XR600B IC4741cp

[corkyperez]

xr600b

Enzo, im sorry is 400bh, i have traced clipping to trans #8 will post results as soon as i replaced it. trans 761.Thanks again.

[Enzo]

OK, 400BH makes more sense than 400B.

For testing, you can remove U2. All it does is the limiting. The amp will run OK without it, you just would not have limiting.

The schematic has DC voltages on it in numerous places. Make sure your DC voltages are reasonably close to theirs.

The 761 was originally a 2N4888, but the most common part to replace it with is the 2N5400 or 2N5401

[corkyperez]

xr600b

I have replaced trans 8,9 and 10. still clipping occurs at about 75% of output. any help would be aprreciated..Is driving me insane. But then again is a challenge.

[Enzo]

Please answer previous questions. Does it clip symmetrically or on just one side? Are ALL the DC voltages in the power amp reasonably close to the schematic?

[corkyperez]

xr600b

yes the dc voltages are very close, and it is symetrical.

[Enzo]

Did you pull U2? Pulling U2 shuts off the limiter, so we can see if it is involved.

Pull the cable that goes to the front panel and clip your test signal directly to the pins on the corner of the 400BH board. Any different?

Before you do that, is there any DC voltage present at the POWER AMP IN jack?

A failing Q17 at the input could clip you, lift it out and see.

I expect this amp to make about 35v RMS at full out or roughly 50v peak on signal. Your are clipping at roughly 25v RMS? or about 35v peak?

And just an oddball idea, check C19 for dried up. That is the 50uf just above the dual diode in the drawing.

Since it has symmetry I doubt they are both bad, but Q1 and Q7 are limiters. You can defeat them by lifting CR10 and CR14. C19 is in common with them, which is why I mention it.

Make sure ALL the large 10w power resistors are OK, none open.

Oh and silly me for forgetting. The two three leg volt regulators on the power amp are for the +/-15VDC for the mixer section only. The +/-15VDC for the power amp is separate and zener derived. Is that TL074 IC actually getting 15v on both polarities?

[corkyperez]

xr600b

Good day, Enzo this is excellent info, will be performing this tests during the afternoon and will post results..Thanks again

[corkyperez]

xr600b

Enzo U2 is not involved. No dc voltage at PA jack. Replaced Q17 and Q18, also replace Q7. Still clipping. Im sure I must be missing something. Do you think I should get a service manual for this PA? just a thought. Thanks

[Enzo]

Service manual? No such exists, the schematic should be sufficient. They were nice enough to add the DC voltages at points around it. It would be impossible to create a If/Then list for everything that could go wrong with something like this. Especially since they have made thousands of products.

I expect this amp to make about 35v RMS at full out or roughly 50v peak on signal. Your are clipping at roughly 25v RMS? or about 35v peak?

Well? That or something else?

REmember the SS amp puts out a voltage signal, the current draw and hence wattage is determined by the load impedance.


Did we sub C19?

Even if Q1 and Q7 are OK themselves, that does not mean the limiter circuits are working right, that is why I suggested unsoldering an end of CR10 and CR14 and lifting them out of the circuit.

For example if excess current flows through R56,57 for some reason, they may be triggered.

[corkyperez]

xr600b

sorry for the delay in response. Amp is still clipping im going over resistance values on board. most of them have to be lifted out for proper measure. I have performed tests lifting cr9 and cr10, still signal clips. Have also replaced c19. A brief history on this amp, maybe I should have posted this first, sorry. This amp had blown power supply area, caps c35 and c36 replaced ,diodes, cr16 -cr19 and cr23 - cr25. All voltages are present. On Q13 and Q3 .62volts. at Q12 -1.21 also Q2.

[Enzo]

I am still wondering WHERE it clips. 10v? 25v? 35v?

The HV rails are +/-52V. May I assume yours are close to that? I would expect then to be able to drive the output to peak at something like 48-50v. That is about 35V RMS on the output, assuming sine wave.

Work with Y2 removed just so the limit circuit it represents cannot confuse things.

[corkyperez]

xr600b

Enzo, Im working with U2 removed. it clips at about 25v. Rail voltage is at 52±. Im checking resistance values and additional caps.

[Enzo]

You mentioned a predriver voltage difference - what was that specifically?

[corkyperez]

xr600b

The base of Q2 and Q12 .66volts.

[Enzo]

Um... instead of -.6 and +1.17?

At the top of C19 there should be about +26VDC - it should be half whatever the + rail is. Make sure both those 2k 5w cement resistors are OK.

If both those bases are sitting at .66v then I'd have to think they are shorted together - either by solder blob or by CR9 and CR13 being shorted.

If they are at +.66 and -.66, that is still not far enough apart. In that case I would check CR9 - lift it due to the 33 ohm parallel. ANd check CR13 - it is on the output transistor board with a two wire connector back to the driver board. Umplug it and make sure it does a double junction voltage drop. it is a dual diode - two in series in one component.