Schematic needed for popping H&K zenAmp Combo!

[pacAir]

I have a Hughes & Kettner zenAmp combo on the bench with a NASTY pop in speakers when the amp is turned OFF (think "gunshot"). The amp seems to otherwise work but the turn-off blast will damage a speaker or two if not fixed! There is no DC present at the speaker outputs and the power supply supplies are balanced so it does not seem to be a power supply-related issue.

I tried to contact H&K through their web site and the e-mail gets bounced back as "undeliverable". They are in Germany and don't provide any U.S. contact info that works or that I can use.

Has anyone seen this problem? Does anyone have the Schematics available? Their mechanical design makes it quite difficult to troubleshoot without a schematic because all the components are buried from view and the amp needs to be almost completely disassembled in order to find the components!

Any help would be most appreciated!

Thanks,

Steve

[Jazz P Bass]

To quote Enzo.
" the POP cannot be louder than the amp at full volume".
The POP itself should do no harm.
You can contact Yorkville Sound, in New York, for info.

[oc disorder]

A last resort method would be to fit a relay on the speaker output
like this
Speaker Protector, Don’t Connect Your Speaker Before The Amplifier Is Powered | Circuit Diagram
which also should disconnect the speaker just before the "pop".

Although possibly harmless it can be a nuisance in a quiet restaurant or
church setting.

[pacAir]

That was the first thing I thought of but I cannot see where any relay was ever used to isolate the speakers AND this at TURN-OFF, NOT turn-on. Typical amplifier speaker relay circuits are time-delay to mask turn-on thump and disconnect speakers if a failure or fault occurs. Turning off the amp when it is otherwise working probably wouldn't trigger a relay fast enough to mask all the hit. According to the owner the amp worked great for years until this cropped up.

The pop isn't an audio tick, it is an extremely fast & percussive DC thump. I placed a test speaker face down on the table and turned the amp off and the speaker literally Jumped off the table! I certainly wouldn't want to subject several hundred dollars worth of Celestion drivers to this abuse! It scared me into disconnecting the speakers the first time I heard it myself!

There is significantly more power in a transient hit like this than in any normal audio signal this amp would generate under normal operation. This is not an audio pop in a preamp stage being amplified by the power amp. This is a DC hit caused by something in the output section that is downstream from the Master Volume control. There is something wrong and a schematic would help me track this down.

Thanks for the responses, though!

Steve

[g1]

They seem to be very reluctant to give their schematics out. That being said the support folks in Germany seem quite helpful. The trick is to get through to Germany. When you fill out the online form it gets routed through to their distributor in whichever country you chose from the drop down box. You want to be from a country that doesn't have a distributor or many "official" repair centers (like Mexico). This way it gets dealt with by Germany. Also be clear that you are a technician working on a customers unit.

[pacAir]

Great idea, thanks! I'll give that a try.

Steve

[J M Fahey]

*Maybe*it didn't pop before but does now because it had an anti pop measure that has stopped working.
The typical ones are a .047x600V (or a UL approved "250VAC" or "X" labelled one) across the power switch terminals and/or across the transformer line wires; and/or a Varistor, typically rated 20% above the nominal line voltage.
Said capacitor might have opened (or simply unsoldered/cracked leg) and said varistors have a definite service life.

[pacAir]

AC Pop-suppression Components

An AC pop-suppressor was the first thing I thought of but kind of discounted after inspecting the amp. First, I could find no suppression components (though they still could be hiding on the circuit board where I cannot see many components) and if the WAS an AC-related problem then it would likely be different every time because sometimes you would a peak in the AC waveform and sometimes you would hit a valley or zero-crossing. I see no such variation. Also, it doesn't happen at all on turn-on, only at turn-off!

This hit is not really a pop but a big scary BANG and it happens pretty much the same way every time under the same conditions. It sounds, looks (on a scope) and feels much more like a DC problem but only occurs at turn-off when everything is dying/discharging.

I'm leaning towards a leaky semiconductor or capacitor in my head. That's why a schematic would certainly make my life easier on this one!

Steve

[J M Fahey]

You don't know where it comes from, so, just to check, short the amp input and test.
Many preamps oscillate briefly when losing power (draining caps at turn off).
Yes, the usual sound is a "chirp", I have suffered an agonizing "Kwiiiiiííírk" myself and heard the occasional "thump"; you seem to have a bad case of the latter.
*If* it comes from the preamp, it can be controlled; if it's generated on the power amp itself, it will be more difficult.
Worst case, write the guys in Germany explaining everything, what you have done so far and an MP3 , plus scope photos (well, send what you can).
The problem *is* unusual, they are a serious company, I'm sure they'll bypass the official service (in this case) and try to find a solution, not only because of you but for themselves.
Whatever you do, please keep us informed.
PS: how many seconds (or milli-seconds) does it happen *after* you hit the power switch?
Do you have an output relay or something like that?
Does it have a crowbar triac to protect the speaker from DC?

[pacAir]

I finally contacted the factory (by telling their website I was from Mexico) and they informed me they only supply schematics to their distributors and gave me the correct contact e-mails for the current US distributor (Yorkville as stated earlier). They were not interested in discussing the problem I reported and offered no additional info.

I contacted Yorkville and described the problem and what steps I had already undertaken and they sent me a nice short reply (no problem info) with an attached zenAmp Service Manual! When I get some time to get back to that project I'll have the right "tool" and I will report back here when I know more.

Thanks to everyone for the ideas!

Steve

[J M Fahey]

Whenever you have some free time, please scan and post it. It will be of great help to others. Thanks.

[pacAir]

I received it as a PDF but I don't see anyway to upload a file to the system. I generally don't like posting a link to a server that could go away rendering the posting meaningless in the future. I run into a lot of dead links after thinking I have found a solution and I know it pisses me off so I try not to do that.

What do you suggest? It seems a bit counterproductive to convert a perfectly good PDF to many less-sharp JPG images just to post them in a forum!


Steve

[defaced]

When you reply, click the "go advanced" button. That'll get you everything you need to upload the pdf.

[pacAir]

Hughes & Kettner ZenAmp Service Manual

I didn't notice the "Advanced" button, thanks!

Here is the file:

HK_zenAmp_ServiceManual.pdf

I still haven't had enough time to get back to the amp but in the next week or so I hope to have the mystery solved now that I have a roadmap!


Steve

[J M Fahey]

Well, the power amp is conventional, thanks God, and it has a JFet mute.
I'd start by temporarily shorting R21 and R23, just to see whether the pop lives before or after the amp input.