Need rare diode for Ampeg B-40 bass head

[Slegwig]

Hi there - I need some help locating a replacement part for an Ampeg B-40 solid state bass amp (circa mid-70s). Specifically, I'm looking for Ampeg part # 530556-2, which is described on the 1978 Ampeg "Replacement Parts Price List" as "1.9V REF."

I contacted Ampeg support and they were only able to tell me that the full part description is a "1.9v reference diode" (and they referred me to this forum).

As an additional piece of info, this component is designated as "D203" on the amp's schematic (Ampeg drawing # 591737-1, the schematic for the B115, B410, and B40 amplifiers).

I really appreciate any help in locating a replacement. Thanks again

[Leo_Gnardo]

Welcome aboard. It sure would be good if you could post the schemo so we can figure out how this "reference diode" is used. It may be simple as replacing it with 3 silicon diodes in series. Each has a voltage drop of about 0.6V, might be close enough for rock n roll.

[Enzo]

It would be so much more helpful if we could see the schematic you have.

The current Ampeg people (Loud Technologies) is not related to the company that made them 50 years ago, they won't have any support there.

Withoput seeing the circuit, in my mind it could be a 1.9v zener, or a multilayer diode with a known junction voltage across it.

[mozz]

Sounds like a Zener diode.

[Slegwig]

Ampeg-B40-Schematic.pdf

Thanks Leo! here's the schematic

[Slegwig]

Thanks Enzo and Mozz - it's not marked on drawings as zener, but look forward to your thoughts. I've been restoring tube amps for a few years now but I'm relatively new to ss circuits.

[Enzo]

A zener would be wired in reverse bias, so it is not one. I think it is similar to the dual diode PV uses, and I agree, you could probably series three regular diodes and see if their drops add up close enough.


But what is wrong with the one you have? That would be an odd failure. Does it not get that voltage drop in the circuit? Of are you testing it like a diode but it reads open? 1.9v may bee too high for your meter to recognize as a diode.

[Leo_Gnardo]

Looks like it's setting bias for Q203 & 204. I'd try digging thru some diodes and/or light duty rectifiers, put 'em on a meter's diode test (shows breakover voltage), try to come up with a trio that adds up to the needed voltage and put that combo in. You're sure the one in the amp is bad?

1.9v may bee too high for your meter to recognize as a diode.

You could test it like a zener, put a couple thousand ohm resistor in series, meter the diode while applying voltage from a bench supply or even a 9V battery.

Looks like that diode has to stand some current, it's in series with a 4K7 resistor, across a 56.5V supply rail to ground. 100 mA light duty diodes need not apply.

[Slegwig]

I was testing it on the diode setting of my multimeter and thought it was bad. Now that you mention it - I'm not 100% sure it's bad. Here's a pic of the reading I just took

[TomCarlos]

Is there any way that you can take a picture of this diode?

[Slegwig]

Is there any way that you can take a picture of this diode?

Tom - here's a closer shot of the diode

[Leo_Gnardo]

Tom - here's a closer shot of the diode

Do I see about 1.5V? That's with the relatively low test current from the meter. Looks like this item passes about 110-120 mA in-circuit, that would likely drive up the voltage. I'm thinking it may be OK.

So . . .

What's the matter with the amp?

FWIW solid state amps suffer from the same problems as tube amps. Old filter caps, in fact any electrolytics are suspect. And I've found lots of drifted resistors in old Ampegs that use carbon comp resistors. Also poorly soldered components on their circuit boards, and old connections becoming corroded and falling apart. But let's not flail around, specifically what it is ze problem?

[52 Bill]

Do I see about 1.5V? That's with the relatively low test current from the meter. Looks like this item passes about 110-120 mA in-circuit, that would likely drive up the voltage. I'm thinking it may be OK.

I'd like to know what happens when it is reverse biased. Switch the two leads and what is the reading?

[Slegwig]

So, here's the full story. Last year a friend brought me the amp complaining of intermittent loud crackling. I opened it up and main filter caps were shot and physically leaking. I replaced them and the amp was in tip top shape. Fast forward to a couple weeks ago. He complained of loud static/hum even with volume turned down, with no signal being passed from input. We fired it up for a few seconds, heard brief crackling, then all sound stopped. I started to smell something hot, but couldn't locate the source exactly and quickly shut it down. Seemed to be coming from output section though. I proceeded to disassemble and test various parts. In output section, I replaced a couple out of spec resistors along with one 470 ohm resistor that was cracked and way out of spec. Replaced smaller electrolytics in output section along with all caps in the output section and all caps in both input channels (most were cracked and disintegrated completely when I pulled them out). Also replaced power transistors with NOS rca 2N4348's. Found NOS direct replacements for most of the transistors in the output section (all except Q302 & Q303). Replaced a few other resistors that were out of spec in input channels. When I reassembled to test, I had substituted the original 1.9v diode in question with a 1N4007 diode (I know, a poorly educated guess). After testing voltage readings shown on schematic I knew I guessed wrong.

As you can see, given my lack of experience in diagnosing the exact issue, I kind of took the shotgun approach and proceeded to replace all that I could in hopes of fixing the problem. Though inefficient, still was worth hours of fun.

I'm wondering now if I put what I thought was the bad diode back in the circuit if it will work.

Look forward to your thoughts...and criticism.

[Slegwig]

I'd like to know what happens when it is reverse biased. Switch the two leads and what is the reading?

Bill - Open line reading with reverse bias