Marshall 8100 Power Amp help

[gibsondude]

Hi there- I was given a Marshall Valvestate 8100 head, but was told it was not working. It does seem to power up OK, lights come on, but not getting any signal out to speakers, not even a crackling or hum. I have taken it apart and there is nothing obvious (no burnt resistors etc.). The "valve" is powered up (it glows), and the pre-amp circuit seems to work- I plugged headphones into the "Line Out" jack and I am getting a signal that responds to channel select, volume, reverb, etc., but nothing from the speaker out connections.

I do have the schematics and some electronics experience and tools, but am new to messing with amps and hoping that someone might help point me in the right direction!

[jaycee]

Going from this schematic: http://www.drtube.com/schematics/marshall/pc0689p.pdf

I can't see where "Line Out" could be...

Check the signal at the point where it says "Input from preamp". If the signal's there then it's time to start looking at the (IMO dreadful) power amplifier stage. Top culprit will be the output transistors TR8 and TR10. You will probably find the BDV devices difficult to get - TIP142/147 should work equally well. You may also find TR4/7 have been damaged - these are MPSA06/MPSA56 and easy to get.

You are probably wondering why I thought the power amplifier was dreadful. The reason is that guitar amps need to be rugged as they cope with a lot of grief such as sustained clipping etc. Even for 100W into 4 ohms, I would expect to see 2 or even 3 pairs of output transistors used.

[davada]

Originally Posted by gibsondude

Hi there- I was given a Marshall Valvestate 8100 head, but was told it was not working. It does seem to power up OK, lights come on, but not getting any signal out to speakers, not even a crackling or hum. I have taken it apart and there is nothing obvious (no burnt resistors etc.). The "valve" is powered up (it glows), and the pre-amp circuit seems to work- I plugged headphones into the "Line Out" jack and I am getting a signal that responds to channel select, volume, reverb, etc., but nothing from the speaker out connections.

I do have the schematics and some electronics experience and tools, but am new to messing with amps and hoping that someone might help point me in the right direction!

Hi gisondude,

I used to do warranty work on Marshall amps. One common fault is the pads and traces on the board where the speaker jacks are mounted can crack and separate. If you have a multimeter you can check the continuity from the jack to some other point on the circuit board where the trace runs to. If not then do a visual inspection. Take the nuts off the jacks and gently wiggle the jacks about and see if the solder is moving around.

If this is the case then scape some of the solder mask away and solder piece of wire across the break to bridge it.

Good luck,

David.

When you say line out do you mean the direct out jack?

[geraldfryjr]

I had Blown mine out aswell back when I had it!
I never did fix it and I always used it as a preamp after that But I do have several sets of the BVD's some where.
That amp has a very whimpy output stage and it only has two transistors in the output.
Although Impressive for the design it is not very durable even though I did abuse it bad, it did hold up well.
Since I don't have it anymore I can try to find them for you if you like,since I will probaly never go through that again.
As much as it would be nice to have it working again as long as the preamp works as required you would be better off just running it through another power amp of your choice.
It was a very clean power amp stage but it doesn't hold up to heavy duty.
Just my opinion, as that is one killer and versital amp !

jer

[jaycee]

Marshall seem to have a habit of this now... they're using nasty little chip amps which die under the abuse :/ Yet just two pairs of MJ15024/5 or even MJ21193/4 on a good heatsink using a standard "Blameless" type circuit would make for a durable 100W guitar amp.

[geraldfryjr]

Yea I was shocked to find out what they used in the newer ones.
The older ones claim that they were stereo versions but they used chip amps.
My friend has one of those newer versions but it does sound sweet.
jer

[geraldfryjr]

I agree ,jaycee,Before mine got stolen, I had gotten the proper output transistors and I was going to repair it and double up on the output stage.

But unfortunatly, I never got that far,it was a very verisital and sweet sounding amp that was unmodified,I do miss it very much!

Just for the record the reason mine blew out was because I was experimenting with ribbon speakers of my own design and it tolerated that .1 ohm load for quite some time and then I decided to use the 2 ohm tap on my ampeg v4-b and that didn't last very long either,Opps!

Live and learn the hard way, I guess! jer

[amptech]

Gibsondude, remove any headphones connected to the o/p just in case you get a loud pop.

It's been years the last time I worked on marshall gear...on looking at the power stage they use darlington power transistors in there much better than 'said' ic o/p. any way moving on.
do you have a scope? if not you can use a signal tracer to follow the signal to see where the signal goes dead. to ensure the pre amp side is working and just because the valve is glowing does not meant it's allowing signal flow...valves work with two supply's so to speak,ht feed of 180volts or more and heater supply 6,3 and in some case's 12v...

to test the o/p side place/tap a signal directly in to the power stage via the cap marked c18 in put cap 0.047 lift it's leg to the left hand side and using a 10k pot to control the tapped signal in and connecting a speaker to the o/p and then apply signal.. you should hear sound? check all track's for dry joints around the front pre amplifier pcb aswel.

[gibsondude]

Thanks everyone for your input! As mentioned I am new to the forum and new to messing with amps (although I have been playing for 20+ years). I have basic electronics from 1st year mech engineering and have a DMM - no scope.

Jaycee: "Line Out" (as marked on front panel) must correspond to "Direct Out" on schematic.

Davada: this model has separate speaker jacks into back panel, connected by wires; all connections look fine.

geraldfryjr: may take you up on offer of BDV64/BDV65!

So I know that testing transistor function in circuit with a DMM set to "Diode" can be iffy, but here is what I got:
TR8 (BDV65)
EB = open
BE = 796
BC = 621
CB = open
CE = open
EC = 492

TR10 (BDV64)
EB = 857
BE = open
BC = open
CB = 661
CE = 530
EC = open

Also tested TR9 (TIP29C, also on heatsink)
EB = 660
BE = 479
BC = 504
CB = 1520
CE = 1646
EC = 865

Also tested TR4 and TR7, and got readings both directions on BE and one direction CE.

Does this mean that all of these transistors are fried? Or is this test method invalid/inconclusive? Any other tests I can do without taking the transistors out?

[bernification]

Hey Gibsondude,

been fixing guitar amps for a few years.

As the amp has pre out, we can assume that the voltage rails are there, but just check them to be sure. The output devices look like they measure good.

Obvious one, just check the insert return. The pre out could have a normaled return to the poweramp input. It's quite common for the out to be returned to the jacks 'switches' to go on, and I have found, countless times, a failure here.

If you have no speaker 'blow' (that faint hiss or hum) Then check the emitter resistors. They are the big 'bathtub' resistors that are close to the output devices. They should read practically short- .33 ohm. They are marked r96 and r110 on the schemo posted.

If you have no scope or generator, try to put your probe, on volts, and referenced to ground at the junction of r98/99 and the diodes. You should hear a buzz. This means it is working. Try at the input to the cascaded/darlingtonesque transistors . If you get a louder buzz, there is more amplification= working. Then try the op-amp. Pin 7 first, then the input, 6. My bets are that it is emitter resistor, or connections out to the speaker if you have no noise at all. Hope that helps.

When you work, be safe. Always know that the amp is unplugged from the mains when you have your hands inside. Always work with just your right hand if possible. Buy an RCD. And don't do it if you are not confident.