How to know a burnt resistor value - Washburn bad dog BD30B

[spunko]

Hello. This mystery amp has no schematic.
How can I check for a value of a burnt resistor.
Maybe someone has this amp and can help me with the color stripes on the resistor, is R38.

[Latole]

Where is this resistor connected?
To a power transistor or somewhere else?

There's a reason why it burned out, and if we don't find the cause, it'll burn out again.

You'll need to draw a circuit diagram of this resistor, as there may be an identical resistor elsewhere

[kagliostro]

This mystery amp has no schematic

Maybe someone has this amp ......

OK, but how can someone identify a mistery amp from the photo of a pair of resistors ???

It will be better if you post a photo of the cabinet (many people don't use to look inside of their amps) and a photo of the whole PCB (for the part of people that looks inside their amps)

I think this is the minimum required if you hope in an identification

----

BTW: if you are lucky and find the value of the resistor and repair the amp to prevent the resistor burns again, I'll mount it with long leads to keep it far from the heat of the large resistor

Franco

[spunko]

Thank you for the responses.

The name of the amp is in the tittle: Washburn Bad Dog BD30B. Also know as Mega Amp BD30B.

One of the power transitors is shorted. It uses two 2sd2058 and I already have the new ones.

I have found a bad pic of the resistor (taken with a nokia 1100  ), but it is imposible to know the colors of it. It appears that is a lonely resistor, I mean no identical at least looking at the pic or the pcb itself I have in hand.

I will try to draw the circuit, bad thing is time consuming for this cheap amp, but it is from a kid that needs it to practice and there is no money for a new one.

[Latole]

I saw those poor pictures of circuit board with Google nothing usefull

Best to do for now is what I wrote before;

"You'll need to draw a circuit diagram of this resistor, as there may be an identical resistor elsewhere

[bmccowan]

I saw those poor pictures of circuit board with Google nothing usefull

Best to do for now is what I wrote before;

"You'll need to draw a circuit diagram of this resistor, as there may be an identical resistor elsewhere

And/or:
Contact Washburn
Check out bass and solid state sites looking for an owner willing to open theirs up.

[Latole]

I saw those poor pictures of circuit board with Google nothing usefull

Best to do for now is what I wrote before;

"You'll need to draw a circuit diagram of this resistor, as there may be an identical resistor elsewhere

And/or:
Contact Washburn
Check out bass and solid state sites looking for an owner willing to open theirs up.

 

[kagliostro]

Just guessing about ......

resistor #39 is connected in the circuit the same way of resistor #38 ?



may they have the same value looking to the photo where resistor #38 is present and intact ?

EDIT: No it is difficult, I gived a better look to the High resolution photo with the burned resistor

Franco

[tubeswell]

Someone over at Music Electronics Forum may be able to help with locating a schematic

[Latole]

I'm afraid the schematic is nowhere to be found, which is often the case for this type of low-cost amp.

What I found ; VB30B preamp

https://schematics.ca/modules/wfdownloads/singlefile.php?cid=70&lid=1532

[Latole]

Someone over at Music Electronics Forum may be able to help with locating a schematic

Great link , I did not know.

https://music-electronics-forum.com/forum/schematic-requests

[tubenit]

If it's a cheap amp and possibly a "throw away" anyhow .............. I'd try a 10k and see how it sounds.  That's a totally arbitrary choice of course. In other words, I wouldn't spend much time trying to figure it out.


With respect, Tubenit

[bmccowan]

I second tubenit's suggestion of trying a 10K. Blowing up the pic of the other board, I thought either 100R, 1K, or 10K. You could clip out that burnt resistor leaving enough of the leads to jump in different resistors, or a pot. An LBL might be a good idea for use during testing. And make sure the proper fuse is in there.
BTW Spunko - nice of you to put in the effort for a kid that needs the amp to practice and can't afford a new one. That's a good reason to not throw it in the dumpster.

[kagliostro]

Did you asked to the manufacturer ?

