Bizarre 5150 volume problem

[PaoloJM]

Anyone ever hear of this?

I replaced two of the post gain pots on a 5150 as they were scratchy.
Temporarily replaced them with 1Ma PCB pots that I had in stock and all was good.
Then the proper replacement ones came in direct from Peavey, needed the proper ones as the 5150 knob does not fit round a normal 6.3mm shaft.
Measured the pots before installing (890k ish) and appear to be audio taper, good enough so I installed them.
Wound the amp right up into an 8 ohm, purely resistive load, and measured 111 Watts clean sine wave. Sweet!!

Eh, not quite ; when dialed down passed 5 on the master volume it is only conversational load, WTF? At 10 there was definitley 111 Watts...

Checked a bunch of stuff (FX loop etc.), checked all the signals against the values given in owners manual (with the settings Peavey recommend). All correct and present.
Traced signal flow everywher and nothing weird. WTF!?!!?
Still no real volume down passed 5. Measured power output at 5 on the dial is .9 watts!!

Anyways as a last ditch I refitted the old knackered pots and guess what, normal volume taper returns. It's spitting out just over 10 Watts at 5 on the dial, which what you'd expect from an audio taper master volume, ie. about half the output volume.

It almost seems like the taper of the pot is log, but to base 100 instead!!

Anyone ever have a problem like this?
I'm still awaiting Peaveys tech support responce, but I would appreciate any input from you guys.

Rgds,
Paul.

[cbarrow7625]

What resistance do the pots measure on 5? Is it normal? Open? x100 taper? Just curious.

[PaoloJM]

What resistance do the pots measure on 5? Is it normal? Open? x100 taper? Just curious.

With audio taper it's hard to measure the exact resistance out of the circuit at 5, as it jumps so much around there. In circuit measurement is difficult in the 5150.
On the scope I measure just under one watt (using Vrms into a purely resistive load) at 5 with the pot Peavey sent, and just about 10 watts with the original pot and a spare 1Ma pot that I have lying around. Both pots measured about 111 watts at 10.
I thought of a base 100 pot, but does such a thing exist? Where you possibly use one of those?

[cbarrow7625]

It is possible that there is some weirdo taper out there but it would be very rare. It is more likely that there is something wrong with the pot you have. Typical commercial "audio taper" pots don't have a real audio taper. They are really two different linear tapers (0-5, 5-10) that are close enough to simulate an audio taper curve for most applications: See here:

Potentiometers (Beginners' Guide to Pots)

It is possible that the pot you received has a discontinuity at the junction between the two linear taper sections or that it just has the wrong resistance for the lower linear resistance (0-5). The discontinuity sounds more likely to me. Try measuring the pot on an ohm meter as you slowly turn down the pot. Does the resistance all of the sudden jump to "infinite" around 5? Is there some anomaly happening around 5? Obviously you just have a bad pot. It would be instructive for the rest of us if you could figure out what exactly the problem is in case any of the rest of us run into it later on.

[jmaf]

Anyone ever hear of this?

I replaced two of the post gain pots on a 5150 as they were scratchy.
Temporarily replaced them with 1Ma PCB pots that I had in stock and all was good.
Then the proper replacement ones came in direct from Peavey, needed the proper ones as the 5150 knob does not fit round a normal 6.3mm shaft.
Measured the pots before installing (890k ish) and appear to be audio taper, good enough so I installed them.
Wound the amp right up into an 8 ohm, purely resistive load, and measured 111 Watts clean sine wave. Sweet!!

Eh, not quite ; when dialed down passed 5 on the master volume it is only conversational load, WTF? At 10 there was definitley 111 Watts...

Checked a bunch of stuff (FX loop etc.), checked all the signals against the values given in owners manual (with the settings Peavey recommend). All correct and present.
Traced signal flow everywher and nothing weird. WTF!?!!?
Still no real volume down passed 5. Measured power output at 5 on the dial is .9 watts!!

Anyways as a last ditch I refitted the old knackered pots and guess what, normal volume taper returns. It's spitting out just over 10 Watts at 5 on the dial, which what you'd expect from an audio taper master volume, ie. about half the output volume.

It almost seems like the taper of the pot is log, but to base 100 instead!!

Anyone ever have a problem like this?
I'm still awaiting Peaveys tech support responce, but I would appreciate any input from you guys.

Rgds,
Paul.

Paul, every 5150 is a maintenance nightmare here at our shop.

Did you check for DC at the pot input? Maybe you have a leaky coupling cap and that's where the scratchy pot burned the track? The DC could also be a cause for the low volume(wrong bias reference ahead). Just an idea.

[PaoloJM]

Unfortunately it's a really boring end to what I'd hoped would be a more interesting little problem.

Peavey shipped the incorrect pots.
They did ship 1M pots as I had measured before installing. They did appear to be audio taper at a quick measure.
However they were the wrong pots intended for a different purpose.

The only pot types I know of that are used in amps are audio taper and linear.
Peavey tried to claim my customer ordered the wrong parts. My customer said he rang and ordered a post gain pot for a 5150, I don't see how they got that wrong?

For completeness sake:

The scratchy old pot had an intermittant track connection, causing what sounded like DC on the pot, even though I hadn't measured any DC.

The incorrectly shipped pot had a value of 6 ohm at a quarter turn and 44 ohm at a half turn. All my other 1Ma pots, including the original Peavey ones, measured 46 (ish) at a quarter turn and 160 (ish) at a half turn.

Still have no idea what the intended purpose of these mystry pots is, and Peavey haven't responded to my mail yet.

For what it's worth, I found the 5150 to be a joy to work on, compared to DSL et al, and I probably know the circuit better than anyone at this stage!!

[bluesfreak]

Good to hear that the "problem" resolved so quick. However, I also do not understand why there are always complains about 5150s to work on, I've had some much worse amps to work on as well *g*

[jmaf]

I also do not understand why there are always complains about 5150s to work on, I've had some much worse amps to work on as well *g*

Perhaps it's the dreaded maze of soldered pcb boxes....
Amp Work :: DCP_0133.jpg picture by dB_AudioTech - Photobucket

[Alex R]

Perhaps it's the dreaded maze of soldered pcb boxes....
Amp Work :: DCP_0133.jpg picture by dB_AudioTech - Photobucket

- looks like a Delta Blues or a Classic 30.

[Enzo]

jmaf, that is not the inside of a 5150. It might be the inside of a 5150 Combo, but that is a totally separate issue. The 5150 is not remotely constructed like that.

[jmaf]

jmaf, that is not the inside of a 5150. It might be the inside of a 5150 Combo, but that is a totally separate issue. The 5150 is not remotely constructed like that.

It is a 5150 combo, perhaps that's where the misunderstanding is. I am talking about servicing 5150 combos. I've never worked on a 5150 head, very rare here in Brazil.