Crate BT220 bias voltage

[stiff]

Anyone know the correct bias voltage for the crate 220, drawings says 2mv + or - 1mv across R32 R33 R38 and R45 iam getting around 20mv rising to 30 mv after 30 seconds i have changed the MOSFETS R10 R16R7 and R29.

Does anyone know if the voltage on the drawing is correct? I have not connected a speaker to the amp as yet

[g1]

Can you post the schematic? Without it I can only guess, but solid state amps that check bias across emitter or ballast resistors usually are in the range of millivolts rather than 10's of millivolts, so I would think the 1 or 2 mV figure would be correct.

[stiff]

posted them in the blog new to this sitedont know how to copy to thread its also on realted searches when dispalying my blog

[stiff]

bit of history got the amp as dead found the speaker had blown then the output MOSFETS, changed the MOSFETS , the AMP worked for around 10 minutes then smoke the 4 resistors had burnt and 2 of the MOSFETS have repalce all, but decided to carry out further checks BIASING seams to be incorrect according to drawing

[DrGonz78]

When you first turn on the amp after replacing parts are you using some sort of current limiter? Personally, I would use that to test for shorts before running the amp full power with replaced parts.

Did you also replace Q12 and Q14? I do believe those are the drivers and whenever you replace the mosfets theses should be replaced too.

Sounds like you looked through the circuit already and tested for bad resistors, but did you test AP1?

i have changed the MOSFETS R10 R16R7 and R29.

Were these resistors testing bad or burnt out?

BIASING seams to be incorrect according to drawing

Why?

[stiff]

resistors were burnt out tested the diodes they seemed ok didnt try AP1 or Q12 Q14 i

When you first turn on the amp after replacing parts are you using some sort of current limiter? Personally, I would use that to test for shorts before running the amp full power with replaced parts.

Did you also replace Q12 and Q14? I do believe those are the drivers and whenever you replace the mosfets theses should be replaced too.

Sounds like you looked through the circuit already and tested for bad resistors, but did you test AP1?


Were these resistors testing bad or burnt out?


Why?

yes they were burnt out, tested the diodes but not AP1 or Q12 Q14 will check these now, been out of electronics for a while, transmission engineer now on electricity distrubution was trained on Valves as well in the seventies but a bit rusty coming back to me now, got the scope out of the box as well, seems these amps are not very well supported

[Enzo]

Not sure what you mean by not well supported. Crate will send you the schematics if you ask, and they have a network of service centers all over the country, so the amps can be repaired easily. Do they have someone there to assist in componenbt level repair? No, no one does. Well, except maybe Peavey. Just as you cannot call up Ford in Dearborn and ask them how to fix your Ford car.

As g-one said, 2mv (give or take a mv) sounds reasonable to me. I have no reason to doubt it. If you are getting 10 to 20 times the current throuigh those resistors, I'd have to agree the amp is not biuased correctly OR it is not functioning right.

Whenever your 240 or 9240 outputs blow, ALWAYS check all those 0.33 ohm resistors a well as all the 47 ohm gate resistors. If one of the 0.33 went open, then it is possible that little limiter transistors Q7 Q8 could be damaged and also the 330 ohm resistors, so just check all the resistors in the circuit area.

Agree with DrG that those drivers ought to be checked or replaced Q12 Q14.

Power amp drawing below.

Even at that high current level, does AP1 make any changes? A useful range, if so? Looks to me like CCW would be the coldest setting, so I might start there. TP6 TP7 show us that about 4v of bias per side is expected, so are your opposing MOSFET gates about 8v apart? That 8v separation is what AP1 adjusts.

And that circuit relies upon good +/-16v, so look at TP4 TP5. Those 5.8v readings won;t happen unless the two 16v rails are good.

[stiff]

Thanks for the reply, i was having trouble getting info from CRATE as iam in the uk and couldnt find a service center, got no reply from E-Mails etc but someone told me they had been taken over a few years ago so my misunderstanding, found links on this forum for the schematics.

Thanks for the advice the 16V rails are good, but ill will check the MOSFET bias and the 0.33 resitors

[g1]

You have had to replace blown output transistors and resistors twice already, so there is most likely something else wrong. This is why the bias is way off. As Dr.Gonz suggested, the driver transistors Q11 & Q14 are the most likely suspects, along with their resistors, R22 & R24.
The schematic in Enzo's post lists the DC voltages for the test points, check them. (especially TP 4,5,6,&7)
If everything checks out, then maybe it just needs the bias adjustment, but make sure those DC voltages are good.

[stiff]

AP1 is good 10ohms to 600ohms, R32 and R38 open cct thanks for the tip Q12 and Q14 seems to test ok but iam gonna replace them, best thing i did was join this forum giving me some motivation for all the amps sat in the shed (Musician at heart) poeple have given to me as they have not been able to get them fixed

[stiff]

Hi managed to get the power amp up and running + and - 65V pp for 1v input could not get the bias right, one of the 16V rails is down to 15.2 at TP16 with no load TP17 is at 15.9V TP14 is at 59V same as TP15 checked the resistors and the diodes any idea's

[g1]

The schematic in Enzo's post lists the DC voltages for the test points, check them. (especially TP 4,5,6,&7)
If everything checks out, then maybe it just needs the bias adjustment, but make sure those DC voltages are good.

How about those TP4,5,6,&7 voltages .