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3300 Control Console pops when switching High Filter switch if Tone is IN.

jd3rdcoast

jd3rdcoast

Super Member
Hi All, I am trying to finish up a Model 3300 Preamp that was restored by someone. Was brought to me not passing signal on right side. All transistors on the Phono/X10/Tone boards had been replaced with KSC1845 and KSA733's. I did locate a PNP transistor that was in backwards on the X10 Board. It was Right Channel Q406 (PNP) that had a KSA733-C installed wrong because it is a E-C-B pinout. All others were KSA733 E-B-C. Also, I touched up all the solder joints for the 3 boards on the Interconnect board. I also was having a popping noise crop up after fixing that transistor on Right side. I noticed the +Vcc was jumping around when hot, traced to the +Vcc regulator on P/S board. The Base leg fell off when desoldered due to corrosion. They were original S36049/S36048 that I replaced with 2N3440/2N5416 I had in stock. +/-Vcc now stable 18V.

Everything seems to be working fine other than when the Tone switch is moved from Flat to IN or IN back to Flat, I get a Positive audible pop which can be seen on scope. Also, I get a pop each time I move the Hi-Filter Switch from Out to either setting. Negative blip going from Out and Positive blip when coming back to OUT position. No other switches cause any audible popping.

I am also seeing on scope about a 20mV high frequency waveform on both channels with VOL at rest if the TONE is IN. When Hi-Filter is engaged, the noise amplitude increases slightly. But If I start to raise the volume the noise quickly diminishes and goes flat by the time VOL reaches the 9:00 position and moving the Hi-Filter or Tone switch does not create a pop.

Wondering if anyone else has ever seen this type of anomaly on the 3300 or have a suggestion? Here is pic of Left side Hi Filter on X10 and Tone circuit. The yellow trace path is when Tone is Flat (No noise and no pops).

3300-L-X10-Tone.jpg
 
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Does that happen in both channels? If the transistors were replaced wholesale with ksa1845 you could be having issues with oscillation. These are not the most approriate to replace SPS1928 which usually have a very high gain. The closest replacement I know of is BC550C. I've found out that in general the US made units don't play well with newer KS transistor types.
 
ocdecio said:
Does that happen in both channels? If the transistors were replaced wholesale with ksa1845 you could be having issues with oscillation. These are not the most approriate to replace SPS1928 which usually have a very high gain. The closest replacement I know of is BC550C. I've found out that in general the US made units don't play well with newer KS transistor types.
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Thanks @ocdecio, I was starting to think the same. I believe it is oscillating. I will get a pic of the hi-freq noise. It looks like a ringing at the front end of a square wave on an amp but dirty, even though no input, then as the Vol starts up the dirty party clears up but the ringing on front end remains when you look at in with very low Volt/Div on the scope and in AC mode.

What about the PNP replacements? Also the BC55 compliment to the BC550C?
 
If you are referring to 2N4250 I believe PN4250 is still being manufactured.
 
Yea I was.
Here is a couple pics of the signal measured at output of Tone board. You can see the noise in both (Right on top). The noise drops out showing the ringing I mentioned in second pic. Scope is set at 5mV/Div and .2uS time sweep. The extra noise is not present on Left side input from x10 but it shows up on Tone output and 3/4 of the amplitude.
 

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Wow that doesn't look good. I would recheck all subs and do the best to have them replaced with ones that match the original better unless you want to experiment with some Miller compensation caps which is a bit of an art.
 
What’s weird is can’t really hear it and it sounds good going into my 250 without destroying it. Lol. But, I’m pretty sure it’s not right and I will get the proper subs.

One more thing. The 1uf coupling caps on Tone were replaced with 3.3uf 63V WIMA’s. One of those were indicated as a polorized caps. Not sure if is of any consequence. One was 250V also.
 
jd3rdcoast said:
What’s weird is can’t really hear it and it sounds good going into my 250 without destroying it. Lol. But, I’m pretty sure it’s not right and I will get the proper subs.

One more thing. The 1uf coupling caps on Tone were replaced with 3.3uf 63V WIMA’s. One of those were indicated as a polorized caps. Not sure if is of any consequence. One was 250V also.
Click to expand...
Even bi polar caps have a negative and positive perhaps this is causing a potential issue and might be good to restore to stock to isolate it
 
Here is what I recall viewing on Mr. Carlson's Lab. You want the capacitor outer plate connected to the lower potential part of the circuit (ground or lower potential). He shows how to test the caps to determine which is the outer plate. xraytonyb also has a similar video.
 
dankik said:
Here is what I recall viewing on Mr. Carlson's Lab. You want the capacitor outer plate connected to the lower potential part of the circuit (ground or lower potential). He shows how to test the caps to determine which is the outer plate. xraytonyb also has a similar video.
Click to expand...
This is a great explanation whilst in most instances it doesn’t matter in circuit designs and situation it makes a difference
 
Well, good news all. I updated the X10 and Tone Board transistors and that got rid of the oscillation noise as well as the popping on Tone Switch and Hi-Filter switches. :banana:

I replaced all the NPN KSC1845's with matched BC550C's and the PNP KSA733's with BC557BTF's. The B has higher gain which is closer to the original 2N4250 high gain transistor. Mouser is out of the BC560's (NPN) and I had the BC557B's on hand which are basically the same. Also, Mouser doesn't carry the higher gain BC557C which I would have preferred but it is listed as obsolete (same at DigiKey).

The Phono board is all original other than new electrolytic Nichicon FW's and sounds good.

So hopefully someone in the future will read this and avoid those lower gain transistors in a 3300 PreAmp. They function but not properly and who knows what additional issues could crop up with extended use.

When I order for restoration of my personal all stock 3300 I'll get some extra proper capacitors to replace the 3.3uf WIMA's that were used in this Tone board. I realize now mine could sound a bit better and more balanced with a proper restore.

JD Out
 
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