Samson XM610 offset

[guitardad]

Fun one!

Had the typical cap issues in the PS (bad 10000/ mfd). Now I have about a neg 110 mv offset on Ch1. Ch2 is fine (8-9mv). Not much to this board so I have no clue why I am chasing my tail. lol. Checked and even swapped diff amps, 4148's (none were leaky). Opened the line from R100 input from the mixer section (just in case) but still manage to have this offset. Odd part is the unit seems to work ok (no clip or dist). I have not cranked it off the variac yet, because that offset difference might prove to be catastrophic. I doubt the bigger semis would cause this.

Taking a break! Just wondering if any tech notes for a cause of this.

Thx in advance

[Enzo]

Tech note? If I find only 100mv offset, I think not bad, and move on to other things.

Is the offset still there when the speaker load is connected?

Not off the variac? Does that mean you are not running it at full voltage?

[guitardad]

About the same no load and loaded. About 15mv higher at full AC.

Came in with the cap issue, noisy hum both ch's clipping dist issues (cust complaint).

Now it has that offset. Ch 1 and 2 should be identical or at least close. I can adjust it down to about 60mv, but the current draw is higher at idle. I don't think that would be right either. Calls for about 7-12mv on the board.

I start to notice a bigger change at Q114/115. E on the good ch (2) are .575 and .535v. On the suspect ch (1) I have -.708 (Q114 E) and .45v (Q115 E). The 110mv starts at the Bases of Q114 and Q115. I would think they should be near zero. That is why I opened the coupling cap and even the resistor R100. IDK, but it would seem more like C-B leakage providing any DC at those bases, unless the schem is wrong. In a perfect world, looks to me as if it should be 0vdc on that base junction of Q114-115. No other DC path or source.

[Enzo]

Now it has that offset. Ch 1 and 2 should be identical or at least close

Not exactly. The two channels should fall within an acceptable range of the various parameters that matter. Being identical is not important. In other words if one was at 23mv and the other was -23mv, they would be the same amount offset.

I can adjust it down to about 60mv, but the current draw is higher at idle. I don't think that would be right either. Calls for about 7-12mv on the board

How are you adjusting offset? There is no offset trimmer that I am aware of. And the board asks for 7-12mv of offset??? If you are twiddling bias, stop, and put it where it should be. Offset and bias are separate issues.

In a perfect world, looks to me as if it should be 0vdc on that base junction of Q114-115. No other DC path or source.

Well, those are transistors, not 12AX7s, so the base is part of a circuit, not a current dead-end. The other DC path for current is through the transistors.

Current source Q112 feeds Q115,Q117. And to the extent Q115 emitter is more positive than the base, currents will flow through R103. However, the flip side, Q113 is the negative side current source for Q114,Q116, and the same story occurs for them going the other way. Ideally we might expect them to balance out at R103. But ideally and really are not always the same thing.

You have 110mv offset at the output and you have 110mv offset at R103? Sounds magical. So look at those current sources. I see four diodes, D105-108. check the voltage drop across each one while it is running. Are they all four dropping the same amount? And so are the bases of Q112,113 at about the same voltage, but opposite polarity?

Look just left of your output drivers. See the two 4.7 ohm 2W resistors? On the right end of each is a main power rail. Are both up to the same voltage? Now the left side is for the low current circuits, do you have the same voltage on the left ends of both - opposite polarity of course.

This is a solid state amp, and so is one massive feedback loop. Q114,115 do not operate alone, they have Q116,117 paired with them, and those are the feedback of the output sample. If the output is not the same as the input, they try to shove the output where it ought to be. SO your ofset can really occur just about anywhere. Don't lose sight of that.

[guitardad]

Maybe this will illustrate things better. I changed all the 4148's, and the transistors in Yel highlight. No change.

[Enzo]

OK, so the foundation along the left is OK. And at the diffy pairs you have a tenth of a volt offset, which is reflected at the output.

I am sure the cause of this could be found, but 100mv on the output translates into a whopping 2 milliwatts excess dissipation into a 4 ohm load. So at that point I consider it an academic exercise, and not a problem to solve. But I do hope you find it.

[guitardad]

OK, so the foundation along the left is OK. And at the diffy pairs you have a tenth of a volt offset, which is reflected at the output.

I am sure the cause of this could be found, but 100mv on the output translates into a whopping 2 milliwatts excess dissipation into a 4 ohm load. So at that point I consider it an academic exercise, and not a problem to solve. But I do hope you find it.

For giggle I swapped the diff/fb xist pairs again with a A1016/C2362 complement and the offset went to -234 mv. Put the orig's back in and back to 110mv. So I can assume the dc at R103 comes from those diff pairs. But like you said that source of dc can come from anywhere in the loop. Being it is neg polarity (offset), I assume the pos side of the circuit is likely the source of the offset condition?

100mv isnt much, I just wanted to be sure. One of those thing you become conscious of and I don't like to be married to units. I am going to run it full load for a day and see what happens.

I had another thread where these kind of offsets drive me nuts! I have seen everything from leaky diodes (4148's esp), to leaky transistors. They test fine, but leak in operation. Even coolant spray doesn't affect them half the time. Short of shotgunning, there is no better way to find the issues if they are semi related.


Thanks so much for looking at the analysis diagram and for your assistance.

One more thing..... is there a good source for KTC3400's? Seem to be a rare bird. I can find the A970's, but the C3400's are next to impossible and only show up as obsolete.

[Enzo]

Careful. Compare the KTC3400 data sheet to the 2SC3400. They are not the same.

If it were me, I'd rummage around my collection for some other 120v parts. But you might just call Samson, and ask if they have them, and ask what they are using in their place if not.

[Jazz P Bass]

The KTC3400 crosses to :

NPN Silicon

2SC2362K, 2SC2631...32, 2SC3800


-/120V, 0,1A, 100MHz

[guitardad]

That's what I did when I changed them to 2SA1016/ C2362 pairs. It didn't like those.

The A970 complement is a C2240

Samson said replacements are 2N5551, N5401 but pinouts are different.

[Enzo]

These are small TO92 transistors. The asian ones have a pinout of ECB while the USA types have EBC. You can replace one with the other as long as you turn the thing sideways and form the legs so they go in the right holes. SImple enough.

[Dr Kaos]

Have you checked/replaced C115. If this goes leaky or short it will reduce the DC loop gain and lead to increased offset.
These capacitors often get over-voltaged or reverse biased when there is an output DC fault.
Its job is to provide a path for the voltage divider R131/R132 for AC signal but not for DC offset signal.