Marshall Valvestate 265r repair

[leydenjar]

This amp has a moderate vol. level hum. I burned up one of the speakers upon initial testing it. Yes there is DC on that pair of speaker leads (red and black leads) (approx. 20 v DC).

I've tested TR 15, 16, 17, and 18 and they test fine on an Atlas DCA55 semiconductor component analyzer. I also tested TR 13,14,2, and 7 and they test fine as well.

TL072 shows + and - 12 v DC NOT the 15 volts I see on the schematic. Same thing at D6 and D7.

I've pulled some of the caps and have yet to find a bad one. My troubleshooting abilities on a solid state amp have just about been exhausted so I'm reaching out for help.

Thanks for any help in advance. Gut shots and schemo attached.

[DrGonz78]

I was working on a vs100 a few weeks back and noticed that the schematic says 15v/-15v. I was really confused by that and my amp was fixed by this point and was just measure voltage to test all the IC's to make sure. So on that amp as I recall are these regulators...

MC7812CT Fairchild Semiconductor Linear Regulators - Standard

I almost asked on a post as to why they have a 12v rating on the output. The rails to the IC components and the MC7812CT were the same - 12v. My thought at the end of it was that this voltage seemed to be correct. I then pondered why the voltage was at 12v and not identical to the schematic. The amp sounds good.

[J M Fahey]

@ leydenjar
The output transistors on the side that feeds 20VDC to the speaker are dead. Replace them.
Build a lamp bulb limiter (search for it) and use it.
Test the amp with no speakers connected until you have no more DC at the output.
Less than 100mV is acceptable.
The 0.33 ohm 5W emitter resistors might have blown open. Replace them too.
Do not pull parts at random to check them outside, unless you have a very good reason to suspect them.
You may do more damage to the PCB by so much soldering and desoldering, plus wiggling parts loose, than you originally had.
An "ENZO LAW": Shorted output transistors can be tested still onboard, since nothing external hides a short.
You may still have other bad parts, but let's start by something.
@ Dr Gonz: +/- 15V is a classic value, but +/- 12V is fine too. Kustoms use +/- 8V !!
Maybe they run out of 15V regulators, no big deal.

[Jazz P Bass]

I then pondered why the voltage was at 12v and not identical to the schematic.

Wow.
I do not think I have ever used that word in a written statement.
"Pondered."

[leydenjar]

@ J M Fahey
If the output transistors are dead why do they test good? I've tested them with the diode setting on a Fluke 83V and the aforementioned Atlas DC55.

I should clarify that I'm testing them out of circuit.

[Jazz P Bass]

@ J M Fahey
If the output transistors are dead why do they test good? I've tested them with the diode setting on a Fluke 83V and the aforementioned Atlas DC55.
I should clarify that I'm testing them out of circuit.

O/k, if they test good move on.
I have found that repairing SS output sections takes a slightly different approach, as compared to tube amps.
You have to keep centered on the fact that the output transistors (in a Class A/B output) should be very slightly ON at idle.
If any condition exceeds this "slightly on" condition, there is a yet unidentified problem.
So you go snooping.
The polarity of the Vdc on the outputs is always a good piece of information to have (which you have not supplied).
That polarity sign may lead to the problem area.
Grap a cup of Java and sit down with the output schematic.
With the unit off & power drained, start measuring things & compare that reading to the schematic.
Often times a screwy reading will show a section with a bad component.
If nothing can be found this way then you have to take some powered up readings.
Check the power rails (+ & -) for stability.
Measure the base voltages of the output transistors.
Compare readings to what should be what on the schematic.(since they are not marked, some thinking will be required)
If a base voltage is off (way high) at idle, work your way back to the fault.
From some of the circuits that I have seen, this circuit has got to be one of the easiest to work on, as far as complexity goes.(or lack of)
Dig in.

[soundguruman]

It's usually one of the outputs shorted, it will get hot when it's on.
The tester may not catch the problem.

[J M Fahey]

Well, a shorted transistor does not get hot.
If anything, it will make *another* transistor to overheat.
Shorts dissipate no power.

