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Old 7th May 2015, 4:46 am   #61
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Default Re: HMV 904 prewar television restoration

Yes, sorry, last week when the focus control broke again, I fixed that but got distracted trying to track down the last remaining picture fault - mains hum. I have not fixed it yet, but I think I've made progress in tracking it down to a specific circuit, after a lot of red herrings.

Anyway to start off from where I left it last week, the work improving the picture quality, and the unexpected reappearance of the focus control fault.

To tackle the focus control first. As mentioned in the last post, on stripping down the focus control pot again, I discovered evidence of arcing between the centre peg and the case. Now this is subtly different to the initial fault I fixed. Unfortunately to describe why it is necessary to describe the construction. The centre tap or peg is internally connected to the pot brush. This peg then sticks out of the case, via a hole in the case, and it is insulated from the case via paxolin. Once out of the case a large metal tag was rivotted to the peg, on which the connection terminal sits. The tag is insulated from the case via something like paxolin but more fibrous.

When the initial short to case via the centre tag was discovered (of the order of 1Kohm), I removed the metal tag and its insulator, and discovered the short went away. Puting a piece of plastic between the case and the original insulator proved to fix the fault. In otherwords there was no short evident between the peg and the paxolin insulation when it went through the case.

The focus pot subsequently shorted to the case through the paxolin. Now at this stage it is difficult to be sure of the sequence of events. One possibility is both insulators had deteriorated, and shorted though the weakest link, once that was fixed, the other insulation soon failed too. Another alternative is the plastic insulator introduced an air gap between itself and the paxolin, and arcing was though that. A third possibility is of course the short was always through the paxolin, but the dismantling temporarily fixed it, giving a false impression.

In any case, the question was what to do now. The first step was discount the idea it was symptom of another fault, which is why I asked about about focus coil resistances. Having got that out of the way, and given the above unknowns as to why it failed a second time, I decided my best approach was to completely remove all the existing insulation, and to replace it with a continuous block of insulation without any air gaps. Chemical metal came to the rescue here, and it enabled me to build up a replacement starting from the peg insulation as it goes through the case, and ending in a built up pad on which the metal tag sits. Photo 1 shows the result. To double check the result I tested it with an insulation tester @1000v, and it showed no insulation leakage.

Getting back to the previous restoration thread. Photo 2 shows the particularly bad picture example given previously. It was a major improvement but there were still a lot of issues. In particular folding at the left hand of the picture, an inability to pull the left hand side further to the left of the CRT leaving a gap (even with the width pot at max setting), line tearing, and generally noisy.

Strangely enough these issues once identified proved to be easy to fix. Which tends to show how sensitive these sets were to perfect operation.

The line tearing was the first issue I looked at. From the symptoms it looked largely a line timebase issue, random sync problems, rather than any signal problem. Given I had gone through the line timebase carefully, checking and replacing all faulty components, I was a little stumped as to what could be wrong. But on the basis the fault is likely in the remaining components, this left the valve or the valve base as suspects for the noise. I replaced the valve with a couple of spare replacements, no improvement. I waggled the valve experimentally in the base and the line tearing went away temporarily. This perhaps showed the valve base was faulty, or at least dirty (even though it had been cleaned once). I examined the valve base, no obvious faults, I gave it a really good clean, and to my surprise it fixed the problem. I then also gave the other valve bases another clean, as it looked like my first clean wasn't thorough enough.

The next thing I looked at was the problems with the left hand side of the picture. The folding proved to be trivially easy to fix once I discovered (looking through the book "Television picture faults" by John Cura and Leonard Stanley) this was a symptom of over damping in the line output circuit. Previously on finding too little damping in the circuit (on an oscilloscope trace), I had simply turned the "form" control too far and over compensated. This is a classic case where many problems become simple once you recognise the symptom, recognising the symptom is often the difficult bit.

This still left the inability to pull the picture fully to the left hand side of the CRT, even with width at max. Now I'd previously hit problems with frame cramping, this proved to be low HT voltage caused by an open circuit height pot. This didn't look like this, and in any case I'd already previously checked the resistances/voltages. But, the literature was full of references to the fact these sets were sensitive to line output valve emission (basically they needed all the emission you could get). It was worth doing a line output valve substitution test.

