Redbear MK 60 Phase Inverter Noise/crackle

[Steve Fundy]

Hello all. Does anyone here have any experience with a Redbear MK 60? yes, the one made in Russia in I believe 90's. I have been chasing down a crackle/dropout and traced it to the phase inverter. I have tried numerous things to try & identify the potentially bad component, but no luck. I am hoping someone here has worked on one before and has some possible insight. Many Thanks in advance.

[The Dude]

Do you have a schematic you can upload? On an amp of that age, my first thought would be bad electrolytic cap or caps.

Edit: From what I can tell the MK120 is the same, except of course 50W. Does this schematic look right?

http://www.prowessamplifiers.com/sch...bear_mk120.pdf

[Steve Fundy]

Aloha Dude. Yes that's the same schematic I worked from. I discovered that if I remove the lead from master volume wiper I still have that crackle on & off at an unsteady interval. I have seen it with my scope in the PI with the rest of the Pre/Tone stack disconnected and also at the output jack. Additionally I found that if i remove the connection to the presence control just before the PI the crackle goes away, but the amp sounds a bit weak.I also tried lifting the .022 uF cap on the Pres control with the 22K pot still connected and the crackle continues. I would agree about potential Electrolytics being dodgy at the age this amp is, but, the resulting crackle & dropout just don't sound like any cap problem I ever encountered. This amp has very unusual filter caps. They are in a can that has a single lead that actually look like a diode but are not. I suppose it's former Soviet technology used to make an amp. I checked the caps w my fluke & they "seem ok". Kinda Baffling. Any thoughts appreciated. Steve

[vintagekiki]

Schematics from prowessamplifiers.com is wrong, NFB from OT (point F) connected over 100k to gnd; presence control shown in the wrong place ... ...

In the original Redbear used in preamp 6n2p tubes, and they are not 12AX7. ¹⁾
For noise / crackle, first look at which tubes are in the amplifier.
Remove V2. If noise disappears replace V1 and V2. If there is still noise replace V3

http://www.prowessamplifiers.com/schematics/misc/RedBear_CubCombo_MK_x50.html

http://www.triodeel.com/images/rbcc501.gif

http://www.triodeel.com/schindex.htm

http://www.chambonino.com/work/miscguitar/misc21.html

http://4tubes.com/SCHEMATICS/BY-BRAND/Red-Bear/

http://web.archive.org/web/20090208122349/http://www.angelfire.com/ok2/joseph3000/redbear/faq.html
Redbear faq

https://www.ultimate-guitar.com/forum/showthread.php?t=1091619
Official Red Bear Amps Thread

https://irationaudio.com/2015/06/03/red-bear-mk-60/
6N2P rewire the tube socket to 12AX7 ¹⁾

https://www.musiker-board.de/attachments/schematics-redbear-mk120_teili-pdf.258689/.²⁾

https://www.musiker-board.de/attachments/schematics-red%20bear-mk120%20teile-pdf.258690/²⁾

[Steve Fundy]

Hi, and thanks for your reply. I have seen the different tube 6N2P designation. The owner of this amp tells me it was working fine & then all of a sudden the trouble started. Perhaps it was already rewired to accept 12AX7. I have removed V1 & V2, as well as tried a brand new 12AX7 for V3, no change. I will look at some of the info you have sent & see if anything pops up at me. I will check that presence control & NFB connection against a correct schematic.
Thanks,
Steve

[vintagekiki]

I have removed V1 & V2, as well as tried a brand new 12AX7 for V3, no change.

Since with the new V3 the symptoms are still the same, most probably some of the output tube (V4-V7)
To find out freely remove all output tubes, and put in the same socket one by one tubes.
Knock them lightly on the balloon so you can determine if any output tube produces a sound sensation (Noise / crackle)

[Steve Fundy]

Already tried that...thanks. Also I pulled PT's out & had my scope on the PI in various places in that circuit. Still seeing those crackles on the scope. It sure seems to be in the PI somewhere.

[vintagekiki]

Look at the pictures
http://www.chambonino.com/work/miscguitar/misc21.html

https://irationaudio.com/2015/06/03/red-bear-mk-60/

Soldering probably doesn't look jubilee after 10-20 years.
There is a good chance that some hidden cold solder may have appeared.
Check / clean from oxide mount fuse holders (in addition to PT)

https://www.musiker-board.de/threads/red-bear-mk120-potis-ersetzen.517881/ ¹⁾

[Steve Fundy]

Thanks Vintage...yeah i agree 100% about the solder joints & I have been re doing joints as I go through the board as well as places where I have opened up a connection for testing. The fuse holder cleaning is a good idea. I haven't been into my shop yet & will be looking more at it later...hey, it is Hawaii and only 12:PM..LOL I have many tabs open with all things Redbear at the moment. I appreciate your help. I'll let u know if I finally fix da buggah later today (or tomorrow?) Steve

[The Dude]

Also check/clean/retension tube sockets.

[Steve Fundy]

Thanks Dude. Already done that on all sockets. No luck. If I wiggle V3 around (or any of them actually) no change. I might try & retension V3 a bit more just in case.
Steve

[g1]

Another thing to consider is that crackling can sometimes be a manifestation of HF oscillation. In such case the presence could be involved as you have found.
Scope the output of the amp with your scope set for high freq. and see if there is any oscillation (while you can hear the crackling).

[The Dude]

For clarification: Can the noise be caused by mechanical vibration? In other words, can you tap on the amp and make the noise? Or, is it random and the noise is there even without any taping or vibration?

[Steve Fundy]

@ g1, I like the idea. I haven't checked for any oscillations. I had the scope on the output and can see the crackle as I hear it out of the speakers. I hadn't been trying to see any HF. I will try. Thanks

@ Dude. Nope it's totally random. I wish it was mechanical, I probably would have it fixed & gone by now..Thanks though

[Steve Fundy]

FIXED! Turned out to be .1uF cap in the PI that is part of the Presence control. Just luckily I had a suitable part! Thanks for all of your help, opinions and just for being here as an asset!
Mahalos from Big Island of Hawaii!
Steve