Need rare diode for Ampeg B-40 bass head

[Slegwig]

Leo - sorry - I meant to answer you original question in my long post.

Currently the amp has zero output (no noise, hiss or audio of any kind)

[TomCarlos]

The schematic has several instances of the IN456 diode. That is an easy one to look up. It is rated at 1.0v forward voltage.

Take a closer look at the 556. Are there any other markings? That seems to be an elusive ID number. But based on what you are reading (if it was measured in the circuit) it might be good.

Just as you would diagnose a problem in a tube amp, use your checklist and take a look around this amp. Do your visual check first. Do you see any bad solder joints? Any loose wires? Anything obvious? Crackling could be a bad connection somewhere.

After that, what about your Power Supply Voltages. Do you see 56 volts at the rail? What about the plus and minus 16 volts at the Zener Diodes D301 and D302?

That is where I would start...

[Slegwig]

Thanks Tom. I went through usual checklist and checked all connections, leads and looked for any visually "blown" components. Fixed a few solder joints but all are solid now. Im getting the correct +56.5v and -56.5v readings from the power supply. However, readings from zener diodes (D301 & D302) are currently +19v and -19v.

Regarding the mystery diode, there's an "S" marking as well, but the part number appears to be an Ampeg one. Hoping from the other posts that it may actually be ok after all. BTW, readings are measured out of circuit.

[TomCarlos]

The voltages on D301 and D302 seem a skosh high... but lets move on.

Reinstall D203. Look at R210 - right next to it. It shows a 1.5 voltage drop. So if you are measure from Ground to the Emitter of Q204, do you get 56.5-1.2v? and what about the emitters of Q201 and Q202? Do you get .5 volts (referenced to ground)? And what about the base of Q205? Do you see .5 or .6V?

You have some check points in the circuit. Let's see if you get some steady and correct voltages. If they fluctuate, then we know there is a different problem to solve.

I'm sure the other MEF members can weigh in too!

[Slegwig]

Thanks Tom - I will reinstall D203 tomorrow and retest the voltages. Have to turn in for the night.

Thanks everyone for all the great help so far!

[olddawg]

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.

Back in the day a lot of older power amps had stacked diodes. Lots of big Pioneers, Sansuis, etc. They could be 2, 3 or 4 junction. They were also quickly obsolete. They rarely failed except when mishandled by inexperienced techs, usually broken leads. The cludge was to stack the appropriate number of sanely sized rectifier diodes in series.

[R.G.]

Yes, as noted earlier, this reference diode is used as a voltage to set the currents in Q203 and Q204. You can do a couple of things. There is a nominal 1.8V zener, the 1N4614, which is available from Mouser. You can stack ordinary silicon diodes to get 1.9V +/- 4% at the working current. Or you can tinker the diode voltage AND the emitter resistors on Q203 and Q204 to make things come out the same.

The whole point of a constant current source is that the external world only sees the current, not how it's made. For Q203, any combination of the base voltage and emitter resistor that makes the current on the collector the same works fine, as Q203's collector is taking up all the voltage from near the positive power rail to the emitters of the input diffamp, so any small difference in voltage on Q203 collector doesn't matter.

Q204 is a little trickier, as it mainly sets the bias point for Q205, and that has to be right to get the correct balance on the output amplification.

A simple thing to do would be to find a rock-solid, easy to replace zener for D203 in a value near 1.9V. Get an industry-standard value, and get couple of them in 2% or so. One will be a service spare, taped into a baggie on the amp with a note for the next service guy, as a kindness. The current isn't much. I make it about 11ma nominal, 54V through 4.7K, so the dissipation is not an issue; it's about 25mW.

With a good reference zener, you then adjust the values of R202 and R120. R120 is easy, fortunately. They wanted 120 ohms with 1.2V +/-10% across it. That makes the current in Q204 be 1.2V/120R = 12ma. So you measure the voltage your new zener gives you at the emitter of Q204, then calculate a resistor that makes the current in the resistor be 12ma. Let's say your new zener gives you 56.5 minus 1.5V at Q204 emitter. The emitter resistor to get the right current at Q204's collector is then 1.5V divided by 12ma, or 125 ohm, and you can pick the nearest 1% resistor to nail this down more finely than the original amps.

Much the same reasoning works for Q203. I'm not sure why they didn't want you to measure the voltage across R202; possibly because it would introduce some BIG transients in the speaker output. If those speakers are replaced with a dummy load, I can't see any good reason not to go ahead and measure the voltage on R202 and do the tinkering to get the correct 0.5V across R206.

