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Old 11th Aug 2019, 12:06 am   #1
em536716
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Default HMV Thorn 1500 Low audio and no video!

Hi all,

Been a year since I posted last. I was given a HMV branded 21inch (or bigger perhaps!) thorn 1500 styled tv. I thought this would be a fun job to commit to since the first 1500 I restored for myself was mainly quite easy... I was wrong.

The set was presented to me with a frame fault, which turned out to be an open section on the mains dropper (HT feed to frame timebase). Then the odd fault of the brightness control. It would magically fade out all of a sudden and it was quite touchy. I tested the pot first and all seemed fine. Both out and in circuit, and letís just say after lots of swearing and tired eyes over the diagram, I went back to the potentiometer, took it apart and found out all the carbon has disintegrated and now was magically testing open.

With the new pot installed I suddenly lost video, with only faint audio from my VCR. Iíve checked the video amp transistor voltages and all seems fine, other than a little low LT feed to it. I also checked VT 4/5/6, all seemed fine other than the emitter voltages which seemed way to high on all of them. I checked the resistors between emitters to ground and all seem fine. I also swapped out the original DC caps along along with the others.

Bob the bodger has been on here before me and it seems itís had some work done to it before. I dislike PCB designs, well early ones, as theyíre so prone to W falling apart!

Iíve verified that video amp is working by connecting a .1 cap between pin5 of V1 and collector then base of VT9/8/7, and I get a form of black bars on screen. This proves at lead that the video amp is amplifying!

Does anyone have any advice on what to do/check? The CRT seemed lovely and bright. Iíd love to get it going!
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Old 11th Aug 2019, 12:04 pm   #2
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Default Re: HVM Thorn 1500 Low audio and no video!

One thing that stands out is VT6 emitter. It is connected to earth/ground/chassis, and can't be high unless there is a break in the print somewhere. Also, those lockfits are reputedly unreliable. Are there any black Callins capacitors still resident?
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Old 11th Aug 2019, 1:55 pm   #3
em536716
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Default Re: HVM Thorn 1500 Low audio and no video!

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Originally Posted by AC/HL View Post
One thing that stands out is VT6 emitter. It is connected to earth/ground/chassis, and can't be high unless there is a break in the print somewhere. Also, those lockfits are reputedly unreliable. Are there any black Callins capacitors still resident?
Thanks for the reply.

No there isnít. The video amp transistor does get a little hot, Iíve put a new one in its place with a heat sink, just in case.

Iím hoping since Iím getting weak audio, this is just a faulty transistor. Whatís a good modern replacement I could use to replace the IF transistors with? I assume problem is with these as Iím getting very weak audio and no video, yet i know the video amp transistor is working fine.

Iíve touched up the solder joints and thereís good ground connections all around.
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Old 11th Aug 2019, 3:10 pm   #4
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Default Re: HMV Thorn 1500 Low audio and no video!

Something rings a bell as a stock fault on these. The audio won,t be from, say a modulator source. But a foreign station, or lots of them at once, and of course non tunable'
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Old 11th Aug 2019, 3:45 pm   #5
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Default Re: HMV Thorn 1500 Low audio and no video!

VT7 in the IF and VT8 video pre-amp both BF197 types used to give trouble, measure between the base and emitter. Anything wide of 0.6v would indicate a faulty device.

John.
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Old 13th Aug 2019, 12:38 pm   #6
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Default Re: HMV Thorn 1500 Low audio and no video!

Have you checked the 1.7 or 1.5 K dropper section? If it goes open the L.T voltage goes sky high then when one or two transistors fail they can short out the LT line to bring the voltage back down and mask the original cause.
This was a fairly common fault.
It's worth checking.
Rich.
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Old 14th Aug 2019, 4:35 am   #7
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Default Re: HMV Thorn 1500 Low audio and no video!

Hiya Rich, Don't remember that one, what would have been wrong with a zener for protection. I think they were called avalanche diodes. Sets of just a few years later used them but I think they were used when the 1500 was introduced.
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Old 14th Aug 2019, 8:40 am   #8
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Default Re: HMV Thorn 1500 Low audio and no video!

I remember having fun & games with that section of dropper. It's at the bottom of the heater chain I think, and it's there to develop a low voltage from which the transistor stages run. It goes o/c and allows the voltage to rise. The exact fault symptoms depend on which components pop!
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Old 14th Aug 2019, 8:47 am   #9
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Default Re: HMV Thorn 1500 Low audio and no video!

