Peavey mace no output

[Simonwill]

Hi
Since my daughter played her iPod through my amp for her birthday party there is no output. Not sure if that is relevant though...

I have checked all power rails - OK
I have injected a signal and traced it to the power amp where it disappears after the tubes. All tubes glow happily.
There are a couple of diodes (7924 zeners) on the outputs that measure about 8 Megohms in the fwd direction when removed (reversed = open circuit).
Doing a diode check with my DMM gives a fwd reading of 2V.

I will replace these as soon as I get a couple nextt week but in the meantime - Anybody experienced this sort of fault before?

Thanks.

[Enzo]

Um, if you mean the diodes from the power tube plates to ground, they are not zeners. If the amp is not blowing fuses, they are probably fine.


Isolate the problem - it is either the amp or the speakers. Yes the speakers AND their cable. If they are OK, it is the amp. Get out the voltmeter and start checking. The fact the tube heaters light up tells us exactly one thing and one thing only: the heaters work. The tubes need operating voltages. There should be something like 450-500v on the tube plates - pin 3 on each socket or on the red. blue. and brown wires from the output transformer. But also there must be that B+ voltage on the screens of each tube - pin 4. If neither, then the power supply has a blown fuse or a problem or both. If there is B+ on the plates but not the screens, then that large 400 ohm (390 ohm) resistor in the power supply is likely open. Or the standby switch may have failed.

There should also be about -55 volts bias on the grids - pin 5 - but if it were missing you's still hear something.

[DDK65JAG]

Just for grins & giggles, connect a jumper cable from your effects send jack to the return
jack. These Peaveys are prone to problems with the jacks. Cured my Artist VT. DDK

[Enzo]

Yes, that is a better first step than to tear into it.

It is not just PVs with this problem, ALL amps with loop jacks are prone to it. PV uses the same jacks as everyone else.

[Simonwill]

Thanks for the suggestions, will have a look asap and report back in a day or two.

[Simonwill]

OK.
The speakers the cable and the interconnecting wires are fine so it's the amp.

I've got 505V on the tube plates and on the outputs from the transformer.
On pin 4 of each tube I have measured 500V too. The resistor also measures 400ohms as expected. The standby switch is OK too.
Pin 5 wrt PA ground is -60V.
Pin 5 wrt Preamp ground is -76V.

DDK65JAG said "Just for grins & giggles, connect a jumper cable from your effects send jack to the return jack"
I'm not sure what you mean by this. The Mace has two Effects inputs and Normal and Automix inputs. On the rear are Preamp Out, Power Amp In and Line Out.

Any ideas?

[52 Bill]

Originally Posted by Simonwill

DDK65JAG said "Just for grins & giggles, connect a jumper cable from your effects send jack to the return jack"
I'm not sure what you mean by this. The Mace has two Effects inputs and Normal and Automix inputs. On the rear are Preamp Out, Power Amp In and Line Out.

Any ideas?

First off, all of the voltages that you have documented seem to be at appropriate levels for the power tubes to be working ok. One test would be, as you are taking these voltage measurements, listen for pops and crackles through the speaker as you probe the different test points. If you hear these noises, then you can assume that the speaker, speaker wiring etc. is ok and passing signal.

As for your question, Peavey and Marshall and Fender, etc. all have either FX loops or pre-amp-power amp jacks, that will over time become oxidized and will often cause signal drop out. This is a very common problem and has been discussed here many times.

What both DDK65JAG and Enzo suggested, was to take a guitar cable and plug it into the pre-amp out and the power amp in jacks, so that the guitar cable will make the connection between the two jacks, thus bypassing the internal switching contact. This way you can see if the switching contact is in fact making contact. If your amp has an FX loop (in and out) try the guitar cable in these jacks as well.

[Simonwill]

Much appreciated, will keep you posted.

[Simonwill]

I tried connecting the output of the preamp to the input of the PA with a cable which made no difference. There is no effects loop as such, just the Effects A and B inputs. The speakers and cables are fine as I played a walkman through them via the jack plug.

I injected a 1kHz 4Vpp signal into the preamp input and traced it as far as the tubes input (R27/30/32/28/34/36/pin 5) where it was 175Vpp. There was nothing on the other side of the tubes (CR16/17 cathodes) and (R29/31/33/35/37/38/pin4) except 520VDC.

Where is the ac component going?

[Enzo]

Pre out/Power in is your loop.

OK, you have a Mace "VT Series" and not a plain old Mace, is that correct?

Your plate screen and grid voltages appear OK. How about all the pins 8? Are they at ground? How about the heaters? Are all the tubes heaters running?

You can't measure it with a meter, but any chance the output is shorted? The AUX speaker jack is a switching jack - lots of extra contacts there to get bent, or worse if someone replaced it with the wrong type or miswired it. You might disconnect the green and white secondary wires from the OT and clip them directly to a speaker load.

[Simonwill]

Yes, it is a Mace VT Series.
Yes, all pins 8 are at ground.
Yes, all the tube heaters are working.
The Aux jack is as it left the factory, nevertheless I disconnected the Green and white wires and connected them directly to the speakers in turn with no joy.

[Garydean]

Have you tried working backwards with your scope and signal generator, tracing the signal path in reverse may isolate or eliminate certain areas.

[Simonwill]

Thanks for the suggestion, though I'm not sure what this might achieve. At the moment I know the signal is OK up to the tubes, then it disappears (see earlier post). Am I missing your point?

[Enzo]

I believe RG has a transformer tester on his Geofex site, you might check that out.

If there is large signal on the grids and the tubes have all the voltages at plate, screen, and bias,and the cathodes are finding their way to ground, it ought to work. Not much left but the transformer. SInce you get B+, the primary is not shorted to frame or secondary. MAke sure the secondary is not shorted to frame. Obviously disconnect the ground wire from the secondary for this test. And a transformer test should reveal shorted turns.

[Garydean]

My thought was to eliminate the OT secondary and 4 & 8 ohm jack network, but I see you've done that with your walkman.

[Simonwill]

Enzo said: "MAke sure the secondary is not shorted to frame. Obviously disconnect the ground wire from the secondary for this test. And a transformer test should reveal shorted turns."

Removed the ground wire and there is no short to earth (with the feedback connection to the preamp it measures just over 100kohm which is correct and with it removed it is open cct).
Had a look at the Geofex site but couldn't find a transformer tester (will carry on looking).
Checking the resistance of the secondary (after allowing for the resistance of the test leads) I get, White-Green:0.1ohm, Green-Black: 0.1ohm, White-Black: 0.2ohm which adds up ok but I've no way of telling if this is too low or not.
How do these figures appear to you?

[Enzo]

The heavy wire and relatively low number of turns means the secondary will measure more or les as a short circuit. AS I mentioned, you can't check it with a meter.

At Geofex, look under: Tech Tips. And within that: Tube Amp Tips