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 > Offered and Wanted Messages > Sets, Parts and Service Information Wanted (private buyers only, no swaps)

Notices

Sets, Parts and Service Information Wanted (private buyers only, no swaps) If you need help obtaining components, sets or equipment, post a message here. Private buyers only - no traders. No swaps.

Reply
 
Thread Tools
Old 11th Jun 2021, 7:59 pm   #1
majex45
Triode
 
Join Date: Jul 2011
Location: Hastings, East Sussex, UK.
Posts: 31
Default 10k log stereo pot

I'm trying to restore a PW Texan amplifier I recently bought and I need a 10K log twin gang potentiometer for the volume control. It is for through hole mounting and a threaded bush. 1/4 inch shaft.
RS do similar one but this is not suitable for through hole mounting.
Also two 15 V zenners - they are small so no great power dissipation.
majex45 is offline   Reply With Quote
Old 12th Jun 2021, 12:23 am   #2
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,917
Default Re: 10k log stereo pot

Blore Edwards do a variety of pots and may make one up for you.

Cleaning the existing one may work if done carefully with electrolube, if you haven't already tried.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 12th Jun 2021, 6:18 am   #3
David G4EBT
Dekatron
 
David G4EBT's Avatar
 
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 4,495
Default Re: 10k log stereo pot

Quote:
Originally Posted by majex45 View Post
.
Also two 15 V zenners - they are small so no great power dissipation.
Yes, Blore Edwards if you really do need the pots.

500 mW BZX55C 15V zener diodes are widely and cheaply available.

EG: 1.97 post free for ten from a UK supplier:

https://www.ebay.co.uk/itm/312810450...Cclp%3A2334524

Hope that helps.
__________________
David.
BVWS Member.
G-QRP Club member 1339.
David G4EBT is online now   Reply With Quote
Old 12th Jun 2021, 9:46 am   #4
majex45
Triode
 
Join Date: Jul 2011
Location: Hastings, East Sussex, UK.
Posts: 31
Default Re: 10k log stereo pot

Thank you both for the information. I don't know, but suspect, that the custom pots will cost more then the amplifier is worth.
Rather than replace the zenners I am going to use a ready-built 7815/7915 circuit board as the regulaton of the main PSU is so poor. With 8 ohm load but no signal the power rails are +/- 32V with infinite load V= +/- 51V. I may put some sort of regulation in the main rails as well. Budget busted!
Here is a picture of the offending volume control. This apparently is a common fault.
All the metal components (fuse holders, cases of the pots, screws, etc) in the amp were covered in a white "fur", some can be seen on the nut in the background. Some sort of oxide I suspect (I hope it is not cadmium).
Attached Thumbnails
Click image for larger version

Name:	P1020037.JPG
Views:	97
Size:	108.1 KB
ID:	235750   Click image for larger version

Name:	P1020038.JPG
Views:	90
Size:	105.9 KB
ID:	235751  
majex45 is offline   Reply With Quote
Old 12th Jun 2021, 10:38 am   #5
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,917
Default Re: 10k log stereo pot

Quote:
Originally Posted by majex45 View Post
Thank you both for the information. I don't know, but suspect, that the custom pots will cost more then the amplifier is worth.
Don't make assumptions like that. It costs nothing to find out, and you might be pleasantly surprised. Your assumption might be right, but if it isn't, you miss an easy fix. I don't remember their prices being all that bad, roughly comparable to RS.

One oddball fact about Zener diodes (only one 'n', named after Clarence Zener) is that they're only made up to a couple of volts. All higher voltage diodes are actually 'Avalanche diodes' and don't use the Zener effect. This is one of those strange things that almost nobody is aware of. You run into the difference if you get involved in their noise generation properties.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 12th Jun 2021, 10:53 am   #6
KeithsTV
Nonode
 
KeithsTV's Avatar
 
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,436
Default Re: 10k log stereo pot

Be aware that the maximum input voltage for the 7815/7915 is +37V/-37V so the +/- 51V would probably kill the regulator ICs and damage the other ICs.

Keith
KeithsTV is online now   Reply With Quote
Old 12th Jun 2021, 10:57 am   #7
frankmcvey
Heptode
 
Join Date: Jan 2015
Location: Cottesmore, East Midlands, UK.
Posts: 665
Default Re: 10k log stereo pot

Agree with the above, check Blore Edwards out. I specified a ganged pair of log pots with an on/off switch for a valve stereo preamp from them and the assembly came to around 35 including VAT and P&P. Not cheap, but excellent reproduction kit, made on the original 1950s (?) tooling.
frankmcvey is offline   Reply With Quote
Old 12th Jun 2021, 10:59 am   #8
Cruisin Marine
Pentode
 
Join Date: Jul 2018
Location: London, UK
Posts: 167
Default Re: 10k log stereo pot

Modification to accept a normal cheap pot is the name of the game- oh, and change those tantalum caps as well, if they aren't shot, they will be.
Cruisin Marine is offline   Reply With Quote
Old 12th Jun 2021, 2:06 pm   #9
G6ONEDave
Heptode
 
G6ONEDave's Avatar
 
Join Date: Dec 2019
Location: Owston Ferry, North Lincolnshire, UK.
Posts: 775
Default Re: 10k log stereo pot

The control looks like it could be stripped down for cleaning, just be very careful not to snap off any of the folded tags. Also make notes on paper re the assembly as you strip it down, so as to avoid any mishaps when re-assembling it, photos are really useful for this sort of thing. You could also make some alignment marks/scratches as you strip it down, which will help when refitting it together and remember if using a permanent marker pen for alignments, that a lot of cleaners will also remove the pen ink.

