Marantz 4300 Amps/Protection

[GeezerB]

Thanks to the help from the folks here... I was able to get my 4300 Preamp/Tone all sorted out and now have a good signal on all 4 channels... no crackling no hiss and sounds nice. Through external amps.

But, now I have moved onto the amps. I have 4 amp units (2 from parts unit) and cannot get it to come out of protect with any.
Even though it won't come out of protect, it does not light up the DBT
No DC coming from preamp.
All of the Output transistors test good.
I have only right amp installed (which looks to have been rebuilt) currently and I have
JN01 4.9V
JN02 5.2V
JN03 6.7 V
JN04 4.9V

I also plugged in one of the old amp boards and immediately smoked R744.

I have kits here to recap the amps, but I would like to try to figure out what is causing it rather than "shotgun" the whole amp?

 

[GeezerB]

Confirmed exactly 35V from the power supply

 

[dankik]

I have only right amp installed (which looks to have been rebuilt) currently and I have
JN01 4.9V
JN02 5.2V
JN03 6.7 V
JN04 4.9V

Those signals basically go to the Main/Remote speaker outputs (Main/Remote).
Here is the schematic tracing a couple of the signals from the P700 board to the speaker switches (PN01 board).
As far as I can tell, those should all be around 0VDC (or at least a few millivolts).
Since you measured a large voltage at each there is something wrong with your P700 boards or you need to adjust the DC offset.

Have you tried performing this?

 

[GeezerB]

Thanks I have been looking at the schematic trying to figure it out... beginning to make a lot more sense.

After setting the power supply to 35V, I did not do the audio adjustment as the DC was so far off that I did not think I could get it down to normal with just the trimmer.

What program are you using to mark up/trace the schematics with the colors and arrows?

 

[dankik]

I'm not saying that you won't have problems setting the DC offset due to some other issue, but you should try it first to see if it will get close. You can measure some of the transformer voltages shown on the schematic and compare to the schematic.

To draw the lines and arrows, I just use a drawing program, Visio. I just put the section of the schematic to fill the screen then take a screen shot using "Snip & Sketch" program, paste it into Visio and use the line drawing tool to make whatever line color/type you want and draw on top of the pasted schematic. The only drawback is if the portion of the schematic is large, when I paste it here, it gets fuzzy to read.

 

[Redkarter]

Does it come out of protection with neither board installed? Doing all this testing in 4ch mode?

I agree with @dankik trying to set the dc offset may at least tell you which side of the amp card is having issues. Edit: I now see you're seeing several volts on each of the jn01-04 you probably won't be able to dial that much dc out.

 

[GeezerB]

Yes, testing in 4 channel mode... unfortunately adjusting the trimpot only changed it a little.

I may be onto something... just didn't make sense that all 4 amps (had 2 spares) would need work... and after testing all the output transistors and reinstalling I tested the voltage on the collector (can) of each output. 2SC1403 had correct positive voltage, but nothing on the 2SA745 !!! So, now I know I need to go back and look at my power supply because I have nothing on the negative.

I will check out Visio, thank you.

 

[GeezerB]

Confirmed! Due to a bad connection at the power supply, I did not have the -36V supply.

Still won't come out of protect (keeps lit on the dim bulb with an amp connected) but I know that was a problem... just not the only problem

 

[GeezerB]

Thanks for the tip on marking the schematic dankik, I use a Mac so figured out how to do that on here and it is gold!

Left channel comes out of protect. Right channel doesn't. I'm hoping I'm learning here so I am comparing the pins on P700 on good channel to bad channel, and I came up with some inconsistencies... I removed and tested these outputs and they tested fine, is it possible they are still bad?
This is with the right amp unplugged.
I'm getting 36V on 25 where I should be getting .55 (base)
And 36V on 35 where I should be getting .01

 

[Redkarter]

You're getting that reading with the amp card removed? If so, sure seems like H006 could be shorted. If getting that with the card in I'd give a close look at H715

 

[dankik]

Left channel comes out of protect. Right channel doesn't.

I'm a little confused. I think that the schematic has some errors in labeling of the FL, RR, FR, RL on the lines going through the speaker switches and on the P700 board schematics.

1. Are you sure that on 35 you are reading +35v, not -35v?
2. Looks like there is a problem with H715, maybe emitter to collector short?
3. That might also be causing the 35v to pass through H722 to H001 emitter.

 

[GeezerB]

Glad you said that about the schematic, I wasn’t sure what I was looking at as far as labeling RR RL etc.

These are the numbers that I’m getting with both boards removed. Mind you this receiver was worked on by someone else and I don’t know it’s history other than it looked real nice.

I will double check with the meter on whether it’s negative or positive when I get back home. But at this point, the only thing that is connected are the Output transistors on both sides, no cards installed.

 

[GeezerB]

The only thing that made any sense was shorted outputs or if somehow the previous guy had miswired,etc. Checked them again, and two outputs were blown! Not sure if I lost track when I checked all 8, if one of the amp units damaged the outputs?... have learned a lot on this one so far. Now clicks out of protection, lucky I had a parts unit here I have needed it.

 

[dankik]

From the Owner's Manual:

 

[GeezerB]

Definitely adhere to that rule!

 

[dankik]

These are the numbers that I’m getting with both boards removed.

I'm not sure what you mean by "... both boards removed."? Do you mean that both of the P700 boards are wired-in but not installed in the chassis or what? If they are not connected, how did you measure any voltages?

So now can you set the DC offset for that channel?

 

[GeezerB]

That was with both of the 700 boards removed from their sockets. Both amp sockets empty.

I checked the voltages by measuring each pin on the blue sockets with the amp board removed and the receiver powered up.

I did that because the same amp board would work in one side but not the other... so I knew that something was going on that was not related to the amp unit.

Ran out of time last night so I will hopefully be able to get the offset/bias dialed in.

 

[GeezerB]

I was able to set the offset/bias for the front half of the right board... then discovered that the rear half of that particular board did not work. And somewhere along the way I have lost one of the 4 preamp channels (all 4 were working) I do have kits to do both amp boards. May have to set this one aside and back away slowly until I learn a bit more... but I have learned a ton on this one so far.

 

[GeezerB]

Decided to have another look at this amp board... discovered that the trace was broken for the input on the rear part of the board! Started using a very simple signal tracer (just a cap with a small amp) and was amazingly easy to find that one... one good side. Was able to set offset and bias for that side.

Tried another amp board and although it initially allowed it to come out of protect, it would light the dim bulb after a little while so that one will need some attention. Thank goodness for the dim bulb.

 

[GeezerB]

See photos to see how bad of a parts unit. Cockroach, there were many and luckily long since dead but I always unpack in the garage now. Thought I got them all and there was still a body hiding in the amp board... yikes.

Also, an unusual way of mounting a transistor with a resistor on it stuck vertically? I've never seen that before.

This board does click out of protect. However shortly after power up it smoked the R770 and R769 resistors. Still trying to learn how to properly read schematics... and learning slowly. I can see that the resistor receives 37V positive then to H727/H728 diode and then the base of H711/H712... does that sound accurate?
So I cant figure out how the resistor was getting cooked unless receiving greater than 37V OR both diodes failed?