Marantz 4300 Update & question.

[mrmodify]

The right channel amplifier in 2 channel mode is finally alive. DC offset floats slowly between 1-4 mv.

I took my scope and found a funny saw tooth type waveform on pin 1 on the power supply board (P800). I did some checking and found H807 shorted.

Now when power is applied relays wait about 3-5 seconds then speakers connect. Dim bulb is very dim.

My question is Pin 1 still has a clean saw tooth maveform. Shouldn't pin 1 be a :40v dc straight line?

New outputs are in and will be install in a few days.

 

[(*steve*)]

I remember saying at one point "I think we need to look at the power supply".

Good job finding the fault in there.

When you say that you see a sawtooth waveform, van you indicate whether it's a small ripple on a much larger DC level, or something more concerning.

Please either provide pictures of your scope's display, or be quantitative (preferably both)

 

[mrmodify]

You did say about the pwr. supply. I am learning.

Thanks very much, I owe you.

I never learned much about my scope I can get a waveform on it and take volt measurements but that is all I know.

I have attached pictures of the traces. One is "before" but not very good and the second is a current picture "after".

Top of waveform is about 40v bottom peak is about 37v.

 

[(*steve*)]

There's not much noise in the former, a lit in the latter trace.

This suggests to me that either you've got it at a more sensitive setting in the latter or you've failed to connect the ground lead.

Can you confirm the vertical setting (V/div)

 

[mrmodify]

volts/div was at 5 on both traces. The probe on the before was @10x and after was @ 1X. The Sweep Time/div was @ 2ms on the after. I think it was on the same setting for the before.

The ground was connected unless on the before it came disconnected and I didn't see it. I just check the trace again with and without a ground and trace does not change. I even went to channel 2 and got the same results. I do have the chassis ground from the scope connected to the receiver chassis. Don't know if this matters.

By the schematic, I should see a straight line on pin 1?

 

[duke37]

In the second picture, either the scope was out of focus or there was a high frequency signal. Try a much higher time base speed to see if you can find it. If it is there, you will need to stop the oscillation.
Some ripple at 120Hz is to be expected.

 

[mrmodify]

Don't know if I explained pictures or not. But the clearer picture with the full picture of the scope was taken after H807 was replaced. The picture of just the screen was before H807 was replaced. I could not get a clear picture on the camera the before picture.

The probe was placed on pin #1 on the output plug of the power supply with the ground attached to the chassis. The power supply was plugged into the receiver.

The 2 signals were simular in shape but the before was erratic & nonlinear. The after had very clean lines and each pulse matched.

 

[mrmodify]

OK spoke to soon, Unit does operate better, but still will go into its surging mode when volume is raised to about 1/3 volume on dummy loads.

 

[(*steve*)]

What I would be doing right now is to look at the power supply in isolation.

Add simple resistive loads to it and ensure that its performance is normal.

A fault in the amplifier could be causing the apparent noise on the power supply, or it may be the power supply that is causing the noise and the other problems.

Since you've found a fault in the power supply, follow through to check that it's actually fixed and ensure that you haven't replaced a component which was an effect rather than a cause of the problem.

 

[mrmodify]

I understand isolating the power supply and what we are going to accomplish. I don't know what size resistors or what pins to place resistor on.
How do I know if performance is correct? Some of the pins do not have voltages on the schematic.

 

[(*steve*)]

The first step is to determine what the output rails of the PSU are.

I'd have to go back and check, but I believe it's probably a +/- 35 volt rails -- measure them when the amp is behaving itself and at low volume.

From the power of the amplifier you can determine a reasonable load to put on the rails. e.g. for a 35V rail, a 70W amplifier should handle a 1A load without problems (it should handle higher, but this should be within the continuous power it can supply)

If disconnecting stuff from the power supply shuts it down, leave things connected but at low volume and no input signal.

Using ohms law, figure out some loads for maybe 0%, 10%, 20%, 50%, and 100% of full load. If you have nothing else then a series of 100W light bulbs in parallel would probably work. Beware that their cold resistance is lower than their resistance when hot.

Load should be between the rail and ground (not across the rails) as that may cause problems in certain fault conditions.

Measure the voltage, current, and look at the waveform at each load.

Don't maintain the load for longer than necessary to measure the performance and stop immediately if anything starts to smoke.

These tests will need to be done with the series lightbulb from the mains removed.

