I have never been able to eradicate audible “thumps” from the outputs of my 902 clones.
The 100 r trimpot at Q8 Q9- SIGNAL INPUT seems critical to eliminate these"thumps" but the adjustment is super fine.
At the right point I can get very faint “thumps” but can never totaly loose them …but if I slightly catch the trimpot they return. It seems an incredible fine adjustment for such an important function.
For trimpot P3 I am using two 1.2 r resistors.
My clones are spot on to Norlin Schematics and all paired transistors are matched.
Is this audible noise normal? Can it be reduced in any way?
On the schematic I have P1 is 10ohm not 100ohm… but that’s probably not the [problem…!?
I have three R.A. 902’s and cant really say the ‘thump’ you mention has really bothered me. There is some, but it seems to go unnoticed once I’m immersed in my beast…;c)
No Tom ,its not P1 its signal input… def 100r…yes, on percussive full tones its not noticeable , but on certain timbres its a very dominant unwanted signal. Maybe thats just the way they are?..theres also a 100 r resistor in parallel with the trimpot and both are valued in a large bold font…is this significant?
Those Norlin schematics are a bit confusing, since they are mostly the last revision made. They are copies of copies and several revisions on top of each other. Best if you can to get a look at some of the older revision schematics to compare those many changes made. “Signal Input” is refering to the circle above where the + input is. The trimmer next to it on the right, is the Q8/Q9 balance trimmer.
Any clicks/thumps on my 902’s were usually the result of voltage offsets, trimmed out via P2 output bias. Looking at a scope, adjusting the P2 will give a zero offset, reducing any pops/clicks/thumps. Just adjust for a flat line with fixed voltage at 6, any offset plus or minus will result in an audible thump.
I’m not really getting any noticable thumps from either +/- outputs.
The rest of the adjustments are pretty well documented on the Norlin 902 schematic. The 100R trimmer with 100 ohm resistor is the adjustment for Q8/Q9 balance. (P5?) The procedure is outlined in step 8 after completing step 7.
The 1969 revision 3 902 VCA here has many different parts and values from those Norlin ones. The Q8/Q9 balance trimmer (P5) is a 2.5 ohm with a large .47 ohm resistor. There is no bridge rectifier. Probably the toughest part of dialing in a 1969 902 is identifing where the trimmers are. My 902 does not have a P4 trimmer, but rather has a piggy-backed resistor selected for the input bias. One could easily confuse where the proper trimmers are, or finding Q8/Q9 and Q6/Q7 for the test procedure.
The 1969 rev 3 902 and trimmer ID:
(Q8/Q9 just above P5 and the large .47ohm resistor, with Q6/Q7 pair further above those.)
Nothing like the trimmer/part locations on the Norlin daigram.
Tom..in fact you were correct..Signal input does have a 10r inp. balance ..but the Norlin Schematic has me mislead …at Q8/9 its labeled SIGNAL INPUT which I now realise of course its refering to Q6/7.
Big thanks also CZ…these early schematics are very interesting having only my Norlin 902.
Excellent photo too ! Maybe I will build a 902 to original layout as my 904a’s and this info is gold dust.
Early attempts to modify the Q8/9 emitter resistors are not good, makes the problem worse.
I wonder if to stick with Norlin mods and put a multi turn Cermet 100r along with the 100 r…I can adjust now to diminish the knock/thumps but I guess putting the module back slightly puts the 100r out again or it may drift .
The thump/knocks are particularly noticeable with triangle waves at low input to 902.
@ rarecomponent, it’s amazing to know that you got rid of your “thumps”, well at least because it gives me some hope. Big thanks to CZ for providing with the info. One question though, I’ve changed my components as suggested in the posts above but I’m really finding it difficult to tune. How do you tune it so that the “thumps” are gone completely?
I used matched Q6-7 / Q8-9. / Q11-12 and also the relevant 100 / 150 /15 R resistor pairs.
But the adjustment of the 100 R trimmer is critical. I have a midi sequence playing repeated short sharp attack triangle waves thru the VCA to be adjusted then holding my breath I adjust the trimmer roughly mid way and then micro adjust either way until the “thumps” are at their least audible. I have replaced my standard trimmer with a Bourne multi turn which makes this more accurate and stable.
Ive also matched the control transistors and used two matched 1.2 R resistors
for P3..all this seems to help eliminate the “thumps” but I’m still looking for any further tweaks as well.
