Marshall Valvestate 8080 has only 12V + and - on 15 volt rails and it has a loud hum

[rpape2601]

Working on a valvestate 8080 1 X 12 combo for a friend. It started out with a loud hum that wasn't affected by turning any of the knobs. I was reading some of the forums and it was suggested that maybe one of the zeners was blown. I checked the zeners and both seem ok, so I checked the 1000uF filter caps. The one for the negative voltage seemed a little dodgy so I changed it. I now have on both rails about 12.8 or 12.9 volts plus and minus but the hum is still there. Please note that when first turned on, the speaker is being pulled back and stays there. Any insight or help with this would be greatly appreciated.

[Enzo]

Speaker pulls back and stays there. That means the amp is putting DC on the output. Probable shorted output transistor.

[Randall]

What Enzo said. Disconnect that speaker and put a meter on the output. What do you see?

[nevetslab]

Working on a valvestate 8080 1 X 12 combo for a friend. It started out with a loud hum that wasn't affected by turning any of the knobs. I was reading some of the forums and it was suggested that maybe one of the zeners was blown. I checked the zeners and both seem ok, so I checked the 1000uF filter caps. The one for the negative voltage seemed a little dodgy so I changed it. I now have on both rails about 12.8 or 12.9 volts plus and minus but the hum is still there. Please note that when first turned on, the speaker is being pulled back and stays there. Any insight or help with this would be greatly appreciated.

After looking at the Valvestate 8080 schematics, as Enzo suggests, you have at least one shorted output xstr. I'm not sure what the nominal power amp stage supply voltages are (not indicated on the schematic), but they're being dragged down by the shorted devices in the power amp. Might not be an output..could be a driver or voltage gain stage, forcing the output voltage to go north or south, and is pulling enough current to drag the power supplies down (as well as causing your speaker to suck in), preventing your +/- 15V supply voltages. And, with that current load, it's causing hum from the excess supply current. Usually, when an output xstr fails short, it blows the mains fuse. Reading between your lines, that doesn't appear to be the case, which makes me think it's up-stream from the final output stage in you 8080 power amp.


8080 VlvState Preamp.pdf

[rpape2601]

Thanks for the input, I'll check this out tonight. I am using the light bulb load box so it probably won't blow a fuse. This is good information. Thank you.
Is there a quick way to check for a blown transistor.
Ok here is a newbie question, how do I know which transistors are considered to be the XSTR or are they all considered to be XSTR because they are used for trimming current/voltage?
The fuse hasn't blown because i only turn it on long enough to check some voltages, and, from reading other posts, I'm leaving the speaker disconnected until I can get some sort of resolve for this issue. Apparently he had given the amp to someone before and it came back like this. The work was really bad that was done. I cleaned it up and re-sweated the joints just to make sure we didn't have a cold solder joint or 2.

[Randall]

XSTR is just another shorthand for transistor.

"Is there a quick way to check for a blown transistor."

Let's not call it a blown transistor, let's call it what it is being suggested you have, a shorted transistor. We ideally test a transistor out of circuit with the diode setting on a meter, but a transisitor that is shorted will show as shorted in or out of circuit. A quick check with an ohmmeter between legs will expose a short. Also, if you have two of the same output transistors side by side you can compare them to each other. They should read the same.

[rpape2601]

Thank you, I'll do that tonight. I'm pretty sure it has to be downstream for the power section like Enzo suggested. Thank you for the help.

[nevetslab]

Is there a quick way to check for a blown transistor.

To elaborate on Randall's reply in testing for shorted or bad transistors (xstr's), most present-day multimeter (or DMM, if you prefer) have a Semiconductor or Diode Test mode. If not, they all have an ohmmeter mode. In Diode Test mode, it provides a 1mA constant current source thru the (+) and (-) leads, with a voltage source, usually the magnitude of the battery. In a DMM not having that diode test mode, the 2k range in OHMS does the same thing. 1mA current source, but with a 2V voltage source behind it. In any case, you're turning on the semiconductor junction and reading that junction voltage with 1mA DC flowing thru it.

Using the Red (+) and Black (-) leads, you can test diodes, zener diodes, NPN & PNP xstrs, and other semiconductor devices to be discussed later. Testing IN-Circuit (parts still soldered in), you begin by testing between the base and emitter, base and collector, then collector and emitter. Of course NPN's use the Red lead at the base, the Black lead on the emitter & collector, while PNP's use the Black lead at the base, and the Red lead at the emitter and collector. A small signal xstr, such as an MPS A06 NPN part TO-92 package, the base is the middle leg, the emitter is the left leg facing the flat, and the collector is the right leg facing the flat. If it's a European or Asian part, such as a 2SA970, also a TO-92 package, the base is the right leg (facing the flat), the emitter is the left leg and the collector is the middle leg.

