UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 21st Oct 2021, 5:44 pm   #1
slidertogrid
Octode
 
slidertogrid's Avatar
 
Join Date: Apr 2005
Location: Peterborough, Cambridgeshire, UK.
Posts: 1,897
Default HMV 1865 Help needed

Hi All
Many years ago I bought a lovely HMV 1865 from a fellow enthusiast . The set had been overhauled and is in lovely condition. I have used it from time to time and it has always given an excellent bright picture. For a while the picture width has been a little low it would fill out a little after a while so I just assumed one of the valves in the line output stage was getting a little tired.
A couple of days ago the set was running happily I left the room for a few minutes to make a cup of tea when I returned the picture had gone. Sound was still there. I switched off and decided to have a look later hoping to find a snow-capped line output valve....
Today I had a look, no snow-capped valves the line whistle is there but much quieter than normal, with no signal this set used to make a fair noise. The line output and boost diode are getting hot but the spark from the top caps is very low. The line oscillator is running and my neon lights when held near the valves. The set has been fitted with a tripler and I think it looks as if the primary of the LOPT has been rewound.
I disconnected the input to the tripler no change. I then tried replacing the PL81 and the PY no change. I tried a second set of valves as I have found new ones faulty, I have now downloaded the circuit diagram from Paul's site.
To make any further checks it looks as if the chassis will have to come out if that is the case I will have to haul it upstairs to the workshop as I don't want to disembowel it on the dining room table...
I just hope the LOPT is OK...

I have found that I have a PCF80 fitted where the diagram shows a PCF82 (sync sep and frame osc part) the PCF80 must be OK there though as the set has worked for years.

Any thoughts as to what I should check next? I don't have a component layout unfortunately just a circuit and valve layout.
Any ideas greatly appreciated....
Rich.
Attached Thumbnails
Click image for larger version

Name:	20211021_143034.jpg
Views:	143
Size:	50.9 KB
ID:	243895   Click image for larger version

Name:	20211021_143055.jpg
Views:	132
Size:	108.1 KB
ID:	243896   Click image for larger version

Name:	20211021_164157.jpg
Views:	121
Size:	60.7 KB
ID:	243897   Click image for larger version

Name:	20211021_165103.jpg
Views:	110
Size:	72.8 KB
ID:	243898   Click image for larger version

Name:	20211021_143048.jpg
Views:	85
Size:	71.2 KB
ID:	243899  

__________________
The rotation of the earth really makes my day...
slidertogrid is offline  
Old 21st Oct 2021, 7:48 pm   #2
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
Default Re: HMV 1865 Help needed

This HMV is the first model to be fitted with a Thorn chassis. It is the equivalent to the Ferguson 306T and is far better known under this banner.
At first thought it appears that the LOPT is faulty. It was a common breakdown so much so that Radiospares supplied an improved transformer [Type LOPT 108] The 306T and it's big brother the 21" 308T were huge sellers for Ferguson.

I expect the boost capacitor and line coupling cap have been replaced. Try running the chassis with the top cap of the PY81 disconnected. If the line output stage livens up the boost cap is S/C.

The original line coupling cap to pin 2 of the PL81 is quoted as .001uf. This was later changed to a .01uf to give better coupling and less sensitive to capacitor tolerances.

Other than that there are no known line faults with the 306T. The scan coils are well made. Worth checking the screen feed resistor to pin 8 of the PL81. It's a very rare failure [if ever] but it's been 57 years since I first repaired a 306T and odd things happen with ancient electronics. Good luck with it. John.

Last edited by Heatercathodeshort; 21st Oct 2021 at 7:54 pm. Reason: Info added.
Heatercathodeshort is offline  
Old 21st Oct 2021, 9:11 pm   #3
slidertogrid
Octode
 
slidertogrid's Avatar
 
Join Date: Apr 2005
Location: Peterborough, Cambridgeshire, UK.
Posts: 1,897
Default Re: HMV 1865 Help needed

Thanks for the advice John, I have noticed that the line whistle is stronger when the set is first switched on and a tiny spark can be drawn from the PL top cap. once it has been running for a minute or so the line whistle gets very weak an there is no spark on the top cap. if left on longer the PL then starts to overheat. So I assume I have either got a fault loading the line output stage or the drive is weak and gets weaker as it runs.
I will check the screen feed resistor but first I have to find it! Looks like the chassis may have to come out..

Rich
__________________
The rotation of the earth really makes my day...
slidertogrid is offline  
Old 22nd Oct 2021, 12:52 pm   #4
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
Default Re: HMV 1865 Help needed

Interesting. Check the boost capacitor C124 .25uf for S/C. The PY81 generally takes longer to warm up due to its enhanced heater/cathode insulation. This may account for the diminishing line drive after a short period of time. With the boost capacitor S/C it represents a short circuit across the HT supply'

If you remove the PY81 top cap you may find the line timebase perks up and you may even get a folded up picture or at least a livened up line output stage.

