Bottlehead Forum

Bottlehead Kits => MonAmour => Topic started by: jjvornov on April 21, 2020, 05:13:00 AM

Title: Assembly Question [resolved]
Post by: jjvornov on April 21, 2020, 05:13:00 AM
I'm on the home stretch of assembly. On page 62, it says "Attach and solder the red wire from A6 to the Kreg position on the A side of the circuit board."

I don't see where that wire would have been put in. There is a white wire at A8, and I don't see where that gets attached to the board. It looks to me like it's A8 To KregA. KregB already is going to A2 via a white wire.

James

Title: Re: Assembly Question
Post by: Paul Birkeland on April 21, 2020, 05:28:39 AM
The white wire at A8 needs to go into that Kreg.  I'll forward this to Josh to get that corrected.
Title: Re: Assembly Question
Post by: jjvornov on April 24, 2020, 05:32:03 AM
Paul

Thanks for the reassurance on my assumptions.

So new issue now. One amp is working great and sounds good. The other, now that fully built is giving me variable read on the bias meter. When I turn it on, the meter goes up to 100 mA then after a minute or less drops down to 30 or so. It will then drift up to the correct 50/60 mA reading, but then pops back up to high readings soon after. Rechecking all of the voltage readings look good, so I'm assuming the problem is in that bias circuit.

My fear is that I damaged the 47uF Solen cap during installation. I've gone over the connections and desoldered everything.

James
Title: Re: Assembly Question
Post by: Paul Birkeland on April 24, 2020, 05:39:13 AM
What DC voltages do you get at A1/A4? 

Some causes that immediately come to mind that would cause this:

1.  Loose 9.09 ohm resistor(s) on the bias meter.
2.  Loose or poorly soldered 249K resistor back by the 9 pin socket.
3.  Loose connection at pin 3 on the 4 pin socket.
4.  Debris dragging down A1/A4 voltage.  The likely spot for this is between the center lug of the hum pot and the chassis plate.

Based on the DC voltages looking good, I would be looking for a bad solder joint around the meter, any maybe reheat the joints on the big white resistors that set the bias.  You can't really have 100mA of current draw without the voltages on the 4 pin socket being way off, or plugging the 2A3 in with the fat pins in the wrong holes.
Title: Re: Assembly Question [Resolved]
Post by: jjvornov on April 24, 2020, 06:32:02 AM
Paul

You're the best! All fixed. I'm not sure what did it, but I worked to get that middle lug clear of the chassis there by the nut cover.

James
Title: Re: Assembly Question
Post by: Paul Birkeland on April 24, 2020, 06:57:43 AM
Oh man, I'm really glad you found that.  Whenever I have discovered that in a repair, it has come with a destroyed plate choke!

-PB
Title: Re: Assembly Question [resolved]
Post by: jjvornov on April 24, 2020, 12:27:36 PM
Paul

I’ve got the amps up and running, but the amp that had the problem has lower volume. I was going to recheck the signal path next week. How would I know if the plate choke was damaged?
Title: Re: Assembly Question [resolved]
Post by: Paul Birkeland on April 24, 2020, 03:15:54 PM
The DC resistance of the plate choke would be low. 

It sounds like you may have a loose solder joint causing some problems.  I would follow all the connections that connect to the two cathode bias resistors and look for one that's loose.
Title: Re: Assembly Question [resolved]
Post by: jjvornov on April 26, 2020, 10:32:17 AM
Paul

Well at least the plate choke resistance looks the same for both amps- 500 ohms between the red and blue wires and the same for each amp.

When you say the cathode bias resistors, you mean the 910 and 270 ohm in series between the potentiometer and meter? Would it make sense to measure resistances to ground?

James
Title: Re: Assembly Question [resolved]
Post by: Paul Birkeland on April 26, 2020, 12:24:34 PM
The cathode bypass cap will make resistance to ground measurements somewhat tough.

Instead, I would look for loose connections on either end and the associated wiring.  You could also just reflow all of those solder joints.
Title: Re: Assembly Question [only partially resolved]
Post by: jjvornov on April 27, 2020, 02:35:32 AM
Paul

I realized I didn't respond about the V at B1 and b4. With the current at 60mA,  B1 is 58.7V, B2 is 56.3V Turning the hum pot has no effect. At 50 mA, they are 56.3V and 60V. If those are the filament voltages, it seems that they are way off, right? I checked the AC voltage at the transformer, and that's right at 6.3V as expected. 

I'm going to recheck all of the connections again today, but at least it looks to me like there is a problem still in that bias circuit as you suspect. Any other voltages that would be useful to check to isolate where the problem lies?

