Valvestate poweramp repair

[ruepselrum]

I got a Marshall Valvestate VS65R, which lost its volume with some noise.
Still both channels are working fine and sound good, even the reverb section, but at very low volume, even with the master set to 10.
I connected send and return jacks, connected another marshall amp, even changed the preamp tube, but the problem remained the same. So I suggest the poweramp is not working and the rest is fine.
I measured 0V between hot speaker connector and ground (no speaker connected).
How can I find the broken components?
Has anybody any idea?

[Mars Amp Repair]

Marshall Valvestate issue

Could you please clarify;
1. Did the other amp that was running off the SEND of your Valvestate sound ok with proper level?
2. Did you also try pumping the SEND of the other amp into the RETURN of your amp & how was the level of the valvestate then?

If you merely jumped the SEND to the RETURN on the valvestate...how was the level of the Valvestate then. Glen

[Enzo]

ANd after answers for Glen - I wondered the same thing - check for poor contact on the CUTOUT contacts of and jacks that can cut the speaker out, such as remote speaker, heaphones.

Make sure none of the large cement resistors have broken free of their solder. And check solder under those afore mentioned jacks.

[ruepselrum]

Hello Glen and Enzo,

Iīll try to clarify what I did:

1. First I connected Send and Return of the Valvestate-nothing changed, level was still low
2. Then I connected Send of Valvestate to the Return of my Marshall 2210, which seemed to have the proper level. But this isnīt as easy to say, because the 2210 is equipped with an active tube driven insert.
3. After that, I connected the Send of my 2210 to Return of Valvestate, which was louder than the Valvestate of its own, but no 65W! I suppose, the active Send of the 2210 is operating at higher level.
4. Then I connected a cable to the Direct Out jack (nothing happened) and to Headphone Out, which muted the Speaker.

As far as I am now, I have soldered out the 3 cement resistors (R21,24,29), which should rate 0,33 Ohm each, as printed on them. My DMM read 0,8 Ohm each, but I donīt know if this "low resistant"-resistors can be measured in this easy way at all.

How could I check for poor contact of the CUTOUT contacts, with the amp not running?

Please excuse my poor English and thanks for help
Sascha

[Mars Amp Repair]

Sascha, Your english is excellent.
I still can't confirm if the problem is in the power section or before it.
Have you tried banging on the amp with your fist to see if it's a connection issue?
Also as Enzo mentioned, it could be as simple as the switch connector in the headphone jack. Repeatedly putting a plug in there & pulling it out could clear that up enough to tell if it's an issue or if it's connections on the jack.

I was just looking at the schematic on Schematic heaven for the VS65 & it would also be good to know if the LINE OUT level is normal.

You mentioned that you connected a cable to the DIRECT OUT jack & nothing happened. I'm assuming that DIRECT OUT is the same as LINE OUT. What did you connect it to? It is designed to connect to a line level input on another amp....you could connect it to the return on the 2110.

In this amp, the line out level is the actual signal that is goes to the power amp section. If that level is good, then the problem would be in the power amp section.

Also another test would be; with the reverb control turned up, move the amp around to get the reverb springs to bang around. Do the reverb springs make a really LOUD sproing noise...as loud as you would expect if the amp were working fine? If so then your problem is probably not in the power amp section, but in or around the op amp IC 70.

Once we confirm where the actual issue is, we can move on....hope that's all not too confusing...glen

[Enzo]

OK try this. At the input to the power amp is a JFET type J111. it is T16 in the circuit. Remove it. Now did the levels return? All that part is is the mute for power on thump. it prevents a loud pop at power on or off. But it is not needed for operation. Your amp might be stuck on mute.

Crashing the reverb looks like a good test since the reverb is after all the other mutes but the one I mentioned above.

[ruepselrum]

Hi Glen and Enzo,

I used my afternoon to reassemble the Valvestate to check your suggestions.
-Banging on the amp didnīt affect the sound or volume.
-I cleaned the headphone jack and found it operting fine.
-I connected the two Marshalls in every possible way and verified what I knew before (but sometimes I donīt trust myself), and youīre right Glen Direct Out is Line Out and connected to the Return of the 2210 it works fine at proper level (approximately the same as Send, a little less brilliant).
-Moving the amp with reverb turned up didnīt make any noise (how hard should I hit the chassis). I canīt find IC70 in the drawing, I guess you mean IC7c.
-And to make it short, a few minutes ago I unsoldered T16 and the VOLUME returned!! Thank You!

But what have I to do next? Can I just replace T16 or are there other damages possible (it didnīt sound so, but who knows)?

Sascha

[Enzo]

I am not looking at the schemo now. JFET T16 is simple shunt across the signal. Either the part has failed, or that circuit below that controls it has failed - perhaps a timing cap on it has gone leaky so it never charges.

I would suspect T16 first. JFETs are cheap anyway. I'd replace it and see. If that doesn't solve it, then look into that circuit that controls its gate. I seem to recall about 3 or 4 small transistors in a circuit all just to control that one JFET. There should be a cap that times out how long the FET is on at power up. That stuff is part of the low voltage power supplies.

Then again, does the amp make obnoxious noises now when you turn it on or off? Noises it never made before? If not, leave T16 out of the amp and forget it.

[ruepselrum]

As there is a loud pop at power on (not at power off), Iīd like to replace T16.
Somebody told me it is a NFET and the same as J112, anyway both types arenīt easy to get in Germany. Does anybody know which types I could also use? Since itīs a simple switching transistor, I think there should be several transistors which fit in this circuit.

Sascha

[Enzo]

JFETs come in two flavors - P channel and N channel. J11, 112, 113 and many other N channel JFETs would work fine. Very uncritical. FInd a common N channel FET in local types.

Watch the leg pinout. Unlike bipolar transistors, these are not all wired the same inside. The gate could be any of the three pins depending upon transistor type. ALl that means is the new part may have to face the other direction or have its legs bent funny

JFETs are ON until turned off by a gate voltage. So sitting on the table in front of you, two of the legs should have a low resistance between them, like 200 ohms or something. The leg with very high resistance to the other two is the gate. SOurce and drain are interchangable in this application.

And if it winds up that the control circuit T13, 14, 15 are at fault, again ANY small general purpose transistors will work - just watch the PNP/NPN and stuff.

[ruepselrum]

I replaced T16 with a 2N4093 (the only N-Channel JFET I got quickly), but the volume loss still remained. I checked components around T16, but everything seems to be in good condition, even T13, 14, 15.
How can I determine the faulty component anyway? Are there any measurements to do?

Sascha

[Enzo]

Did you make sure the pinout was right? With the FET on the bench all alone, there should be two legs with a relatively low resistance between them, and the remaining leg hi-Z to the others. That hi-Z one is the gate. MAke sure that leg goes into the proper hole on the circuit board. THEY ARE NOT ALL THE SAME.

But assuming you have it right, what voltage is on its gate? AFter a second the gate should receive voltage and turn utself off. Follow the current logic through those other small transistors, and there is a electrolytic cap that has to chare at the base of the first one - that is the timer. If that cap is bad, the circuit will never charge and turn off the JFET.

[ruepselrum]

Hi Enzo,

Iīm sure the pinout was right, I had to bend the GATE to get it in the right position. Meanwhile I also replaced T13,14,15 without fixing the problem. What about Zener Diode D8?

Here are some voltages:
Gate T16 0,7V (I guess there should be about 10V)
Plus of C15 39V (I think this is ok)
Base T15 -6,3V
Rectifier BR1 +-40V DC

Iīm going to replace C15 tomorrow, but I only got one with 10u rating. This shouldnīt be a problem, only alter charging time.

Sascha