The line output valve is a KT63, and here we run into the problem these are used in valve amplifiers and it is almost impossible to get a NOS valve. I had 6 KT63s, two originally from the set, 1 NOS, and 3 used matched valves (sold as a unit). All tested at various grades of good, and I had the NOS valve in the line output stage. Doing a substitution test with the other 5 valves, showed a major effect on the line output, even with valves which tested roughly the same (on the valve tester). One refused to work at all, and It so happened the NOS valve was one of the worst performers in the line output stage. One of the original valves was stunning, and gave immense output. Selecting the best performing two for both the frame output valve and the line output valve gave the effect shown in Photo 3 with height/width pots at max. There was now more than enough frame/line output.

As a slight aside, this brought to mind something that Henry Holden said in his restoration of a HMV 904 (http://www.earlytelevision.org/holde...storation.html)

"In my set the height is only just adequate, with all voltages correct, how is yours? I'd bet it would have been better with a choke for the anode load in the frame opt tube rather than that 10K resistor! "

Well, I can't say I have any problems with the height or width

The other thing he says is regarding video bandwidth "The way they configured it is fairly restrictive on bandwidth, only about 1.8 to 2 MHz", and indeed the test card picture shown on that site doesn't reproduce the 2Mhz square (nor the 1.5Mhz square well either).

But the 2Mhz square is reproduced clearly on this set, and that's without any realignment.

The last thing was the noisy signal. Given I'd discovered valves can have a major effect, I embarked on a substitution test of the vision detector/IF valves. The vision detector substitution had no effect. But, I discovered the noise was due to one of the IF valves. Curiously I had bought a set of 3 military NOS KTZ41 valves, and one of those proved to be producing the noise, but all the military valves proved to be slightly noisy, and I replaced them with other commercial NOS valves. In fact I've not had a good success rate with NOS military valves, and I don't really know why that is.

Once all this had been done, it left the picture shown in the first post in this thread (Photo 4).

I still have one elusive problem, mains hum. But that's for another day.
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Old 8th Oct 2015, 4:03 am   #62
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Default Re: HMV 904 prewar television restoration

Hi Guys,

Sorry for the long time since the last post, this was never my intention, but the best laid plans often go awry.

When I last posted I thought I had almost finished the restoration, I had some annoying mains hum, and the long running issue than sound didn't work (not a squeak).

The mains hum (at 100 Hz) eventually proved to be external to the set, it was a burst of signal modulated onto the mains by some horrible ethernet over power devices. So that proved to be a red-herring.

The sound problem proved to be much more difficult, initially as the radio worked and vision worked, I hoped the problem was fairly simple (maybe a fault in the sound take off circuit in IF valve 2), but no. This turned into a kind of long running saga, every time I fixed something, something else proved to be broken or broke. Once I had sound working, and I connected up the CRT, I discovered the LOPT primary had gone open circuit. Once re-wround (Mike Barker did an excellant job as usual), I discovered I had sound on vision, and this wouldn't go away.

Everything is finally working I have uploaded some pictures. The first picture is the test card as of now. The second picture is the original test card before re-alignment and fault fixing. I think you can see the test card is much better, the overshoot (white after black ghosting and vice-versa) which is always a sign of misalignment (response curve peaking) has completely gone away.

The next three pictures are the pictures I took when I got the set. When I initially posted this thread I had lost these pictures and so had to make do with some later ones when some work had already been done. As usually happens I found them on a flash card which I put away for safe keeping

I have also uploaded a couple of videos to youtube. These show sound and video, the sound I think is of excellant quality, which is one of the things the clip mentions television programmes were known for. The clip is "Television is here again part 1", it is recorded in three parts because the Android phone I recorded it on crashed after 5 minutes (the marvels of modern technology).

https://www.youtube.com/channel/UCKJ...GINBGfcQEJXolA

Hopefully I'll write up tomorrow some of the problems I had with the sound.
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Old 8th Oct 2015, 4:18 am   #63
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Default Re: HMV 904 prewar television restoration

