HMV1902 - CRT Substitution, and restoration tips - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 6th Sep 2007, 10:26 pm

The TA10 and TA15 aluminised CRTs were fitted to 1948 models 1805 and 1806 and derived models like the 1902 and 1851. The EHT was supplied by the EHT transformer only, no EHT boosters are required.

In the case of earlier models like the 1804 and VT50. The original pre-war designed CRT was replaced by the TA10 and an EHT booster device was recommended. The timebases required some modifications to increase power to accomodate the higher EHT.
Early fifteen inch models would use the TA15 as the replacement CRT along with the EHT and TB modifications.

My Marconi VT50 is fitted with the TA10 CRT and displays a bright picture without any modifications.

DFWB.

Heatercathodeshort · 7th Sep 2007, 2:44 pm

The queries relating to the TA10 and TA15 have come up before in these threads. David [Fernseh] is correct in what he says. It would be very unlikely to find a set fitted with the original 3/6A crt as these were prone to a short life and due to the very low EHT, any drop in emission would have resulted in a drastically reduced picture brightness.
All early post war EMI sets [excluding the 1803/4] were fitted with the TA10/15 crt that required around 7kv due to the aluminised screen. They were a very efficient tube and did work with reduced EHT but the performance was a little under what the tubes could produce. When operated at 7kv these receivers had a 'search light' performance, truly amazing. The mains EHT unit fitted to these receivers produced over 6kv for these tubes.
When used with the 1803/4 it was recommended that the EHT boost circuit was fitted together with the increase in scanning power modifications required due to the higher EHT. As mentioned in my earlier notes, EMI supplied all the parts required in a 'mod' kit for each model. The 1805 series was very much a pre war set and it was much behind in technical design for its day. Nothing wrong with that of course because it certainly was a reliable series due partly to its very robust oil filled EHT transformer.
The TA10/15 were of course PYREX tubes and therefore difficult [tell me about it, I regunned one back in the 60's with the help of my mate who owned a regun plant] to rebuild. There is no dirrect replacement due to the very low EHT potential. Any 15" tube that is capable of showing a decent picture is a rare one to find these days.
Nice looking chassis, pity about the horrible 'bolt together' appearance of the cabinet.[s]..Very much a marriage of convenience...Regards, John.

6th Sep 2007, 10:26 pm	#21
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: HMV1902 - CRT Substitution, and restoration tips The TA10 and TA15 aluminised CRTs were fitted to 1948 models 1805 and 1806 and derived models like the 1902 and 1851. The EHT was supplied by the EHT transformer only, no EHT boosters are required. In the case of earlier models like the 1804 and VT50. The original pre-war designed CRT was replaced by the TA10 and an EHT booster device was recommended. The timebases required some modifications to increase power to accomodate the higher EHT. Early fifteen inch models would use the TA15 as the replacement CRT along with the EHT and TB modifications. My Marconi VT50 is fitted with the TA10 CRT and displays a bright picture without any modifications. DFWB.

7th Sep 2007, 2:44 pm	#22
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: HMV1902 - CRT Substitution, and restoration tips The queries relating to the TA10 and TA15 have come up before in these threads. David [Fernseh] is correct in what he says. It would be very unlikely to find a set fitted with the original 3/6A crt as these were prone to a short life and due to the very low EHT, any drop in emission would have resulted in a drastically reduced picture brightness. All early post war EMI sets [excluding the 1803/4] were fitted with the TA10/15 crt that required around 7kv due to the aluminised screen. They were a very efficient tube and did work with reduced EHT but the performance was a little under what the tubes could produce. When operated at 7kv these receivers had a 'search light' performance, truly amazing. The mains EHT unit fitted to these receivers produced over 6kv for these tubes. When used with the 1803/4 it was recommended that the EHT boost circuit was fitted together with the increase in scanning power modifications required due to the higher EHT. As mentioned in my earlier notes, EMI supplied all the parts required in a 'mod' kit for each model. The 1805 series was very much a pre war set and it was much behind in technical design for its day. Nothing wrong with that of course because it certainly was a reliable series due partly to its very robust oil filled EHT transformer. The TA10/15 were of course PYREX tubes and therefore difficult [tell me about it, I regunned one back in the 60's with the help of my mate who owned a regun plant] to rebuild. There is no dirrect replacement due to the very low EHT potential. Any 15" tube that is capable of showing a decent picture is a rare one to find these days. Nice looking chassis, pity about the horrible 'bolt together' appearance of the cabinet.[s]..Very much a marriage of convenience...Regards, John.