VOX Buckingham V1121

[DckTech]

Hi,
I'm back with another problematic amp, at least to me.

It's a VOX Buckingham V1121, the original problem is:
Normal + Reverb = overall sound level drops, when reverb blend increases.
Brilliant * Reverb = overall sound level drops, when reverb blend increases.
OFF = normal operation, good sound.


I have:

     Replaced the original "1122" reverb tank with a "P-MOD-4AB3C1C" = best spec I could find to equal original.  The original tank had an open reading on one end and needed replacing.
     Replaced all of the electrolytic capacitors. (based on internet research)
     
Now:

Normal + Reverb = loud buzz/noise (can't even leave it on)
Brilliant + Reverb = overall sound level drops, when reverb blend increases. (same)
OFF = normal operation, good sound.

I have attached the schematic, parts list & documentation.

I am not sure where to start, the layout of this circuit is very tight, older and not labeled.
But, I believe someone here can help, based on my last problematic amp.

Any ideas would be greatly appreciated.

Thanks,
Dan

[phatt]

""Replaced all of the electrolytic capacitors. (based on internet research)""
opps?? might have been better to find the problem first.  8|

Obviously reverb is causing the issue so check all the connectors for continuity and make sure the pickup end is grounded to case.
Also what happens when Blend knob is rolled back to zero?,, still buzzing?

The schematic is a little hard to read but I'm sure some clever minds will know there way around the circuit and will reply with other thoughts.
Phil.

[JHow]

I have a Berkeley and a pathfinder, and one that that often happens when you touch anything in them is that wires break.  Since you replaced caps, double check you aren't missing a ground wire connection somewhere.  Also, even though you don't have a layout can you measure E B C voltages for each device ? Reverb recovery  is likely to have a heat sink.  Other reverb transistors will be nearby on the board, also you can look near the reverb lines in and out to identify which devices are related to reverb.

[DckTech]

Hi,
I check out a few things, thanks for the ideas.

phatt:
I checked the rca reverb connectors = good
According to the schematic reverb out is grounded = confirmed on amp
Buzzing sound was a pulled off wire = fixed now = no more buzzing
Just back to original problem now, glad that buzz is gone.

JHow:

I did find a broken wire - thanks
Could not see any missing grounds.
Did manage to measure some voltages in the reverb section:
Q201 - E=.82v, B=.80v, C=2.3v
Q202 - E=1.73v, B=2.3V, C=27.6v
Q203 - E=1.86v, B=2.85v, C3.5v (schematic shows cap on C to gnd actual cap was C to B)

I hope this info offers some insight to someone, the help is appreciated.
Thanks,
Dan

[Roly]

""Replaced all of the electrolytic capacitors. (based on internet research)""
opps?? might have been better to find the problem first.  8|

Yeah, there is what almost amounts to a "craze" for replacing electros on spec, without any diagnostics or even sound reason to do so, apart from them being a bit old.  It's this audiophile infection of picking up a stray fact and becoming a self appointed expert.  The problem with this "blunderbuss" or "shotgun" approach is that it often introduces new faults that greatly complicate the situation, may hide the original fault, and worst of all, don't actually fix anything.

"My amp is/is not doing X"
"Replace all the electros/transistors/valves/transformers/resistors/knobs..."

And after all that is done...

Just back to original problem now

(and the "expert" adviser is nowhere to be found)

So having well muddied the waters, we come full circle to actually doing some technical diagnosis.


A couple of common causes of weak/noisy reverb problems are;

- the reverb line has been re-installed backwards by somebody at some time (in->out, out->in), had this in a Gibson one time that took me quite a while to spot 'coz it all looked original (and took quite close inspection to see that it wasn't).

- springline is grounded to frame at the wrong end.  Normally springline frames are grounded at the output end only; grounding both ends, or the wrong end (i.e. line backwards) can give buzzing due to earth loops or incorrect grounding.

If you short the springline output, P202, with a cliplead or similar, does that shut up the buzzing?

If you disconnect the springline drive, P201, does that shut it up?

Please post the type numbers for Q201,2,3.

Q201 - E=.82v, B=.80v, C=2.3v

This is of course unlikely, the Base being at a lower voltage than the Emitter, and most likely the result of the meter loading the Base connection; however the voltage on the Collector is a fair bit lower than the circuit value or what we would logically expect, around 5 volts, so it is possible this transistor is duff (since the voltages around Q202 look reasonable).

[JHow]

Re transistor types the OP probably won't have any more info available than "TO 98 with yellow and green paint dot", "TO 39 with heat sink".  RG Keens geofex site has info on likely transistors used.  I want to say 2N2925 for the small signal one.  You could probably get any reasonable small signal device to work if you mind the pinout (change pin order to fit board by careful bending).  I will look at home and see what I have in the Berkeley.  It is similar and I had to replace some transistors in it, including reverb driver.

[DckTech]

Hi,

Roly:
OK, I deserve that, no more "shotgunning". I got it.

There is no more buzzing - that part was a broken wire.

the original problem is:
Normal + Reverb = overall sound level drops, when reverb blend increases.
Brilliant * Reverb = overall sound level drops, when reverb blend increases.
OFF = normal operation, good sound.

