Speedball small board voltage higher by factor of 2 [resolved]

[Mark Ferring]

No worries coming back to the same thing. I get your logic completely and am happy to keep trying things. I really like following any leads that would have potential voltage effects on both the A and B side.  I have built a few kits in my day, but I have tremendous respect for your history and experience. I will gladly follow any lead you suggest.

To that end, unfortunately, T7 and T9 were both in the 2930-2500 Ohm range. I wiggled the big cement resisters around (and every other wire going into T3) and could not get the reading to change.

Quick question. One of the leads on each of the 237 Ohm resistors is really close to one of the leads of the 2907 transistor. Are there any potential issues if the solder on those two joints touch each other? I don't think they do but my old eyes are having a hard time telling (and this is adding my reading glasses to my driving glasses for extra magnification!).

One other quick question for the learned master......I don't remember the math from my old engineering circuits class 40 years ago, but is there any single place in the circuit where a bad connection or component could cause this almost  perfect doubling of all the voltage readings?

[Doc B.]

Can you power the amp back up and measure A3 and A8? should be around 1.5-1.6VDC.

[Mark Ferring]

Hmmmm

They both read 2.5

[Doc B.]

OK, let's take a small sideways step. Carefully check the AC volts across power transformer terminals 4 and 5, which supply the heater current. Should be around 6.3VAC. Just want to verify the meter calibration.

If that is OK, I think maybe it is time to consider replacing the PN2907 that was in backwards. I don't have an exact scenario in my head of how one 2907 would throw both sides off and pull the cathode LEDS up from 1.56V, but that should more or less take you back to where things went haywire.

Another thing you can try is pulling the 6080 and remeasuring DC voltages OA, OB and A3 and A8. I don't imagine they will change, but it might give some more info.

[Doc B.]

Maybe one more step before replacing the 2907 -

I keep coming back to T3 connections being the culprit. One more reflow of all the connections on T3 might be worth trying. If that doesn't help you might want to try temporarily disconnecting IA/IB, OA/OB and reattaching the original 22K resistors to verify that voltages go back to normal. That would narrow the issue down to the board itself and not the ground connections.

[Mark Ferring]

Transformer across T5 and6:       Measured 5.3V for two different meters

6080 pulled:     No voltage change at OA/OB or A3 or A8 (actually small drop of voltage on OA/OB to around 141V)

Reflowed T3 one more time.         Did not change anything.

Reconnect original 22K resistors:     I foolishly clipped them off at a point where I cannot use them.

I would be happy to go back to the original 22K resistor config or try a new 2907. Is there a way to source those parts from you? I live in the middle of nowhere in the mountains of New Mexico and don't have access to an electrical supply house. I would be happy to pay of course.

[Doc B.]

Transformer across T5 and6:       Measured 5.3V for two different meters

Hopefully that's a typo and you measured from T4 to T5. 5.3VAC is kind of low. What is your AC mains voltage at the wall outlet?

If there is just a little stub of lead left you could temporarily clip lead or tack solder the 22K resistors to the red and white IA/OA pair and IB/OB pair of wires after disconnecting them from the PC board.

You can order parts by emailing [email protected]

[Mark Ferring]

No typo.  Measured again (after going back to original configuration) and still got 5.3V. Outlet is 119.8V

Thanks for the experienced tip (regarding resistors). I got them hooked back up and .........holy cow....Here is what I got.

I reran all of the original resistance measurement checks. All were as expected per the manual.

Now for voltage. I think we found our general problem. The below seems to imply it is not the board.....I assure you that originally they all measured normally. I then listened to my HD650s (with much pleasure) for over a month. However, now they are all messed up. I will only provide the measurements that were not "expected"

T1    163
T2    385
T4    385
T5    161
T7    226
T9    226
T13  385
T15 431
T21 478

A1  162
A6  163
A9  5.3 vs 0

B1  164
B2  391
B3  230
B4  161
B5  390
B6  233
B7  5.4  vs 0

[Doc B.]

No typo.

Sorry, I think I wasn't very clear. I was asking for an ACV measurement from T4 to T5 on the power trans and you said you measured 5.3V from T5 to T6 on the power trans. What I meant was I hoped that T6 was a typo as it should be from T4 to T5.

Add to that that you are seeing 5.3V (is this AC? DC?) at A9 and B7 and I am further confused. T4 of the power transformer is one end of the 6.3VAC heater secondary winding. It should be connected to ground at either T22 or T14. And that should mean that the other end of the heater secondary at T5 of the power transformer - which is connected to A9 and B7 - is effectively connected to DC ground as well and should also read 0VDC. If you are seeing 5.3V it is either 5.3VAC and you need to switch your meter back to DC or something is floating that should be tied to ground.

All of the other readings should be DC volts too.

See if you can sort that heater supply stuff out first and then we will move on to the other high readings. At this point and assuming the meter readings are all DCV I would be inclined to look for a loose ground connection somewhere.

[Doc B.]

OK, if the meter has been set correctly to DC on the last round of measurements and you were in fact measuring 5.3VDC at terminal A9 I have another thing to try. This would assume that the 12AU7 might have a short from heater to cathode.

Pull both tubes out of the amp. Power up the amp and measure AC volts from power trans terminal 4 to power trans terminal 5. Should be 6.3VAC or a little higher. Then power down and unplug from the wall.

Switch meter to DC volts. Connect black test lead to T12, connect red test lead to A9. Power back up and measure DC volts at A9. Should be 0VDC. Be careful going into the amp after this, the filter caps will store some charge for a while without the tubes being in there to drain them down at shutoff.

[Doc B.]

Another possibility is terminals or leads attached to terminals on the A socket touching each other, that shouldn't be touching.

[Mark Ferring]

Weeeelllll. I think I have likely been a prize idiot. I was measuring all of the terminal voltages in the AC setting. Now that I went correctly to the DC setting, all of the voltages were normal. I would guess that the small board is also OK and that I was using AC vs DC as well.

I will now add the board back to the circuit and see if this is indeed correct. And if so, my sincere apologies for the wild goose chase......

[Mark Ferring]

Went back with tubes out and measured across the transformer t4 and t5 (the 6.3V heater circuit). I measured 6.1ACV

And yes, I did use AC previously and it was across the correct terminals T4 and T5. But I used a different meter which perhaps is a bit off calibration. It still measures around 5.3. But the other one measured 6.1.

I am off to add the small board back to the unit and recheck voltages in DC.

[Doc B.]

OK, that's great. This makes for a useful lesson that used to be pounded into our heads when I was a physics major - a number is worthless without units. I will remember to remind about the proper meter setting up front on these threads.

[Mark Ferring]

So amazingly enough, this has been fixed since I swapped the transistor....many days ago......Lesson learned also.

Put the board back in. It passed all checks. And I listened to a song or two on my HD650s. Marvelous improvement just with the input stage mods.

Well, at least I had time to put together the big board already (along with extremely careful construction of the heat sinks per the directions), so I ought to be able to complete the rest of the kit in a few hours.

I cannot begin to express my appreciation for your hanging in there with my wanderings when all it required was a rotation of my meter. Your patience and forbearance was beyond kind, and you have my eternal thanks.

And I will be certain to continue to shop here and recommend it heartily to my friends!!

Now, off to complete the kit....with much lighter spirits!