That's an amazing improvement in the picture, from photo 2 to photo 1! Well done And for so many things to go wrong, each one as you've just fixed another ..... It's all paid off in the end, though
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Old 8th Oct 2015, 8:59 am   #64
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Default Re: HMV 904 prewar television restoration

Wow! I just found your topic tonight, and I was reading through the posts, and I'm stunned! You did amazing work! I just joined the forum recently, but after reading Mark Pirate's topic and now yours, it's amazing how you guys are able to take these antique, rusted, beat-up televisions and restore them and get them to work again! In the picture you posted of how yours looked when you first got it, it would have seemed hopeless that you would ever even get it to power up again...nevermind actually getting an image on the tube!

I didn't even know television sets were available to the public in 1939. I thought they were still experimental then.

- Chris
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Old 8th Oct 2015, 9:58 am   #65
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Default Re: HMV 904 prewar television restoration

Hi Chris,

The BBC opened its television service in November 1936 and continued to broadcast a good range of programs until the commencement of WWII in September 1939 by which time 19,000 television sets had been sold. The same 405 line service continued after the war until the early 1980s.

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Old 9th Oct 2015, 1:20 am   #66
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Wow! I just found your topic tonight, and I was reading through the posts, and I'm stunned! You did amazing work!

I didn't even know television sets were available to the public in 1939. I thought they were still experimental then.
Welcome to the forum, and thank you for your kind words.

Most people don't know that because pre-war sets were so expensive they were only available to the wealthy metropolitan elite in London. The break-through in television adoption in the UK is often said to have been the coronation of Queen Elizabeth II in 1953.
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Old 9th Oct 2015, 7:05 am   #67
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With sound not working at all, and having no idea where the problem lay, I started testing methodically from the sound take off circuit and worked backwards through the stages.

The sound take-off circuit is a tuned circuit in the anode of IF stage 2 (V7), obviously tuned to the sound intermediate frequency of 4.5 MHz. Injecting a 4.5MHz modulated frequency there resulted in a good response on the speaker. Moving the sigal generator to the grid of V7 I got a surprise, the signal was now weaker. Moving back to the grid of IF stage 2 I got nothing at 4.5 MHz but a weak signal at 5 MHz, the same at IF stage 1.

OK, this looks like misalignment in the IF stages. What about the HF and frequency changer stages, are they working? Given the IF stages seem to be working at the (wrong) audio IF of 5MHz it's not surprising audio wasn't present using the Aurora box. But we should be able to inject a modulated HF frequency into the aerial which will result in a 5MHz audio IF frequency (given the sound carrier of 41.5MHz results in a IF of 4.5, 42 MHz should give 5MHz, with the local oscillator at 37MHz). But 42MHz gave nothing.

Hmm, lets move over a much larger range of input, from ~65MHz to ~30MHz, given 2nd/3rd etc.order harmonics from the mixer stage, I expected to find lots of frequencies generating a 5MHz IF frequency, but almost total silence. There was only one weak signal at 35MHz

So the IF stages appeared to be misaligned, and the local oscillator seemed to running at 30MHz ... Not good.

At this time I began to be hampered by my test equipment, which had always been adequate for radio work, but couldn't handle the frequencies for television. I would have liked to hook up the oscilloscope and inspect the local oscillator frequency directly and to verify 41.5MHz was indeed giving an IF frequency of 11.5MHz (41.5 - 30), but my old 10MHz Scopex oscilloscope obviously couldn't handle it. I did the next best thing and injected frequencies into the aerial which should give an IF of < 10MHz, and confirmed the local oscillator did indeed seem to be running at 30MHz.

The next thing I decided to do was to buy a second hand 60MHz scope. Deciding to delay checking the alignment until that arrived, the next thing was to strip down the local oscillator stage and check the components.

Going through the circuit I unexpectedly discovered 2 of the low value mica capacitors were way out of spec. The 65pF capacitor across the local oscillator coil was 3 times over value, and a 0.0023uF bypass capacitor was open-circuit. Could that explain the oscillator running at the wrong value?

