: Amstrad TVR2

Quote:
Originally Posted by Restoration73 View Post
The only one I saw had dry joints on the crt base panel rgb transistors but this may be
useful;
https://www.americanradiohistory.com...on-1996-08.pdf
Hi,

Thank you for that link I think it’ll come in very handy as it shows the bridge rectifier section and i will need to work on the VCR so i’ll keep it in mind.
Thank you!

I’ll take a look tomorrow as i’m not home until then. I’ll report back whether i’ve found it or need some help.

Thank you for your help! really appreciated!
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Old 21st Jun 2019, 9:54 pm   #6
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Default Re: Amstrad TVR2

Hi, from memory the ceramic capacitor which your found cracked will have been part of the snubber network for the SMPS, it's highly likely this capacitor failing will have caused failure of the STK (I can't recall the number) SMPS IC and this in turn will make the relay click due to the protection circuitry.

It's been more than twenty years since I encountered one of these sets so my diagnosis may be incorrect

Andrew
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Old 21st Jun 2019, 11:00 pm   #7
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Default Re: Amstrad TVR2

I used to see these things from time to time, but I'm sure it was usually VCR problems. I seem to remember that the VCR was pretty much the same as the Amstrad VCR4600 and used a whole bag full of belts.

I have an old magazine from about 1986 that has a full page ad on the rear cover featuring Amstrad products. The TVR 2 is one of them. I'll try and dig it out.

Good luck. I think you'll need it if it's a switch mode power supply fault!

Cheers
Nick
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Old 22nd Jun 2019, 1:04 am   #8
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Default Re: Amstrad TVR2

First things first, remove the line o/p transistor and check for shorts, power up the set with a bulb across c-e. If it lights then try isolating the secondary loads off the line o/p transformer (RGB, frame, audio etc).

That will tell you if the PSU or line stage need further investigation.
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Old 23rd Jun 2019, 2:52 pm   #9
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Default Re: Amstrad TVR2

Hi,

Thank you for all the replies, and sorry for a late response

I have had a look inside the TV and I couldn't find a white unglazed 1 cm x 1cm x 5cm porcelain resistor, so I think it may have been changed before.

I looked on the schematic in the link Restoration73 sent. I found the only 7 watt resistor (R309) was 39kohms so isn't the one i'm looking for.

In the actual service manual I cannot find any reference to a 10ohm resistor. The closest (not in value) is 10 KOhm.

I'll take another look on the schematic and look for any STK chips and see if I can order a new one to try, it wont hurt to replace it any way, I wouldn't have thought.

Edit: Does STK7348 ring a bell?

The VCR in this is a Funai 1400 and is based of the VCR4600, as for the belts, those are a whole other issue, which i'll sort once I fix this relay issue.

If all else fails i'll give bens idea a go as i'd rather try the somewhat easier things before going the much harder way and potentially finding out that it was an easy fix otherwise. Sorry if this is a stupid question, but what do you mean by "c-e" I cant think what it could be.

Thank you all again for your help, I really appreciate it

Last edited by 19Seventy7; 23rd Jun 2019 at 3:01 pm.
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Old 23rd Jun 2019, 3:12 pm   #10
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Default Re: Amstrad TVR2

They are not that scary. C-E merely means check from the line transistor case down to deck (chassis). You should get drastically different resistances. The tv side of these were quite tame, this is only a guide line but the line transformer was also iffy on these. As was the line transistor.
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Old 23rd Jun 2019, 3:27 pm   #11
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Default Re: Amstrad TVR2

"C-E" just means collector to emitter. With power transistors, the collector is always the metal mounting flange or tab (also connected to the middle lead, if it's in a plastic package) and the base and emitter are the two (outside, if plastic) pins. They are always the same way around (not like plastic TO92 packages, which exist in all six possible variants!)
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Old 23rd Jun 2019, 3:39 pm   #12
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Default Re: Amstrad TVR2

Thank you for explaining that, I wouldn't have ever guessed that.

