It is currently Tue Mar 17, 2020 2:15 am

All times are UTC - 7 hours



Post new topic Reply to topic  [ 5 posts ] 
Author Message
Post subject: Please Help Output Problem With A Blues Jr. NOS
Posted: Tue Jun 19, 2012 12:14 pm
Offline
Hobbyist
Hobbyist

Joined: Tue Jun 19, 2012 11:28 am
Posts: 3
Hi everyone. I recently traded a very nice Strat for a Fender Blues Jr. NOS. The amp operated great for about three days, sounded wonderful, then the sound would sound almost like frying an egg, and then go clear as a bell. My first suspect was the v-1 12ax7
swapped it for a new one, amp sounded fine for a while and then started the noise again, second I swapped the output tubes, for two older tubes that I believe were good (not 100% positive) amp sounded great for a while and went back to the same noise, this time the noise stayed.I removed the back and did a few minor tests and found that on D11 the VAC reading was out of range on the high end (more than 600v) I am not an amp tech , but have some knowledge concerning tube amps.
Bad output transformer? Any help would be greatly appreciated


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Please Help Output Problem With A Blues Jr. NOS
Posted: Tue Jun 19, 2012 3:27 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
Hi ToeTappinTone,

D11? How were you measuring? There isn't any voltage anywhere near 600 volts in the BJr. 600 mV DC sounds about right for a voltage drop across a diode, if that is what you were measuring. But D11, that is the flyback diode for one output tube, connected to the Plate, in which case there is no AC voltage there, only DC, and you should measure the full DC Plate voltage of approx 330 VDC across that diode (it only conducts when powering off). Selecting AC on the meter will result in a false reading. With power off, amp unplugged from electricity and caps discharged, check diodes for a short with an ohmmeter. If not shorted, diodes are likely OK.

Intermittent problems usually indicate a poor connection somewhere, and the first suspect in that case is solder joints. Quite possibly at or near the tube socket joints. Maybe even a socket problem, but I would re-flow any questionable solder joints first. I use a magnifier to get a real close look, as some poor joints can be difficult to spot. Of course, amp off, unplugged from electricity and make sure that the caps are discharged before soldering or touching components.

BTW, you still haven't eliminated V2 and V3 as the possible source of the problem. You can use the same tube, try exchanging it in each position one at a time, checking for the noise after each change.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: Please Help Output Problem With A Blues Jr. NOS
Posted: Tue Jun 19, 2012 6:05 pm
Offline
Hobbyist
Hobbyist

Joined: Tue Jun 19, 2012 11:28 am
Posts: 3
Thank you for the reply. On D11 I measured from the voltage side of the diode to ground. Also I did swap tubes in v2 and v3, I failed to mention that in the first post. Also when I removed the back of the amp (cover) I tried the wooden dowel trick and did not get any results ,but as I allowed the amp time to warm up I noticed a spark at D11 that is what drew my attention to that component. When checking the diode is it necessary to de-solder one end? Is there a work around? I will also check my solder joints as recommended
Thanks again for your great input. :D .


Top
Profile
Post subject: Re: Please Help Output Problem With A Blues Jr. NOS
Posted: Tue Jun 19, 2012 6:45 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
Ok, seems that it is not the tubes. D11 will normally have approx 330 VDC across it while amp is on. D12 (or D10, depending on which revision) should be roughly the same. You can check these diodes while in the circuit, power off, unplug from electricity, and caps discharged. Actually D11 and D12 (D10) diodes have no affect on sound unless shorted (no sound, blown fuse), but an arcing solder connection could generate some noise, maybe like a sizzle sound, like you described an egg frying. You have to pull the main circuit board to check the solder joints for D11, and check all joints on that board while it's out. Be sure to support the pots inside, to prevent them from moving and breaking the legs or circuit traces, while removing the knobs as they can be difficult to remove. I use two small pieces of wood that fill the space between the pots and chassis when removing the knobs. When replacing the knobs, support the pots from the other side (underneath).

Notice that the tube socket solder joints are on the other circuit board right below D11. Is it possible that the spark came from somewhere else? If the spark came from D11, you might have found the problem area.

Here's a great site with lots of tips and details about the BJr that you might find helpful.
http://billmaudio.com/wp/

Edit:
I notice that those two Plate diodes are located on the main board for some revisions, and on the tube socket board on other revisions. Which do you have?

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: Please Help Output Problem With A Blues Jr. NOS
Posted: Tue Jun 19, 2012 8:50 pm
Offline
Hobbyist
Hobbyist

Joined: Tue Jun 19, 2012 11:28 am
Posts: 3
The plate diodes are on the mainboard, which are d11 and d12.
The spark came from d11 I happened to be looking right at it when it arced. Thanks again.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 5 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: