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**Enzo** · 07-07-2009, 05:23 AM

Try attaching the drawing again, it seems all I find there is the tiny picture itself. I already have the schematic, but others may not.

As far as I know, the symmetry control is basically just a DC offset. Except with a single supply, DC is always offset on the output. You use it to set the output bus to 1/2 the +V rail. That allows maximum signal excursion, and thus centered would cause any clipping to be symmetrical.

OK< so you apply an input and nothing comes out. before we worry too much with voltage minutia, lets just check out the basics.

All those 32v, 17v, whatever are for the preamps, and have nothing to do with power amp problems if you are feeding the PA directly.

Looking at the schematic: the output comes off the board, through an inductor with a bypass switch, then back to the board. So you get signal at the output bus or not? if you do, then the amp is working and your output wiring is faulty. or the output cap.

Your output bus SHOULD sit at half +V more or less - does it? If it doesn't, then make it do so before worrying about signal. There are DC voltages in little boxes all over the schematic. I;d concern over everything to the right of Q401/402 before worrying about them. But do a quick check of them in circuit with diode test and see how they stack up.

If +V does settle right, then trace signal through the amp.

You got signal at collector Q401, base Q403, collector Q403, bases Q404,405?

**gbono** · 07-07-2009, 08:21 AM

Okay, uploaded another copy of the schematic - BTW Q401/2 are new but I'll start looking at the output bus voltages.

Any idea what CR401-4 are for?

Attached Files

470PA[1].pdf (127.7 KB, 779 views)

**Enzo** · 07-07-2009, 10:35 AM

If Q401,402 are new, then all the more reason to suspect everything to the right of them first.

Since one diode junction drop is about a half a volt, then two in series is about a volt. meaning they won;t conduct until there is more than a volt across a pair of them. COnsidering that CR401-404 are right across the input, I;d have to say they were there to prevent the input signal from getting any larger that 1v peak.

**gbono** · 07-08-2009, 03:47 AM

Yes the rail voltage is not correct - I don't even get 6V at the output (node R432/1). Going upstream things aren't any better with only 4.6V on the bases of Q412 and Q413 (BTW Q412/13 are new devices).

The voltage at R417 is 92V on one side and 64V on the other side of the resistor but I only get 4V between R418 and CR411. R418 seems to ohm out on the the board. Don't get it?

Could CR408-9 be damaged? They test out as .5V in one direction and .95V in the reverse direction???

**Enzo** · 07-08-2009, 05:39 AM

Have you doen a search here for other recent discussions of the 470 and 370? We have gone over all this exact stuff.

Q412,413 are just limiters. If they are OK, let's get rid of them. Lift one end of CR411,410 from the board. That effectively removes them from the circuit.

You are not blowing fuse, so the output is stable, just yanked down way towards -V, which in this amp is ground. On the output you have output, driver, predriver. On the top side that is Q409,407,405. Similar lineup on the bottom side. The predrivers control all that.

Now the bases of the predrivers are held close to each other by Q415, the bias transistor. It keeps them a stable distance apart, in this case somewhere in the 2-3VDC range. Think of it like the steering in your car. The front wheels pivot (outputs), controlled by the radius rods (drivers), controlled by the steering link. (predriver.) Then the left and right steering links are both attached to the steering assembly - the rack of a rack and pinion. Now whatever you do to move the rack, the wheels both move that way. No matter how you aim them, they both point the same way.

SO just like steering your car by moving the rack back and forth with the pinion gear, we steer the output by moving the bias circuit up and down. If the voltages around Q415 are about 4.6v, then so will the output be. That is how it works. Seems to me your Q415 and everything after it is working. I could be wrong, but it seems so.

Normally that bias circuit sits at half +V. Q403 is the pinion gear that moves that bias up and down from rest. The harder Q403 conducts, the more it pulls that bias transistor down toards ground. And if Q403 turns off, then that Q415 will get pulled right up to +V by R417,418.

SO it sounds to me like either Q403 is shorted or stuck on hard, or R417,418 might be open.

Wait, do you have the 170045 board? And you have CR408,409 and R416 all instead of Q415? if you do, then substitute CR408,409 and R416 anywhere I mention Q415 above. The amp works the same, regardless of how they establish bias.

Look at Q403, is it shorted? What is at its base?

**gbono** · 07-08-2009, 06:18 AM

I have the 170045 board - I even removed Q403 from the PCB and tested it and it's a pass. Already checked R417/18 and as I mentioned before ....

"The voltage at R417 is 92V on one side and 64V on the other side of the resistor but I only get 4V between R418 and CR411. R418 seems to ohm out on the the board. Don't get it? "

I doubt that CR408/9 are damaged.

**Enzo** · 07-08-2009, 06:43 AM

SO you are dropping 60v across R418. If I calculate a voltage divider across the +92v with those two values, I get about 63v.

That still looks to me like Q403 is conducting hard. If it is not shorted, fine, then I'd look to see if it is being turned on at its base real hard. What is there?

CR406,407 open? R411 open? C405 shorted?

Unlikely, but C406 shorted? That would make Q403 appear shorted in circuit , but not of course when out.

By the way, did you lift the two diodes at the limit transistor colletors - CR410,411?

