Marantz model 4070 low volume [Solved!]

[Alex_Baker]

Hello everybody, I just picked up this amplifier as a side project to the HP 1727a scope that I am working on, I don't think it will be finished anytime soon. 

The amplifier in question is a Marantz Model 4070 console amplifier. I picked it up for what I would usually say is a rather high price but I have seen that these Marantz amps can sell for a lot on the used market, which is my intent once I am done.

It was sold to me as not working, which is kind of true. Everything seems to work fine(after cleaning some stuff), the only issue is that overall the volume is kind of low on all channels and all inputs (as far as I can tell). I don't know exactly what the volume should be but compared to most other amps I have messed with this one is quiet.

This unit has rear preamp to main amp jumpers so of course I put my signal in there and the volume+quality is great (on the crap speakers I have anyways). This narrows the possibilities down some.
So now that you have the background does anybody have any suggestions on what to test/look for?

[Audiorepair]

The amplifier is rated at 15 Watts per channel into 8 ohms.

Put an 8 ohm dummy load on one channel, and see if you can get 15 Watts out of it.

Chances are you can, and there is nothing wrong with it.

[TimFox]

And, if you can get 15 W mean power into your 8 ohm load (10.95 V rms or 31 V pk-pk), see how much voltage at the CD or AUX input, or at the preamp/main amp jumper, was required for that output.
You may find that the amplifier cannot generate 15 W continuously:  if not, repeat the test at maybe 5 or 10 W.

[bob91343]

From what I gather from your description, the power stage is working fine but isn't getting enough signal.  Measure how much signal it takes into the rear jacks and that should tell you where to look.

I presume you have a service manual.  Since all channels are weak it points to a power supply issue.

[Alex_Baker]

I do have a service manual.

With the limited checks I did The power supply seems fine, it is pretty simple compared to some other amps I have messed with. I think I will test power output first just to make sure U am not going down the rabbit hole...

I have a pair of 6 ohm 25W resistors, could I do the power test that way? What do you mean how much voltage at the inputs/jumpers? like with a multimeter on RMS?

[bob91343]

Yes a multimeter on ac should indicate how much signal you are getting into the power section from the preamplifier section via the rear jumpers.  If not sensitive enough, use a 'scope or audio voltmeter.  Six Ohms is fine for a load but keep an eye on the temperature of both the load and the power amplifier.

[Alex_Baker]

I will test signal voltage tomorrow but I just did a quick test with the resistors and, although I got a faint tone from the speakers, I measured like 7mv on my meter.

My meter is a cheap one but I verified that my test setup works by bypassing the preamp with the signal. By doing that I was able to get about 3.2V RMS (1.7W) with the volume cranked all the way up on my phone using the AUX jack and a 1kHz sine wave.

So this points to a preamp issue, and one that is common between both channels... Is there a way to make sure none of the filter/loudness/speaker switches is causing the problem?

[TimFox]

When you drove the AUX jack to get 3.2 V rms output, what was the input level at the jack?

[Alex_Baker]

This morning I tested it again and I got 5.57V RMS on the output with an input of 0.257V from my phone. When I use the aux input on the amp and put the jumpers back in I am only measuring 0.6mV on the jumpers with the same 0.257mV input. I only measured 13mV on the output of the amp as well.

I am not quite sure why I am getting a little over 5W this time compared to 1.7 last night but nonetheless I think there is still a problem with the preamp.

[Audiorepair]

If you haven't already, check the frequency response of your meter on AC.
Many cheap meters have an upper limit of 1kHz or even less.

You might get more accurate results using 100 - 500 Hz from you phone instead of 1kHz.

[bdunham7]

This morning I tested it again and I got 5.57V RMS on the output with an input of 0.257V from my phone. When I use the aux input on the amp and put the jumpers back in I am only measuring 0.6mV on the jumpers with the same 0.257mV input. I only measured 13mV on the output of the amp as well.

I am not quite sure why I am getting a little over 5W this time compared to 1.7 last night but nonetheless I think there is still a problem with the preamp.

