MAC 4C A52 (schematic could really help..)

[ricothetroll]

Hi,

I got my hands on this amp :
http://i1299.photobucket.com/albums/...ps0fbc0371.jpg
http://i1299.photobucket.com/albums/...psf367a220.jpg
http://i1299.photobucket.com/albums/...ps51775cea.jpg
http://i1299.photobucket.com/albums/...ps12b7da43.jpg
http://i1299.photobucket.com/albums/...ps27d620c8.jpg
http://i1299.photobucket.com/albums/...psf6bc5abb.jpg
http://i1299.photobucket.com/albums/...ps7adb9c96.jpg

I couldn't find the schematic for this one. It seems to be a pretty rare amp.

The tubes are : 4x EL84 output tubes (the ines put in a square position) and the rest is all 12AX7.

The standby switch isn't actually a real one as it grounds the input of the phase inverter instead of cutting B+)

The amp seems to have been made in the beginning of the 70, as it is mostly tubed but has some transistors too. The reverb drive/receive circuit is transistorised, the rest is all tubes.

As you can see of the pics, I recapped it completely.

It works perfectly, excepted there's a slight but annoying hum (50 Hz) on the output. Most of the hum seems to come from the phase inverter, as it's still present when the input tubes are removed and disappears when th PI tube is removed. It is still present with the vibrato oscillator tube removed. I swapped the PI tube with some new 12AX7, problem still remains.

I first suspected the heaters, as those aren't fed with twisted wires but directly wired on the PCB. I cut those traces on the PCB to keep the AC far away from the PI circuit, and fed the heaters with an external +/- 6,3V DC supply, but the hum is still present, unchanged.

There is a ground loop on the main circuit, as the PCB is surrounded by an uncut ground trace. I cut the trace to try removing the loop, but the hum got only stronger

I must admit that I don't know what to try now...

If anyone here has the schematic for this amp, I would be really happy to get it !

Any idea to go on the toubleshooting would also be really appreciated.

Thanx in advance !

Best regards.

Eric

[Jazz P Bass]

Can you try & measure the hum at the speaker leads?
Set your meter to read Volts/ AC.
Ideal reading would be 4 mv's.
Anything over 9-10 mv's is a problem.

[ricothetroll]

Hi, thanx for your answer !
I actually did all my tests, after listening to the problem with a speaker, with an oscilloscope connected to a 8R/50w dummy load resistor. If i remember correctly I measured about 300mV P-P...
So yes there is a problem :-)
Best regards
Eric

[nickb]

Down in the dustiest of the achieves....ah ha!



Good luck

[ricothetroll]

Hi !

Thanx a lot for the schematic ! It is really helpful, as it allowed me to identify the multiple inputs that feed the phase splitter (actually an amplifier before the concertina circuit).

I tried grounding each input just before the summing resistor and it appeared that each has its own small contribution to the output hum ! With every input grounded (channel 1&2, reverb and tape), there is still some hum left. The only way to suppress the hum is to ground the grid itself, after the summing resistors, like the "standby" switch does.

Looking at the general grounding network, it appears to me that it's quite a messy one : every PCB corner connects the PCB ground to the chassis, and every PCB is linked together with a piece of wire. And every PCB has a ground loop becaiuse the ground plane surrounds it without interruption... That's a lot of possibilities for stray ground currents to flow !

If the hum is caused by those loops, I guess I have 2 solutions : either live with the hum, or redo the whole grounding of the circuit, and replacing the metal screws by plastic ones to get rid of the chassis connections.... Then I'll be able to star ground the whole circuit and... see what happens then

Any opinion on this ?

Thanx again for taking time to help me.

Best regards.

Eric

[nickb]

It's hard for me to imagine that the manufacturers would have settled for 300mV of hum on the output so I would be of the opinion that there is a fault rather than grounding being the reason, bad though it might be.

One of the replacement caps could be bad - it happens. A solder joint might be bad. What effect do the volume controls have? Could the heater center tap be open etc...

[ricothetroll]

Hi,

I checked the solders, they all look just fine.

I tried to disconnect the other circuits from the main PCB, ground the unused inputs in order to not leave them floating, remove the bolts from the main PCB and unbolt the input jacks, so that there's only one connection to the ground, that is the return to the PSU caps (star ground). I checked each step to see if the hum decreases : nope. With the ground disconnected everywhere excepted at the PSU cap return, things got even worse, the hum trippled in volume, and with the volume of input 1 even slightly up, I got some nasty HF oscillations...

Correct me if I'm wrong, but I think that if the PSU caps are bad, then I should get some 100Hz buzz instead of my 50Hz hum, right ? Before the cap job the amp already had this hum issue.

I even tried wrapping the PSU trafo with laminated steel (stolen from the core of a sacrificed toroidal trafo for shielding a PSU trafo on a mic preamp project), without any result.

With channel 1 volume control all the way up, the hum doubles. Volume control of channel 2 has little or no effect on the hum.

Here is an audio sample of the hum :
https://drive.google.com/file/d/0B6K...ew?usp=sharing
Here is a close up one the waveform, and the FFT (made with LTSpice) :
https://drive.google.com/file/d/0B6K...ew?usp=sharing
https://drive.google.com/file/d/0B6K...ew?usp=sharing
If ever that can ring a bell...

I'm running out of ideas right now...

Best regards.

Eric

[nickb]

Agree that bad PSU caps would give 100Hz.

I never would have thought have using LtSpice's wav file reading tool to calculate an FFT. That's one for the books Anyway. It proves it's 50Hz. It seems you have hum getting in at multiple places so you have a battle on your hands for sure. I'd set the volumes to zero and fix the hum remaining, and then turn the "guitar" volume up and fix that and so on, a section at a time.

Thinking about the heater supply, have you checked for 3.15VAC on either side i.e. the center tap is good?
You could try isolating the AC heater supply and use and external DC PSU.

On the multiple PCB to chassis connections could you add insulating washers and/or nylon screws on all but one mounting hole.

[TimmyP1955]

The heater center tap is not drawn as a center tap, whereas the 8 Ohm is drawn as a center tap. Perhaps the heater tap is off center to counter the load of the pilot, which is not across the heater winding, but instead is from one side to ground. As nickb said, check the voltages there. Maybe even disconnect the light and the 'center' tap, and put in a faux CT (a 100 Ohm resistor from each side to ground).