C2a EQ section idle current, one channel

[Komodo]

Hello,

I've recapped my C2a, replaced one RCA board with a new one, reflowed all solder joints and now am doing setup via the SM. I've left the MC section alone for now, and don't have an oscilloscope to do that adjustment anyways.

-PS voltage set at 40v
-DC offset in EQ section set and holds ~100mv. (SM says 0+-200mV). I'd love to replace those trimpots with new multiturn.

My problem is in the idle current adjust in the EQ section.
R channel is right around the 23mV +-10mV and stays there.

The L channel will read 0, or if the unit is power off for a couple minutes and turned on, the DMM reading climbs up to ~23mV then drops down erratically. Sometime down to ~10mV and back up, sometimes it holds around 23mV then down again, usually it ends up diving down to 0. This is with no source or amp hooked up.

I double checked all the solder joint for the L channel VR205, the TP6 and TP7 test points. I'm thinking it may be an issue just upstream but again this is where I'm no good at troubleshooting. There is a series of 4.7 resistors (R287, R291, R289, R285) just above the test points, a diode and double diode (D211, D213), and a small group of capacitors/resistors (C241, R283, R281, C239).

I've read some threads about replacing those little capacitors as they crack and cause issues, as well as the double diodes being troublesome. I'm not opposed to ordering everything and swapping it all out, but wonder if someone more familiar with this pre and this symptom could shed more light.

Thank you much in advance for your time and help.

 

[avionic]

I've read some threads about replacing those little capacitors as they crack and cause issues

Are you sure those little capacitors are not actually flat resistors.They crack and cause popping and static.

 

[avionic]

There is a series of 4.7 resistors (R287, R291, R289, R285)

Emitter resistors..May have a bad solder connection. Or one thats drifted out of tolerance. Or may have and issue with base resistors R281 or 283. Or transistors TR227 and/orTR231.

 

[clinic-audio]

Hello

I've left the MC section alone for now, and don't have an oscilloscope to do that adjustment anyways.

you need a distortion meter to make this adjustment not a scope . I have made this adjustment in many C2A and I can say that this adjustment is very accurate .

The adjustment , you talk about in EQ board , is always fluctuate , so it is normal in my point of view .

I've read some threads about replacing those little capacitors as they crack and cause issues,

These are resistors (as avionic says) and I did the "warning" in a previous post about C2A cracking , and giving DC voltage to any amplifier connected to it . The noise is sometime very impressive in speakers and may damage your system.

You have to be sure that the final adjustment of DC Offset and Tone control section 0V (+ or- 25mv ) is well made because it is very important for all your system .

 

[Komodo]

Are you sure those little capacitors are not actually flat resistors.They crack and cause popping and static.

Of course you are correct. Late and tired. My main goal for this was to replace the dodgy RCAs (x2) and lucked into finding a new phono side OEM RCA board. There's one more on ebay if anyone needs one. Also, to replace the electrolytics and refresh any solder joints. The pre actually sounded great before with only the dodgy loose RCA jack. I haven't tried any actual audio through it yet.

you need a distortion meter to make this adjustment not a scope . I have made this adjustment in many C2A and I can say that this adjustment is very accurate .

I really need to drink more coffee before I post. Yes, I didn't touch the MC section as I only have MM carts, no distortion meter, or experience using one. I figured this section wouldn't affect the rest of the preamp and would start with the meat and potatoes first. I can revisit this as stage 2 if you think it may require it.

The adjustment , you talk about in EQ board , is always fluctuate , so it is normal in my point of view .

I considered this, but isn't it odd that one channel is stable and the other goes up and then dives to 0?

You have to be sure that the final adjustment of DC Offset and Tone control section 0V (+ or- 25mv ) is well made because it is very important for all your system .

Roger that. Two more things of possible interest, the copper rail had a sketchy join which I reflowed, will double check that again. Also, when i first opened it up VR203 looked like it was turned more than would be typical (which is also L channel). Usually these trim pots seem fairly symmetrical in orientation. The original measurements were strange, going up to 10v and down to -6v on VR204. After reflowing, checking over everything again, etc. the readings were down to mV and I could them to under 200mV, though difficult to get good accuracy under that.

While it doesn't look like someone was mucking around in there before, that one trimpot was a bit odd. I'm starting to wonder if a previous owner thought they had issues with the L channel RCA when it was really something internal. They had carved out the RCA with a meat clever trying to get some kind of connection (I'll post pics, it's ridiculous).

Thank you much for your help. This preamp is the best I've ever had and I really want it to be back in service with it's M4 brother!

(edit) And the plot thickens. This morning I looked at the big transistors closer, and compared to the schematics. It looks like TR228 originally was 2SC1913Q but was replaced with B556A. TR232 was 2SA913Q and was replaced wth D476A. Now all bets are off.
(edit again) Hmm, now I see on the Amb8 site an original C2a and those transistors seem to be stock but are not the ones listed in the SM. Confused.

 

[avionic]

It looks like TR228 originally was 2SC1913Q but was replaced with B556A.

There are 6 transistors mounted on that heatsink. Two are for the power supply pass regulators (B556A and D476A) and the other 4 are (2 each C1913Q and 2 each A913Q)

 

[avionic]

Looking from the front of the unit. Left to right......

B556A - D476A - A913Q - C1913Q - A913Q - C1913Q

 

[avionic]

These are resistors (as avionic says) and I did the "warning" in a previous post about C2A cracking , and giving DC voltage to any amplifier connected to it

Thats where I saw it posted .

 

[avionic]

These are resistors (as avionic says) and I did the "warning" in a previous post about C2A cracking , and giving DC voltage to any amplifier connected to it .

Patrice. Did the volume control exaggerate the DC voltage ?