Your power amp has a gain that seems within reasonable expectations and the signal definitely isn't getting through the preamp, so that's where to look.  I have no experience with that specific model of Marantz amp, but I've seen issues with other quadraphonic stuff where the proprietary quad stuff loses signal internally.  As a next step I'd put all the jumpers back in and test all of the inputs to see if any of them work.  Also, try it in the 4-channel mode (switch on back) if you aren't already and just test every single pair of inputs except phono.  I'm guessing your tape and tuner inputs may work properly.  Once you've done all that and you know what works and what doesn't, open up a schematic and figure out where the problem may be.

[Alex_Baker]

OK, so I found the problem already... I am surprised it didn't take me longer than just 2 days. 

The issue was that a couple of matsushita (I think that is the brand) caps had shorted. Weirdly it was the exact same cap in the exact same block of the preamp that shorted for both channels. I haven't come across any dead shorted caps in my limited experience let alone two identical ones in two identical circuits. I hope this doesn't mean that there is still an issue upstream but given the circuit I doubt it.

Now the question is what to do about it. It is obvious that some caps in this amp will need to be replaced, the question is how many and which ones. The two main amp boards have nippon chemicon caps, but the other boards have a mix of Matsushita, Elna, and nippon chemicon caps.

[bob91343]

My first thought is to replace the bad parts and let it run.  It may be fine.  If you want to drive yourself nuts you can replace all the electrolytics but I think that's a waste of time and money.  Not to speak of the possibility of making errors.

[Alex_Baker]

My goal is to fix and clean this amp up and sell it, as I got it non-working I might be able to make a profit  . My concern is that I just replace the bad caps only for it to break again a little down the line, I think what I will do is replace all of that type/brand of cap, of which there are only about 15 of them.

Does anybody know where to get a replacement light bulb for this? It is one of the ones that is the same shape as an AGC fuse (1/4x1-1/4).

Lastly does anybody have any tips on cleaning pots and switches? just about all the ones on this amp need cleaning badly. I have used deoxit in the past but I don't like the oily residue it leaves behind.

[Alex_Baker]

I just put my scope across the main filter caps to test for ripple and this is what I got.



Is that about 90mV of ripple? This is with a 10x probe and the scope AC coupled. This ripple seems a little high to me but please let me know what you think.

[bdunham7]

The ripple is normal and will get much larger under load.  As far as what capacitors to replace on vintage Marantz, more experienced audio techs know which ones are likely to go bad--I think certain Sanyo and Elna models are known to fail, I'm not sure about Matsushita labelled caps.  Typically the filter caps are OK, caps on things like tuner boards (you don't have one and if you did you wouldn't want to just dive in and start replacing stuff), tone boards and preamps are usually OK and the caps on the auxiliary power boards are often marginal or bad.  User mzacharias would likely know a lot more specifics if he is still around.

[TimFox]

That's a classic 120 Hz serrasoid ripple waveform.  Without further information, 90 mV pk-pk seems reasonable.  If you know the current draw and the filter capacitor, the calculation is simple.

[Alex_Baker]

I measured the current draw on the positive rail to be 100mA, and the formula says that with 10000uF caps 90mV is about right. Good thing I don't have to replace those caps, that would be expensive!

[bob91343]

Those lamps are 8 Volt.  I have a few of them that I wisely took home when I closed my repair shop in 1985.  I think they may be available on the auction site.

[Alex_Baker]

I dint know what the search term for them is, I have looked but didn't get any results.

[bdunham7]

Fuse lamps.  LED replacements are very common.

https://www.ebay.com/itm/224133195543?hash=item342f612717:g:kr8AAOSw6EhfRVMF

[Alex_Baker]

Thanks!

[Alex_Baker]

The new caps came in this morning, I put them in and this amp sounds Great! I had to borrow some speakers from my brother because mine are garbage, LOL

Thanks everybody for your help, Hopefully I will be listing this on ebay next week when the light bulbs and some deoxit I ordered arrive.