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Crate BT 350 Troubleshooting

Started by jukelemon, January 11, 2025, 04:56:56 PM

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jukelemon

Hi all! 1st post and happy to be onboard. Thank you.

I am a tube tech for many years, know my way around a scope and sig gen but am just starting to further my knowledge into SS.

I have read many articles/educational topics on general Transistor theory, amp design etc but far from an expertise level of tube topology/design.

I was given a BT350 that was "dead".  There were several things going on and I will list them here just to give some back drop.

1- Both low voltage power supply fuses were blown
2- 22 ohm fusible resistor was blown on the preamp board C7
3- IC1 was found to be inop
4- Several diodes on the preamp board were bad
5- Of course...very little signal would pass

I have resolved all the above and even replaced IC2 given I had more than one 5532.  I have a  clean/strong output to the speaker jack and this can be seen both on scope and practical playing a bass thru it. 

However, it has a lot of static noise/some crackle pops and some hum that increases via the Master Level with or without an input plugged in

I have bridged the Effects jacks. No change. 

I have tested the signal at J3 pin 2 on the power amp and can see (scope) static/noise.  I have also sent the Effects out to another amp and get a clean signal BUT the same noise albeit less than what I see/hear at the speaker jack.

I have tested both power rails at every TP and they are solid at +/- 70 ish volts and +/- 16 ish volts with "ish" = tenths of volts and negligible I think

With no speaker load, I am getting -0.2 vdc at the speaker jack with Master at 0 and wild jumps from -36mvdc to 16vdc RMS via my meter when I turn up the Master Level.  In other words, voltage is bouncing after the master pot leaves ground

With an 8 0hm dummy load, it is the same +/- mvdc bounce

Any suggestions would be greatly appreciated and thanks in advance!

I don't see a way to post a schem file here.  But I have them




Jazz P Bass

Follow the noise backwards from the Master control.

jukelemon

Thank you sir and I am on the same page.

This board is a major PITA to test given its orientation. I guess I could mark all the test points with a sharpie and then put it back on.  Is that what you all do?

Or do you test the board outside and run any applicable grounds?

Do any of you use a listening amp for tracing on preamp boards?

The ground symbols are not conventional on this amp which makes it that much more difficult :(

Lastly, and based on what I am describing i.e. full power/all preamp functions work/burnt C7 (which I cannot even find on the schem), does anything look like suspect?

Jazz P Bass

Narrow it down.
Do any other controls affect the noise?
If so, then the issue is before that control.
An audio probe would help immensely.

jukelemon

Thanks

None of the controls affect the noise.  I can pull the 12ax7 out and the noise is still there.

I have a listening amp at my other shop that I am going to grab in a few weeks.

I can see the static on my scope/sine wave all the way back to the input..which has me a little confused.

If I were to close my eyes and someone said...what is this...I would say it is an open jack/failed jack.  I have heard this enough times in tube amps and it is like the ground reference is compromised.  But SS...not so sure given all the other complexity.

Given how clean and strong the output is it doesn't seem like a component failure.  It is like a global/general failure but nothing looks out of place as far as grounds.

jukelemon

Ok..

I ran two probes into the preamp with one being at the very end of the preamp (Pre Amp Sign Out) and then used another to test from input and up to the Master Level send.

By looking at the end of the preamp, I could see the noise that occurs once the master send is anything other than grounded i.e. anything other than 0.  That noise/whatever type of signal the scope is picking up became my visual reference point.

I used the other channel to probe from input all thru the circuit and up to the Master Level pot.  There is a j112 (Q4) that is the only other place that is showing the same noise.  All other test points are low level noise/likely normal noise floor.

So, I tried to test the JFET with my meter.  Discharged any gate voltage by jumpering G/S and then took ohms readings.  It did not test out like I have read it should.  At no point did I get an OL reading and when I did get a reading and expecting low resistance (50-200 ohms ish), I got 1k+

I did not remove it so I am testing in circuit but that seems suspect to me.

Jazz P Bass


jukelemon

Ok thanks.

What is the function of Q4 in that it could just be removed?  It has a Neg gate voltage so it is off/not doing anything when the amp is on right? And takes the preamp signal to ground when Amp is turned off?

Jazz P Bass

It appears to be a mute circuit.

jukelemon

Yep..agree.

I removed Q4 and the amp is 100% now!

So, I will replace that FET and all is well.

Thanks for the assistance.

Jazz P Bass