@ leydenjar.
Please measure them both ways in the diode test setting.
Post the values.
You will have 6 pairs; 2 BE , 2 BC , 2 CE for each transistor.

As I said, there may be other problems, but I want to *fully* discard the output transistors first.
Letīs go step by step.

Thanks.

[DrGonz78]

Wow.
I do not think I have ever used that word in a written statement.
"Pondered."

Like in an old dickens novel I ponder my own pondering. Like other writers it would be "To ponder or not to ponder."

[km6xz]

esting the transistors in-circuit by testing the device for operating level values is different than a junction test at low current like a meter. As others have mentioned, speakers only get connected to SS amps after it is proven to have no offset, and has run into dummy loads for an extended period. Replacing speakers in each repair can be expensive.

The random static tests of parts is a slow go and is not needed since all the test conditions are present with live power if the there is no high current being drawn. As Juan mentioned, when transistors get hot, they are usually the ones working but shorted transistors are usually cold since a shorted junction is the one with the least voltage drop across it.

You have two channels, one is your test reference because it works. Remove the load, measure the base-emitter drop of each transistor in the output and drivers and write them down, circle any that are different compared to the working side.

If the voltage across the speaker is 20 volts, it means the outputs are probably OK. I do not have the schematic in front of me but the power output supply rails would have to be higher than 20 volts for 65 watts, so the power transistors are probably doing what they are supposed to be doing, generating an output in proportion to their drive current.

Measure the rails, a shorted transistor would cause the speak output to be the same voltage as the rail of the polarity of that of the speaker offset. The circuit, if I remember is really simple, a diff amp and drive, level shifting and power devices. If 20volts is there but the rails are higher, the more likely areas to look at are the level shifting and diff amp. Look for opens in the level shifting section that establishes the biasing for the two halves of the power amp with a set offset.

[leydenjar]

@J M Fahey

TR 15, BE- OL, .802 BC- OL, .624 CE- .508, OL

TR 16, BE- .746, OL BC- .603, OL CE-OL, .466

TR 17, BE- OL, .811 BC OL, .628 CE- .509, OL

TR 18 BE- .748, OL BC .606, OL CE OL, .467

@km6xz

I think I'm going to need an explanation as to where in the schematic the "level shifting" and "diff amp" are.

I apologize for not knowing protocol/convention when posting yet. I'll catch on.

Thanks all.

[J M Fahey]

No protocol breach at all.
Simply language is not that accurate and we all want to be certain we are talking about the same.
As an example: over 10 posts and I still donīt know whether the bad power amp is the right or the left one
Post that and Iīll suggest some voltage readings you *should* find, per km6's instructions.
Which includes the amp being complete and turned on.
Through bulb lamp limiter and no speakers connected.
As he said, meters apply a small voltage to parts being measured (after all, they are battery powered, they usually donīt apply more than a couple Volts at best), while the real World amplifier supply will apply 40V , 80V, whatever it uses.
Good luck.
PS: and yes, the output transistors look good, so far.
Donīt forget proper micas and grease and check that collector tabs are not shorted to the heatsinks or ground.
EDIT: and thanks God this is a simple amp, meaning it's very straightforward.
There are amps with lots of add ons, and improvements which complicate things.
This one does not even have a short circuit protection, go figure.

[Enzo]

According to the schematic, the main rail filters are only 50v caps, so left to my own guesses, I'd be expecting 35-40v maybe.

I am assuming those power rails are OK if the other channel works, since there is a common supply to both channels. But i don't recall a report of the other side working or not. SO verify both main rails are about the same voltage and present at the powr transistors.

I agree with Stan, the outputs are probably not shorted. Let us not forget how these things work. Using the left channel as the example, TR17 and 18, since we don;t know which one is bad. The output transistors are putting the 20v on the speaker, yes, but I suspect they are being directed to do so. Whatever appears on their bases is pretty much what will appear on their emitters, which is the output. SO what DC voltage is on those base legs? I will wager about 20v. Of course other possibilities exist.