Dave
__________________
Quote "All is hyperthetical, until it isn't!" (President Laura Roslin, Battleship Galactica)
G6ONEDave is offline   Reply With Quote
Old 12th Jun 2021, 8:15 pm   #10
majex45
Triode
 
Join Date: Jul 2011
Location: Hastings, East Sussex, UK.
Posts: 31
Default Re: 10k log stereo pot

Quote:
Originally Posted by G6ONEDave View Post
The control looks like it could be stripped down for cleaning, just be very careful not to snap off any of the folded tags. Also make notes on paper re the assembly as you strip it down, so as to avoid any mishaps when re-assembling it, photos are really useful for this sort of thing. You could also make some alignment marks/scratches as you strip it down, which will help when refitting it together and remember if using a permanent marker pen for alignments, that a lot of cleaners will also remove the pen ink.

Dave
I like this idea, however once I have disassembled the chassis, removed the pot, disassembled the pot, cleaned it and reassembed everything, only to find that it is still faulty or worse then it was before, it seems an awful lot of effort to resurrect a 10 to 20 amp.
Has no one got one of these in their "junk box"?
Otherwise I'll get one from the company mentioned (but not for 35!) or RS.
Any ideas on the white "fur"?
majex45 is offline   Reply With Quote
Old 12th Jun 2021, 8:16 pm   #11
majex45
Triode
 
Join Date: Jul 2011
Location: Hastings, East Sussex, UK.
Posts: 31
Default Re: 10k log stereo pot

Quote:
Originally Posted by Cruisin Marine View Post
Modification to accept a normal cheap pot is the name of the game- oh, and change those tantalum caps as well, if they aren't shot, they will be.
Change them for what?
majex45 is offline   Reply With Quote
Old 12th Jun 2021, 8:22 pm   #12
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 6,755
Default Re: 10k log stereo pot

If you can't or don't wish to fix the original through hole mounted pot, fit a common or garden one with solder tag terminals upside down and make the connections to the PCB via flying (insulated) connecting wires.

Quote:
Change the Tantalums for what?
Replace them with like for like if you are concerned about keeping it as internally original as possible, otherwise replace them with good quality standard electrolytics with the same value / voltage rating.

Tantalums of a certain age, especially those used as supply decoupling capacitors, have a notorious habit of going low resistance / short circuit.

Last edited by SiriusHardware; 12th Jun 2021 at 8:28 pm.
SiriusHardware is online now   Reply With Quote
Old 12th Jun 2021, 8:50 pm   #13
Restoration73
Nonode
 
Restoration73's Avatar
 
Join Date: Jun 2013
Location: Surbiton, SW London, UK.
Posts: 2,303
Default Re: 10k log stereo pot

Your pot is an AB metals type and as usual is problematic as the design has no carbon
brush which reduces wear. Whilst there are other suppliers you might be able to alter
the tags on the Citec one to fit the pcb;

https://uk.farnell.com/citec-te-conn...opotentiometer
Restoration73 is offline   Reply With Quote
Old 13th Jun 2021, 10:37 am   #14
G6ONEDave
Heptode
 
G6ONEDave's Avatar
 
Join Date: Dec 2019
Location: Owston Ferry, North Lincolnshire, UK.
Posts: 775
Default Re: 10k log stereo pot

If the pot problem is due to it being internally dirty thus causing noise, then a full strip down clean and retension of the wiper contacts will cure the problem 95% of the time. If however the internal carbon tracks are worn away or open circuit (see below), then the only cure is to replace the part. Given that obtaining an exact like for like match will be somewhat difficult, your options are to try a strip down/repair or replace with a standard part that is wired to the pcb. Personally I would go for the strip down method, since this keeps everything original but if the part is too far gone then replacement is the only way forward. Even if a strip down fails, at least you will have a better idea of how the part functions and if failure is due to an error in the cleaning process, you'll know what not to do next time round.

Re damaged tracks, occassionally the problem is a break in continuity at the very end near to the rivot connection and if so after a good clean it may well be possible to effect a continuity repair with a drop of silver loaded conductive paint, although this won't work for long if it's in an area where the wiper travels over the new paint

It's really a case of how you value your time, as it is seldom financially viable to do a repair job to resell the equipment for a profit. This is a situation that we all find ourselves in and why this sort of work has become known as a labour of love.

At the end of the day it's your personal choice of what action to take, all we can do offer our own views and suggestions about it.