But you can also just find/make any stack of diodes that will give you 1.9V +/-4% at 11ma, too, and that would eliminate any resistor tinkering.

[Slegwig]

Thanks old dawg and R.G. and I appreciate greatly the time you put into the thorough response RG. You gave me the in depth theory on the circuit design that I was lacking and I learned a lot from your post.

Thanks again all - MEF is by far the most helpful forum I've found so far and wish I joined sooner.

[Slegwig]

Hi all - I'm about to get started on working on the amp again this afternoon and had a few more questions about diodes:

1. D208, D209, and D210 all call for 1A diodes on the schematic. In cross-referencing the 1978 Ampeg parts list, these are part # 530555-1, which are listed as "1A 800 PIV". If I was to replace these, is it OK to use 1N4007 diodes (1A 1000V), or should I stick with the 1N4006 (1A 800V)?

2. To replace D301 and D302, I purchased some Fairchild 16V 1W Zener diodes (Mouser part # 512-BZX85C16). Can you verify these are the correct replacements?
(I also plan on replacing R305 & R306 (680 Ω, 5W) adjacent to D301 & D302, as the originals read about 710 Ω)

3. The original D204, D205, D206, and D207 diodes (1N456) appear to test fine. Is it worth it to replace them as long as I've got the amp open and I'm replacing everything else? ;o)
Just don't know how long these last and if it's a good idea to throw some new ones in.

Thanks!

[Tom Phillips]

My approach would be.

...1. D208, D209, and D210 all call for 1A diodes on the schematic. In cross-referencing the 1978 Ampeg parts list, these are part # 530555-1, which are listed as "1A 800 PIV". If I was to replace these, is it OK to use 1N4007 diodes (1A 1000V), or should I stick with the 1N4006 (1A 800V)?

No problem. In fact I just use 1N4007 any time a 1N400X needs to be replaced. Easier & cheaper just to stock 4007's bought in bulk.

...2. To replace D301 and D302, I purchased some Fairchild 16V 1W Zener diodes (Mouser part # 512-BZX85C16). Can you verify these are the correct replacements?

Any good quality 16V 1W zener will be fine.

...(I also plan on replacing R305 & R306 (680 Ω, 5W) adjacent to D301 & D302, as the originals read about 710 Ω)

The 710 Ohm reading is only 4% high with respect to 680 Ohms and likely doesn't need replacement at all.

...3. The original D204, D205, D206, and D207 diodes (1N456) appear to test fine. Is it worth it to replace them as long as I've got the amp open and I'm replacing everything else? ;o)
Just don't know how long these last and if it's a good idea to throw some new ones in.

They last almost forever if not over stressed electrically or physically damaged. I would not proactively change them or any other parts that you cannot identify a really good reason to replace.

The more you work on the PC board the more chance there is to do collateral damage or make part replacement mistake that could snowball. I understand that it's really tempting to do extra work just because you are there but, in the big scheme of things, minuscule changes like the resistor value will not improve the amp.

[Leo_Gnardo]

1: 1N4007 will do fine. Nothing wrong with a little extra margin of safety for the PIV rating.

2: Those Fairchilds will do fine, of course (said the former Fairchild employee.)

I'd suggest mounting them so they're a little above the circuit board, say 1/8 inch or 3 mm. So the air can get behind them, keep 'em a little cooler.

3. The last 4 I'd leave alone. I think they were spec'd as 1 volt, a little unusual but it will save us searching for a modern equivalent.

[Slegwig]

Thanks - I appreciate the quick responses Tom & Leo. I hope to power it up and retest the voltages soon.

[Slegwig]

Hi everyone - SUCCESS!!!

All the voltage readings were within acceptable range once I got it all put back together. So I adjusted the bias pot to .011V, plugged in my bass and it sounds like a brand new amp! (which it ought to, given how many parts I replaced) )

Huge thanks to everyone for helping me through this! Now I get to call my friend to give him the good news...after I play it for a little while longer.


P.S. Leo - the Zener diodes I changed in this amp are mounted on a terminal strip far away from the circuit boards, so they have at least 1/2" clearance all around.

[TomCarlos]

Excellent news Dan... nicely done.

Now help me fix my Crest Power Amp !! :-)

Tom

[Slegwig]

Hahaha - bring it on!