Fender Bassman 200 crackling noise

[christarak]

A mere 8 years ago there was help for a member with a Bassman 200 with loud crackling noises. Maybe this link will take you back http://music-electronics-forum.com/t19721/ and jog your memory, Jazz P Bass (Mind you, I can't remember a repair I did a week ago :-)

I now have the same problem. The clues for my learned friends:

Q10 & 11 are very hot to touch. Cooling them down with compressed air stops the crackling noise very quickly. Dead silent for a minute till they heat up again. These 2 transistors were replaced by a previous Tech by snipping them above the PCB and soldering the new legs to the stumps of the old. This repair worked for a while according to the customer, but the problem has returned. (see photo)

When there is no noise the Vbe on Q11 is 0.98v. When the crackling starts the voltage plummets to a jittery 0.5V.

The +/- 57v rails are at +/-61v

The No Noise Conditions are: TP6= -10.49 TP7 = -52.8 TP11= 60.45 TP12= -60.5 TP13= - 450mV

The Crackling Noise Conditions are: TP6= -10.49 TP7 = -53.0 TP11= 61.72 TP12=-62.3 TP13= - 800 to 1050mV (jitters like crazy in time with the snap crackle and pop)

When I was measuring TP11 (D18 cathode) very often the noise would stop. With chopstick there was no effect. I repeated this test a few times. Only the Fluke 87V would dampen the noise.

Out of boredom, I cooled down the RT2 PTC thermal sensor, but it gave me no joyous outcome in doing so.

Any suggestions on what causes Q10 and Q11 to start running hot which in turn creates the noises?

[MarkusBass]

Are Q10 and Q11 soldered correctly? Are they MPSA92 and MPSA42? I think that originally MPSW42 and MPSW92 were used but they are the same as MPSA*. On the PC board they are marked reversed. Can you check it?

Mark

[Enzo]

The MPSW are the 1 watt series. They are TO92, but taller, they are heavier duty than the plain MPSA42. DO not put the smaller ones in place of the MPSW.

http://www.onsemi.com/pub/Collateral/MPSW42-D.PDF

[MarkusBass]

That's true. The MPSW series has much higher total power dissipation than the MPSA series. MPSW transistors shouldn't be replaced with MPSA series (as it is in your amp).

[g1]

Agree with the others that these new ones are failing because they are not the correct type. They can't handle the heat/current.
As far as the freeze spray, unless you have a signal going through, you can't be sure if you are curing the crackle, or shutting down the transistor and killing all audio. May not matter in this case, but just something to note about using freeze spray to identify noises, it's best to be running signal so you know if you are killing all audio and perhaps the problem is upstream.

[christarak]

Thanks Mark. The 2 that a previous tech used are the MPSA. They are installed as shown on the PCB graphic. I will check the pin outs.

[christarak]

The MPSW are the 1 watt series. They are TO92, but taller, they are heavier duty than the plain MPSA42. DO not put the smaller ones in place of the MPSW.

http://www.onsemi.com/pub/Collateral/MPSW42-D.PDF

Thank you Enzo. You're right they are the MPSA. I will see if I can find an Aussie supplier of the MPSW and install them properly.

[christarak]

Agree with the others that these new ones are failing because they are not the correct type. They can't handle the heat/current.
As far as the freeze spray, unless you have a signal going through, you can't be sure if you are curing the crackle, or shutting down the transistor and killing all audio. May not matter in this case, but just something to note about using freeze spray to identify noises, it's best to be running signal so you know if you are killing all audio and perhaps the problem is upstream.

Ha Ha! I was accidentally smart, G1! I wasn't using freeze spray. I was only using a slight trickle of air from my compressor. But I will store that snippet of information in my brain cell for a future memory struggle.

[christarak]

Not only can I not find an Aussie supplier, the only supplier I can find of these obsolete transistors is in China with a 45 day delivery time. I want to get a few of each in case I accidentally solder one in upside down or twist the legs together before soldering. Does anyone know of a supplier in the USA? I tried Mouser, RS Components, Farnell and WES (Australia). I can get the MPSA, but now we know that's whats causing the problem.

Oooh. A thought just entered my brain cell: Do Americans "solder" or "sodder"? I keep hearing "sodder", but in Oz we pronounce it "soul-der".

[Jazz P Bass]

Have you tried the NTE brand?
NTE255 - NTE ELECTRONICS - TRANSISTOR,BJT,NPN,300V V(BR)CEO,1A I(C),TO-237 | Newark element14

There is a UK seller that has the proper MPSW42.
https://www.ebay.com/itm/MPSW42P-Tra...wAAOSwAClZ4KBF

[mozwell]

You could always try MJE340 & MJE350 300V 0.5A 20W hfe maybe around 100 at the operating point in this amp
Also BF469 & BF470 250V 0.1A 1.8W hfe min 50
Both these types are good for the Vas stage in a power amp

MPSA42 is 200V 0.5A 1.5W (at 25C) hfe around 40 or so

Jaycar is AU have both the MJE and BF types

[christarak]

Thanks fellow Aussie, Mozwell. I think the MPSA versions are 0.6w. That's what I have cooking in the amp now. If I can't source and MPSW42&92's I'll give the MJE or BF a nudge. It won't be a neat job with the different case size and pin configuration, but the higher wattage is attractive.

[christarak]

Found 'em. They're coming on a slow boat from China (literally) but I've paid extra for a slow plane. I'm getting 10 pairs of the MPSW's from good ol' eBay for less than the price of a coffee and a muffin. No wonder the Chinese are taking over the world. Who can compete with that pricing!! https://www.ebay.com.au/itm/10pairs-...AAAOSw-YVXlv-u

Thank you all for your amazing support once more. I'll report back once they arrive and are installed.

[g1]

Sorry to say this but I hope they're legit.
Obsolete parts sourced from China often are not.

[christarak]

Sorry to say this but I hope they're legit.
Obsolete parts sourced from China often are not.

Bugger. Well, they've got my money now. Lets hope they are legit, g1, or it will be a slow repair and a re-order from the UK supplier .