Marantz 4300 dim bulb test

[mrmodify]

I am the working on the above amp and would like input on my thinking.
I have it plugged up to a dim bulb tester. I have a 7.5 ohm dummy load on the main front speakers. When volume is turned up 1/4 of the way the bulb will go bright and start to flash & the relays will start to make and break and the front panel lights will go out. If load is removed all is fine.
If load is placed on left channel and volume is increased the light will start to flash and relays cut in & out. If I remove the load from left channel and placed on right channel bulb will slowly brighten as volume is increased. When the main rear speaker are connected to a dummy load they act the same as the front right channel does All the output transistors are new along with a lot of other part. I am thinking the problem is related to the outputs on the front left channel ?

 

[(*steve*)]

OK, so you have a light bulb connected in series with the mains lead to limit current?

During these tests, do you have a signal present on the input of the amplifier?

Grab a multimeter on a DC voltage range and measure the voltage across the dummy load at all volume levels. It should be zero (or very close to zero).

Check the output transistors and see if any are significantly hotter than the others.

You may have a fault in both channels (that happens to be worse in one than the other), or yes, it may be limited to the left channel.

The rear channel(s) may be separate amps, or they may be simply connected in series across the two main (front) amplifiers.

 

[mrmodify]

Evening Steve, First two questions are yes. I have a 100w bulb in series on the hot (black) wire. I did have a radio station tuned in on FM. I connected dummy load to amp and placed amp in aux 2 mode (no input). Connected multimeter to main front left channel. Powered on and the front left went to flashing bulb and relays started cutting out soon as the main speaker button was depressed. I could not get any reading on my multi meter because of pulsing to fast. I checked front right with meter connected across dummy load. I got a floating reading from -20mv to about 20 mv at all volumes. Then after at full volume for about 3 sec, the relays went to pulsing and light flashing just like the left channel. I transfered the dummy load to the Main rear speakers and connected multimeter and obtained a rock solid measurement of 0V on right & left channels. I did the audio adjustments as the manual procedure calls for last night before I posted the first message. There are four sets of NPN & PNP's outputs on this amp. I am pretty sure that there are four amps in this unit. I am going to check for a short some how in the spkr protection board. This unit was in very bad shape when I started. Someone went in and I don't know what they were trying to do. I thought I checked everything very thoroughly. Guess I didn't do to well.

 

[mrmodify]

I just found that if the volume control is advanced at a very slow pace you can get full volume with volt meter reading from 20mv to -20mv and is floating like crazy but the relays are not cycling. Any movement other than very slow in up or down rotation the relays will start to cycle. When relays start cycling you get a peak voltage of 15 to 20V pretty sure DC on my scope.

 

[(*steve*)]

Are you looking at the output (across the dummy load) on your scope?

It sounds like you have no input to the amplifier.

It may be worth shorting the input to ground to help ensure you're not picking up hum etc.

Does the amplifier show any output?

I would be expecting a small ripple and maybe a very tiny DC offset. The DC offset should not change with volume setting but any hum (should be predominantly mains or 2x mains frequency + noise) may increase with volume setting.

If you're seeing a large signal or offset then you have a problem.

Once the speaker protection is triggered, the oscillation you see is almost certainly caused by the high supply impedance (due to the light bulb) and the filter capacitors. There is almost no point in recording what's happening whilst it's doing this. But finding out what happens just before it first triggers most certainly is important.

Measuring the current through the output transistors might be useful. It's possible that someone has "dicked around" (that's not a technical term ) with the biasing and you're seeing some form of runaway behaviour.

 

[mrmodify]

Yes sir the scope is connected across the dummy load. I have no input on amp. The selector switch is on the aux 2 position with nothing connected to RCA jacks. I get a .1V sine wave on the main front channels if volume is move slow and steady. The remote front channels the traces are the same except for the left channel is at 50mv sine wave. I do have output on the scope when I switch to FM. Do I still need to measure the current on the outputs? If so how? The remote front channels have started to do the same as the main fronts are doing. Think the dim bulb is causing this?

 

[(*steve*)]

I asked previously if any of the transistors get significantly warmer than the others. Can you check that?

Just leave the unit operating at a low volume level and without the speaker protection activating.

 

[mrmodify]

I meant to. I will do that tomorrow and let you know. I will have to check temps with a infrared temp gun which I have. The outputs are tucked between the amp board and there heat sinks. I am assuming I do need to have the dummy loads connected while the unit is running and with a signal on the input? Do I need music or a solid signal injected into a input?
I greatly appreciate all the help you have giving me.

 

[(*steve*)]

No, do it first under quiescent conditions. No need to a dummy load at all. and no input either.

I want to see if the biasing of the amplifier is so bad that the output transistors are getting hot.

 

[mrmodify]

Good evening Steve. I ran receiver for 1 hour with no input and no dummy loads connected. I did however forgot to turn main and remote speaker buttons on. Does it matter?. I have attached what I did get.

