Crate BT220H Schematic

[AliG]

Hi there.
Im new here. Seeking help for repairing my Crate BT220H. I disassemble it and find out that on PCB S-11/94V-0 there are burnt off resistors: R10 and R24.
So could you tell me the parameters of these components please? Or scheme / i found some old threads of this issue but the links are "unopenable".
Thanks for responses.

Mira

[doctor]

Hi there.
Im new here. Seeking help for repairing my Crate BT220H. I disassemble it and find out that on PCB S-11/94V-0 there are burnt off resistors: R10 and R24.
So could you tell me the parameters of these components please? Or scheme / i found some old threads of this issue but the links are "unopenable".
Thanks for responses.

Mira

Both are 47 Ohm, 1/2W.
See schematic.

[Enzo]

And if they are burnt up, it is almost certain Q6 is shorted, and probably Q14 as well.

[AliG]

Heeey.
Thank you both. It will really help me.

[AliG]

So it is not R24 but R16 - R16 was totally melted. R10 was just slightly brown. Do you have the other piece scheme? 07-662. There are two resistors R26 and 29 and are extremely hot (even when the other PCB is unplugged) -are white ceramics.

Thank you very much.

[DrGonz78]

This is it right?

CrateBT220Hpre.pdf

[Jazz P Bass]

Those 10 watt resistors do get smoking hot normally.
You need to check the voltage rails that they are associated with.
If the voltages are o/k, then most likely you are following the wrong path.

[AliG]

Those 10 watt resistors do get smoking hot normally.
You need to check the voltage rails that they are associated with.
If the voltages are o/k, then most likely you are following the wrong path.

OK
I found out that the output (input) J1 on the first dwg has values of +/-67,7 not +/- 55 as it is on the DWG. Could it be problem?

Edit: The transformer gives output of +/-50 V AC but output of bridge is +/-67,9 V DC - this isnt normal, is it? Should I buy a new one, what is the specification of it?

Thanks again for responses. Its new for me and I enjoys it a lot.

[Enzo]

50vAC once rectified and filtered makes about 70vDC. Take away a couple of volts for diode drops and other losses, and your 67.7 looks perfectly normal.

[AliG]

Well.. allright. So just laast question, before I take some books and literature and start studying electronic problematic and basics, when on dwg they say output or input is 55 and my measurement is cca 68V its OK? It violates the dwg - like in drawings for machined and forged parts,which I know better. BTW is there some kind of interpretation for electronic circuits like SC6500/7000/7500 for machined parts?

[Enzo]

In the world of guitar amps, schematic information can change. You stated you had 50VAC, that will make the 67vDC you report. Note on the powr supply schematic it calls them +50 and -50. Those are test points TP14, TP15. But on the power amp schematic those same rails are now +55 and -55, TP12 and TP13. They are not implying that the voltage will be those two levels or that some shift occurs. They are only approximate.

To actually get 55 volts DC, your transformer would need to produce about 40vAC.

These circuits are not precision things. I would not expect your high voltage rails to be 10-15v higher than the drawing, but it is not a super concern either. The circuit should work fine. It is possible they made a change during production of the amp. Every time they change something they don't necessarily publish new schematics.


I expect the 15v rails to be prety close. As to the high voltage main power rails, I usually do not concern myself with their voltage so much as whether they are both about the same voltage as each other and free of ripple.

If you are looking for some rule about what percentage off spec can schematic voltages be, you won't find one.


In the old days, we had some amps that had single supplies. Like +90v. The output would sit at +45v and a big cap would let the signal pass without the DC to the speaker. But OMG, sometimes the supply came out at 96v, and the output was at 48v. The thing to get from the schematic was not that the output should be at 45v, but rather that the output was at half the power supply voltage.