ANOTHER dead valvestate !

[Valvehead]

Just got yet another Marshall 8100 that has similar problems as my last one....which i never did fix.

So, this one , when plugged in through my limiter , shows a bad short. The amp also hums VERY loud at a low freq , around 100hz or so.

Its not the power trans.

i corrected any bad solder joints

I had lots of (new) leftover parts from the last amp so i replaced a bunch of things while the board is out.

Replaced :
TR8/ 9/ 10 on the output heat sink

RESISTORS - R 115 / 118 / 119 / 120

Everything else looks ok visually

I tested the 3 output chips and found this -

testing the legs left to right

tr8 = -8vdc / +25vdc / -8vdc

tr9 = -9vdc / -9vdc / -9vdc

tr10 = -9vdc / -9vdc / -8 vdc

shouldnt these have around 38-40v at 2 of their legs ??

ALSO--- with amp on -- limiter in the ac power - shows a nasty -11vdc at the speaker jack !! Should be around 25mv


scem- http://www.drtube.com/schematics/marshall/8080-pwr.gif

[Enzo]

Unless you measured them as bad, it is not a good idea to start a repair by wholesale replacing a bunch of parts. If you were replacing bad parts, tell us which ones were bad, and in what manner were they bad.

For 38-40 volts to appear on circuit parts, it is important to first know we even HAVE both 40v rails. (or 35v or 25v or whatever they are) Are both rails present to the power supply? ANd free of ripple?

-8v on the base of TR8 leads me to suspect that TR7 is not doing its job, or it is not being driven by TR6.

Humming VERY loud tells me you have a speaker connected. DO NOT connect a speaker until the amp is stable and NOT putting DC on the speaker output.

[Valvehead]

Unless you measured them as bad, it is not a good idea to start a repair by wholesale replacing a bunch of parts. If you were replacing bad parts, tell us which ones were bad, and in what manner were they bad.

For 38-40 volts to appear on circuit parts, it is important to first know we even HAVE both 40v rails. (or 35v or 25v or whatever they are) Are both rails present to the power supply? ANd free of ripple?

-8v on the base of TR8 leads me to suspect that TR7 is not doing its job, or it is not being driven by TR6.

Humming VERY loud tells me you have a speaker connected. DO NOT connect a speaker until the amp is stable and NOT putting DC on the speaker output.

I dont have a scope--cant measure ripple ?

for the power rails--that would be after zd1 and zd2 yes? (with the current limiter) , i have -2.2vdc at zd1 , and +2.7vdc at zd2

and at the bridge rectifier BR102, i have +12vdc and -12vdc out of it

[Jazz P Bass]

I dont have a scope--cant measure ripple ?

You can very successfully measure voltage ripple using an everyday volt meter.

Set it to read Volts AC.

[Valvehead]

ok , would i place the probes on each side of the bridge rectifier.....or have the neg probe to ground ? and we are talking about the outs of the bridge rect. (power rails) ?

sorry guys--in all seriousness, i do have memory problems. I think that Mr Enzo probably told me how to do this in the past

[Jazz P Bass]

When taking power supply voltage measurements it is best to use ground as one reference point.

[Valvehead]

ok... measuring ac..

at zd1 and zd2 the meter just jumps around .

at BR102 one side says 0.3mv ac , other side the meter jumps around. So the meter doing that...does that show ac ripple ??


i can swap out TR6 / TR7 now as i have many parts left over from the last valvestate fiasco...might as well since the pcb is out

[Jazz P Bass]

I don't know what you are on about with those measurements.

They make no sense.

They may indicate that you need a better volt meter.

[Valvehead]

You might be right. I measured with my $5 meter and got this=

r101/102 the ends coming from br102 , 0vac / 26vac

the ends going to zd1/zd2 0v / 5vac

i also notice that tr8 is getting hot , while tr10 is cool

im not good diagnosing ss circuits..thats why im here If anyone could tell me where to start taking voltage readings , id love to dive into this amp... keep in mind, the amp has to be powered on with my current limiter to take measurements---as it shows ive got a short somewhere---i use a light bulb old school limiter , and its glowing bright when powered up !

[g1]

TR8 is turning on hard, that is probably what would be your short (and blow) if you didn't have the limiter lamp. If it has not blown, something is making it conduct heavily. You need to verify all transistors in power amp (9) are good before you turn it on and do voltage checks.

[Valvehead]

ok, tr8 is brand new. i hope the brief heat spells didnt hurt it. i never have it on more than a minute. ill start testing transistors

[Valvehead]

the site seemed to be down for a couple days.. so i tried a bunch of things.

i had a lot of leftover / new parts so this didnt cost me anything but time.

replaced =

TR4 / 5 / 6 / 7 / 11 / 12

still has a big short.

I clipped out some parts to see where the short goes away -

clip out R 83 , no short
clip out r78 - short (r83 back in)

clipped out D5 where it meets r99 = short

clipped out r99 where it goes to TR8 = no short

i had another new bdv65 so i replaced TR8 again just to try it = short.

could IC8 be bad ? would it cause a huge short if so ?

Im lost guys......any suggestions where to look now ?

[Jazz P Bass]

Have you tried it without the lamp limiter.

These ic driven output stages do not like a limiter.

[Enzo]

One note first: when your bulb limiter lights up brightly, it is not telling you there is a shorted part. it is telling you the circuit wants to draw a large current. Certainly many times it IS a shorted part, but many times it is not. For example, if your bias transistor goers OPEN, not shorted, it usual;ly allows both sides of your output to turn on at once and that draws huge current. But nothing is shorted. And in a case like that, you could remove all manner of stuff and it would not clear the bright bulb condition.

So, a shorted transistor can blow fuses, but an open resistor can do it as well.

Did you ever determine that your ZD1, ZD2 were making 15v? They need to do that.


I mentioned before NO SPEAKER. Are we now trying to run the amp that way? If an amp has DC on its output, you can run it that way as long as no speaker is conneted. Connecting a speaker to an amp that makes DC will light your bulb hard. NO SPEAKER until we know there is no DC on the speaker wires.

Clipping parts out is not effective troubleshooting. The whole circuit needs to be there.

[Valvehead]

yes. fuse blows instantly