Blown 300B, help requested [resolved]

[Bobinsms112]

Hi there,

For the past 3 years I have had a nicely operating Kaiju with DC filaments.  The other evening the left channel 300B went neon and croaked.  Put some new 300B tubes in and the left channel is more or less out with just some faint sound coming from it.  Swapped tubes and the issue doesn't follow the channel so I think it is definitely an issue on the left channel due to the tube blowing.  There are no obvious issues looking at the underside of the chassis on the left side.  Any idea where to start troubleshooting this with my volt-meter so I can get the Kaiju back in action?

Thank you in advance,
Matt

 

[Paul Birkeland]

Measure the DC voltages on the 4 pin socket on that side.  Also measure IA, OB, KregA, and KregB on the driver board on that side.

 

[Bobinsms112]

Hi there, thank you for the reply.  Left side measurements as follows:

C1 74.2
C2 426
C3 0
C4 69.2
IA 455
OB 211
KregA 3.5
KregB 7.6

Thank you,
Matt

 

[Paul Birkeland]

Everything is working properly on that end.  If you have garbled sound, I would look at the speaker wire connections between the binding posts and the OT and the jumper(s) on the OT itself.

 

[Bobinsms112]

Thank you Paul.  I checked and re-heated the OT tabs and the binding post tabs. After hooking it to the speakers it was maybe not as bad as before but still a very noticeable imbalance.

After getting it on the bench again, I noticed that 46 U and 32 U were out of wack so I also adjusted the trim pots so 46U and 32 U both = 175.  I also reheated IA/OA and IB/OB.  Fired it up again and it still sounds out of balance, like there is info missing from the left channel with vocals more prevalent on the right side.    Measurements for the left and right channels still seem close except for the following:

Kreg A (Left): 2.55
Kreg A (Right): 3.5

OA (Left): 206
OA (Right): 299

IB (Left): 206
IB (Right): 298

Nothing else stands out to me at the moment.

Thank you again for your assistance in troubleshooting this.

Matt

 

[Paul Birkeland]

The OA is definitely indicative of what you're hearing.  Oddly enough nothing as of yet really has anything to do with the 300B. 

I would recommend swapping 5670s between channels just to see if the problematic voltage follows a tube. 

If the problematic voltage stays put, it would help to know what OA is on the offending side with no 5670 in that socket after the amp has been on for at least 20 seconds.

-PB

 

[Bobinsms112]

Hi there,

I tried another pair of Raytheon 5670s while hooked up to speakers and got the same audible issues with the 5670.  I put the stock GE's back in on the bench and the voltage stayed put when i switched them around.  I then did as you said and removed the left channel 5670 and waited a minute.  The OA reading on the side with the 5670 removed was 358.  Will await next steps. 

Thank you again Paul, I appreciate your time.

Matt

 

[Paul Birkeland]

I would replace the 431 regulator on the B side of that C4S board. 

 

[Bobinsms112]

Thank you again Paul, I will send a message to the queen shortly seeing if that can be sent.

Best,
Matt

 

[Bobinsms112]

Hi Paul,

Received the new regulator and swapped it in.  Voltage readings really haven't changed from my 6/5 11:27 post.  Not sure where to go next. 

Thank you,

Matt

 

[Paul Birkeland]

Are you able to adjust your OB voltage at all?  The KregA being about 3.5V and OB being over 200 is a bit odd.  If you are unable to alter the voltage at OB by turning the blue trim pot, that would be an additional symptom to list and would help explain what's going on with the high voltage regulator.

 

[Bobinsms112]

On the left side (where the regulator was replaced) OB is 175 and is adjustable.  Same with the right side of the chassis. I just re-measured everything again, values below.

left

C1 72
C2 421
C3 0
C4 67
IA 450
IB 204
OA 204
OB 175
KregA 2.54
KregB 6.89

right

B1 70
B2 422
B3 0
B4 75
IA 452
IB 297
OA 297
OB 174
KregA 3.5
KregB 6.63

 

[Paul Birkeland]

Can you post some photos of the 9 pin socket on the side giving you problems?

 

[Bobinsms112]

Hi Paul, see attached. 

Thank you,
Matt

 

[Paul Birkeland]

What's the voltage at the X pad on the B side?

From what I can tell, the B C4S that loads the 5670 works, the 431 bias circuit where the blue trim pot is works as well, you have a new 431 regulator on the other side of the C4S board (the B side, hopefully you didn't put the replacement in the A side), and with no 5670 the OA/IB voltage pops up to 360 which is what we would want. 

The kinds of problems that this would leave would be solder joint and/or resistor problems in the regulator portion of the B side of the board, some kind of issue with the wiring on the 9 pin socket (if the grid stopper that goes from the 9 pin socket to the middle lug of the 5 lug strip behind the socket with all the zener diodes breaks off, you'd get some wonky annoyances like this), or possibly a broken wire going to Kreg on that B side of the board, or even a broken wire jumper on the PC board.

 

[Bobinsms112]

x pad B side = 204 (left channel) 
x pad B side = 297 (right channel)

I reheated several joints on the board and checked the resistances on the sockets and they match the other side. 

Thanks,
Matt 

 

[Paul Birkeland]

This is the pad of interest.  The voltages you reported wouldn't make a lot of sense for this spot in the circuit.

 

[Bobinsms112]

Hi Paul, you are correct.  I re-checked and the x pad = 2.49 on both channels.

Thank you,
Matt

 

[Paul Birkeland]

Well, the last debugging step is to trade boards between channels to see if the problem follows a board (it probably will), and if so I would just order a replacement. 

 

[Bobinsms112]

Hi Paul, looks like it followed the board.  Will order a replacement board and post back once i can get it hooked up. 

Thanks
Matt