So just replace it then.
Seems to be a jump in one of the tracks.
But yes graphs are fun and found it
impressive and neat to see.
Its panel mounted pot, ahhh the good ol days.
So pop in a new 2 gang with switch.
seems to be few caps on the switch
to eliminate popping. could just be leaky
old guys or switches.
" noise" or hiss sounds more like transistor.
DC on a pot is more crackle when rotating
Seems to be a jump in one of the tracks.
But yes graphs are fun and found it
impressive and neat to see.
Its panel mounted pot, ahhh the good ol days.
So pop in a new 2 gang with switch.
seems to be few caps on the switch
to eliminate popping. could just be leaky
old guys or switches.
" noise" or hiss sounds more like transistor.
DC on a pot is more crackle when rotating
The high cut behavior suggests an open in the left channel, most likely in the switch, but also C610, C612 or their paths. Do you find that the high cut switch also fails to reduce treble content in music? I suspect the momentary changes you describe happen when C610, C612 get grounded and momentarily disrupt the bias. I bet that if you quickly cycle the switch, the glitches won't occur because the caps will retain their bias charge. That you experience both high cut failure and bias glitches suggests a dirty, intermittent switch. Deoxitol?
Did cleaning the volume pot solve the "scratchy" problem? I reiterate, if my noise current theory is the issue, replacing the pot with the same 250k, $70 control will not improve the hiss complaint.
I neglected to mention that resistor thermal noise is also a contributor and will be worst at the same wiper positions. But I suspect that bipolar transistor current-noise is the dominate mechanism. I looked more closely at the circuit, and believe it's relatively easy to install a J310 JFET instead of the existing bipolar. No guarantees, but I believe there would be a significant improvement. I'll provide details if you want to experiment.
Yes, I'm willing to experiment for sure, because I'm near the end of my own expertise. To your questions, deoxit work on the pots early on tamed the scratchiness, but I don't think I applied any to the switches before today. On a closer listen this morning, initially the high cut was working intermittently on the left side, but never remained in effect for more than a few seconds. I deoxit-ed the switches, and since then the high cut is not working at all on the left side. On the right side, where it does work reliably, it reduces highs with an audio source, and it also cuts the hiss we're talking about at 25/50% volume.
I have not replaced any resistors or any ceramic caps in the unit, but many of the transistors and nearly all the electrolytics are new. All the electrolytics and transistors in the control amp unit section are new, now that I've swapped out the 2SC871 and 2SC872 on both sides with KSC1845FTA.
Just a minor detail, odd numbers are used for the left channel, even for the right, so the two ceramics in the control amp for the high cut are C609 and C611. After work I'll pull them out and test them out of circuit.
I wish I'd tried your experiment when the volume pot was out! I did go easy on the solder putting it back, since I thought it might be coming out again. But that's a very helpful suggestion. When it was out, I did measure resistances between all terminal pairs at every position, so I should be able to do the exercise on paper. I'm not an expert but as a former physics major I reckon I can figure it out.
I have not replaced any resistors or any ceramic caps in the unit, but many of the transistors and nearly all the electrolytics are new. All the electrolytics and transistors in the control amp unit section are new, now that I've swapped out the 2SC871 and 2SC872 on both sides with KSC1845FTA.
Just a minor detail, odd numbers are used for the left channel, even for the right, so the two ceramics in the control amp for the high cut are C609 and C611. After work I'll pull them out and test them out of circuit.
I wish I'd tried your experiment when the volume pot was out! I did go easy on the solder putting it back, since I thought it might be coming out again. But that's a very helpful suggestion. When it was out, I did measure resistances between all terminal pairs at every position, so I should be able to do the exercise on paper. I'm not an expert but as a former physics major I reckon I can figure it out.
I'm glad contact cleaner improved the pot. Now there's added incentive to improve the hiss level without replacing the pot. 😀
If your pot data includes the resistance from T to G and T to P, I'd be interested in knowing those two values.
I recommend getting all the tone control switches, etc, working to your satisfaction before experimenting with a JFET.
Would you measure the base, emitter, and collector voltages on the first stage following the volume control? My schematic is too marginal to read with confidence, but the nearby resistors on the base are R601 and R603.
I'll write some notes about how to install the modification.