Q201 & Q203 = (green/yellow dot) = 2N2925 - correct JHow.
Q202 = 2SC4793 or 2SC5171 (recommended replacements)
        = it is a TO39 pkg  (tin can with heat sink) - correct JHow.

Any ideas on this:
Q203 - (schematic shows cap on C to gnd actual cap was C to B)?

I have confirmed the reverb rca cables, they are correctly soldered to the circuit board.
Reverb "IN" comes from Q202 & transformer side & not grounded.
Reverb "OUT" goes to Q203 and is grounded.


Looking for suggestions, thanks for all your input.

Dan

[g1]

Replaced the original "1122" reverb tank with a "P-MOD-4AB3C1C" = best spec I could find to equal original. 

  May I ask where you got your information regarding this? 
"AB" tanks are for tube amps.  Transistor reverb circuits usually use EB or FB for 2nd and 3rd digit of tank number (which indicates input and output impedance of tank).
  I'm not sure if this would cause the volume drop issue, but I certainly wouldn't expect an AB type tank to work properly in your amp.

[Roly]

Any ideas on this:
Q203 - (schematic shows cap on C to gnd actual cap was C to B)?

Well it amounts to something very similar.  Base to ground is of course a simple shunt across the signal path.  From Collector to Base is a negative feedback path and will have a similar top-cut effect, except that because of the active negative feedback the value of the cap required will generally be smaller (or if it's the same value, maybe they decided they needed a bit more top-cut and this was how to get it in production without having to buy in ten-thousand caps of a different value.  When applied like this it is sometimes called a "Miller Capacitance", although this is more strictly a stray capacitance between input and output nodes.

That's a good spot @g1, a wrong impedance tank could possibly explain a weak reverb.

As a matter of curiosity, if you "crash" the springs can you get an almighty loud noise, or is it weak?

Accutronics 4AB3C1C Spring Reverb Tank. Long (16 3/4") 2 Spring Unit, Long Decay. Input Impedance 8 Ohms, Output Impedance 2250

Given those impedances I'm hoping that it is mechanically sensitive, just not being driven hard enough.

[J M Fahey]

Just rechecked the schematic:
that reverb is transformer driven from a small Class A amp, similar to what Fender did, so it *might* be a low impedance (8 ohms) reverb after all.
Of course the driving amp may be bad.

I'd apply 100mV 440Hz or 1 KHz at the input http://www.amptone.com/amptonemp3s.htm , scope the signal along the way, and try all positions of the reverb selector switch.

You may have a bad/dirty switch, bad wiring (wires cracked or corroded green) or the Reverb module itself may be bad, because bypassing it works without problems.

[DckTech]

Hi,

I have tried to back track and find the info on the original reverb tank.
It was stamped with a "1122" and other faint info on the underside "AO-23580-14"
It was bad - I could not even get an ohm reading.
I emailed Tubesandmore, did an exhaustive search on the net, after searching even more now it may look like the output impedance is too high.  I can't really find a straight answer anywhere about a proper replacement tank.

The "crash" test does give a loud noise in normal and brilliant.

Also noticed that a "HUM" gets louder as the music level goes down as the blend is increased - in a very linear fashion.

I have cleaned the reverb channel switch, and checked for bad wires.

I do not have a scope.

Thanks for all your input,
Dan

[Roly]

The "crash" test does give a loud noise in normal and brilliant.

Okay, well that strongly suggests that everything downstream from the tank is okay (and it's a bit hard to have too high an output impedance on the output).

I'm now beginning to wonder if the driver transformer has developed a shorted turn, possibly as a result of being driven when the tank driver coil went open circuit.

If you connect a loudspeaker to its low impedance side, the tank driver side, and inject a signal you should get a small but clean output.

Here's some information on testing the driver transformer;
http://www.ozvalveamps.org/repairs/trannytest.htm

[JHow]

 Roly:  How do you feel about Q201 emitter having .82 volts?  If that is true and if emitter resistor is small (I can't read schematic but in super Beatle it's only 100 ohms). Isn't that more current through Q201 than we would expect?  Shouldn't it only be a mA or so?

[DckTech]

Hi,

Roly:

I connected an external speaker directly to the "reverb in" rca using aligator clips:

Reverb channel = normal = no audio, increased blend = louder hum + output audio down
Reverb channel = brilliant = no audio, increased blend = louder hum + output audio down

Transformer markings:
80-5056-1 (VOX Part#) = Reverb Transformer
--606-646-

Bad Reverb Transformer?

I wish there was a better schematic, I agree some values are hard to read.

Thanks,
Dan

[Roly]

Roly:  How do you feel about Q201 emitter having .82 volts?

Yeah, not happy, I commented on Q201 in post #4 above.  At face value these voltages can't be right because the emitter is more positive than the base, so this transistor should be cut off, however the low collector voltage suggests that it is conducting, so the base reading may be due to meter loading, however this transistor should get some attention, come out and be tested for gain and leakage, or possibly just by substitution - just about anything should work in that position.

It seems pretty clear at this point that the problem is lack of drive into the springline, so the finger is pointed at the driver stage, the two transistors and the transformer, but the hum still has me a bit confused.