James
Title: Re: Assembly Question [only partially resolved]
Post by: Paul Birkeland on April 27, 2020, 05:13:29 AM
Those bias voltages are about what I would expect, though I would expect the higher bias voltage to go with the lower current.  Are these the voltages you get at B1/B4 when the meter is acting up?
Title: Re: Assembly Question [only partially resolved]
Post by: jjvornov on April 27, 2020, 06:00:15 AM
The meter is actually perfectly fine now. And I double checked that the voltage drops when the current is increased via the toggle.

At this point its just low volume out of the amp

James
Title: Re: Assembly Question [only partially resolved]
Post by: Paul Birkeland on April 27, 2020, 06:17:20 AM
Are all of the DC voltages correct?  If so, can you post a photo of your meter, then I can give you some instructions to follow signal through the amp.
Title: Re: Assembly Question [only partially resolved]
Post by: jjvornov on April 27, 2020, 09:33:11 AM
Paul

All of the voltages that get checked in the manual are all fine.
Sorry to be thick, but which meter? The amp’s meter or my DMM?

I’m going to spend some time looking at solder joints through the signal path to see if I can find the culprit.
Title: Re: Assembly Question [only partially resolved]
Post by: Paul Birkeland on April 27, 2020, 09:36:42 AM
Your DMM.
Title: Re: Assembly Question [only partially resolved]
Post by: jjvornov on April 27, 2020, 09:56:31 AM
Here it is
Title: Re: Assembly Question [only partially resolved]
Post by: Paul Birkeland on April 27, 2020, 11:00:13 AM
So the idea here is to download a tone generator app to your phone, then play that through the amp.  Your meter is autoranging and can resolve low AC voltages, so you have what you need to start. 

For the first measurement, the amp can be off, and all you need to measure is the AC voltage across the RCA jack, then post that here.

-PB
Title: Re: Assembly Question [only partially resolved]
Post by: jjvornov on April 27, 2020, 11:47:06 AM
400mV from the iPhone headphone jack volume up all the way
Title: Re: Assembly Question [only partially resolved]
Post by: Paul Birkeland on April 27, 2020, 11:54:31 AM
OK, you can leave the black probe clip leaded to the shell (outside) of the RCA jack, keep the tone playing, then measure the AC voltage at terminal 10 with the amp running.
Title: Re: Assembly Question [only partially resolved]
Post by: jjvornov on April 27, 2020, 11:56:04 AM
Terminal 10  (output transformer) is just bouncing around from nothing to OL and there's 61V of DC on it.
Title: Re: Assembly Question [only partially resolved]
Post by: Paul Birkeland on April 27, 2020, 12:11:21 PM
Terminal 10 by the 9 pin socket, not terminal 10 on the output transformer.
Title: Re: Assembly Question [only partially resolved]
Post by: jjvornov on April 27, 2020, 12:44:44 PM
Okay. Terminal 10 reads 12.75 V
Title: Re: Assembly Question [only partially resolved]
Post by: Paul Birkeland on April 27, 2020, 01:41:46 PM
OK, so that rules out everything going on at the 9 pin socket.

Now check to be sure you also have 12V at pin 3 on the 4 pin socket.
Title: Re: Assembly Question [only partially resolved]
Post by: jjvornov on April 27, 2020, 01:47:40 PM
Pin 3 is the same 12.75V
Title: Re: Assembly Question [only partially resolved]
Post by: Paul Birkeland on April 27, 2020, 01:50:30 PM
As much as we could continue following this around, I think you should post some build photos.  Specifically it would be good to see the area where the 47uF cathode bypass cap connects, as well as the cathode resistors.

The possibilities I see for what's wrong are:

1.  The 47uF cathode bypass cap isn't properly connected.
2.  The secondary wiring on the output transformers wasn't done correctly (in the manual where it says "to wire for 8 ohms, do blah blah blah).
3.  One of the wires going to terminal 5 or 10 on the output transformer is loose or missing.

-PB
Title: Re: Assembly Question [only partially resolved]
Post by: jjvornov on April 27, 2020, 02:11:32 PM
Paul

So it was a miswire at the output transformer. It makes sense that it was playing but at low volume. Now they are matched in volume.

Thanks for your help. I've often said that half the fun of the hobby of kit building is the building, but the other half is troubleshooting and the satisfaction of getting the assembly working at the end.
Title: Re: Assembly Question [only partially resolved]
Post by: Paul Birkeland on April 27, 2020, 03:07:11 PM
I'm glad you found the issue, and I'm thankful it was something simple and accessible.