Just want to clear things up, does ben mean I remove the transistor, then I put the bulb where the transistor was, connected to where c is attached, and then to where e is attached, with the transistor removed?

Thanks for your help
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Old 23rd Jun 2019, 3:59 pm   #13
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Default Re: Amstrad TVR2

Quote:
Originally Posted by 19Seventy7 View Post
I have had a look inside the TV and I couldn't find a white unglazed 1 cm x 1cm x 5cm porcelain resistor, so I think it may have been changed before.

I looked on the schematic in the link Restoration73 sent. I found the only 7 watt resistor (R309) was 39kohms so isn't the one i'm looking for.
According to the schematic I'm looking at the bridge rectifiers surge limiter is circuit ref. R1501 and it's shown as a 8.2 ohm 5 watt resistor (cement type in the parts list)

Lawrence.
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Old 23rd Jun 2019, 4:05 pm   #14
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Default Re: Amstrad TVR2

Ah, I have seen that, many times as I think that is right next to the ceramic capacitor I had to replace, which would make sense.

I'll replace that and see what happens from there.

Thank you for your help
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Old 23rd Jun 2019, 4:13 pm   #15
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Default Re: Amstrad TVR2

dont want to insult your intelligence but make sure the ceramic cap you changed is an identical one, ISTR they're quite high voltage?
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Old 23rd Jun 2019, 4:22 pm   #16
19Seventy7
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Default Re: Amstrad TVR2

Not insulted at all, I did get the right one, 10nF, 1KV.

I made sure to consult the service manual before going any further

Thanks
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Old 23rd Jun 2019, 4:49 pm   #17
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Default Re: Amstrad TVR2

Test it in situ first! Any more than 9Ω, it's definitely high: replace it. Even if it looks about right, unsolder one end and test it again. If it reads less than 7.2Ω or more than 9Ω, it's already 10% out; replace it anyway. If it's between 7.2 and 9Ω, it is in tolerance and probably fine -- unless it is only misbehaving under power. Which is not altogether inconceivable, but it's still statistically more likely to be something else causing the fault. In which case, remember it, and consider replacing it if subsequent voltage measurements suggest R1501 could be faulty.

Are there any voltages marked on your circuit diagram? Has anyone else got a similar set in working order that they could go over with a multimeter?

Re-reading Ben's post, I think he means to remove the transistor (CRT line output transistors often contain an internal resistor and diode, so don't be too alarmed if it tests a bit funny with a multimeter) and connect a (presumably, tungsten filament mains voltage; no doubt Ben will be back around if I'm wrong there) light bulb to where the collector and emitter used to have been. This will complete the circuit through the primary of the transformer . If the HT supply is present and the primary winding is OK, then there should be enough voltage to get the filament to glow, although it probably won't reach full brightness. It needs to be a filament bulb because LED bulbs only work on alternating current (which sounds a bit unusual for something famous for only conducting in one direction; but they use a capacitor as a voltage dropper and then either a bridge rectifier around the LED string, or special LEDs having two diodes wired back-to-back in the same package so one or the other will always conduct and light) and compact fluorescents are much more fussy about power supply, refusing to light at all rather than glow only feebly. Don't leave the set in that state for much longer than it takes to get a result; it probably won't do it many favours having steady DC through the primary like that.

Obviously, be careful when testing anything powered up. Make sure everything is firmly in place and supported well enough for you to hold test probes in place without it moving. And if anybody in the house is using a computer, make sure they saved whatever they were doing -- if you do trip the RCD, believe me, it is much easier to elicit sympathy from someone who has not just lost two hours' worth of hard work!
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Old 23rd Jun 2019, 7:13 pm   #18
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Default Re: Amstrad TVR2

Just an observation, the IC number given in that servicing article link is wrong so far as I can make out, it should be MN14831 not MN14381.

Relay coil supply voltage (24volts) aside, for the relay to be on (as controlled by that IC) pin 8 has to be high.

If that's any use.

Lawrence.
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