A typical reading on a MPS A-06 is 0.658V (B-E, B-C). Reading between C-E or E-C would be open circuit, but only if the xstr is NOT soldered in. Readings on Power xstrs, on the other hand, such as MJ15022 NPN 150W/250V, which has much larger silicon in it's chip, the readings are lower. Typical reading is 0.517V B-E, 0.507V B-C, and open circuit C-E or E-C.

Sometimes a part will fail with open junctions, so there is NO reading between B-E or B-C. Hard shorts will usually result in shorts at B-E, B-C & C-E

Armed with the schematic, and sometimes the data sheet(s) on the parts used in circuit (for package lead locations, etc), you can move thru the PCB trying to get your probes onto the diodes, xstrs to see if there are shorts. Often when there are, you may find burnt resistors and even scorched circuit board material in the area, depending on how severe the failure was. Testing Diodes is much easier. The band at one end of the diode marks the cathode. Red lead to the Anode, Black lead to the cathode, and typical reading on, say a 1N4004 is 0.582V, while on a small signal diode, such as a 1N4148, it reads 0.612V. More silicon in the die the part will read lower voltage, but all still within the range of 0.5 to 0.7V on the meter.

While you can't measure the Zener voltage directly on zener diodes (with your DMM), where there is a positive current-limited potential applied to the cathode thru to the anode, producing a breakdown at the zener voltage, when you measure between Anode and Cathode like normal diodes, it will read like a diode in that direction.

In Circuit, you have multiple connections, and, for example, the readings on multiple output xstrs, where their collectors are common, their bases are common, and their emitters connect to an output bus thru low ohm resistors of, say 0.33 ohms, the readings found measuring B-E will usually be the value of the emitter resistor of the preceeding stage (that of the driver). A short between C-E on one of 4 output xstrs, you'll read that short on all of them, but, reading B-E, the one that's shorted will usually read slightly lower. A shorted part elsewhere in the circuit will have affect on the readings of adjacent parts, so it takes some getting used to, and interpreting the readings. We'll help you thru all that. When in doubt, make a list of each xstr and the B-E, B-C, C-E readings, and get back to us. Good luck.

[rpape2601]

Before I had a chance to check what NEVETSLAB asked, I was checking the volts across TR8 and my hand slipped and now it is showing a short with my limiter circuit. The transformer is ok, checked that by removing the output leads. Disconnected R102 & R101 and the short is still there and now I'm not getting anymore than 5 volts. Something is taking down my transformer.
Marshall-Valvestate80-80W-8080-8100-8412-Schematic.pdf
here is the schematic I'm using. Can't believe I did that.

[Enzo]

The transformer is fine, shorting something fora moment does not kill transformers. Transistors are diffrent, you can kill them in an instant.

[rpape2601]

I guess it was a good thing that I bought some spares then. I pulled the bridge rectifier to see if that was the issue but still not getting voltage from the transformer. The short went away when the rectifier was removed but still the voltage is low, around 5 volts. I'm thinking I blew the door electrics on the 2200uF caps.

[nevetslab]

Were you checking the semiconductors for shorts with the DMM in Diode Test or 2k OHMs mode with the power on? I see none of us made mention that checking for shorts is done with on power applied, and that the power supply caps need to be discharged. Important steps....our apologies.

OK, with the bridge rectifier removed, so you now have the heater supply & the autoformer still attached to the secondary that creates the power amp's +/- voltages and the preamp lower +/- voltages, you are still seeing low voltage there? Unplugging the preamp tube will remove the heater load, and should also remove the HT load. I'm assuming you've shorted TR8, and that's dragging the supply down. But with the bridge rectifier removed, there's no DC load on the transformer.

What is the AC Secondary voltage, open circuit (which is what you have with the bridge removed, tube removed...minor load on the HT filter section?

I also assume you've checked the AC mains fuse FS1. I'd guess if all this happened with the power on, that fuse opened following the inflicted short on TR8.

[J M Fahey]

checking for shorts is done with on power applied

I think you meant:

"with NO power applied".

[nevetslab]

I think you meant:

OPPPS! Exactly.....sorry about the manual dislexia on the keyboard

[rpape2601]

I had read in another thread somewhere that the person had removed TR and then powered up the amp, when I did that The resistor R82 fried, not sure if this is indicative of anything but it was another annoying incident. Going thru the circuit board, I noticed that C44 was changed to a resistor by someone other than me. Could this cause some of the issues?
Seriously thinking of changing all the xstrs. I'm a little worried about the ICs, they can be a little delicate when the voltages go nuts.