Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: Psabin8951 on April 11, 2018, 09:36:42 AM

Title: Marshall 8080 combo blowing fuse and more
Post by: Psabin8951 on April 11, 2018, 09:36:42 AM
I have a poorly Marshall 808 combo come my way.
It was blowing the internal fuse when received and by isolating the circuit I know the +15v and -15v areas of the circuit are ok as is the tube heater portion which leaves the power amp stage. With R84, 85 and TR8 removed from the circuit it powers up fine.
With R84 reintroduced again it powers up fine.
With R85 added too, that resistor severly overheats but doesnt blow fuse.
If TR9 is out of circuit R85 doesnt overheat.
With R85 back out and TR8 refitted the fuse blows.
Can anyone point me to what is likely to be the cause?
I have attached a pic of the circuit in question
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: g1 on April 11, 2018, 01:34:05 PM
I've attached a better copy of schematic.
Did you check the transistors?  TR8 & TR10 good?
TR6,7, & 9 good?
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: J M Fahey on April 12, 2018, 05:37:23 AM
Agree.
Resistors overheat because *something else* is applying too much voltage to them or passing too much current through them.
First suspects are shorted transistors so check those suggested by G1.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: Psabin8951 on April 17, 2018, 11:51:54 AM
Ok after a delay due to tonsilitis im back on this. TR4 - TR12 now checked. All ok except TR8 which has now been replaced.
Fuse instantly blowing now.
Any suggestions where to go from here?
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: g1 on April 17, 2018, 05:52:06 PM
There should be no load (speaker) connected to the amplifier, is this the case?
Do you have a light bulb limiter?  (see here: https://www.ssguitar.com/index.php?topic=2093.0).
Something is turning on TR8 too hard, and appears to be related to the TR6 area, as disconnecting R84 was making a difference.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: Psabin8951 on April 18, 2018, 08:09:49 AM
Correct, there is no load currently connected.
I haven't got a bulb limiter but can construct one fairly quickly.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: Psabin8951 on April 26, 2018, 03:19:49 AM
So what now guys?
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: g1 on April 27, 2018, 08:25:33 PM
Fire it up on the bulb limiter with no load (using 40 to 60W bulb).
Take DC readings on TR4 to TR12, E,B,  & C.  Post results.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: Psabin8951 on May 01, 2018, 11:09:49 AM
Ok results are:-

TR4 B 0.2v, C 0.75v, E 0v
TR5 B 2.9v, C 0.2v, E 3.2v
TR6 B 3.9v, C 3.4v, E 6.79v
TR7 B 6.8v, C 1.02v, E 7.13v
TR8 B 0.89v, C 7.09v, E 1.03v
TR9 B 0.83v, C 1.03v, E 0.76v
TR10 B 0.17v, C 0.02v, E 0.76v
TR11 B 0.35v, C 0.52v, E0.03v
TR12 B 0.7v, C 0.53v, E 0.86v
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: g1 on May 01, 2018, 09:11:11 PM
You have the output transistor b & e's mixed up?  How are you determining e,b,&c for all the transistors?  The bc's & bdv's, and mpsa's have different pinouts.
Correct data is essential, remote troubleshooting is tough enough as it is.  ;)

The best method is showing the voltages on a schematic copy.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: Psabin8951 on May 02, 2018, 05:52:47 AM
Sorry G1. I tried to make my life easier by getting the wife to write voltages down as in probed components.
Obviously that didnt pan out! Anyway i've now redone them. Pinouts were taken from datasheets, my "helper" put some voltages in the wrong columns doh.

Right take 2:-

TR4 B 0.019v, C 0.75v, E 0.07v
TR5 B 2.93v, C 0.02v, E 3.25v
TR6 B 3.97v, C 6.86v, E 3.42v
TR7 B 6.86v, C 10.2v, E 7.19v
TR8 B 1.03v, C 7.18v, E 0.9v
TR9 B 0.89v, C 1.03v, E 0.76v
TR10 B 0.76v, C 0.02v, E 0.17v
TR11 B 0.35v, C 0.52v, E 0.03v
TR12 B 0.71v, C 0.85v, E 0.53v

Title: Re: Marshall 8080 combo blowing fuse and more
Post by: g1 on May 07, 2018, 08:20:32 PM
Ok, some of those are still wrong.  Example, TR11 emitter is connected to TR12 emitter.  They should both be the same voltage.
Now I hope I don't sound harsh, but if there are errors there, then there may also be errors when you are testing the transistors with your diode checks.
I'd suggest you go through the diode checks again with amp unpowered.  Triple check, and remove any suspicious and check out of circuit.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: J M Fahey on June 03, 2018, 04:47:02 PM
Those output transistors, specifically Tr10 are VERY dead, VERY shorted, not sure about your measurement technique.
TR10 must show like a dead short CE ... yet you declare it "good".

