1967 Valco SS 62-9049A

[terminalgs]

I'm restoring a 1967 Montgomery Ward Airline 62-9049A solid-state amp.  Valco built, point to point.  Looks just like one of their tube amp builds, just an extra 7-8 pounds of wire. 










above pic doesn't show it, but C105 was also replaced (C105 as orange and white wires, next to the noval socket used to house the tremolo bulb and LDR).


http://i.imgur.com/gWmpNRw.jpg


The trouble I'm having is.. it doesn't work :-)   In the above schematic,   I've replaced the yellow highlighted items.


Instead of +65V from the power supply, I've got +70V (maybe a 117v vs 120v supply product).  On the bottom side of the C107 cap at top left, I've got ~65V instead of 36.8V, so there is 65V on the base and emitter of Q14,  the emitter of Q8.    collector/Q15.   Also, around 63V on base/Q8 and collector/Q9.


i checked all transistors like this:  using diode setting on DMM, checked base to emitter and base to collector bias (both pos+ to neg- and neg- to pos+ to make sure it looked correct in terms of PNP or NPN. checked to make sure no emitter-collectors were shorted;   I only found two Q's to be a problem, and both are in the preamp (Q2 and Q13), all the Q's in the power section checked okay with this check.
the trim R110 is set at about 100ohm


I suspected the PNP Q8 at first, but it seems to test ok.  I already blindly replaced the two power transistors (first thing I did), but that didn't change anything...  I'd like avoid replacing all the  Q's one by one..


Any suggestions or hints to get the voltages correct?   


full schematic


http://el34world.com/charts/Schematics/files/airline-wards/Airline-Wards_gvc-9049a.pdf

[shooter]

I've got ~65V instead of 36.8V,

a quick glance, I'd suspect and open path to ground, possibly Q9, does Q9 have 2.5v on emitter?

can you break the pre/power signal path and plug un the pre to a listening amp, or even jack into a tube amp, that half work?

[PRR]

> C107 cap at top left, I've got ~65V instead of 36.8V

And why would it ever want to be 36V?

Because the resistors and input transistors are set to balance there. IF all works.

Come back when you have measured all the voltages around Q10 Q9 and marked them on the image. That shudda been first-thing, even before replacing outputs.

[terminalgs]

thank you!





http://i.imgur.com/ly9miiX.jpg


FWIW: I am running a signal into the instrument jack,  and I get a faint audible output.  When I probe the voltage on the base of Q10, I get a pretty loud noise.

[shooter]

Q9 still looks like it's not conducting, or conducting well, you do get Vdrops at the diodes so I suspect they're workingish.  Q10 looks to me to be wired as an emitter follower, which makes sense, then Q9 does the "driving" for the power amp section.  with 62volts on 9's collector she's still "railed", no room for a signal to swing.  that can still be just symptoms and not the problem, but you about have to change it to prove it.

[PRR]

What did you cut off on the left? Missing capacitor.

Why would it want to bias to 36V? R112 and R111 should drop the other 36V, so flow around 5mA. R105 then drops 2.5V. Vbe of Q9 and Q10 demand another 1.1V-1.2V at Q10 Base, 3.6V. Where does this come from?

There's DC NFB through R101 R102, also R118. There's AC NFB through R108 R104 R103 and this also passes some DC. This is a mess. But to a first approximation, ignore R103 and beyond (this is >>R118), then (R101+R102+R118)/R118 is 227K/27K or 8.4. So the top of R101 should be 8.4 times the Q10 B voltage, 3.5V. 8.4*3.5V is indeed 29V, and base current probably accounts for another few volts.

But you are getting 62V to 1.8V, a 34:1 ratio, not 8.4:1. How can that be? Could a resistor have drifted? Maybe. Could a capacitor be leaking? You bet your favorite shirt! C101 is an obvious suspect. The missing cap off-left of this diagram is another.

[terminalgs]

PRR &  Shooter, thanks for look at this.


Here's the full schematic: http://el34world.com/charts/Schematics/files/airline-wards/Airline-Wards_gvc-9049a.pdf


I do have some 2N2102's and 40233's in the reverb and trem circuits.  I can swap them with Q9 and Q10 to see they are culprits.


It's a 50 year old amp, so someone might have snipped a wire or something. I'll trace the circuit more thoroughly. 

[PRR]

I do not suspect bad transistors (yet).

C13. C14. C101.

[terminalgs]

C101.


I replaced it, and viola the 62v side of C107  dropped to 39v, and thus all transistors in the power section straightened up. amplifies as it should.  !!! YAY !!! I'm very happy.    I still have an input channel to work on, and the trem and reverb., but this is was big big milestone.


Thank you guys for paying attention to a SS thread on a tube amp forum!!!