Marshall 8080 - Lack of volume on boost channel.

[Trevor]

Hi All,

I am hoping that one of you may be able to assist me. I recently purchased a second hand Marshall 8080 that had been in storage for a while. It had very scratchy pots and a lack of volume on the boost channel.

I cleaned up all the pots and they are all fine now. No more cracking etc.

However, I am still having a volume issue on the boost channel. If I turn the gain and volume up to the max I only get a very modest volume out of it. Loud enough to hear i.e. average T.V. listening volume but no louder.

The clean channel works great and goes plenty loud.

I re-floated all and any suspect solder joints and I replaced the electron tube with a new one. This has no impact on the lack of volume.

Can anyone suggest where I should look next...I'm running out of ideas...

Many thanks in advance.

Trevor

[J M Fahey]

In the 8080 the clean chanel goes straight, the distortion goes through that 12AX7, if memory serves me well.
Does it light?
Does it have over 100V on pins 1 and 6 ? Post what you measure on pins 1 - 2 - 3 - 6 - 7 - 8 , DC volts referred to ground.
Pull and reseat that tube, with the amp off.

[Trevor]

I put a new 12AX7 in there today. I have seated and re-seated it many times and also re-floated the solder joints on the tube holder.

The electron tube DOES glow dimly.

I measued the voltahes on all 9 pins. Bear in mind the amp was turned on but not being run with a guitar connected etc. Not sure if that makes a difference.

Pin voltages (DC) grounded to chassis as follows:

1: 0.00V
2: 0.00V
3: 0.00V
4: -0.16V
5: 0.00V
6: 0.00V
7: 0.00V
8: -0.22V
9: -0.22V

I noticed that the voltages I did measure were falling as time passed...

Many thanks,

Trevor

[Jazz P Bass]

Marshall 8080

You do not have any high voltage.
The tap coming off of the power transformer should be 170 Vac.
Goes through diode (D3) then R111 , R110 & R109.
I have attached the schematics.

[Trevor]

I have measured the voltage at the diode and all three resistors (small ones). I am getting 0.00V.

How can I test the power transformer? Are we talking about the main transfermer that plugs into the mains? There are a number of different leads of different colors coming off of that unit. A few of them measure 120VAC on the right hand side. On the left hand side I measure 2 white wires going onto the main board measuring 33VAC.

Many thanks,

Trevor

[Jazz P Bass]

Marshall 8080

The stripe on the diode (D3) should measure Vdc to ground.
The other end should read Vac to ground.
That is the high voltage tap from the transformer.
One wire.
It looks like 171Vac on the schematic.

[Trevor]

I have tested the voltage before and after the diode (D3)

There is NO voltage, DC or AC on either side of the diode.

Where should I test next. There seems to be a small board mounted transformer right next to the diode. Can I test that? If so, how? How can I be sure that board mounted transformer is getting a voltage?

See attached pic of the area we are discussing.

Many thanks,

Trevor Keast

[Trevor]

I did a little bit more testing this evening and this is what I have found:

Before the small PCB mounted transformer there seems to be a bigger white rectangular resistor (R114 - see picture in previous post). This resistor is being fed 36VAC from a main power bus line that leads all the way back to the main (mains) transformer. Up until that point in the circuit things seem to be ok as long as that voltage is correct...I'n not sure what the correct voltage should be...

That above mentioned white rectangular resistor (R114) leads directly into the small board mounted transformer. The voltage at the transformer leg (and AFTER R114) is 31VAC.

That is where the trail goes dead on me. There is no voltage coming out of the transformer on any leg. And therefore no voltage at the diode D3.

I am not expert but has the small transformer given up the ghost? If so, what are my next steps?

Many thanks again,

Trevor

[J M Fahey]

Hi Trevor.
Excellent picture, it helps a lot.
It's showing some strange things:
The large cylindrical resistors are R112 and R113.
They should be 180 ohms: brown-white-brown, yet I see brown-white-red=1800 ohms.
That alone means the tube should not light or *very* faintly, if at all.
I would have expected to see rectangular ceramic 7W ones there, not what looks like 2W or 4W regular carbon film ones.
You should have around 30VAC on one side of each of them, that's connected to your working main power amp, almost the same on the other side *with no tube plugged* and around 6VAC on said side with tube connected (and glowing), you should measure around 12VAC across pins 4/5 on the tube.
Now to the mystery (value not visible) rectangular ceramic resistor R114, which should be a 100r, 5W ceramic resistor.
Measure that you have around 30VAC on the side connected to the main transformer, and somewhat less (20/25VAC ??) on the side connected to the small autotransformer.
This one has 3 pins: one grounded, one tap connected to R114, and the high voltage tap (be careful), around 150VAC , which should reach the anode (unstriped end) of D1 (1N400x).
On its cathode (striped end) you should measure high DC voltage.
I estimate around 250V, Don't remember well.
Any failure there will kill your tube section, either for lack of high tension os filament voltage or both.
I guess somebody saw two "overheated resistors" or three, and "replaced" them.
The autotransformer itself might have some open winding, bit is much less suspect than those power resistors.
Another typical Valvestate problem is split/cracked (thin) tracks *just* by the soldering pad.
Good luck.

[J M Fahey]

Hi Trevor, you posted just while I was writing my answer.
To solve one doubt, *with the amp unplugged, not merely off*, measure resistance between all three pins on the autotransformer.
I think my tip is correct, if you have 31VAC on the R114 track, you *should* have that same voltage on the middle pin of the autotransformer, even if it were open, so it must be happenning what I commented you: that track cracking *just* before reaching the solder pad (maddening).
In this case the solution is easy, just solder a small piece of wire straight from pad to pad and f*ck the track.
Re measure those AC voltages and if not happening, measure winding continuity.

[Trevor]

Thanks for the reply,

With the tube connected and glowing I can confirm that the voltage before both of those large cylindrical resistors (R112 and R113) is 36VAC before and 6VAC after. They heat up quite warm and I can feel the heat coming off of them but they don't seem to be overheating...

Now, the rectangular ceramic resistor R114 has a voltage of 36VAC coming from the main power transformer and a voltage of 31VAC coming out of it and going into the small autotransformer.

The small autotransformer has no output voltage. The high voltage tap reads 0.00VAC...

[Trevor]

I measured the resistance between all legs of the small autotransformer and I don't seem to have any continuity between them.

Just to clarify, I do indeed have 31VAC at the middle leg of the transformer. I have 0.00VAC on the ground leg (as you would expect) and I also have 0.00VAC on the high voltage output leg.

I am uncertain that it is a crack in the track as I am getting that 31VAC right on the solder pad going into the middle leg of the auto transformer. I am more inclined to believe the auto transformer is malfunctioning internally.

[Trevor]

Just to correct my previous post regarding the small auto transformer.

I seem to have 30 ohms of resistance between the ground pin and the central input pin.

The other pins have no continuity between them...

[Trevor]

And just one last piece of information before I go to bed...

I measured the resistance of those large cylindrical resistors R112 and R113. They do in fact have a value of 180 ohms. Perhaps the color has faded with age?? Anyway, they seem to be the correct values according to my DMM.

[oc disorder]

Don't mean to butt in but.. you may be lucky and find one of the fine gauge
wires on the transformer has come adrift and can be reterminated.
If you remove the transformer carefully and look on the under side with a magnifying glass there may be an end that can be scraped to remove the
insulating coating and can be resoldered back to the terminal.
If not I guess its a new T5899 auto transformer which hopefully Marshall still carry.