Yes they could have easily provided protection but I suppose every penny pinched was a penny saved...
I think usually the transistors went open circuit I can remember having to change all of them in most cases . Very occasionally some would survive probably "protected" by one going short and reducing the LT voltage...

The firm I worked for had a lot of 1500s and ran them into the ground. The boss could see the end of black and white, certainly as far as rental was concerned so he didn't invest in any new sets, the old single standard hybrids were kept going with Unitra Tubes and Pinnacle valves . They still had lots out on rental when I left towards the end of 1980.

Rich.
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Old 14th Aug 2019, 10:19 am   #10
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Default Re: HMV Thorn 1500 Low audio and no video!

Quote:
Originally Posted by Andrew2 View Post
I remember having fun & games with that section of dropper. It's at the bottom of the heater chain I think, and it's there to develop a low voltage from which the transistor stages run. It goes o/c and allows the voltage to rise. The exact fault symptoms depend on which components pop!
Me and my Dad rescued one (a 24" plastic cabinet version with the square tuner buttons with the screw slot for tuning) from a skip and sure enough, this section of dropper had gone open circuit, the symptoms being faint shortwave radio sound and a light grey raster. In my case though, just one transistor had gone - I forget which. The only other fix was gluing one of the CRT mounts with some special plastic melting type of glue.
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Old 14th Aug 2019, 1:13 pm   #11
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Default Re: HMV Thorn 1500 Low audio and no video!

Just for info, 1500 chassis supply arrangements:

https://americanradiohistory.com/hd2...earch=%22thorn 1500%22

Lawrence.
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Old 14th Aug 2019, 1:38 pm   #12
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Default Re: HMV Thorn 1500 Low audio and no video!

Quote:
Originally Posted by slidertogrid View Post
Yes they could have easily provided protection but I suppose every penny pinched was a penny saved...
I think usually the transistors went open circuit I can remember having to change all of them in most cases . Very occasionally some would survive probably "protected" by one going short and reducing the LT voltage...

The firm I worked for had a lot of 1500s and ran them into the ground. The boss could see the end of black and white, certainly as far as rental was concerned so he didn't invest in any new sets, the old single standard hybrids were kept going with Unitra Tubes and Pinnacle valves . They still had lots out on rental when I left towards the end of 1980.

Rich.
Some companies were still renting large B&W sets into the 1990s.

https://www.vintage-radio.net/forum/...33&postcount=4
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Old 15th Aug 2019, 2:02 pm   #13
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Default Re: HMV Thorn 1500 Low audio and no video!

Radio Rentals recalled and crushed their 1615 solid state sets because of a slight fire risk (dry joints on the linearity coil and scan coil plug) and replaced them with trusty old 1500s. To be honest, they were a better set anyway.
With regard to the OP, it's a good idea to change all the lockfits in the IF strip even if the dropper's OK.
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Old 15th Aug 2019, 10:58 pm   #14
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Default Re: HMV Thorn 1500 Low audio and no video!

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Originally Posted by Welsh Anorak View Post
Radio Rentals recalled and crushed their 1615 solid state sets because of a slight fire risk (dry joints on the linearity coil and scan coil plug) and replaced them with trusty old 1500s. To be honest, they were a better set anyway.
With regard to the OP, it's a good idea to change all the lockfits in the IF strip even if the dropper's OK.
I did wonder how long 1500s stayed in rental use after being reconditioned to replace the 1600s.

Certainly there seems to be more of them surviving compared to 1600s, but I imagine a fair amount were private purchases.
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Old 15th Aug 2019, 11:11 pm   #15
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Default Re: HMV Thorn 1500 Low audio and no video!

Was Thorn the last maker of UK large screen BW televisions?
We did sell a few 1500 sets because we couldnít get any other UK made large screen in the middle 70ís. Those sets had the Unitra CRTís, not in my opinion the best but we had to use what was available. I didnít repair a lot of 1500ís but what I did they were always straightforward.
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Old 15th Aug 2019, 11:18 pm   #16
Richard_FM
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Default Re: HMV Thorn 1500 Low audio and no video!

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Originally Posted by Nuvistor View Post
Was Thorn the last maker of UK large screen BW televisions?
We did sell a few 1500 sets because we couldnít get any other UK made large screen in the middle 70ís. Those sets had the Unitra CRTís, not in my opinion the best but we had to use what was available. I didnít repair a lot of 1500ís but what I did they were always straightforward.
From what I remember on previous threads Ferguson was probably the last British maker of large screen BW sets, but Portuguese made Pyes were made well into the 1980s.
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