Crate BT350 Issues

[jukelemon]

Hi all!

Well..I have spent many years on the forum (tube related) and did not know this section existed lol.  So please bear with me as I am not as versed in SS.

I have a BT350 that was given to me.  Early 90s black faced version.  Attaching schematic.

The unit came to me non working per the giftee :)  So I am going to use it as a test bed/learning amp to start understanding SS.

I have spent many hours over the last few weeks getting to know basic transistor theory, basic SS theory etc. So, not a complete newbie but far from what I know about tube amps.

So, opened the Crate up, turned it on, power light comes on, I hear a click, see smoke right under the board/under the preamp tube socket, and then turned it off lol.

I disconnected the preamp board to power supply wires and turned it back on.

The low voltage supply is good coming off the PT and stable at 26v.  But both power supply fuses are shot now (ordering some more).

High voltage supply (265vdc) is good as well and assume is only there to drive the plates of the preamp tube.

I have tried to inject a signal into the Return jack of the Effects loop.  Nothing comes out to the speaker jack and it is my understanding that this would, regardless of the preamp operation, tell me the driver/output side is good.  So...maybe an issue there as well.


Soooo...I know I have issues in the preamp and it looks likely to have issues in the output as well.

Just looking for some suggestions and hand holding :)

Thank you!

[shooter]

you're correct in your 1/2 splitting the system, so you have toast at both ends 


you need a pretty good working knowledge of SS to have any hope, scope is almost essential.


while I did help out a small music store fixing a dozen or so crates, the next place I inquired about an Amp Tech job, owner said he had a great tube guy, I asked about the SS stuff, he said "take as many as you want, they're out back in the 10 yard dumpster"  I laughed n asked why, "Well I take them in trade for a tube amp just so I can have a satisfied customer, then just pitch them in the trash"


I left them all in the dumpster.

[jukelemon]

you're correct in your 1/2 splitting the system, so you have toast at both ends 


you need a pretty good working knowledge of SS to have any hope, scope is almost essential.


while I did help out a small music store fixing a dozen or so crates, the next place I inquired about an Amp Tech job, owner said he had a great tube guy, I asked about the SS stuff, he said "take as many as you want, they're out back in the 10 yard dumpster"  I laughed n asked why, "Well I take them in trade for a tube amp just so I can have a satisfied customer, then just pitch them in the trash"


I left them all in the dumpster.

LOL.  Fair enough :)

But I'd like to try at least.  It can be fixed and I used to feel this way about tube amps i.e. it was a mystery.  But that changed with time and just digging in.  I am OK with failing and OK with buying what is needed as far as tools...just don't want to give up because it is foreign or hard to do :)

I found one smoking gun at least but cannot find the darn thing on the schematic :(  R7 is toast...quite literally.  BUT...for the life of me I don't get why.  I cannot find it on the schem but I traced both ends.  One is to the GRND terminal coming from the power supply and the other side of the resistor is to a ground on the preamp board.  Huh?

I am assuming this is because what I think of grounds are a little different in SS amps?  Has to be because something came into this resistor and smoked it i.e. something on the power supply board GRND side/terminal/circuit is shorted?

So..the resistor in question is R7.  It is burnt.  It attached to the 10uf cap (C68) which I cannot find on the schem as well of course lol.

[shooter]

...

[jukelemon]

Thanks man.

The C68...I saw it on the parts layout (?), but not in the circuit schem.

On the R7...thank you but your pic is for the Power Amp board not the preamp board?  I think they are unique/different meaning both have an R7?

I should have told you what my pic was from ie R7 is burnt on the preamp board and I cannot find it on the preamp schem (separate page) that I posted.

A little extra troubleshooting details now and after poking around...

I pulled the preamp power supply board (which oddly shows like it is ON the board but is separate).  I tested all the diodes and three of them (D19/17/18) fail to reverse bias test. In other words, they test fine in forward (566mv voltage drop on each) but ALSO show rising voltage in reverse which should not be the case. it should test OL on my meter and like the other diodes.

I drained all caps (tested them as well) so there is no voltage on the board and those 3 diodes seem to be shot unless I am missing something.  Those bad diodes connect to the low voltage AC supply and, I assume, rectify to DC and to the preamp.  The other good diodes rectify to 265 vdc and are good...which makes sense since I can see that voltage coming out off the board.

Appreciate the help Shooter!

[jukelemon]

Yeah..kind of at a stand still now given I cannot ID the value of R7.

I am optimistic that I can at least get the preamp power supply board live and thus the preamp but not without the darn R7 value.



WTH...I mean I cannot see it anywhere nor C68 on the preamp schem.

[shooter]

how did you determine "R7" since the snippet you posted doesn't appear to be silk-screened with call outs, (R7 R 8 C14...)

[jukelemon]

how did you determine "R7" since the snippet you posted doesn't appear to be silk-screened with call outs, (R7 R 8 C14...)

Hey there.

R7 is listed right next to C68 and that whole pic/layout matches to my preamp board and the actual burnt resistor pic I attached.

The R7 in the schem you posted is on the power amp board. That part of the schem file (5 pages total) also has an actual listing of all the values.  The preamp board/sche section does not.

I just saw that the preamp layout states 1 of 2.  That file is missing the other page I think (another issue) and I bet that 2 of 2 page actually listed the the components and their values :(

Take a peek at the file again and you will see what I mean.

I have looked 10 times now over the preap schem and I cannot find R7.  It is connected to the GRND terminal coming off the preamp power supply board and then connected to a ground section on the preamp board i.e. lots of other stuff connected to it like the C68 Neg side of the cap, the Neg side of the input jacks etc.

I think it is a fuse/designed to short if there are issues with the preamp power supply board.  I cannot think of any other reason to have it there.