 

[(*steve*)]

These are degrees F on the transistor case I presume?

What are the "outputs" and "driver"? Do 1, 2, 3, and 4 represent individual transistors?

Running it for an hour is good. If these are as they seem then the temperature rise is not excessive and stabilises fairly quickly.

Now can you do the same with a dummy load attached? Leave the volume low and have no input.

It will be important to have one set of speakers turned on for this (the ones connected to the dummy load) It may also be useful to measure the dummy load temperature.

 

[mrmodify]

Sorry the temps are in degrees F. Each number does represent a transistor. The bottom output numbers are missing because of there location. (Can't get to it to measure)The left channel and right channel outputs have a TO3 case mounted to the largest heatsinks. They are connected to the amp boards by wires. The left and right channel drivers are mounted on the amp boards with a smaller heat sink. By the manual the left and right channel drivers drive the outputs. The drivers are a TO220 case. The emitter of each channel driver is connected to the base of the each output. A NPN driver goes to a PNP output. And a PNP driver goes to a NPN output. On the outputs the numbers are from top to bottom of the heat sink. I haven't traced the physical location of which driver goes with which output. The channel drivers are from front of receiver to rear. I know that doesn't help you much. If needed I can get a relationship between which driver goes to which output. The smaller heat sink has the heat biasing diodes (STV-03's no longer available) mounted to it. I have had to build a replacement for these diodes. Will check unit tomorrow with temp gun with temps on the heat sinks. Going to hit the sack and get ready for work. Thanks Again.

 

[(*steve*)]

OK, if you've replaced the biasing diodes then the biasing is likely to be wrong. There should be trimpots to adjust it and it is advisable to turn then to the minimum bias, then increase it until the quiescent current reaches the correct value.

(What "correct" is, I can't say)

Also whether the biasing is the root of the problem, I can't say.

 

[mrmodify]

I have attached the temp readings with no input, a 7.5 ohm dummy load, and a 1/4 turn on volume, main spkr switch on, the bottom half of attachment is the latest readings. I found by accident that when the voltage feeding the house is disturbed (electric dryer starting a 1 volt line drop) will trigger relays to cycle in and out. When house voltage stabilizes receiver protection circuit will not activate. I have adjusted bias as per manual. One setting is set to 0V and the other is set to 12mv. I can achive both of these settings. If I understand even if protection is triggered by line voltage or not, my dc offset is to high. Adjusting the bias will drop the dc offset? The biggest problem is that I can look at a schematic but don't get the whole picture.

 

[(*steve*)]

Interesting that supply variations so small will trigger the fault.

That turns my suspicion to the power supply.

Your last test indicated that DC offset is also unlikely to be a major issue as the temperature rise with the dummy load is pretty much identical to that without it.

It might be time to take a good hard look at the filter capacitors in the power supply. I would like to see what ripple is present on the power supply, but for that you'd need an oscilloscope.

The bias may affect the DC offset, but not directly. Can you tell us what the instructions are (briefly) for setting the bias? You say one is 0V and the other 12mV -- what are these? Are they 2 points in one circuit, or values for (say) the left and right channels?

Do you have a copy of the schematic?

 

[mrmodify]

Hello Steve, I have attached a text which was typed straight from the manual. It is the description, adjustment procedure and schematic. This is probably kindergarten for you but for me it is part Latin. I had to chop the schematic down to one amplifier (1/2 the board) to accommodate the download size. On the schematic the square boxes on the right is the J036 connector. Will check power supply with scope tomorrow. I had to replace alot of parts in the power supply in the beginning. I was starting to lean toward th P.S. last night.

 

[mrmodify]

The filter capacitors for the outputs have a straight line trace on my scope. +28.6 VDC and -28.6 VDC. Voltage does drop about 1.6V from 0 - 100% volume with dummy load. Was checking a few thing and accidentally plug receiver into my 120VAC non dim bulb plug. Smoked R745 but did not open it. Dim bulb now stays lit. Checked a few parts and found H003 & H004 I think open. On 1 of my transistor testers neither transistor will light the bulb and the other tester shows H003 with a flashing 3000 hFE & H004 just shows a 0 hFE.

 

[(*steve*)]

I'm afraid I'm hindered by the difficult to read schematic.

Can you post a larger (and preferably clearer) one?

I can't actually find R745

 

[mrmodify]

Try this one, My schematics are not a good quality. I have enlarged it as much as I can. I am very limited by download size. If there are any other options or if I am doing something wrong please let me know. R745 is between H721 & H713. Would a open output transistor cause the relay issue? The outputs are new but I was wondering if they had opened before my last night.

 

[mrmodify]

Is it ok to power up receiver with amp board removed and outputs installed on heatsink? And Can I power up receiver with outputs removed and amplifier board installed. I am tying to locate my fault if there is one.