Broken valvestate 8080

[JamesAlexander93]

Hi just joined this forum as I'm having trouble with my amp and thought someone might be able to give me some insight into how to fix it. I recently bought a Marshall valvestate as not working (only £20 so worth a shot). The overdrive channel was very quiet but would sometimes cut back in at regular volume, splutter a bit and then cut out again. The pots were noisy too so I  sprayed some cleaner in them and replaced the ecc83. This fixed the problem and it started working fine, except for when the volume would occasionally drop as it did before the valve was replaced. Hitting it always worked to make it cut back in though. A couple of days ago during a rehearsal there was a popping sound from the speaker and the amp cut out entirely. It powers on okay but the only sound I can get from it is a low hum/buzz from the speaker which remains constant regardless of what any of the controls are set to (volume and gain controls neither reduce nor intensify the hum, it is just one constant volume). Plugging a guitar into it makes no sound at all and also does not affect the hum. Touching the tip of the jack lead whilst the other end is plugged into the amp also makes no sound and I've tried a couple of cables. I've checked the bottom of the pcb for any dry or cracked solder points and can't seem to see anything wrong. Any ideas as to what could be wrong with it?

[Roly]

First thing, is the speaker cone moving hard forwards or backwards when the amp is switched on, or staying roughly in the rest position?  If it is moving to one end of its travel and staying there it must be disconnected to avoid damaging it while other tests are carried out.

Second thing; take a known good lead and bridge the Effects Loop Send and Return - any change?

Try feeding a signal into the Effects Return, say from an MP3 player or the like - any response?

Try feeding the Effects Send into another amp - any response?

These test will help narrow down which section of the amp is playing up.

[JamesAlexander93]

Hi, thank you for your response. I just checked your first suggestion and yes the speaker moves back hard when the amp is switched on. Bridging the fx loop send and return makes no difference at all. Plugging an MP3 into the fx loop return does nothing until the MP3 player is turned up past a certain volume, where the amp begins to crackle. The crackling intensifies slightly as the gain is increased and is worse on the boost channel, but disappears when the 20db cut is engaged. When plugging the fx send into another amp it just produces the same buzz as the valvestate. Does this mean it's a problem with the speaker? Should I disconnect it? Thanks for your help.

[Roly]

Absolutely - disconnect the speaker before it gets damaged.

This is a classic symptom of a failed output transistor.  With the speaker disconnected, measure the voltage on the output connection (to chassis/ground).  The two supply rail voltages aren't marked on the "pwr" circuit but the cap voltages of 50 volts suggest they will be 30-40 volts.  When you measure the voltage on the output I'm sure you will find that it's something of this order, either +ve or -ve, and the polarity will give you a clue as to which transistor(s) have failed.

At this point you will need to make up a limiting lamp lead for testing after the faulty transistors have been located and replaced.  See;

http://www.ssguitar.com/index.php?topic=2093.0

Post what you find, and please confirm the actual supply voltages you measure across C61 and C62 (2200uF/50V, in power supply).

[J M Fahey]

In the 8080 combo rail voltage is slightly more than +/-40V (usually 42).
On the 4100 head, which is the exact same schematic but meant to drive a cab or two, voltage is somewhat smaller (+/-36 to 38V).
Output transistors can be replaced by TIP142/147 .

[JamesAlexander93]

okay, I've measured what you said and I'm getting +46.7v across C61 and -46.9v across C62. But on the output I'm only getting -8.3v.
Pretty sure I'm measuring it correctly but then I'm a bit of a novice so just to double check it's + to the speaker output wire and - to the chassis (ground)? Sorry if I'm being an idiot. Thanks again for the help though.

[J M Fahey]

- 8 or 9V DC at the speaker terminals is *bad*.
Normal is less than 0.1V DC , go figure.

[Roly]

No, you are not being an idiot, those are good measurements.

We have good supply rails, but the offset on the output tells us there is a problem in the power amp section, so we are on circuit 8080-pwr.gif (I'm appending pc0689p.pdf which is clearer).

Now this one is a bit tricky because the output devices are not normal transistors but a dual composite called a Darlington where the driver and main output transistor are in the same package and testing them is not quite as straightforward as normal transistors.

What seems to be happening is that the "lower" transistor TR10 is pulling the output down a bit, but not fully as one would expect with a shorted transistor in the TR10 position, so I'm inclined to suspect that the opposite "upper" transistor TR8 has gone open circuit.

If the board has component identifiers printed on it, find the diode D5 and measure the voltage on each end (w.r.t. ground).