Replacement was obviously the next step, but here I hit a problem, you can't restuff mica capacitors! The only modern components I could find were day-glo yellow poly caps. They looked horrible. I finally found someone abroad selling NOS mica capacitors of identical look for the 65 pF capacitor, and repaired the other mica capacitor (under a magnifying glass it became apparent the failure was the leaves making up the capacitor had become unstuck from the leg due to corrosion, a clean and a blob of solder fixed it). All I then needed to do was wait for the weeks the NOS capacitors took to arrive.

The new 60MHz scope duely arrived, and I tested the individual alignments of each of the IF stages (by injecting a signal at the grid and testing the signal at the grid of the next stage). Here I wanted to double check the stages were misaligned (rather than any other fault giving the appearance of misalignment) before realignment, and I was rather curious to discover how they were aligned for video too (there was a mystery how a local oscillator running at 30MHz was working for video). Of course it was now easy to directly measure the local oscillator frequency and confirm it at 30MHz.

I got two responses for each stage, corresponding to the individual tuning for video and audio. IF stage 1 4.9 - 5MHz and 3.3 - 3.9MHz, IF stage 2 3.8 - 3.9 MHz and 5.5 - 6.5MHz, IF stage 3 4.9 - 5.0MHz and 6.5 - 8.5 MHz.

The ranges of ~100KHz are obviously the audio tuning, and the wider responses the video.

The first thing you'll notice is they're completely all over the place, and wrong. Once of the audio tunings is down at the 3.8MHz range and so too is one of the video tunings ... Of course none of the ranges overlap with the sound IF of 4.5MHz which is why it ddn't get through, and a signal barely gets through at 5MHz. The video tuning is mostly within the range 5.5MHz - 8.5MHz, when it should be centred around the vision IF of 8MHz.

For a while the video tuning had me stumped and I firstly attributed it to random twiddling of the coils. Then I discovered in 1956 Crystal Palace replaced Alexandra Palace transmitting on vestigial side band (lower side band), rather than the double side band transmissions of Alexandra Palace. So it looks like someone tried to realign the set to vestigal side band working and made a mess of it. It also suggests the set was in use as late as 1956.

Satisfied the problems were due to misalignment, the next stage was to realign. As the Aurora box is double side band it made sense to return it to the original alignment given in the service manual.

I realigned the set and it didn't go too well The last step in the alignment procedure is a simple sanity test to check the frequency response is correct/adequate. Tune the signal generator to 8MHz (centre frequency), and note the output level on the test ammeter in the audio detector anode without signal, increase the signal generator input to get an increase of 3.0mA and note that value. Then increase the signal generator input by twice, and retune to 7.5MHz and 8.5MHz and ensure the output level is at least that you previously noted ...

Of course I couldn't get that... Now did I make a mistake in the alignment procedure, is there an undected fault in the IF stages or something wrong with the signal generator (considering it didn't have the capacity to increase the output by 3.0mA and then double the input signal). Turns out all three.

But I've run out of time.
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Old 22nd Oct 2015, 5:57 am   #68
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When I left off, I had discovered faults in the local oscillator stage (which I was waiting for NOS components to arrive), and I'd done a none too successful realignment of the IF stages (having discovered they were badly misaligned).

My next step was obviously to discover what had gone wrong with the realignment. Going back through the alignment process in my mind, I decided there was some odd about the tuning of the tuned circuit in IF stage 3, the other stages showed definite peaking at the input frequency, in contrast the tuning in the last stage was somewhat flat. Could this explain the lack of gain? Testing the voltages on the valve in stage 3, I discovered the screen voltage was far too low. Resistors gone high, or the valve drawing too much screen current? I pulled out the valve, screen voltage OK, put the valve back in and this time checked the cathode voltage, again too high. So the valve was drawing too much current and it wasn't amplifying. When I pulled out the valve to my annoyance I discovered it was one of a batch of NOS military KTZ41's which the others had already proved bad. This was was still in the set because IF stage 3 isn't involved in sound amplification which I'd been working on previously. I replaced the valve with a good used valve, retested the voltages, all OK.

