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Old 14th Jan 2018, 8:27 pm   #1
ManxDave
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Default Hacker Sovereign II RP25 noisy amp

I know there are a few threads about noisy Hacker amplifiers ( I used them to prioritise the fault tracing ) but I thought I'd add my experience.
About 5 minutes after switching on there was a background crackle that wasn't affected by the volume or tone controls, not loud but annoying in quiet programs intervals.
I started by cleaning the presets RV3 & RV4 with a drop of Servisol. I then changed T1 & T2 which were BC148 Lockfits. I had S8050Bs which seemed to work fine but didn't solve the noise.
Then changed R1 & R3. At this point I realised it was probably going to be a blanket change starting with all the electrolytic caps as I wasn't sure about the reliability of ERIEs, followed by the rest of the resistors. After each component change I switched the radio on until the noise reappeared as I was interested in the culprit. Finally after changing R8 it remained silent. Most resistors were high in value by varying degrees but the amp was working fine apart from the noise. I think the lesson learned by me is that although there are common faults on many of these radios almost any component can introduce noise. The second lesson - the amp works fine with T1 the wrong way round, C & E reversed, oops!
Finally C5 is marked as 40mfd on the circuit diag but mine had a 6.4uF fitted. To my untrained ear the bass did't sound so good when I fitted a 40uF so I replaced it with a 10uF, anyone know which is correct? There is no mention of it in the modifications.
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Old 14th Jan 2018, 8:30 pm   #2
Tim
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Default Re: Hacker Sovereign II RP25 noisy amp

Well done Dave, I'll put that down in the Hacker RP25 of my faults book.

I have had a few of these where noise/ intermittent signal has been caused by damage to the amplifier board by gunge from leaking batteries. It is unfortunately quite common to find whole tracks eaten away!
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Old 14th Jan 2018, 8:40 pm   #3
ManxDave
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Default Re: Hacker Sovereign II RP25 noisy amp

Thanks Tim. There was a small amount of something on the board between the B & E of T4 which I was hoping was the problem but cleaning it didn't make any difference.
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Old 14th Jan 2018, 9:42 pm   #4
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Default Re: Hacker Sovereign II RP25 noisy amp

C5 should be 6.4uF (10uF will be close enough). Details are given in the manual addendum that covers the change to the A model, but in short, it was reduced in value to reduce instances of noisy bass controls.

I don't think I've had R8 specifically cause me trouble, but just about every other resistor in the front end has! As far as those old carbon-comps go, trust none of them!

Well done for sticking with it so systematically
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Old 14th Jan 2018, 9:58 pm   #5
ManxDave
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Default Re: Hacker Sovereign II RP25 noisy amp

Thank you for confirming the value of C5. After finding the culprit I changed all the rest of the composite resistors to prevent future problems, except R14 & 15 as I didn't have anything low enough. I may do them in the future as I am planning on keeping this radio.
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Old 14th Jan 2018, 10:04 pm   #6
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Default Re: Hacker Sovereign II RP25 noisy amp

Quote:
Originally Posted by mhennessy View Post
As far as those old carbon-comps go, trust none of them!
I agree with that Mark.

When I have to replace any of the carbon composition resistors, I'll try to use the Piher carbon film type which have a red/brown colour body, are of a similar size and don't look out of place. I'm lucky to have a reasonable stock built up over the many years of hoarding

Regarding the noisy composition resistors, it would be great to have a test instrument that could detect noisy operation. If it could be achieved I would imagine it would rely on the use of an oscilloscope display for assessing the component's noise properties. Could it be possible to devise such an instrument, if so by what principle?

Regards
Symon.
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Old 14th Jan 2018, 10:11 pm   #7
mhennessy
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Default Re: Hacker Sovereign II RP25 noisy amp

Quote:
Originally Posted by Philips210 View Post
When I have to replace any of the carbon composition resistors, I'll try to use the Piher carbon film type which have a red/brown colour body, are of a similar size and don't look out of place. I'm lucky to have a reasonable stock built up over the many years of hoarding
My favourite resistors!

Sounds daft, but I think that's because I spent ages as a kid staring longingly at the wonderful pictures of them in the Ladybird books


Quote:
Originally Posted by Philips210 View Post
Regarding the noisy composition resistors, it would be great to have a test instrument that could detect noisy operation. If it could be achieved I would imagine it would rely on the use of an oscilloscope display for assessing the component's noise properties. Could it be possible to devise such an instrument, if so by what principle?
I think that would be quite simple - just an amplifier that can connect to a 'scope would do. It might be necessary to put some DC bias on the resistor - or perhaps not (I haven't tried it). For some time now I've been thinking about a similar jig to measure Lockfit transistors for noise - perhaps the same thing could do both...

Of course, when interpreting the results we'd have to remember that even perfect resistors make thermal noise. But the difference between regular noise and the noise of a duff carbon-comp is usually massive.
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Old 14th Jan 2018, 10:20 pm   #8
Philips210
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Default Re: Hacker Sovereign II RP25 noisy amp

That sounds promising and would make an excellent project to build, very handy for checking those Lockfits as well.

Regards
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Old 14th Jan 2018, 11:43 pm   #9
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Default Re: Hacker Sovereign II RP25 noisy amp

The old BC Components PR01, PR02 and PR03 metal film resistors (1-3W respectively) were nice visual replacements for older resistors, having dark matt reddish-brown bodies.
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