Dave
__________________
Quote "All is hyperthetical, until it isn't!" (President Laura Roslin, Battleship Galactica)
G6ONEDave is offline   Reply With Quote
Old 13th Jun 2021, 10:58 am   #15
knobtwiddler
Hexode
 
Join Date: Mar 2016
Location: London, UK.
Posts: 261
Default Re: 10k log stereo pot

Assuming that you have sufficient space above or to side of the pot, you could fit a new pot with the same bush diameter, rotate it by 180-degrees (or 90), and have soldered flying leads between pin / PCB. I often do this. If you put some heatshrink around the tags it'll look professional. If you don't mind spending about 12, the Alps Blue pot has fantastic channel matching (9/10 do - to within about 0.5dB at most levels, excluding deep cut-off range).
knobtwiddler is offline   Reply With Quote
Old 13th Jun 2021, 11:51 am   #16
slidertogrid
Octode
 
slidertogrid's Avatar
 
Join Date: Apr 2005
Location: Peterborough, Cambridgeshire, UK.
Posts: 1,328
Default Re: 10k log stereo pot

I have this one it has a switch on the back that could easily be removed of course. You are welcome to it if it will be suitable. I can put it in a jiffy bag and send as large letter so the postage will only be the cost of a suitable stamp
1.29.
Pm me your address and I will pop it in the post.
Rich
Attached Thumbnails
Click image for larger version

Name:	20210613_124302.jpg
Views:	41
Size:	61.0 KB
ID:	235820   Click image for larger version

Name:	20210613_124251.jpg
Views:	23
Size:	39.0 KB
ID:	235821  
slidertogrid is offline   Reply With Quote
Old 14th Jun 2021, 7:24 pm   #17
majex45
Triode
 
Join Date: Jul 2011
Location: Hastings, East Sussex, UK.
Posts: 31
Default Re: 10k log stereo pot

I think the pot has a broken track as when I first tried the amp it was very loud and turning the volume pot didn't make much difference.
I am rather worried that the rails reaching 51V may have fried everything but I will try it and see.
The ICs are TL071 whereas they were 741 or 748 in the original design. Will this help?
I will put some form of voltage regulator in the supply rails using 2N3055 and MJ2955 with a resistor & zener (sp) is this sensible? then the 7805/7905 board for the pre-amp section.
majex45 is offline   Reply With Quote
Old 14th Jun 2021, 8:35 pm   #18
KeithsTV
Nonode
 
KeithsTV's Avatar
 
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,436
Default Re: 10k log stereo pot

The maximum supply voltage for the 741 and the TL071 is typically +/-15V so 51V across the ICs would probably have fried them which ever type has been fitted.

Looking at the Texan circuit the supply to the output stages is +/-25V wrt 0V . The supplies to the op-amp stages are derived from the +/-25V supplies via 680 ohm resistors and the 15V zeners.

Was the 51V measured between the +ve and -ve supply rails or between 0V and either of the supply rails?

It may be worth measuring the voltage between pins 4(-ve) and 7 (+ve) of the TL071, anything over 30V could have damaged the op amps.

Keith
KeithsTV is online now   Reply With Quote
Old 15th Jun 2021, 2:42 pm   #19
G6ONEDave
Heptode
 
G6ONEDave's Avatar
 
Join Date: Dec 2019
Location: Owston Ferry, North Lincolnshire, UK.
Posts: 775
Default Re: 10k log stereo pot

Although uncontrollable audio level can be due to an o/c track, it could also be due to the rotary wiper not connecting to the track properly. IMHO It's worth opening up the control for investigation. If the control has a broken track then replacement is the only cure but whether you use a track from a donor pot or replace the whole unit is your choice.

Dave
__________________
Quote "All is hyperthetical, until it isn't!" (President Laura Roslin, Battleship Galactica)
G6ONEDave is offline   Reply With Quote
Old 15th Jun 2021, 8:05 pm   #20
majex45
Triode
 
Join Date: Jul 2011
Location: Hastings, East Sussex, UK.
Posts: 31
Default Re: 10k log stereo pot

Thanks for the input.
The voltage was measured from the centre of the smoothing caps to the positive or negative terminal respectively. I am hoping the 680 ohm resistors will have limited any damage to the op-amps.
I do not intend to switch it on again without a load (I am using a dummy load of 8 ohms made from an old fire element).
The pre-amp supply rails appear to be at 1 to 2 volts and the 680 ohm resistors are getting hot, I think the zeners may be dead but I will replace them with regulators. I am not exactly trying to make this a restoration but to get the amp working thus saving it from the dreaded WEE vandalism.
I believe I now have the pot so thank you all for your help with my mission.


Quote:
Originally Posted by KeithsTV View Post
The maximum supply voltage for the 741 and the TL071 is typically +/-15V so 51V across the ICs would probably have fried them which ever type has been fitted.

Looking at the Texan circuit the supply to the output stages is +/-25V wrt 0V . The supplies to the op-amp stages are derived from the +/-25V supplies via 680 ohm resistors and the 15V zeners.

Was the 51V measured between the +ve and -ve supply rails or between 0V and either of the supply rails?

It may be worth measuring the voltage between pins 4(-ve) and 7 (+ve) of the TL071, anything over 30V could have damaged the op amps.

Keith
majex45 is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 8:01 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 - 2021, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.