If your pot data includes the resistance from T to G and T to P, I'd be interested in knowing those two values.
I recommend getting all the tone control switches, etc, working to your satisfaction before experimenting with a JFET.
Would you measure the base, emitter, and collector voltages on the first stage following the volume control? My schematic is too marginal to read with confidence, but the nearby resistors on the base are R601 and R603.
I'll write some notes about how to install the modification.
T-G resistance is 78/85k, T-P resistance is 180/186k.
That first transistor is Q601 (indeed hard to make out on the schematic but readable on the layout), and with a diode test in circuit I get B-C 0.668 V, B-E 0.675 V.
That first transistor is Q601 (indeed hard to make out on the schematic but readable on the layout), and with a diode test in circuit I get B-C 0.668 V, B-E 0.675 V.
Thanks!
Here's a recipe for for trying a JFET.
1. Remove/detelete R601.
2. Replace R603 with 1M.
3. Replace Q601 with a J309. Data sheet attached for pin outs. Base to gate, collector to drain, emitter to source.
4. Trim R607 to adjust the bias current, the goal being to develop roughly 10V between drain and source of the FET. I suggest initially removing the 1k resistor installed and solder tacking resistors on the bottom of the board, trial and error fashion. Try 100 Ohm initially; if the drain-source voltage is too low, increase the resistance. Alternately, tack a 1k pot with 0 Ohm initially and adjust for 10V drain-source; measure the resistance and install the nearest 5% value at the R607 site.
Modify a single channel first to determine if hiss is improved. Hiss will be worse at the same volume positions, but I hope will be improved. Of course, the mods are reversible if the results are disappointing.
One other curiosity is the combo of R607 and C605. It has a "corner frequency" of 159Hz, but instinct tells me it should sub 20Hz. I think you'll be able to assess noise performance without addressing this subtlety. Once R607 is determined, you can procure appropriately larger caps. A 6V voltage rating should be plenty to help package size.
Here's a recipe for for trying a JFET.
1. Remove/detelete R601.
2. Replace R603 with 1M.
3. Replace Q601 with a J309. Data sheet attached for pin outs. Base to gate, collector to drain, emitter to source.
4. Trim R607 to adjust the bias current, the goal being to develop roughly 10V between drain and source of the FET. I suggest initially removing the 1k resistor installed and solder tacking resistors on the bottom of the board, trial and error fashion. Try 100 Ohm initially; if the drain-source voltage is too low, increase the resistance. Alternately, tack a 1k pot with 0 Ohm initially and adjust for 10V drain-source; measure the resistance and install the nearest 5% value at the R607 site.
Modify a single channel first to determine if hiss is improved. Hiss will be worse at the same volume positions, but I hope will be improved. Of course, the mods are reversible if the results are disappointing.
One other curiosity is the combo of R607 and C605. It has a "corner frequency" of 159Hz, but instinct tells me it should sub 20Hz. I think you'll be able to assess noise performance without addressing this subtlety. Once R607 is determined, you can procure appropriately larger caps. A 6V voltage rating should be plenty to help package size.
Hey, truly appreciate these ideas. I'll try and sort out this defective high cut on the left side before I proceed. Have not looked at those caps yet. Dog ate my homework.
I fixed the high cut problem. I was measuring resistance across the terminals of the high cut switch, and something seemed off, so I removed all the connections and pulled the switch out. Sure enough, there was no connection on the left side with the switch engaged. I gave it more shots of Deoxit Fader (now with the benefit of gravity to flow into the switch) and cleaned the contacts, and the connection on the left started to work. I reinstalled the switch, and the high cut is now working on both sides. I'd already pulled out ceramics C609 and C611, so I measured those and put them back as they seemed fine.
The unit is sounding really good. All I have left is to replace the cord with one that has a PE connection to the chassis, and try taming the hiss. I wouldn't make that big a thing of it, but it's especially prominent with headphones (present through speakers but barely audible), and I will be using this unit with headphones a lot.
The unit is sounding really good. All I have left is to replace the cord with one that has a PE connection to the chassis, and try taming the hiss. I wouldn't make that big a thing of it, but it's especially prominent with headphones (present through speakers but barely audible), and I will be using this unit with headphones a lot.