 

[mrmodify]

Thanks Steve, I am going to remove the rails from the amplifier sections and perform the load test. I am confident that the outputs will fry again if I apply full line potential to the receiver with the line connected.
I am going to leave everything else connected to the ps. I will try to perform the test Sunday evening.

 

[mrmodify]

Steve the rails are @ 38.2 volts.

I did some calculations and i know what resistance I will need.

Does the resistors need to be rated @ 100watts?

 

[(*steve*)]

What power is your amplifier rated at?

I'm guessing 100W.

I'd use some 100W light bulbs as loads. Several reasons, the exact resistance is not important, and they can definitely dissipate 100W, and they're cheap.

Set up a bulb with a series ammeter and a parallel voltmeter so you can measure both current and voltage. Connect the scope across the power supply.

Either change the bulbs (25W, 50W, 100W, ...) or connect several bulbs in parallel. If you've got 110V mains, expect each 100W bulb to dissipate around 40W at the lower voltage (it's not a trivial calculation because the resistance of the filament is non-linear).

I'm not an absolute specialist at determining the load you should use to simulate a full power test on the amplifier's power supply. There are many variables and the level is strongly influenced by the design shortcuts which may have been made. Following I have listed some of these issues. Proceed with care. If there are fuses in the secondary, that may help indicate a safe load (it will be some fraction of their value -- certainly no more than half of them)

Because you're providing a constant load, you do not want to exceed half the amplifier's rated power on each rail. Amplifiers generally have power supplies capable of less power than you might think. This is based on the general assumption that no music is a constant tone at one level, it is highly dynamic with large peaks and a much lower average. The capacitors (and they're generally large) provide the power for those peaks. Because of these peaks can be quite large, the peak output of the amplifier may well exceed the rated power (it may be 175W for a 100W amplifier) but it may be limited to (say) 4 cycles at 1kHz (1/250th of a second). Another thing is that due to various losses, for 50W of energy output to the speakers, somewhat more than 50W of power from the power supply is required. For all of those reasons, any full load testing of the power supply should be kept short (several seconds) and only attempted if there are no signs of distress at lower loads.

 

[mrmodify]

The power rating is 4 @ 40 watts or 2 @ 100w.

I will be connecting a bulb between the 38v + rail to ground then 38v - rail to ground.

There are no fuses on this part of the power supply. Only fuse for this is on the primary side of transformer (8 amp normally) I currently have a 3 amp fuse in.

Amp meter is no problem. Voltmeter and scope will connect across 38v + rail to ground. Then from 38 v -rail to ground? Not connected across rails correct?

 

[(*steve*)]

That sounds right.

edit: just to be doubly sure, measure the cold resistance of the light bulb filament to make sure it isn't going to grossly overload the power supply before it warms up (which it should do very quickly)

 

[mrmodify]

I have attached the results from the test.

No smoking parts.

I used a variac with no current limiting devices to reciever.

Amplifier boards and outputs are removed.

Could not achieve a good amperage draw using light bulbs. So I used my dummy loads in series with output of the the power supply. (2.5 ohm 50w resistors x 6)

Correct me if I am wrong but this clears the power supply for the amplifiers and the outputs.

Can I use the same procedure to check other outputs of the reciever?

 

[(*steve*)]

OK, now you have determined that the power supply gives you good DC at significant load, reconnect the amplifiers and run your other tests again.

This time, measure the current drawn from the power supply as well as the voltage across it (with a scope too).

If you see something that is not DC, measure the frequency and note the current.

Anything other than a frequency of 2x your mains frequency indicates something odd is happening (that odd thing may be something oscillating)

 

[mrmodify]

I hate to bombard you with more questions but,
Do I need to install amplifier boards & outputs and repeat the previous test?

The problem of frying out puts may or may not be corrected. ( dont know if pulsing fried outputs or previous part replacements fried the outputs)

Do I need to use dim bulb because of parts frying without use of dim bulb?

The last post I should have said Can I use the same procedure to check other power supply outputs especially the preamp feed?

 

[(*steve*)]

If you have some confidence that the problem causing the output transistors to fry is resolved, then try without the series bulb. But be prepared to replace them again.

An intermediate point may be to use a larger bulb (200W)

You can use the same method to test the other power supplies, but the current you need to draw will be far less (and not a simple value that I can quote either).