Have you got any 405 line output transformers to hand? It does not matter what it belongs to. A simple test is to disconnect the top caps of the PL81 and PY81 valves and connect the test transformer using the relevant connections to the two top caps leaving everything else in circuit other than the top cap connections of the original transformer.

If the LOPT is faulty, healthy EHT will be developed in the test transformer. This will confirm if the LOPT is faulty. John.
Heatercathodeshort is offline  
Old 22nd Oct 2021, 8:06 pm   #5
slidertogrid
Octode
 
slidertogrid's Avatar
 
Join Date: Apr 2005
Location: Peterborough, Cambridgeshire, UK.
Posts: 1,897
Default Re: HMV 1865 Help needed

Thanks John
That makes sense it certainly has more life when it first warms up. If the boost capacitor is OK I will try your suggestion with a trial LOPT. I have a spare for my Dynatron which I know is ok I will give that a try.

Rich.
__________________
The rotation of the earth really makes my day...
slidertogrid is offline  
Old 22nd Oct 2021, 8:26 pm   #6
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
Default Re: HMV 1865 Help needed

Ekco/Dynatron transformers make good test LOPTs.
Just to confirm. Disconnect top cap connections of PL81 and PY81 valves and fold them away to stop them shorting to chassis.

Connect line output and boost diode test transformer leads to the correct valve top caps in the HMV. Do not disconnect any connections from the HMV transformer other than the two top cap connectors.

You should get a good discharge from the overwind EHT connection [rectifier anode] to the blade of a insulated screwdriver. If you plug the correct EHT rectifier into the test transformer [U26?] you will probably see the heater light and with the EHT connection in place a distorted raster should appear. Hope you get this action useful. John.
Heatercathodeshort is offline  
Old 23rd Oct 2021, 5:33 pm   #7
slidertogrid
Octode
 
slidertogrid's Avatar
 
Join Date: Apr 2005
Location: Peterborough, Cambridgeshire, UK.
Posts: 1,897
Default Re: HMV 1865 Help needed

Well it's not looking good as far as the LOPT is concerned I took John's advice and wired connected the test LOPT as suggested. The line stage seems a lot more lively and the EHT rectifier on the test LOPT lit up.
I tried connecting the top cap of the boost diode to the input of the tripler to see if I could get anything on the screen I turned the brightness right down in case there was any problems with the scanning as I didn't want to risk any damage to the phosphor. I let the set warm up and then advanced the brightness, but nothing on the screen. There is a nice healthy spark from the top cap of the EHT rect though so I think it has proven the point.
I intend next to remove the original LOPT and check the winding resistances against the ones in the service sheet and check for any shorts or leaks between windings. Unfortunately this means removing the chassis from the cabinet so it may have to wait for a while.
Thanks John for your help I have learned something about fault finding!
Rich
Attached Thumbnails
Click image for larger version

Name:	20211023_145304.jpg
Views:	78
Size:	56.8 KB
ID:	244047   Click image for larger version

Name:	20211023_145127.jpg
Views:	69
Size:	40.0 KB
ID:	244049  
__________________
The rotation of the earth really makes my day...
slidertogrid is offline  
Old 23rd Oct 2021, 6:13 pm   #8
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
Default Re: HMV 1865 Help needed

I think that has proved the point. Shame! The A1 voltage on pin 10 of the CRT is probably absent due to the main transformers wiring being out of circuit.

Don't take LOPT resistance values as fact! They are notoriously incorrect due to the fact that they used different gauges of wire and were often modified during production. Some are wound with resistance wire to reduce capacity. They are very much a guide but can lead you down the wrong path. Regards, John.

PS Due to the boost supply being absent it might be unwise to attempt to get screen illumination. The frame timebase oscillator is fed from the boost rail that will not be available with the test transformer likewise there will be no line scan. This might create a bright spot that will damage the screen [burn]. take care.

Last edited by Heatercathodeshort; 23rd Oct 2021 at 6:19 pm. Reason: Added text.
Heatercathodeshort is offline  
Old 24th Oct 2021, 11:50 am   #9
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
Default Re: HMV 1865 Help needed

This is a set that must be saved. The quality of the cabinet alone justifies this.
Almost any line transformer taken from a set with a 70degree tube will work in this set. The only proviso it has to be of the autotransformer type.
The good thing about the 300 series chassis is that it has a separate line oscillator so there is no feedback pulse from the line output transformer to initiate the end of the scanning stroke.

DFWB.
FERNSEH is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:57 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.