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

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Reply
 
Thread Tools
Old 26th Jul 2019, 11:39 am   #21
Mike. Watterson
Heptode
 
Join Date: Dec 2014
Location: Limerick, Ireland.
Posts: 873
Default Re: Ultra 761. Actually a T671.

Yes, green and red came in with brown for tolerance. Gold is more common but suggests old stock.
It's rare now for small resistors of metal film to be other than 1% The 1% usually have three digit bands but not always.
Mike. Watterson is offline   Reply With Quote
Old 27th Jul 2019, 3:58 pm   #22
RobertF
Triode
 
Join Date: May 2019
Location: Cavan, Republic of Ireland.
Posts: 22
Default Re: Ultra 761. Actually a T671.

Thanks for the advice guys.
I removed the lamp limiter and replaced the reservoir cap which was a bit suspect.
I am now getting 226v at the rectifier, which seems a bit off 240v.
I am getting lower readings than on the service sheet.

V4 Anode 222v Screen 211v Cathode 6v
V3 37v
V2 214v 102v 2v
V1 215 102v 2.2

The radio is silent. There is crackling on the speaker when I change bands but nothing up and down the scale.

Are these lower voltages normal or should I try and improve them before looking at the sound.

Any advice appreciated.

Robert
RobertF is offline   Reply With Quote
Old 27th Jul 2019, 4:04 pm   #23
Mike. Watterson
Heptode
 
Join Date: Dec 2014
Location: Limerick, Ireland.
Posts: 873
Default Re: Ultra 761. Actually a T671.

Replace Screen Grid decoupling caps and any other HT decoupling paper caps as that kills gain and pulls down HT.
Mike. Watterson is offline   Reply With Quote
Old 27th Jul 2019, 6:06 pm   #24
RobertF
Triode
 
Join Date: May 2019
Location: Cavan, Republic of Ireland.
Posts: 22
Default Re: Ultra 761. Actually a T671.

I have already replaced all the paper caps and electrolytics.
I didn't think the smaller pf caps had a reputation for failing.

Excuse my lack of knowledge, but are the grid decoupling capacitors the ones between the grid and the negative rail.
RobertF is offline   Reply With Quote
Old 27th Jul 2019, 9:09 pm   #25
Mike. Watterson
Heptode
 
Join Date: Dec 2014
Location: Limerick, Ireland.
Posts: 873
Default Re: Ultra 761. Actually a T671.

g2, screen grids. The voltage on them sets the gain. They usually connect to 0V / negative/chassis.

Check resistor feeding triode anode of V1. It's a 6C9, or there may be an ECH41 or ECH42 there. You only list one set of Anode and Screen voltages for V1. It's a Triode Hexode.
The triode is the local oscillator and the Hexode g1 is RF in. Its coil is on Anode pin 3 and feed back to triode grid pin 4, internally connected to Hexode. The resistor from the coil to HT is typically 33K, I don't know what it is on the T671. If that resistor goes high resistance the Local Oscillator won't run.

Pin 5 is the Hexode screen grid, to HT via a resistor and typically a 50nF or 100nF decoupler to 0V

The Mica caps are generally fine. They are rectangular and up to 2000pF

It's rarer that paper caps are less than 2000pF, though 1000pF and 470pF are sometimes used.
Mike. Watterson is offline   Reply With Quote
Old 27th Jul 2019, 9:25 pm   #26
cathoderay57
Heptode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 507
Default Re: Ultra 761. Actually a T671.

Hi Robert, compared to the quoted voltages on Trader #1017 the voltages are reasonable, a bit down but not sufficient to prevent the set from working satisfactorily and so something else is wrong. The voltage measurements show that the valves appear to be drawing some current which is a good thing but doesn't prove they are working properly in every respect. To answer your question on the decoupling capacitors, yes that's correct. The capacitors between grid 2 and chassis carry a high voltage and will pull down the HT if leaky as Mike says. Also, decoupling capacitors appear between grid 1 and chassis (sometimes not completely obvious as they are often connected to grid 1 via an IF transformer coil or resistor); only appears to be one such on this radio and it's C6. If this is leaky then the radio may distort on strong signals as it preserves the negative grid bias on grid 1 under automatic gain control. Clearly not a problem on this radio at present! Crackling on rotating the wavechange switch is a good sign. So what to try next? Assuming you don't have a signal generator here are a couple of useful tips. It sounds like the audio amplifier is working but to be sure switch to gram and touch the upper gram socket with your finger. As you increase the volume you should hear a loud buzz from the speaker. Then if you have a portable transistor radio that receives AM tune the 2 radios close to the same wavelength and spin the tuning knob of the Ultra backwards and forwards across the dial. If at certain points you hear a whistle on the transistor radio then it indicates the Ultra oscillator is working. If not then a rethink will be needed. Just occasionally dirty valve pins or loose sockets can be an issue so give the valves a waggle while the set is on and see if you get a signal. Don't do this with the rectifier V5 or V4 as they get very hot. Oh one other tip: these type of valves have a locating pip on the base. If this is part of a metal shroud then no probs but if it is a moulded glass pip be careful pulling the valves out as there is a wire spring at the base that can all too easily shear off the glass pip, loses vacuum and the valve is then useless. if you can carefully release the spring before pulling the valve out. Cheers, Jerry
cathoderay57 is offline   Reply With Quote
Old 27th Jul 2019, 9:43 pm   #27
cathoderay57
Heptode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 507
Default Re: Ultra 761. Actually a T671.

Assume you have an aerial wire plugged in?
cathoderay57 is offline   Reply With Quote
Old 28th Jul 2019, 3:49 pm   #28
RobertF
Triode
 
Join Date: May 2019
Location: Cavan, Republic of Ireland.
Posts: 22
Default Re: Ultra 761. Actually a T671.

Hi guys,
Thanks for all the advise.. I was going through all your suggestions one by one until I got to the resistor feeding the anode of V1.. This resistor I had already replaced as it was very high. I then noticed my schoolboy error. It is a 68k resistor and I had replaced it with a 680k. Red and orange look very similar when you get to a certain age. I think I will measure them in future.
The set is now playing very nicely..

I have learned a lot working on this set.
Thank you again.

Robert
RobertF is offline   Reply With Quote
Old 28th Jul 2019, 4:22 pm   #29
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 6,957
Default Re: Ultra 761. Actually a T671.

Quote:
Originally Posted by RobertF View Post
Hi guys,
I think I will measure them in future.

Robert
Worth doing even if the colours are easy to see, not be the first time the odd wrong one as been mixed with correct ones in the packaging.
__________________
Frank
Nuvistor is online now   Reply With Quote
Old 28th Jul 2019, 7:12 pm   #30
cathoderay57
Heptode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 507
Default Re: Ultra 761. Actually a T671.

Nice work. Well done. Cheers, Jerry
cathoderay57 is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 11:31 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 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.