TR14 and a couple resistors maintain a voltage "space" between the bases of TR17 and TR18. Since they are darlingtons, I am going to guess maybe 2v between them. Normally at rest that would mean about +1v on the base of TR18 and -1v on the base of TR17. MORE OR LESS. No matter what the signal does, that TR14 keeps the base of TR18 about 2v more positive than the base of Tr17. And whatever the voltage is on those bases, is what will be on the output. Well really, the center voltage betwen what the bases are - an average if you like. Whatever. The output sits there balanced, poised to be told where to go. Just like what my wife expects.

Music is a steady stream of voltage changes going positive then negative. To get the output to send that to the speaker, we grab TR14 and pull it up and down - voltage wise. That is TR7's job. The harder TR7 conducts (the more it turns on), the more it pulls TR14 down towards the V- rail. SO the output follows. And going the other way, the more we turn TR14 OFF, the more the voltage at TR14 can rise towards the V+ rail - and output follows.

Well what if TR7 can't turn on very much? Then TR14 winds up at some positive voltage instead of around zero. MAybe +20v? Now TR7 doesn't act alone. The current path through it starts at V+, flows through R18, R28, D12, TR7 and finally R15 to the V- rail. If TR7 can;t turn on - bad transistor, then the TR14 parts will rise to unwanted positive volts. (And if TR7 shorted, then they'd be stuck at some negative amount) What if R15 down there goes open? That breaks the circuit so TR7 CAN'T pull TR14 back to zero.

Just to remind, zero volts is the middle between V+ and V-. It is what we want on the speaker except when it is moving to the music.

SO TR7 has to be on a little to bring the TR14 guys to the middle ov the voltage continuum. So to speak.

Like all transistors, TR7 is controlled by its base... by TR9, through D9.

We need to have TR7 turning on farther than it is, in my scenario, so check both it and R15. I bet R7 is OK, or we'd have -20v instead. An open D9 would leave TR7 unable to turn on much.

Of course TR9,10,11 and stuff have to be good, but at this point, I think we ought to check the bases of all those parts I mentioned. If the output bases are at +20, then they are just doing their jobs. If they are about 2v apart, TR14 is doing its job. What voltages are on the legs of TR7? And explore the voltage around TR9,10,11. Remember the other channel is sitting there - I assume working - so you can use it for comparative voltage readings. The two channels are identical, only the transistor anad resistor numbers have been changed to protect the innocent.

Oh, looking at the front of the power transistors - the flat face with the printing - the legs are left to right: base, collector, emitter. Any exposed metal on the mounting tab is also the collector.

[Enzo]

If my voltage space deal is not speaking to you, think about how your car steers. The two front wheels both have to move the same direction at the same time, but they also have to be a steady 5 feet apart or however wide your car is. Ther is a tie rod between them that makes sure they always stay the same relation to each other. Now to steer, you don;t push each wheel side to side, you push that tie rod side to side, and the wheels follow. That tie rod is TR14.

[leydenjar]

Ok. I'm going to try to answer all questions I've been asked to the best of my ability.

These are my readings. No speakers connected.

1. I have 32 v ac coming off both halves of the transformer.

2. The power rails, if I understand correctly, are at + and - 12 volts dc.

3. TR15 B -1.14 v dc C -41.82 E -.03 TR16 B +1.02 v dc C +41.68 E -.02 TR17 B -1.11 v dc, C -41.79, E -.02 TR18 B +1.06 C +41.73 E 0.0 v

4. TR13 B -1.13 C +1.02 E -.02 TR14 B -1.10 C 0.00 E 0.00 TR7 B -40.1 C -1.11 E -41.42 TR2 B -40.1 C -1.13 E -41.44

5. R7 = 671r R15 = 21.75r

6. TR9 E .68 C -40.0 B .05 TR10 E .o7 C -40.0 C .05

7. TR3 E .07 C -40.0 B .69 TR4 E .06 C -40 B .69 TR11 E 40.66 B 40.13 C .66 TR6 E 40.66 B 40.13 C .66

Hopefully that will narrow it down.