My failure to realign also made me want to double check my previously trusty Heath Kit signal generator, which so far had never given me any problems on radio work. Was the output level adequate, was it accurate? Hooking it up to the oscilloscope, I discovered the output level was adequate, but it varied depending on the frequency, and the output frequency was randomly +- 5 – 10% off the dial indication. Googling obviously brought up the fact this is to be expected on basic hobbyist equipment, and you get what you pay for. Given the alignment called for quite strict tolerances, alignment to 7.3, 8.0 and 8.7 MHz, this wasn't good.

So next I decided to buy a second hand but calibrated Marconi TF2016A signal generator and Racal frequency counter, and obviously to delay the next alignment until they arrived. Incidentally just starting off in television work, it would have been useful to have found a thread describing the obvious pitfalls, and maybe recommends for good second hand equipment. I'm sure I'm not the only one that has discovered they needed to buy a whole load of better test equipment. To my shame before I bought the Marconi signal generator, I bought a new Digimess SG100 and found it wasn't much better than the Heath Kit.

Whilst waiting for the equipment to arrive, the replacement NOS mica capacitors for the local oscillator stage arrived. Duly replaced, I re-tested the frequency changer behaviour. The replacement capacitors as expected brought in much more RF frequencies that generated the local IF frequencies (now at 4.5MHz for sound due to the realignment), rather than the single frequency previously seen, and the local oscillator frequency shifted as expected too. In fact it now behaved as I expected, except for one snag. The local oscillator shifted from 30MHz to ~33MHz, closer to the correct 37MHZ, but still not correct.

I unclamped the local oscillator trimmer, and checked what range it could trim the circuit, +- 1MHz, not enough, so there was clearly still something wrong with the circuit. But I'd checked everything.

In the best tradition of Sherlock Holmes, once you've eliminated everything else, the one remaining thing must be the culprit. I had checked and replaced/repaired everything wrong. The local oscillator coil, as I mentioned in a previous post, had been damaged, I had repaired it and checked the coil spacing against an undamaged coil in an identical Marconi 706 television. But could the damage despite the repair have changed its value? In fact could the coils be variable anyway. That was the only thing left that could be wrong.

I experimentally squeezed the coil “tighter” a couple of millimetres, keeping the spacing between the windings the same. This reduced the local oscillator frequency. Stretching the coil brought the frequency up to ~36.5MHz, at which point I could use the trimmer to get the final 37MHz frequency.

It did cross my mind to wonder if this was why the coil had been “damaged” in the first place. The supposed damage was actually to fix it. But comparing original photographs of the coil when getting the set it doesn't look to be the case. The coil on getting the set had been massively compressed, and it was the compression that had caused the damage.

So now the local oscillator/mixer should be finally working, and a sound IF frequency of 45MHz on the aerial should produce something on the speaker. Connecting the Aurora box up to the set, and yes, sound at last.

Next step to do a better alignment of the IF circuits.
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Old 22nd Oct 2015, 10:58 am   #69
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I can't think how, but I have only just spotted this very interesting thread. Firstly, may I say congratulations on such a thorough write-up and on the methodical restoration that you are doing. Excellent!
It is also very good timing for me as I have just bought an extremely rusty Marconi 707 for my collection. I already had a 706 so I now have the pair. Did you find that the dial cord pulleys had rusted solid and are the brass tags on the tag strips and pots OK? Mine have dezincified to the extent that just moving the wires attached to the causes them to snap?
Your thread has pre-warned me of the types of problems that I will almost certainly encounter and should help me a lot. Thanks
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Old 22nd Oct 2015, 2:12 pm   #70
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Default Re: HMV 904 prewar television restoration

Same here Brian! Lord only knows how I missed it too.
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Old 23rd Oct 2015, 4:24 am   #71
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I can't think how, but I have only just spotted this very interesting thread. Firstly, may I say congratulations on such a thorough write-up and on the methodical restoration that you are doing. Excellent!
Thank-you, I'm glad the posts are being found interesting

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It is also very good timing for me as I have just bought an extremely rusty Marconi 707 for my collection. I already had a 706 so I now have the pair.
Is this a new find? If so it is nice to know new finds are still comming up. The rate of new finds do seem to have been decreasing over the last year or so? (or maybe I'm looking in the wrong places ). A 7" 707/905 would be very nice.

