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 Radio (domestic)

Notices

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

Closed Thread
 
Thread Tools
Old 16th Feb 2018, 1:19 am   #21
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
Default Re: Hacker Black Knight RP74MB Quiet Audio

BC559 (in the more modern TO-92 package, but with the base in the middle) should be a direct sub.
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments.
julie_m is offline  
Old 16th Feb 2018, 9:15 am   #22
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
Default Re: Hacker Black Knight RP74MB Quiet Audio

Quote:
Originally Posted by mhennessy View Post
The direct replacement would be a BC548,
Oops - my turn to make that particular typo

Of course, I meant BC558. Yes, BC559 would also be fine (it's the lower-noise version) or BC557 (the higher-voltage version), though in this role, neither property is required (but won't hurt). Also, the BC327 would be fine. Honestly, just about any PNP small-signal transistor will do the job - it's really not critical. More here.
mhennessy is offline  
Old 16th Feb 2018, 2:02 pm   #23
OldTechFan96
Octode
 
OldTechFan96's Avatar
 
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
Posts: 1,177
Default Re: Hacker Black Knight RP74MB Quiet Audio

I removed TR4 and tested it with my component tester and it tested bad. It was replaced with a BC327 and now the radio is working! I'll do some more checks later on.

Thanks for the help!
OldTechFan96 is offline  
Old 16th Feb 2018, 3:51 pm   #24
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
Default Re: Hacker Black Knight RP74MB Quiet Audio

Result

Transistor testers (and other component testers like capacitor ESR meters) are very handy, but with experience you won't need them (beyond perhaps just confirming the diagnosis).

What led me to T4? The base-emitter voltage. That should never be any more than 0.6V, but here it was about 7V (strictly, I should really put minus signs on those numbers as the transistors is PNP). The fact there was 0V at the collector shows that no current was flowing from emitter to collector, despite there being more than enough Vbe (and then some), so that's a dead transistor by any definition. All other voltage readings were in line with this diagnosis, so it was a fairly safe bet...

These sorts of testers are a relatively recent phenomenon - back in the day, we had to arrive at our diagnosis from observations made from the circuit. Something I often find myself saying when someone asks for advice about buying a transistor tester: there is no better test for a transistor than the circuit it works in.

With experience, the above will make sense. In the meantime, apologies if it seems a bit tangential.

It's also worth saying that the Hacker circuit is about as complicated as audio amplifiers get in a radio, with all 6 transistors being DC coupled with 2 feedback loops. So if you don't yet fully understand the circuit, don't worry! For some time now I've been working on an article for my website that explains how this works (and therefore, how to troubleshoot it), but it's surprisingly hard to do. One day...

Again, well done!
mhennessy is offline  
Old 16th Feb 2018, 8:09 pm   #25
OldTechFan96
Octode
 
OldTechFan96's Avatar
 
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
Posts: 1,177
Default Re: Hacker Black Knight RP74MB Quiet Audio

Thanks for the helpful post mhennessy. I was going to ask how you arrived at the conclusion of T4 being bad. I think that I'm starting to understand.

The Black Knight is back together for the first time in a while.

It is certainly worthwhile to stress the importance of measuring voltages first when faultfinding. When signal tracing with the scope I kept this in the back of my head and only made the measurements when you asked. Since the signal disappeared by the base of TR2 I assumed that TR1 was faulty. It looks like the scope would have led me to the wrong diagnosis.
OldTechFan96 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:22 pm.


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.