5150 bias mod

[tedmich]

I finished up a bias mod on my resurrected 5150 that had a toasted PA board.



Sweet sealed vintage Allen Bradley RV4 pot from R5-D3 Surplus here in Portland ~11k in series with 4.7k resistor, Teflon leads tacked to capacitor and tucked under my home made 6505 PA board.

Nice and clean up top, locking of course.



Had a vexing problem of ultra low volume, could hear guitar, channels switching just way way LOW

swapped out preamp tubes, cleaned FX send and return..still barely louder than my TV...

See the IDC connector on FX board coming from main board? Guess what happens when you plug it in UPSIDEDOWN? super low volume...

I found this mod pic when googling pictures of the main board, gotta love the daring...

[tedmich]

I'm going to use a nice anodized aluminum 2U rack vent to replace the plastic one that always breaks ($16 vs $12)



I should countersink it but maybe I'll just bolt it down... it only sits up 1/8"

[Valvehead]

looks good !
hey i found some great mods for this thing--a couple preamp mods by Mark Cameron--really improves the tone . Google cameron 5150 mod if you are interested

[tedmich]

no mods for this badboy, I'm gonna sell it with a new foot switch I just made



toner transfer onto raw aluminum with clear lacquer overlay looks pretty good!

[The Dude]

All the work you have in that one, I think you should pour yourself a strong one, strap on a guitar, sit back, and abuse it for a while before you sell it.

[tedmich]

it sounds pretty good but simply must be LOUD, the XXX its currently sitting on sounds fricking awesome even at low volume. I'm gonna have them stacked like this for prospective customers

6505+
XXX
5150

and maybe the fugly old Ultra 120 in the corner... I'll snap a pick for the memories

[The Dude]

......and maybe the fugly old Ultra 120 in the corner...

One of my favorite amps despite their fugliness......oh, and the weight! I still play through a Peavey Rockmaster preamp. It's basically the same pre as the Ultra's.

[tedmich]

yah the transformer on the Ultra 120 is HUGE, mine says Triumph on the PCBs

Well my 5150 footswitch has fucked up the amp, its a pretty simple design with 2 SPST footswitches with 12V LEDs downstream.

The LEDs light up just as expected, and the front panel LED shows the switch from LEAD to RHYTHM but theres no actual switching, no relay click and it stays in RHYTHM no matter what you do. FX footswitch also doesn't bring loop in.

Another symptom is that with footswitch removed there is a very loud crackle when front panel channel button is pushed, but channels do switch.

Maybe the 12V LEDs are dropping too much voltage for relays? But why front panel LEDs switch without relays confuses me... gotta revisit schematic !

DOH!

I did surgically remove the "Mudflap Chick" from my XXX



what a waste of nice aluminum...

[tedmich]

I dropped a 470 ohm resistor across both LEDs and it passes enough mA to switch channels now, gotta test the FX but I think the relays just needed more current.

They're supposed to have 200mW coils but the 12V LEDs pass about 12mA, less than the ~16.7mA the relays need... They were not very bright to begin with and they're even less now...

In the future I'll use the following T 1 3/4 (5mm) LEDs
WP7083ZGD/G
green, 2200 mcd, with 1k ohm resistor in series and 470 ohm in parallel
WP1503SRD
red, 1000 mcd with 1.2k ohm resistor in series and 470 ohm in parallel

that way they'll only see their 3.2V and 1.85V respectively, pass >20mA for the relays and should still be plenty bright, 60 degree view angle etc.

I suppose a 470 ohm series and parallel on each LED might do it too...

[Enzo]

What is a 12v LED? LEDs generally have a junction drop of a volt or two.

The stock FS is just two switches with a plain old LED in series with each. No resistors.


Go to the very bottom of this thread, and download the file "wiring guide", which covers many PV footswitches. The 5150 is not far down the list.
Tech Tips/Tricks/FAQ- READ THIS FIRST! - Peavey Forum

[tedmich]

Thanks Enzo, I think I've come to the conclusion the K3 relay is bad, it never puts the FX loop in even without the footswitch where it should default ON. Maybe it cooked being constantly ON without a FS for many years?... the datasheet on the Omrons says this will stress them. I took the LED out of the circuit and it just makes a hum with FX loop jumpered with or without switch. I can't see the 0.33MF caps (C53, C65) going bad so my next thought is the relay. I doubt its JFET Q7, cause the channel switch is fine so next up is K3
I've cleaned the FX/footswitch jacks like 5 times so I doubt its the contacts. although some recommend a 30 caliber gun brush...

Luckily I have several NOS P&B relays which should be compatible

12v led has (tiny) built in dropping resistor, figure it pulls only 12mA, I should have bought standard one but Frys had limited choices and I was momentarily dumb. ;(

I'm sure glad I found the 6505 schematic as its so much easier to read than old fuzzy 5150, has BOM etc.

[Enzo]

You can always get free factory clean complete schematics right from Peavey just by asking customer service for them.

The built in resistor is killing you, just use a plain LED. Steal LEDs from some other circuit even. Go buy a couple from some repair shop.

Before you go changing a relay, find out if it is getting energized or not, when the foot switch is stomped. The coil will always have - or at least SHOULD always have - the 24v or whatever on one end. When it is NOT eneergized, the 24v will be on both ends. (Ohm's Law - no current flow, no voltage drop) If the thing is energized, then ther will be roughly 12v across the coil. Now th coil is inconvenient to get at, but ther is a parallel diode, see the schematic, so just measure voltage across that diode.

A relay can be bad in the coil, and not move, or the coil might be fine but the switches don't move, or the switches move but don't make contact. OR... the circuit that turns on the relay might be broken.

[tedmich]

The diodes were easy to get at and showed about 12v with the switch, I replaced the relay and its switching the FX loop fine now.

Even my nicest digital delay sounds pretty noisy on the LEAD channel though, this thing is a br00t and I'm ready to sell it to some obnoxious teenager!

Thanks Enzo!