and now for something completely different part 3

[Pars]

I take it that you have checked to make sure all components are the correct value and in the right place? From your voltage measurements, everything looks more or less identical until you get to the servo area. What are the voltages on the inputs to the opamp?

Also, in your picture previously, 2 of your output devices appear to be missing screws into the heatsink.

At this point, I would probably replace all the 15030s/31s given your probe slip just to make sure. These boards are pretty easy to rework.

[Satyrnine]

1 hour ago, Pars said:

I take it that you have checked to make sure all components are the correct value and in the right place? From your voltage measurements, everything looks more or less identical until you get to the servo area. What are the voltages on the inputs to the opamp?

Also, in your picture previously, 2 of your output devices appear to be missing screws into the heatsink.

At this point, I would probably replace all the 15030s/31s given your probe slip just to make sure. These boards are pretty easy to rework.

Yes, I've visually verified all parts, and ohmed out each resistor to verify. Checked for cold solder joints, polarity/orientation/etc.  N's and P's in right spots. 

Good eye Pars! Yes, in that pic the two toasted output devices had the screws removed, I've since replaced with new official distributor parts. I do have spares, I'll just replace all the 15030/31's as you suggest. Those are easy as you say, esp with my new Hakko desoldering gun. 

Inputs to opamp would be the feed/sense from the output right? 

I should add, I have the amp working in SE mode now, with 120ma bias or so, and it absolutely blows my mind. Unreal power and slam, and the dynamics and detail are just wild. Raw dogging for now without protector board with my LCD-2's and LCD-X's. 

Edited June 8, 2023 by Satyrnine

[dip16amp]

If the 5k pot measures the same as the good board but it's position is at it's end, maybe replace the pot.

 

[simmconn]

@SatyrnineWhat does it mean by the minus sign on the voltages? It wouldn’t make sense unless you mark which pin had the red probe on it and which one had the black one. You might instead mark the actual polarities, like the pin with higher voltage with a (+). For voltage to ground such as the servo output, it should be the actual voltage. You marked -10V on the schematic but said 10V in the post, which one is true? Based on the voltage drops on R19/R21, it should be 10V, so the lower current mirror should have more current than the upper one. But that’s not the case when you compare the voltage drops across R25 and R31. So something must be off there. The transistors seem very mismatched. Btw, most of the reference designators are hardly legible on your hand-marked schematic. If you expect people to help you, at least make it easy for them to do so.

[Satyrnine]

3 hours ago, simmconn said:

@SatyrnineWhat does it mean by the minus sign on the voltages? It wouldn’t make sense unless you mark which pin had the red probe on it and which one had the black one. You might instead mark the actual polarities, like the pin with higher voltage with a (+). For voltage to ground such as the servo output, it should be the actual voltage. You marked -10V on the schematic but said 10V in the post, which one is true? Based on the voltage drops on R19/R21, it should be 10V, so the lower current mirror should have more current than the upper one. But that’s not the case when you compare the voltage drops across R25 and R31. So something must be off there. The transistors seem very mismatched. Btw, most of the reference designators are hardly legible on your hand-marked schematic. If you expect people to help you, at least make it easy for them to do so.

You're completely right, arbitrary to have a negative since I was just measuring drop across the resistors. Dukei recommended measuring drop across resistors vs voltages to ground. I mindlessly added the negatives when I was jotting it down, just a matter of probe position in this case. +10v is the proper voltage after the servo in this case.

Thanks for the input, I'll investigate further. Sorry about the size. The resolution is decent, but on a phone or such device it may be tough. The schems I was jotting on were small, tried to keep everything visible. I def often write too small. 

Edited June 8, 2023 by Satyrnine

[Satyrnine]

Swapped all 15030/31’s and the bias issue is solved! Behaving nicely. I tested all the removed transistors with a cheap ebay tester and none tested “bad” but I realize thats a limited test. Thanks for the help all. I can finally try this beast in balanced mode!

Still waiting on tube input boards from Dukei who had spares. Also need to wrap up dc connector details and order chassis yet. 

Edited June 9, 2023 by Satyrnine

[mwl168]

16 hours ago, Satyrnine said:

Swapped all 15030/31’s and the bias issue is solved! Behaving nicely. I tested all the removed transistors with a cheap ebay tester and none tested “bad” but I realize thats a limited test. Thanks for the help all. I can finally try this beast in balanced mode!

Still waiting on tube input boards from Dukei who had spares. Also need to wrap up dc connector details and order chassis yet. 

Assuming all the removed transistors are indeed all good, this leads me to wonder if the original issue was caused by cold-soldering?

[Satyrnine]

2 hours ago, mwl168 said:

Assuming all the removed transistors are indeed all good, this leads me to wonder if the original issue was caused by cold-soldering?

It's possible, although my soldering skills are pretty developed. I like use Kester 186 liquid flux, Kester 44 60/40 solder and a decent Hakko Iron at 750*. Taught by a guy who helped make some electronic parts for the F117 fighter as a civilian contractor even. Everything is always nice and shiny and liquid looking. I wonder if like tubes, a tester only tells you a part of the picture. I've had numerous tubes test OK on a tester, but not in circuit.

[Satyrnine]

Trying to diagnose some excessive offset on one board. Likely culprit is unmatched N/P pairs I assume. I ordered 20x spare of each bc556 and bc546 but there's no way I can match these. 546's are all about 215hfe, and 556's are all about 120hfe. Same with ptza06 and ptza56, at least 50hfe difference between N and P types, but tight matching within their own type.

