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scott_fury View Drop Down
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    Posted: 15 August 2021 at 6:04pm
Hi,

Had a channel develop some hiss and crackle on one channel last weekend. 

Set it up and tested and all was fine, came back the next day to get it running for the first day of a 3-day festival and the A channel had background hiss, with or without a signal, that would get louder as the gain was increased, and a slight crackle when the pot was adjusted.

Luckily I had a 1200 to run as a replacement for that channel, but need to get it sorted. Had an issue with the other channel before here….


Think it’s time for the old girl to get a full service so my questions are…..

1 - what’s likeliest cause of the hiss?
2 - what else should I do/replace whilst I’m at it?

As in the previous thread with the issue with my other channel, I have replaced the op amps as Simon suggested. Jbl_man and Gaz both suggested replacing the big dropper resistors on R45 and R55, which I may as well do.

Any other tips or suggestions would be very welcome . Cheers!
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djeddie View Drop Down
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Points: 4119 		Post Options Post Options   Thanks (0) Thanks(0)   Quote djeddie Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2021 at 8:49pm
Possibly some old, dried-out caps? Mine's doing the same but I have other priorities at the mo'.
The crackle is probably dirt in the pot, could try some contact cleaner in it or replace it.
Chas n Dave : it's like Drum and Bass but with beards.             E=mc² ±3dB
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scott_fury View Drop Down
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Points: 316 		Post Options Post Options   Thanks (0) Thanks(0)   Quote scott_fury Quote  Post ReplyReply Direct Link To This Post Posted: 12 September 2021 at 9:35pm
Thanks for the reply

Originally posted by djeddie djeddie wrote:

Mine's doing the same but I have other priorities at the mo'.


I know that feeling. Taken me almost a month just to get the cover off.

I've opened it up and run through a few of the resistors and zener diodes with a multimeter, but there's no smoking gun.


I also ran both channels through REW and recorded the levels of both with the gain up full to see the difference in noise.



It doesn't really help me in anyway, but when I had connected up to my hifi speaker at home using the wire binding posts and not the XLR, I couldn't hear any major difference. But running it through my soundcard using XLR's shows there is a huge difference, so maybe the problem is connected to the XLR?

I also did a frequency sweep on both channels for a bit of a laugh as I was at it.


The orange is the faulty channel.

Anyway, the only guy I know who fixes amps in Bristol is chokka with repairs and so it's going to be a few weeks before he might be able to get round to look at it. I'm keen to see if I can find the fault but don't know where to start, other than looking at the schematic and systematically going through each component.


It would be really helpful if anyone could give me a pointer in the right direction, such as the first logical place where to look. Thanks!!






Edited by scott_fury - 12 September 2021 at 9:37pm
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kedwardsleisure View Drop Down
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I would look at the output on a scope and see if the noise is actually a bit of HF oscillation (regular wave) or white noise (spiky rubbish). White noise could be generated by leakage in a component eg semiconductor and is quite distinctive to an experienced engineer, compared to noise generated by say a faulty resistor or capacitor which is slightly more random, like a thin rustling.
Also, small dc offsets at any point may be blocked by a capacitor downstream, and the hiss is the slight leakage through the capacitor.

HF oscillation can be anything from a leaky output transistor (unlikely with those hitachi's but you never know) to a high ESR decoupling capacitor that is allowing a feedback loop through the supply rails. Check the LV rails for such noise with the scope, suspecting the dropper resistors, zeners and rail decoupling caps.

With care you can use thermal probing (freezer spray/hot air) to see if that affects the noise but unless you're careful you could cause the amp to run away, so to be used as a last resort.

I'll omit the scores of obvious things as I'm sure you've already checked those.
Kevin

North Staffordshire
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scott_fury View Drop Down
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Points: 316 		Post Options Post Options   Thanks (0) Thanks(0)   Quote scott_fury Quote  Post ReplyReply Direct Link To This Post Posted: 12 September 2021 at 10:49pm
Unfortunately I don’t have access to an oscilloscope so I’m having to rely on listening to what is coming out of my speakers. I was having trouble describing the noise, but a thin rustling sounds about right.

Originally posted by kedwardsleisure kedwardsleisure wrote:

I'll omit the scores of obvious things as I'm sure you've already checked those.

Please don’t, as I’m a novice and a lot of what you suggested went right over my head LOL

I’ve checked the voltage across the zener diodes, especially the 15v ones and they seem good. Checked the wire wound resistors and the values are all okay. I had replaced the IC’s with new chips previously, but may have to get a tester, or do some A/B testing with the ones on the other channel.

Looking at the schematic, could you tell me which ones are the dropper resisters?
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scott_fury View Drop Down
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Edited by scott_fury - 12 September 2021 at 11:06pm
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toastyghost View Drop Down
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Points: 10866 		Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 13 September 2021 at 7:08am
Originally posted by scott_fury scott_fury wrote:

Unfortunately I don’t have access to an oscilloscope


Are you sure?

https://www.roomeqwizard.com/betahelp/help_en-GB/html/scope.html

I’d still recommend getting a proper scope, but the software ones will do the trick. Just be wary of knowing your audio interface’s own noise floor or quirks. Plus, measuring the output of the amp or components in an amp directly into your audio interface can very easily make many computer things go bang, unless you’re careful to check the voltage - or make a little resistor-based voltage divider for the purpose.
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