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Loud crackle and hiss at volume 0

H

HRC88

Active Member
Hello,

I am restoring a Revox b750, I am almost there but an issue remains:

When I initially turned the amp on, I had a hiss even at volume 0. The hiss was not getting any louder when the volume was turned up, and it;s on both channels. I've dismantled the tone control, reflowed some joints and cleaned every pots. It's still there but not gone.

However I have another issue that arised, and that's strange. When amp on, volume 0, it seems to now pick some interferences. I have some random loud crackle at volume 0, I believe from both channels. It could be relatively quiet and all of a sudden it crackles very loud like if the volume is at max. The volume pot has been cleaned, I did not have the issue before but I don't rule this out. It seems like it could be bleeding signal? I would trace it to the tone control board maybe.

I have an oscilloscope on the way, but in the meantime I would like to see if it can be easily fixed.

Thank you
 
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No transistor replaced other than the 4 output.

I was thinking the volume control could have gone wrong too, but it didn’t happen before. So I’m thinking it may come from somewhere else. Very strange
 
Does the condition occur with the tone controls switched off?

Does it occur with the pre-out/main-in jumpers disconnected, or "separated" switch off?

Determining that can eliminate several circuits from the "suspect" list.
 
Very good question! I've just tried, and no it does not happens when the tone defeat is on (the loud crackles.) Or very very faintly. The hiss at 0 remains there That would lead to the tone control boards, correct?

As for the power/pre, it is a switch on the revox. When pre is out, I have some sort of hum that I can not hear when it is in. Does that mean something is noisy with the power amps too?

Thank you
 
The crackles/static burst noise could be an amplifier stage in the tone control circuits, or the connections/switches. The Philips BC series are supposed to be low-noise amplifiers, and are not widely reported to be problematic like some other types, but there may be one or more failing. A fixed sine wave input to the amplifiers and an audio circuit tracer (or an oscilloscope) could be an easy way to isolate that, monitoring a clean sine wave through the various stages until it reveals noise on the waveform.

As for the hum in the main amp stage, that could be due to grounding problems or a power supply issue, although a power supply problem would likely also be present regardless of the pre/main switch selection.
 
I have received an oscilloscope today, so I can diagnose a bit further. Although I have never used one before.
I need to clarify something, what I wrongly called a hum at volume 0 is more a hiss. Like coming from a transistor more than a grounding issue.
For the hum when the pre/main are separated I think is is normal.
For the crackles, I should check the tone control amplifier then. But honestly can't see any bad solder/trace on that board.
 
HRC88 said:
I have received an oscilloscope today, so I can diagnose a bit further. Although I have never used one before.
I need to clarify something, what I wrongly called a hum at volume 0 is more a hiss. Like coming from a transistor more than a grounding issue.
For the hum when the pre/main are separated I think is is normal.
For the crackles, I should check the tone control amplifier then. But honestly can't see any bad solder/trace on that board.
Click to expand...
Take a look at what transistors are used on the tone board to start and replace any known problematic ones.
I have Noresco branded Rotel rx-602 that had his and pops at zero volume and replaced all the transistors during the restore and it is now dead quiet and sounds fantastic.
 
Hi,

Going back to this, I haven't really worked on it for a little while. I managed to sort out the crackles, it was some transistors in the tone control. The amp works fine.

Now what I didn't sort out is that hiss. It is much more noticeable on headphone of course. Not as much through speakers.
Just to recap:
- The hiss is some sort of white noise.
- It is present at volume 0 and does not get louder with volume. After volume pot?
- It goes away when power/pre switched on. Not the power boards?
- It seems equal both on left and right. Once again, not the power boards? Output transistors are new.
- No difference when tone defeated. Most likely not from tone control?
- It is present on all inputs - phono, aux, tuner. All transistors changed on input board
 
Unfortunately I sold mine to a friend about a month ago so I can't check inside.
I had fully recapped and I think I did most transistors except for the outputsaccording to some old notes.

2SA564A - KSA992FBU
2SC1567 - TTC004BQ-ND
2SC644 - KSC1845
2SC828 - KSC1815

I likely replaced these too but don't have the substitute listed in my notes so I may not have.
2SC1384
2SA794
 
I would be tempted to replace all transistors in my Revox, but that's probably overkill. To be honest, I have purchased lower noise modern replacement for all transistors, so I may go that route board by board at some point but I would like to first figure out where the hiss comes from.

The fact that it seems to be from both channel (at least on headphone, I need to double check on speakers) makes me think it may not be a transistor.
 
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