Wouldn;t matter if CR408,409 are shorted. The amp would still work, you'd just have more crossover distortion. They are not open or they couldn;t help bring the end of R418 down to 4v.

Oh, this matters - Note there are two grounds. the main power amp ground and the signal ground. For example Q401 collector through R410,411 go to power amp ground, while Q402 collector references to the signal ground. Those two grounds must be connected together somewhere. They may not have a direct connection anywhere, but they may bond perhaps at different chassis points through the ground bushing of some jack.. So if the inoput jack for example is not grounded well to the chassis, that ground could be floating.

I don;t care is some hum results, get a clip wire and jump those two grounds together if there is not a low resistance between them.

I have learned that lesson the hard way on some various powr amp modules. I recall a Yamaha I spent way too much time trying to figure out a large DC offset. I thought I could just unplug the input cable. Nope, needed to ground the input ground pin.

**gbono** · 07-08-2009, 07:40 AM

I'll check out the base bias on Q403 after I solder it back into the board. I haven't lifted the diodes on the limiter but will also try that.

Will check out R411 and C405 as well.

Funny you should mention signal ground and main ground. I noticed that the output jacks are isolated from ground by plastic washers. The holes in the chassis look like they were purposely drilled for isolation washers.

**Enzo** · 07-08-2009, 10:39 AM

Only takes a second - check to see if those two grounds are connected somewhere. That could be your whole problem, at least the DC offset. Well, the amp wouldn;t amplify anything that way either.

**gbono** · 07-08-2009, 11:15 PM

That still looks to me like Q403 is conducting hard. If it is not shorted, fine, then I'd look to see if it is being turned on at its base real hard. What is there?

Base of Q403 has .85V and the emitter is at .23V

CR406,407 open? R411 open? C405 shorted?

CR406/7 are not open and R411 tests okay and I replaced C405.

By the way, did you lift the two diodes at the limit transistor colletors - CR410,411?

Lifted the limiter diodes with no change to the bias.

Oh, this matters - Note there are two grounds. the main power amp ground and the signal ground. For example Q401 collector through R410,411 go to power amp ground, while Q402 collector references to the signal ground. Those two grounds must be connected together somewhere. They may not have a direct connection anywhere, but they may bond perhaps at different chassis points through the ground bushing of some jack.. So if the inoput jack for example is not grounded well to the chassis, that ground could be floating.

I ohmed out the power and signal grounds and they are connected - looks like one side of R414 is the connection/node for both grounds on the pcb.

I'm still stumped. This is getting really frustrating since it looks like there shouldn't be any issues with this board.

**gbono** · 07-31-2009, 06:52 AM

Okay, I finally started back on this amplifier. After replacing the transistors in the differential amp, I found that Q404 was damaged and this device was then replaced. The node voltages now appear to be correct but I've run into a problem when a sine wave (1 kHz 150-200mVrms) is applied to the input of the PA board at the 47K resistor.

I have no output at the negative side of C409 - chasing the signal from the input to the output I get correct waveforms until I look at the base node of Q403. Here is the wave form at that node:

http://i284.photobucket.com/albums/l.../DSCF05211.jpg

I've tried to adjust the R406 ("symmetry") but I don't get any change or lose the signal all together. I haven't chased the signal past Q403. I assumed the output of the differetial amp Q401/2 should be the same as the input with some gain. Is there a feedback loop I'm not seeing - i.e. why is the waveform non-symmetric at the base of Q403?

Also noticed that the node voltages on the differential amp and other points of the board (NOT the 90V supply or the voltages at C415, etc) take almost 15 seconds to stabilize/settle to the correct value.

What's causing the the voltage ramp?

**gbono** · 08-04-2009, 01:28 AM

Back to the 470 - here are some of the node voltages. Almost every semi has been repalced on this board (except Q403/MRF431 - it does test okay with a DMM):

Device Base Emitter Collector Node Voltage Comments
(V) (V) (V) (V)
Q401 23.8 24.5 0.6
Q402 23.8 0.6
Q403 0.6 0.6
Q404 55.1
Q405 57.1
Q406
Q407 48.2 48.2
Q408
Q409
Q410
Q411
Q412 48 56.5
Q413 48.2 48

V420/18 57.2 node voltage between R420/418
Vsupply 91 PS voltage on board

The differential pair have an issue with the collector voltage - all devices check out okay and all the resistors (R410/11) also measure good with no open contacts/solder joints. Q403 is also not biased correctly. The output from the diff pair should be a current which drives Q403 (VAS). I can only assume that the feedback signal to the base of Q402 is the issue??

**stosostu** · 08-04-2009, 07:11 AM

I recently fixed a 270 which has a similar power amp, that had a similar problem. It turned out to be the feedback resistor (equivalent to R412 in your schematic) that had gone high, in this case it was a 270kOhm that had gone to 750kOhm..

**gbono** · 08-06-2009, 06:26 AM

No R412 is just fine. I'm wondering about the output capacitor C409. The "can" type electrolytic was replaced with a axial leaded part now on the board. I have to assume that the schematic polarity is correct for this ref designator?

The output jacks have been isolated from the chassis (ground) as well - not sure where this is indicated on the schemo.

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