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 Audio (record players, hi-fi etc)

Notices

Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

Closed Thread
 
Thread Tools
Old 24th Apr 2018, 3:58 pm   #21
Techman
Dekatron
 
Techman's Avatar
 
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 4,985
Default Re: Hacker GP15 Cavalier

On the face of it this fault does seem to defy all logic.

I'm thinking in terms of some oscillation going on. Perhaps it's out of the range that the scope can see, or perhaps the scope probes are killing it.

The conflicting meter readings could be due to the fact that if there is some un-seen oscillation going on that the meter is reading normally at the lower voltage cathode, but can't resolve it at the anode side of the triode. If you're using DVMs then I would resort to an Avo instead.

We all know that standard foil wound coupling type capacitors are non-polarised, however, they can be regarded as polarised when used in certain critical and very sensitive circuits. You probably already know that the outer foil is connected to one lead out wire while the inner foil is obviously connected to the other lead out wire. There is a little test that you can do to find out which way round they are, but in most circuits it doesn't matter which way round they are connected. I think at this stage I would be thinking about whether one of the coupling capacitors was connected the wrong way round and there was some stray capacitance effect going on between the outer foil of one of the capacitors and either the chassis or another component or wiring. I would either test them to see which end the foil is connected to if it was obvious that one particular way was going to give the problem, or I would reverse each one in the circuit one at a time to see if this resolved the problem. I think it's important to do them one at a time or you could have one that was right cancel out one that was wrong.
Techman is offline  
Old 24th Apr 2018, 4:05 pm   #22
Biggles
Rest in Peace
 
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
Default Re: Hacker GP15 Cavalier

Try slugging the triode anodes to chassis with something like a 0.1 capacitor. This should rule out any possibility of oscillation, which may be quite high frequency, perhaps too high to see on a scope. I would also try taking measurements with a trusty Avo 8 or similar. You are right, it doesn't seem to add up. Don't you just hate faults like this? I would also have both valves on the tester if you have access to one. I would even go as far as constructing the circuit on a tagstrip or something and patching it in to see how it behaves. I wonder if having the two electrode structures in one bottle can cause leakage? I have never seen this before but there is always a first time. I hate being beat by a circuit. I would have to chase it to the end.
Alan.

Post crossed with Techman!
Biggles is offline  
Old 24th Apr 2018, 4:41 pm   #23
Michael Maurice
Moderator
 
Michael Maurice's Avatar
 
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,219
Default Re: Hacker GP15 Cavalier

Thank you both for all your help, but that amplifier WAS working with all the new components fitted a couple of years ago. As these parts are new good quality Vishay components, both resistors and capacitors they should not have failed in such a short time, and even if they had, why would BOTH triodes behave like that.

The new valves are of a completely different manufacture to the originals so for both sets of valves to have the same fault is somewhat unlikely.
__________________
Forum Moderator

http://www.michaelmauricerepairs.co.uk/
Michael Maurice is offline  
Old 24th Apr 2018, 4:51 pm   #24
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Hacker GP15 Cavalier

Were the current figures of 2.2mA and 0.022mA measured or deduced?

Lawrence.
ms660 is offline  
Old 24th Apr 2018, 4:52 pm   #25
Michael Maurice
Moderator
 
Michael Maurice's Avatar
 
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,219
Default Re: Hacker GP15 Cavalier

deduced using ohms law
__________________
Forum Moderator

http://www.michaelmauricerepairs.co.uk/
Michael Maurice is offline  
Old 24th Apr 2018, 4:56 pm   #26
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Hacker GP15 Cavalier

I would measure the cathode current(s) and at the same time measure the cathode voltage, using a scope if you don't have a spare meter.

Lawrence.
ms660 is offline  
Old 24th Apr 2018, 6:55 pm   #27
John10b
Nonode
 
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,869
Default Re: Hacker GP15 Cavalier

I don’t have in front of me a circuit diagram of your Hacker, so I hope you don’t mind me commenting on your problem, but I did have a problem with amplifier (not Hacker) some years ago and after much frustration it turned out be the output transformer.
I was lucky in that I had a pair of amps on the bench so I was able to swap the transformers over to prove the fault before purchasing a new one.
Cheers
John
John10b is offline  
Old 24th Apr 2018, 11:49 pm   #28
Techman
Dekatron
 
Techman's Avatar
 
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 4,985
Default Re: Hacker GP15 Cavalier

I also haven't looked at a circuit for this but I was also wondering about possible problems in the NFB line and agree with John's thoughts as regards the output transformer. It's possible that although it's still working, that somethings gone a bit funny with it, so I would be thinking about either removing the feedback connection from it, or temporarily replacing it with a similar one borrowed from another amplifier.
Techman is offline  
Old 25th Apr 2018, 12:04 pm   #29
Michael Maurice
Moderator
 
Michael Maurice's Avatar
 
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,219
Default Re: Hacker GP15 Cavalier

Its certainly an idea, I'll investigate later
__________________
Forum Moderator

http://www.michaelmauricerepairs.co.uk/
Michael Maurice is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:45 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.