I replaced the input pair and the ptza's with the closest hfe match I could find, helped offset ever so slightly, maybe 20ma. 

Should I be matching vbe instead of hfe? Sorry, tube guy learning the ropes.

Edited June 10, 2023 by Satyrnine

[dip16amp]

What is the offset without the op amp? It should be around 40mV. Then the op amp will bring it down to near zero.

Don't think the transistors hfe would be the cause of high offset.

What are the other boards offset without the op amps?

 

 

[Satyrnine]

1 hour ago, dip16amp said:

What is the offset without the op amp? It should be around 40mV. Then the op amp will bring it down to near zero.

Don't think the transistors hfe would be the cause of high offset.

What are the other boards offset without the op amps?

 

 

Without opamp, about 400ma 😬 The others are around 40ma as expected. 
 

I Just replaced all the output trannies, the input pair, and the ptza’s with as close matched parts as I could, no luck. 

[dip16amp]

I'd measure the voltage across each 1 ohm resistors, then the 1m, 100, and 560 ohm resisters.

 

[simmconn]

Is it the 'ok bias' channel in your schematic that has large output offset with servo disabled? I gave my analysis in my post. Q24 and Q25 form a current mirror. The current through R21 should be the same as the current thru R31 and thru the R26. Apparently it's not the case based on your measurement. I would focus on the related few components.

[Satyrnine]

1 hour ago, simmconn said:

Is it the 'ok bias' channel in your schematic that has large output offset with servo disabled? I gave my analysis in my post. Q24 and Q25 form a current mirror. The current through R21 should be the same as the current thru R31 and thru the R26. Apparently it's not the case based on your measurement. I would focus on the related few components.

Ack, I apologize simmconn, yes its the “ok” board. I’ve been wracking my brain on this and failed to recall you’d already pointed out that the OK is def not OK. Thank you for your help, will investigate there, makes perfect sense.

[Satyrnine]

Well, the problem is I’m an idiot apparently. After going over the board carefully so many times, I came back to it after a breather and there it was, Q8 in backwards. I’m so sorry for wasting everyones time with this. 😫

[dip16amp]

Good to hear the problem is solved. Sometimes stuff happens, but at least it can be corrected.

 

[migo]

I have some NEW unused 2SA1860 and 2SC4886 to share here. Located in EU. If someone interested PM me please.

[justin]

Been working on this for a looong time. CFA3 from scratch mixed smd/thru-hole design. Currently assembled with NOS Toshiba 2SC3381/2SA1349 inputs, NOS MPSW TO-92L thru-hole transistors and bcxxx for the smd. Power consumption 75W no fan. Linear PSU with 20,000uf/63V filter capacitance — custom golden reference regulators. Final dimensions are 12x9x4.5” / 22lb+

 

the motorized 48 step attenuator w/ remote and display was a last minute addition. definitely interested in finding out the feedback for that, but would want it built backwards for a final design, I think.

Edited June 24, 2023 by justin

[chinsettawong]

Is that a stepper motor connected to the volume pot?

[justin]

Yes

[Satyrnine]

20 hours ago, justin said:

Been working on this for a looong time. CFA3 from scratch mixed smd/thru-hole design. Currently assembled with NOS Toshiba 2SC3381/2SA1349 inputs, NOS MPSW TO-92L thru-hole transistors and bcxxx for the smd. Power consumption 75W no fan. Linear PSU with 20,000uf/63V filter capacitance — custom golden reference regulators. Final dimensions are 12x9x4.5” / 22lb+

the motorized 48 step attenuator w/ remote and display was a last minute addition. definitely interested in finding out the feedback for that, but would want it built backwards for a final design, I think.

A friend is at canjam right now and just sent me a photo of this beast there. Great work!

[dip16amp]

Here is my new CFA2 LargeT version
up and running with large triple output transistors pairs
it first started up at 200ma bias and turned it down to 150ma
offset is less than 1mv with the servo

The big resistors are 1 ohm 5 watt 2% Metal Oxide Film MOSX5C1R0G
the other resistors are RN55 & CMF55
the caps are
560pf 100v FG28C0G2A561JNT0
47uf 25v FG26X5R1E476MRT0

 

[dip16amp]

I noticed that 1 of the 3 bias was lower than the other 2 after warming up.
Had to add a couple of 20 gauge copper strips to thermally connect the 2 heat sinks together.

 

[Satyrnine]

Does anyone have any reading/data/thoughts of case temperature of the 15030/1's at various bias levels? I'm on temp less than ideal heatsinks, but direct mount with thermal paste. Getting about 50*C across the board after and hour or so with 30v supply and around 120ma bias. (Planning on 200ma once I'm on the dissipante sinks) Using a 4 channel thermocouple.

[dip16amp]

On 7/7/2023 at 10:37 PM, Satyrnine said:

Does anyone have any reading/data/thoughts of case temperature of the 15030/1's at various bias levels? I'm on temp less than ideal heatsinks, but direct mount with thermal paste. Getting about 50*C across the board after and hour or so with 30v supply and around 120ma bias. (Planning on 200ma once I'm on the dissipante sinks) Using a 4 channel thermocouple.

ran my CFA2LT for 70 minutes at 140ma and it got up to 49c
used a fluke 87v with thermocouple probe on the middle 2sa1860 mounting screw
minute temp
0  23
5  34
10 39
15 42
20 44
25 46
30 47
45 48
48 49
60 48
70 48