Let us know what you find.

{JM - you got any suggestions how to tackle this one?}

[J M Fahey]

Well, step by step.
I'd start by measuring all transistors (with amp unplugged from the wall outlet) for shorts.
We are looking fror gross shorts here, so we can start by measuring them still on circuit, needlessly pulling all them for measurement will do more harm than good.
Builda lamp bulb limiter, you'll need it later.
The original Marshall 8080 power amp is somewhat tricky, don't know what they tried to achieve, but it's still repairable, of course.
Later Msh amps went back to more conventional circuits.
Google how to measure transistors for shorts, I guess Rodd Elliott has a good tutorial.

[Roly]

No, not Rod that I can see, but here are some others;

Making Electrical Measurements Part 3:
Testing Diodes and Transistors
http://www.elexp.com/t_test.htm

Meter check of a transistor
http://www.allaboutcircuits.com/vol_3/chpt_4/3.html

Rudimentary transistor test (diodes and TO3 power transistors)
http://www.youtube.com/watch?v=wFHAGd_Tcvw

Google how to test transistors
http://www.google.com/q=how+to+test+transistors

[JamesAlexander93]

Okay, I've gone ahead and replaced both TR10 and TR8. For what it cost I thought it was worth it just to be sure. However, the problem is still there with no change whatsoever. I've read elsewhere that bad filter caps can cause similar problems. Could this be the case? If it looks like there's component damage that'll be difficult to isolate or that I can't fix without taking it to a tech, I may give up on it. There's only so much money I can put into it before it becomes false economy, becuase I've decided that regardless of whether or not I can get it working I'm still going to sell it afterwards as I simply don't trust it enough to gig with given the problems it's had. I'd rather sell it, make my money back and get something newer and more reliable. Whether or not I can fix it will be the difference between selling it for parts or repair, or selling it as working, with the latter obviously being preferable as I'll make more money from a working amp (I obviously won't sell it without mentioning its history). But yeah...if you have any other suggestions let me know. Thanks.

[Roly]

Firstly; you must feed back your findings and measurements to us so we have some idea of what is going on at your end.  There is no point in randomly replacing components unless they actually test faulty.

Secondly; for problems in solid state output stages making up a limiting lamp and using it as directed when bringing up the amp after repair is not optional (unless of course you enjoy replacing brand new transistors you have just fitted).

http://www.ssguitar.com/index.php?topic=2093.0

Thirdly; no, this is dead silicon, not leaky caps; it's just a matter of locating it all.

Okay, now, given that you have already tried the amp without a limiting lamp, what voltage do you have on the output now that you have replaced the main power transistors?

Did you test the ones that came out?  With what result?

[Enzo]

My thoughts here are that this is not an output stage problem, I suspect TR8 TR10 are doing what they are told.


As Roly said, filter caps won;t put that DC on the output.  They will make loud hum if they are bad, but not DC.  DC on the out will ALSO make loud hum.  And so will a bunch of other things.

You had about 46v on the main rails, close enough.  But you also have to check the low voltage supplies.   This power amp is driven by an op amp, and that needs both +15 and -15 up aqnd running.  Pins 4 and 8 of the IC.  Is it there?

If one of your 15v rails has failed, it will leave the IC lopsided power wise.  It won;t be able to center itself and thus center the output at zero.   FInd R101 R102, 5 watt resistors.  SHould have that 46v at one end and 15v at the other on each one.  One will be negative and the other positive.

It may be a coincidence, but if one of your 15v rails is missing, your op amp will try to center on what is left, and that would be just about 8 volts.

And along with the op amp, TR5 TR6 need the 15s so THEY can center as well.

TR4,5,6,7 are cheap common types, and wouldn;t hurt to replace them, but if I recall your outputs were not really shot, so likely did not damage those smaller ones.    I could be wrong, but slapping the whole deal over to a rail is a more likely result of one of those failing, not 8 volts.


Right now, my money is on a bad zener.

[JamesAlexander93]

Okay Enzo R101 and R102 aren't giving me what you say they should. R101 is giving me +47.3V on one end and +16.2V on the other. On R102 I have exactly -47V on one end and -0.08V on the other.

Regarding the op rails, which IC is it that I need to measure pins 4 and 8 on...Is it IC8B? 

[J M Fahey]

That's what Enzo suspects.
A shorted Zener killing the -15V rail.
INCREDIBLE marksmanship, as usual.
Replace that Zener, check whether you recovered your -15V rail and whether your amp now works with little output offset.