Please measure it out of circuit (pull it out), both CE and EC , on Diode scale, NOT resistance scale, buzzer or any other, just the one which has a small diode printed on it, and report what the screen shows, just the raw number, not interpretation, no "it reads fine", not "it shows no resistance" or whatever, just the digits on the screen. Thanks.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: Psabin8951 on June 09, 2018, 12:08:32 PM
J M Fahey, only reason they were stated as being "ok" was they had been replaced with new components.
Anyway two discoveries now - yes TR10 is dead CE & EC both read 0.02 so that needs replacing. I have also noticed that for the emitters on TR11 & TR12 then one must be fitted the wrong way around. Turns out TR11 is wrong but is fitted as per silk screen which seems odd. Checking all other connections for BC212s apart from that one they fitted correctly which seems most strange. I will make those corrections and report back
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: solderer25 on June 09, 2018, 02:37:12 PM
Hi. Proceed with caution if you suspect a wrong pinout with a transistor as many of them come in different packages. The BC212 for instance, has a different pinout to the BC212L. My data book shows this transistor comes in three different packages, depending on the last letter (or letters) of the type code.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: Psabin8951 on June 17, 2018, 06:45:12 AM
Ok pinout issues resolved and TR10 now replaced.
TR readings now are:-

TR4 B 0.01v C 0.01v E 0.07v
TR5 B 3.33v C 0.01v E 3.73v
TR6 B 4.35v C 7.76v E 3.8v
TR7 B 8.21v C 0.58v E 3.47v
TR8 B 0.53v C 8.41v E 2.14v
TR9 B 0.2v   C 0.53v E 0.08v
TR10 B 0.08v C 0.02v E 1.42v
TR11 B 1.59v C 0.03 E 1.76v
TR12 B 1.54v C 1.55v E 1.76v

Title: Re: Marshall 8080 combo blowing fuse and more
Post by: Jazz P Bass on June 17, 2018, 10:55:35 AM
It appears as if the B- rail is missing.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: J M Fahey on June 17, 2018, 04:05:04 PM
Please check that Tr10 collector is insulated from heatsink and chassis.
Did you use mica+grease or a Silpad there?
Collector shorted to ground would kill -V .

Or: check for continuity between Tr10 collector and -V terminal at main filter caps (C61) and/or - end of rectifier bridge (Br102/DB1).

Or maybe negative-pointing part of bridge (Br102/DB1) is open.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: Psabin8951 on June 18, 2018, 11:40:21 AM
Thanks for the fast reply.
I used a mica sheet plus grease.
Collector shows no connection to heatsink or chassis on meter.
There is continuity back to -V on C61 and DB1
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: J M Fahey on June 19, 2018, 11:59:32 AM
OK, let´s separate power supply and amplifier and measure.

With a sharp boxcutter cut tracks leading from +V and -V to amplifier at points 01 and 02.
Cut a narrow 1mm strip or so and remove copper , later you can scratch soldering mask on both sides and rejoin with a drop of solder or a little bit of wire.

1) Measure +V and -V to ground at points 04 and 06 .
If normal (35/40V ) , also measure +/- 15V rails at ungrounded ends of ZD1 and ZD2 .

2) with amp OFF measure resistance to ground from points 03 and 05.
Both ways and using resistance (say 200 ohm or 2k) and diode scales.
Open circuit is no problem, I´m trying to find some continuity to ground which shorts raill voltage.

I also suspected some open track or connection, but you confirmed
Quote
There is continuity back to -V on C61 and DB1
so that seems to be working.
Title: Re: Marshall 8080 combo blowing fuse and more
Post by: MasterVolume on June 23, 2018, 12:14:03 PM
I had an 8040 that had similar issues.
Picked it up for $10 a market. First thing I realised researched / worked out was that the output transistors were probably blown as I could still get a signal through the preamp.

Once I’d replaced those, if I recall correctly it blew fuses . Measured the voltages and worked out that something was pulling down the voltage on the preamp. Replaced the preamp transistors and still happened. Inspected. Carefully resistors around the power rail for the preamp and replaced any that looked the slightest discoloured, and may have replaced capacitors too. Don’t recall.

Anyway that fixed it. Other things that were wrong were a broken channel switch that I then replaced. Found after awhile of playing it would start to cut out. Hoped it wasn’t the problem that killer the amp before. Tested it with cable in the preamp out / power amp in and eventually carefully managed to squash the jack contacts so that they had a strong contact when there was no cable in it.

Tried other speakers but found that the original speaker suited the amp best ( what do you know). Fixed the rattles in the cab with the amp chassis with foam strips that you put on the bottom of doors, no more rattles or buzz. Added a jack for speaker out and did s couple of mods to reduce the gain from the gain control and Merlin’s mods for cathode followers from his preamp book.

In the end worked out pretty well. Was going to keep it but too many amps, kind of surprised myself at how well it cane out and that I was able to work out the faults and fix each one.

Sold it for probably half the going second hand price of one in good nick. To someone who is now very pleased with it.