---

Some years ago I asked to Fender for the schematic of a Squier 10, the replay was they can't send me the schematic because they didn't had it but sent me a new Baffle with Grill Cloth and a PT all for free

Franco

[Latole]

Did you asked to the manufacturer ?

---

Some years ago I asked to Fender for the schematic of a Squier 10, the replay was they can't send me the schematic because they didn't had it but sent me a new Baffle with Grill Cloth and a PT all for free

Franco

Fender and its service can't compare to Washburn in my opinion or the amps have to come from Asia made by who knows who.
Fender makes its own amps, as we all know.

That being said, it's a good idea to contact Washburn.

[kagliostro]

The Squier was something that Fender didn't build by themself and so is explained the missing of a schematic on their archives

However may be that Washburn asked about will send you the schematic

Send them the more documentation you have (serial number and so on) and the photo, also ask to know the resistor value (if they don't want to share the schematic that is reserved to only their assistance tech may be they will send only the resistor value)

Franco

[bmccowan]

Agree - there is no downside to contacting Washburn. You may find a sympathetic staffer who is willing to search the dusty files.

[Latole]

The Squier was something that Fender didn't build by themself and so is explained the missing of a schematic on their archives

 
Franco

These small, low-end amps (Squier, Washburne and others) are perhaps built by the same Chinese manufacturer ?

[bmccowan]

These small, low-end amps (Squier, Washburne and others) are perhaps built by the same Chinese manufacturer ?

My understanding is that just like guitars, there are dozens of firms in China making guitar amps. They produce amps under their own names but also
 private label amps for international mfgs. Its just the way of the world these days.

[pdapalma]

Hi everyone, I know this post has been inactive for over a year, but since the OP's question hasn't been answered yet, and for all those like me who have come to this post with the same question, I decided to reply and share my experience.

At the end of 2024, a Washburn BD30B from the music school where I study got to my hands completely dead. I had heard it a few days before with a clipping sound, not from a saturated speaker but rather from an overloaded power supply, even with a passive bass and the volume set very low.

When I took it apart and checked the PCB, in addition to the transformer with a burnt primary winding, I found at first glance this R38 burned and almost illegible, except for the first green band.
After checking the transistors, I also found burnt Q34 (C1815, NPN) and Q33/Q35 (A1015, PNP), which I later replaced with BC337 and BC327 respectively, despite the pinout.
The power transistors were two 2DS2061 and were in good condition.
The transformer had lost its label, so judging by the power supply filtering capacitors (2200uF 36V), I replaced it with an 18V+18V 1A transformer.

I took the time to draw the schematic of the PCB power stage to simulate it and determine the appropriate value of R38 to achieve linear zone biasing of the Darlington pair.
At least in the simulation, with completely generic transistor models, I was able to achieve a current of 6 mA through R38 and 64 mA of bias current through the 2DS2061, with about 500 mV at the speaker output.

Based on trial and error in the simulation, it seemed that the value of R38 could vary within a range between 1 ohm and 240 ohms, maintaining a bias current of 60 ~ 70 mA. Values ​​above 240 ohms increased the bias current to over 100 mA and more, that could lead to unnecessary dissipation with idle input.

Long story short, I replaced R38 with a 50 ohm (two 100 ohms in parallel) since I couldn't get a 56 ohm one, following the assumption of the first green band.
On the PCB, the measured current of R38 resulted in values ​​very close to the simulation (approximately 6 mA), but the darlington bias current was much lower (about 10 mA), with an idle input speaker output voltage of about 110 mV (closer to 0 V than the simulation).

The amplifier sounded clean even at high volume settings, although it probably had imperceptible harmonics due to the lack of fine tuning of the bias current. It's back to music school to give (I hope) a few more hours of students practice.

I hope this analysis, the schematic, and even more so the simulation file (created with SimulIDE-1.1.0 under Ubuntu, .sim1 extension renamed to .sch) will be useful to others who stumble with a fault in one of these amplifiers.

Regards,
Pablo