Hurb's Vintage DMM

[Progrockabuse]

got this in trade from the good nickd of this parish. used to belong to our champion noise bring hurb.

whilst it has that nice warm analog delay sound, it has it's drawbacks.

60 cycle hum at all times
Bypass isn't very good
Random Switch that's not actually connected
Modulation Section doesn't work.
Slightly Ropey Mains Lead with separate earth cable to plug.

I'm planning to work out what has been change and see if i can restore it to it's former glory.

Plans are to

Try and quiet down the hum and noise
Convert to True Bypass or decent buffered Bypass
Get the modulation section working
Replace Mains Lead/maybe transformer.

As far as i know it was a 120v one converted to 230/240v.

if Hurb or anyone could shed light/help me get this thing sorted it'd be greatly appreciated

[NickS]

From what I read, I don't think the original DMM was mains powered, so I think that internal transformer is a possible source of your hum. The schematic I'm looking at >>here<<shows it as having an internal discrete-component -15V regulator; an IC regulator would give you better regulation.

[Progrockabuse]

so what i want to know is, how hard would it be to return this back to the original schematic?

i plan to run it in a loop so the bypass might not be a problem. is is cost effective to repair this one or look into a reissue memory man.

[Hurb]

I wish I could help matey, it came to me like that and I looked inside and got scared, I wonder if that green switch used to turn the modulation on or off and that's why it doesn't work correctly now? or perhaps it maxed the feedback at a click of a switch....well that is what I would of liked to do to it.

I also can't remember it being noisey when I had it.

[NickS]

so what i want to know is, how hard would it be to return this back to the original schematic?

i plan to run it in a loop so the bypass might not be a problem. is is cost effective to repair this one or look into a reissue memory man.

You'd have decisions to make about whether to revert it to exernal power, what to do about the extra switch and so on. The original switch may have been disconnected because it's faulty, so you may have to source a replacement. Whether it's cost effective is entirely up to your personal view of what it's worth. It's certainly a project I'd consider.

[Mike]

I think NickS is your man with this stuff. He's a pro.

[NickS]

Hmm, more research - US models used to have mains power, and used to suffer from hum problems. So to be totally orginal, I guess you'd keep the transformer and suffer the hum.

[Progrockabuse]

Thanks for your replies.

Nicks, i think i'm gonna look in to external power, don't mind a wall wart as long as it cures the hum. i admit i'm not that good with electronics so i could use all the help i could get.

i'm gonna return 90% to stock, just upgrade the power supply and bypass. get rid of that green mod button and clean up the case a little. defo wanna go down the repair route.

[NickS]

Tips:
1. Be static-safe. CMOS chips such as Bucket Brigade Delays (BBDs) and their clock generators are easily damaged by charge but won't necessarily fail immediately. Take care to make sure everything you're doing is at earth potential (some say work on an aluminium foil mat - I have a conductive plastic one). So, don't wear your nylon fetish gear, don't scoot around the room on your wheely chair stroking the cat, etc., etc.; regularly touch the soldering iron to the foil before applying it to the PCB, keep an elbow on the foil etc.
Or buy a static-safe kit

from >>Amazon affiliates<< or whoever.
2. If you've found a >>schematic on-line<<, you'll see that the power is -15V derived from a rectifier bridge and a pretty crap transistor regulator that provides a fraction of the rectified input voltage. (Later versions used a single diode and 30V Zener fronting a -15V regulator) Be sure that your wall wart doesn't run too high a voltage; I see genuine EHX ones are £14.99 at Andertons. You could mod it to the later design for the safety of the chips.