C-4 Pre with HF Oscillation

willygene

New Member
Hi all. This is my first post here, though I have received a lot of valuable information from this site.

I recapped my C-4 pre a while back and after words noticed some HF hash on the output at around 400kHz. I didn't look at it before the recap, so don't know if it is the result of the recap or if it was there before. It seems to be all over the place, though the 30 volt rails look clean IIRC.

I'm not sure how to about troubleshooting this problem. Any ideas anyone?

Bill
 
C4 Pre with HF Oscillation

Hello BILL !

Welcome on AK !

You have this trouble on both channels ?
 
Thanks for the welcome clinic-audio, and your time.

Yes, both channels. Amplitude is dependent on volume setting. At first I suspected something with my test equipment or environment, so I moved to a different location in the house and it measures the same (using an O'scope). It is there with or without a test signal, though I think one channel may have a higher amplitude than the other.

It seems that no matter where I measure, the oscillation is there, except for the 30 volt rails.

I will check again this evening to be sure , though.

Bill
 
What does this 400khz oscillation sound like?:scratch2:
 
What does this 400khz oscillation sound like?:scratch2:

Dental work:yikes:

I guess if you can't hear it, then it shouldn't be an issue, but I would like to eliminate it.

As for the symptoms, I ran home at lunch and re-measured a couple of points just to refresh my memory (which was wrong, BTW).

Photo 1 is Both Channels at Pre-Out 1. 20mv/div. 1us/div.
Photo 3 is the -45volts (looks the same on +45) 1v/div. 2ms/div
Photo 7 is the +30volts. 5mv/div. 1us/div.
Photo 9 is the -30volts.

So, it looks like the oscillations are in the 30volt p/s.

This evening I will unsolder the 30v p/s jumpers from the flat amp board and see what the supply looks like then.

Bill
 

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Last night I tried a few things. I removed the 7155 board (bass/treble turnover and head phone amp) and the 7157 board (mono, reverse, filter switch), and disconnected the +/- 30, RE, RL, L, E, and R from the 7149 function board. With this done, the HF hash and p/s ripple were still present. So, the problem is not on any of these three boards, and I left them disconnected.

I then lifted the +/-45, -21 and +/-30 between the 7153 p/s board the 7151 board, and measured them.
+45 measured +49 and was clean
-45 measured -48.5 and was clean
-21 measured -25.5 and was clean
+/-30 measured zero which was expected

So, it looks like the HF hash and ripple are generated by the 7151 board where the +/-30v, power for the lamps (now LEDs) and the relays are produced.

Vint Age suggested I add 0.1uf caps across the +/-30 filter caps, so I may try that tonight, as well as further troubleshooting.

Bill
 
Continuing on (should anyone still be interested:)):

I reconnected the +/-45 and -21 to the 7151 board, then tried bypassing the +/-30 volt filter caps with 0.1uf caps, but there was no change in the HF hash (photos 7 &9) or 120Hz ripple (photo 3).

The 120Hz ripple is what you could expect if a power supply was being loaded down excessively, but the voltages don’t change that much. The +/-45 dropped from 49 to 47 volts and the -21 dropped from 25.5 to 22 volts. On the other hand, the only load on them at the moment is the 7151 board.

The problem is definitely generated on the 7151 board, so my next step is to re-install the old caps. It may be that the new ones have an impedance profile that contributes to the oscillations, thou I doubt it. If that doesn’t solve it, it looks like replacing active devices is next on the list.

Bill
 
Hi Vint Age,

Nope, no help at all. I would have thought there would have been some improvement, but I didn't see any.

Bill
 
Maybe you're chasing a ghost in that the oscillation is only present when you connect your 15 or so pF scope probe ...
 
I hadn't considered that exactly. However, it is present on the pre outs when hooked directly to the scope, even with about a 600 ohms load if I recall.

I'll investigate it.

Bill
 
C4 Pre with HF Oscillation

Bill

What happen if you disconnect all your "LED" ?

:scratch2:

attached file of the section "flat amp 1/2 "
 

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wow i guess it is possible for leds to generate high frequency noise what about adding a bypass cap of 47- 100pf ceramic disk cap across each led.
 
I didn't get much of a chance to work on it this weekend, but to answer your questions: Yes, the record out is affected as well, however, I have that board disconnected from any power so it is not the cause of the problem. I also disconnected the 21 volts supply which is required for the LEDs and relays, and still have the junk on the supply lines, so the LEDs are not the cause either.
 
Have you tried shorting the phono input while you look. The phono amp has a lot of gain and its input impedance might be high enough that it's picking up a wifi or other RF signal and passing some harmonic thereof ...
 
It has taken me a while to get back to this project, but I did find the problem - sort of.

I recapped the whole pre and ended up with HF oscillations on the +/- 30v (see beginning of thread). It turned out to be the +/- 30v filter caps I used as replacements. I pulled those and installed the originals and got a good, clean +/- 30v. Now here is the interesting thing about it: If I used one original and one new replacement cap in either position there were no oscillations. Only when I used both new caps did the oscillations occur :scratch2:

The caps are C323 and 324. Originally 47uf/35v Nichicon LN 85degC.
The replacements were 47uf/50v Nichicon KA 105degC.

Any ideas what may be going on here?

Bill
 
It has taken me a while to get back to this project, but I did find the problem - sort of.

I recapped the whole pre and ended up with HF oscillations on the +/- 30v (see beginning of thread). It turned out to be the +/- 30v filter caps I used as replacements. I pulled those and installed the originals and got a good, clean +/- 30v. Now here is the interesting thing about it: If I used one original and one new replacement cap in either position there were no oscillations. Only when I used both new caps did the oscillations occur :scratch2:

The caps are C323 and 324. Originally 47uf/35v Nichicon LN 85degC.
The replacements were 47uf/50v Nichicon KA 105degC.

Any ideas what may be going on here?

Bill
Weird.. Consider trying a new pair of 47µf caps such as Nichicon KL's or Panasonic FM/FC's. The KL's have been working great for me.While your in there replace D311. Its a VD1212 in the same circuit as the two caps. Use a couple 1N4148's in series . I've seen/heard this VD varistor go real noisy ,injecting a hissing sound into the pre-amp.Theres also about 6 more of these VD's elsewhere within that get eliminated as well.:thmbsp:
 
Thanks, Avionic. I'll order both the Panasonic and Nichicon, and the diodes. We'll see where that takes me.

Bill
 
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