test problem

[KevO]

Greetings,

I ran the first power test and had a failure with C4S board. Found poor connection, repaired, and passed voltage tests.

Hooked up to play! Noisy, scratchy, popping, quite dynamic sounding. Swapped tubes, noise did not move. Both channels.

Read and reread 'Basic Troubleshooting' section. Spent next hours re-flowing solder per instructions. Tested each component. resistors, caps up to 10 uf, transistors where the lowest ohm load I found was 109 ohms. But same on both transistors. Slept.

Went back through resistance checks. With tubes.

Reading on 4 was 5.3 ohms but the reading on 19 was 120 ohms.

Swapped 300b's and reading on 4 was 178 ohms and 19 was 5.3 ohms.

And I was beginning to think I had killed an electrolytic cap....

then again....

Warmed preamp up 3-10 minutes each time before turning on amp. Always had 2 loud, weird thumps from left channel and scratchy noises from right channel. Low to no volume each time.

Just a bad tube or a cap and a tube?

Thoughts?

Kevin

 

[Paul Birkeland]

I would reheat the center pin of each MJE5731A transistor.

Having scratching/pops from both channels really narrows things way down, there aren't many components common to both channels.  Do all the inputs have these issues?

 

[Paul Joppa]

...
Reading on 4 was 5.3 ohms but the reading on 19 was 120 ohms.

Swapped 300b's and reading on 4 was 178 ohms and 19 was 5.3 ohms. ...

4 and 19 should both read around 5 ohms, it's the sum of 4 ohms from the parallel3ed 8-ohm resistors plus the cold resistance of the 300B filament. This data suggests one of the 300Bs has a dead filament. You should be able to measure around 1 ohm from pin 1 to pin 4 (the fat pins) of the tube, with the tube out of the socket. If the tube is really bad, then that's the problem. If the tube measures OK then the problem might be with the socket not getting a good contact (or the wrong pins in the socket? That happens!)

 

[KevO]

Thanks gentlemen,

I will follow your recommendations. I ordered a new tube since the bad reading followed the tube swap. I will test as outlined also.

Since the noise was loud, even with no volume, I will check the transistors.

-Kevin

 

[johnsonad]

Kevin to piggy back on your noise, I'm having a popcorn, crackling noise in one channel that doesn't follow the tube but is specific to one tube.  I reflowed the tube pins and the corresponding joints and tightened the tube pins.  It went away for a while but if I move that tube at all as I did when taking measurement today, that crackling sound comes back for a few seconds and then disappears on and off throughout the day. 

Thoughts?

 

[KevO]

Gentlemen,

I tested the 300b's. One was 1.1 ohms and the other 36 ohms. New one arrives Monday.

Reflowed solder on transistors. While doing so, I found a cracked wire going to IA on C4S board. That was probably the source of the noise! Probably killed the tube too.

I hope that is all I have to fix. We'll see.....

I drank less wine while assembling this BeePre and I won't make that mistake again.

Thanks for the help.

-Kevin

 

[KevO]

Greetings from Napa.

Scratching noise is gone. Probably the cracked wire.

Now I am experiencing a major oscillation. Both channels. Woofers are dancing! I powered it off quickly.

I will start again and recheck all my wiring. Could any particular component be the cause?

Any special test I need to perform?

I did use a Goldpoint switch and volume. (bad Kevin!) but I think I have them wired correctly.

Boy am I learning with this build.

Thanks,
-Kevin

 

[Doc B]

Is the oscillation like 60Hz, or higher or lower? Is it a smooth tone or buzzy? Or perhaps is it a ringing sound?

 

[KevO]

Doc,

It is low frequency. I can see the grill cloth pulsing.

I am a bit afraid to hook it up again, but my amp can't kill my speakers, so I can if necessary for testing.

-Kevin

 

[Doc B]

Was the volume control on the preamp turned down all the way when this happened?

 

[KevO]

Yes it was.

 

[Doc B]

OK, assuming the attenuator is installed correctly (which i can't say for sure since it is not stock) being turned down all the way should eliminate anything upstream of the preamp.

The oscillation is in both channels for sure? The preamp is more or less dual mono, there is very little that is common between the channels. You should probably recheck all of your voltages for starters. You might check your output jacks too, to see if there is measurable AC voltage across them. Let us know of any measurements that look way out of spec.

 

[KevO]

Thanks Doc.


I will recheck and document all readings. When I check the outputs, should I have volume up or off or both?

-Kevin

 

[Doc B]

Turn the volume all the way down. To be extra sure you can short the input jacks too. If you have significant AC across the output jack with no signal on the grid of the tube then there is definitely some kind of signal being generated by the preamp itself. Open grounds, backwards caps or a super sensitive amp and speaker setup that is amplifying the microphony of the 300Bs to feedback levels are some possibilities. 

 

[KevO]

Here are the results. Input shorted, volume 0.

T1 - 5.09          T2 - 99.1        T3 - 0        T4 - 9.8        T5 - .007    T6 - 145.6      T7 - (-.1)      T8 -              T9 - 0          T10 - 184.7

T11 - 145          T12 - (-.05)    T13 - 0      T14 - 0        T15 - 184.7  T16 - 5.06      T17 - 101.3  T18 - 0        T19 - 9.74    T20 - .008

T21 - .012        T22 - 211.5    T23(-.006)  T24(-.006)    T25 - 211.5  T26 - .05      T27 - .034    T28 - (-.054) T29 - 6.75    T30 - 6.75

T31 - 13.54      T32(-.002)    T33 .001    T34 - 13.58  T35 - 6.77    T36 - 6.77      T37(-.041)  T38 - 0        T39 - .035    T40 - 211.6

T41 (-.009)      T42 (-.009)  T43 - 211.3  T44 - 0        T45 - 0        T46 - 211.3

A2=0, A3= 95.7, B2=0,  B3=100.5

C2 (-.006)    D2  (-.009)
C3 - 4.02,    D3 - 4.13
C7 - 145.6  D7 - 144.9
C9 - 145.2  D9 - 144.6

VOLTAGE ACROSS OUTPUT SWINGS (-.075 TO +.075) VDC WITH OUTPUT SHORTED AND VOL ON 0

Thanks!
Kevin

 

[Doc B]

Can you measure AC voltage with your meter? An AC measurement across the output could be helpful.

 

[KevO]

It reads 0.022 VAC

 

[KevO]

Doc,

I think the fuse blew before the AC readings I just took. I tested when I did the DC readings and it was very low.

-Kevin

 

[Doc B]

Does the fuse look really blowed up inside, or like it went fairly gently?

 

[KevO]

Gently. a quiet demise. I just measured with a new fuse and it is zero VAC.

-Kevin