 

[Komodo]

Follow-up.
I've checked and rechecked every solder joint globally, reflowed many. I've given a thorough visual inspection of all the little red resistors globally under high mag and they all look like new.

I've rechecked the idle current on the Right channel and it's rock solid, right at 23mV with the slightest fluctuations. The L channel comes up to around 23mV after a couple minutes, then dives down, bounces 1-2mV and then 0. If the unit is shut off for at least a couple minutes and back on, it repeats that. I'm convinced it's a thermal event, and that it should stay at 23mV per the SM. Still haven't tried a source or output to amp. Is it OK to try?

Also, if I physically press down on the board just to the left and slightly down of the L channel TP6 and TP7 test points - the mV rise back up to 23mV. So, a thermal solder connection problem? I reflowed everything in the area. Interestingly, directly behind the board "tab" where I press is the solder pads for the TR227 / A913Q. I reflowed those solder joints and still have the same result. Could the legs be moving inside the transistor and causing an intermittent issue? I guess next step is to replace that one?

(I'm pretty sure I found an error in the SM schematic which lead to some confusion. The TR227 and TR231 are mislabeled/swapped.)

 

[avionic]

if I physically press down on the board just to the left and slightly down of the L channel TP6 and TP7 test points - the mV rise back up to 23mV. So, a thermal solder connection problem? I reflowed everything in the area. Interestingly, directly behind the board "tab" where I press is the solder pads for the TR227 / A913Q

May have a hair line crack in the printed circuit trace around the emitter resistors and output transistors. You could try to ohm out the traces. Or possibly a solder bridge across adjacent traces.

 

[avionic]

. Is it OK to try?

It won't work in the channel with zero bias.I'd get the bias(idle current) squared away first.

 

[Komodo]

You could try to ohm out the traces.

Could you elaborate a little on this? I'll get back to it after coffee tomorrow.

 

[clinic-audio]

Patrice. Did the volume control exaggerate the DC voltage ?

Hello Dave
I don't remember and I don't have a defect C2A actually to check that

 

[clinic-audio]

Hello Komodo

Also, if I physically press down on the board just to the left and slightly down of the L channel TP6 and TP7 test points - the mV rise back up to 23mV. change your adjutment pot

So, a thermal solder connection problem ? no , it is mechanical trouble and it will take time to find why

Could the legs be moving inside the transistor and causing an intermittent issue ? no , not possible

(I'm pretty sure I found an error in the SM schematic which lead to some confusion. The TR227 and TR231 are mislabeled/swapped.) sorry but it is wrong . You compare the pcb drawing in SM with the unit in front of view . This pcb is view from components side and the unit is view from solder side that's why you think there is a mistake , but there is not .

 

[Komodo]

Hello Komodo
(I'm pretty sure I found an error in the SM schematic which lead to some confusion. The TR227 and TR231 are mislabeled/swapped.) sorry but it is wrong . You compare the pcb drawing in SM with the unit in front of view . This pcb is view from components side and the unit is view from solder side that's why you think there is a mistake , but there is not .

I was comparing the SM PCB drawing with the SM schematic. The transistors should be in a row, B556A, D476A, A913Q, C1913Q, A913Q, C1913Q. But in the schematic, TR227 is labeled as C1913Q which is why I was confused.

Tonight, I will look for PCB cracks and order new trimpots for the DC offset and idle current spots.

 

[Komodo]

With a loupe, I examined this who area from the VR points to the bottom by the transformer, and out to mid-board. I've reflowed many solder joints to be sure nothing is hiding. I even reflowed the legs of the big transistors, and looked for any hairlines, but see nothing. VR205 holds its adjustment perfectly, and when the mV rise up they always hit the point it's adjusted to.

The behavior is the same. As it powers up the mV rise up to the adjustment point, then dive down, usually going to ~17mV and "fluttering", then back up to 23mV, then down with little bounces back up, until it works it's way to 0 and then stays. It will only go back up if the unit is shut off for a minute or two (thermal?), or if I press on a point right in an area by the jumper between C273/C274, or even right on C273. [this flexes the whole side of the board up, which would seem to open hairlines]

In my search for help I found this (on this site):
"The other thing to remember about adjusting the bias current is that there is a very slow feedback loop that involves the heat of the transistors (usually). To prevent thermal runaway, most class A-B amps will use a bias circuit that senses the heat of the output transistors (or heat sink) and reduces the bias current as they get hotter. So if you turn the bias pot to increase the bias current, this will heat the output transistors and the circuit will react to this after a few seconds or even minutes and reduce the bias current, somewhat offsetting your adjustment."

Is it possible this is what's happening? If so, and since it's one channel, I should direct attention to TR227 / TR231 attachment? Or could the TR themselves be running too hot?

 

[avionic]

Could you elaborate a little on this? I'll get back to it after coffee tomorrow.

It is a very time consuming process to check continuity of the printed traces from one solder pad to the next along that particular trace. By pressing down on the circuit board you are slightly flexing the board. That slight flex can either " make or break " a cold solder connection or cracked copper trace. As your unit heats up during operation. The slight thermal heating of components is enough to expand the copper trace and exploit the bad trace or connection.

 

[Komodo]

Boy, I’ve been over this thing endlessly now. The behavior is the same, except after it warms up flexing the board doesn’t make the mV go up anymore. It stays 0.

I’m going to replace the double diodes that are right by this area but that’s just a shot in the dark that many people have reported issues with. I’m getting close to looking for outside help, but someone competent who knows what this preamp is!

 

[slimecity]

m going to replace the double diodes

Whats the number of this double diode? Is it a small coloured blob? If so, many of these types are known to cause problems, including the VD1212.