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Did you find that the dial cord pulleys had rusted solid
Yes, everyone completely solid.

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and are the brass tags on the tag strips and pots OK? Mine have dezincified to the extent that just moving the wires attached to the causes them to snap?
Yes! I found an annoying tendency for some of the tags on the pots ontop of the chassis to snap off with just movement of the pot/wires, or be rather delicate. If you look at one of the photos on post #49, the left most tag of the right pot snapped off simply removing it from the mount point (prior to de-soldering). I just thought I was being clumsy, as I'd never come across the concept of dezincification, but googling it suggests this could well be what happened.

There is a curious feature of the set, which the photos may show. The top of the chassis was incredibly rusted and corroded, but the underside of the chassis was a different story, much less rust and corrosion. So the tag strips on the underside of the chassis are generally OK.
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Old 23rd Oct 2015, 4:29 am   #72
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So now the local oscillator/mixer should be finally working, and a sound IF frequency of 45MHz on the aerial should produce something on the speaker. Connecting the Aurora box up to the set, and yes, sound at last..
Obvious typo, I meant a sound RF frequency of 41.5MHz, not an IF frequency of 45MHz.
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Old 23rd Oct 2015, 12:08 pm   #73
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There is a curious feature of the set, which the photos may show. The top of the chassis was incredibly rusted and corroded, but the underside of the chassis was a different story, much less rust and corrosion. So the tag strips on the underside of the chassis are generally OK.
I'm pretty sure that this is due to condensation when the water on top of the horizontal surfaces hangs around and runs down the sides causing the rust to form. Underneath is a different matter unless it's the top of pots etc. The underside tag strips on mine are shot - this may be caused by a polluted atmosphere during its storage years. I'm researching into having a thousand or so tags specially made. It all depends on cost, of course.
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Old 24th Oct 2015, 1:22 pm   #74
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As a followup on the tag search, I have asked a couple of pressings companies for a quote based on this drawing:
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Old 24th Oct 2015, 2:04 pm   #75
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I suspect the tooling costs will be very high.

It might be cheaper to have the brass laser or water jet cut and do the bending yourself.
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Old 24th Oct 2015, 4:02 pm   #76
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We use RB punching http://www.rbpunchingservices.co.uk/ at work for lots of short run parts.
 
Old 24th Oct 2015, 6:22 pm   #77
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Thanks guys. I will follow up both those suggestions.
I actually stated in my inquiry that flat parts would be acceptable if cost was saved.
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Old 27th Oct 2015, 5:35 pm   #78
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Default Re: HMV 904 prewar television restoration

I've had success with the eyelet tag search. I have ordered 1000pcs (moq) from "The Hampton Works (stampings) Ltd." who gave me a reasonable price with a very low tooling charge. It does mean now that if anyone needs to replace broken ones in their valuable pre-war TVs, there is a source of the necessary parts. I will have some in about 4 weeks time.
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Old 27th Oct 2015, 5:45 pm   #79
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Default Re: HMV 904 prewar television restoration

Nice to know there are some proper manufacturing companies still in existence. Do they punch them or use water/laser cutting?
 
Old 27th Oct 2015, 6:05 pm   #80
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Default Re: HMV 904 prewar television restoration

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Originally Posted by brianc View Post
I've had success with the eyelet tag search. I have ordered 1000pcs (moq) from "The Hampton Works (stampings) Ltd." who gave me a reasonable price with a very low tooling charge. It does mean now that if anyone needs to replace broken ones in their valuable pre-war TVs, there is a source of the necessary parts. I will have some in about 4 weeks time.
Well done Brian.

Make sure the tooling doesn't get scrapped when the job is completed. Maybe take into your own safe custody?

I've known firms who've had their tooling stolen or have "gone under" and had the receivers sell everything to the scrap man.
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