2250B protection relay

[MikeGarofalo]

Finishing up final component changes on this 2250B. Replaced R807 150 ohm 5 watt with 7 watt, and H808 and H811 with new BYQ28E-200E,127 rectifiers. Powered up fine with no issues (although didn't run up volume, just checked for 35VDC). Next I replaced all 6 transistors on the phono board with new KSC1845's. Tested TT and as volume turned to about 50%, unit cut out and I can see small sparks at relay contacts. Switch to FM and same result, relay cut out at higher volumes. Returning volume back down and relay releases. Re-checked every solder joint and every new piece for correct orientation, everything looks perfect. Even put the original rectifiers back into PS board, no help. What causes the protection relay to engage-disengage?

 

[sanjib]

Have you checked the bias and DC offset? Probably the bias current is too high.

 

[MikeGarofalo]

I did, and the bias was a bit high (20-25 mv) so I adjusted back down to 10 mv and tweaked the offset back to zero. No help, relay clicked over as volume came up. I used my 2275 preamp into the 2250b mains and the problem went away. This would suggest the issue has to be on the preamp/tone board. That board was recapped and the four transistors replaced with the suggested replacements. I tested the unit after those mods, but admit never ran the volume up and may have missed the issue.

 

[sanjib]

Yes, you are right, the issue is with the pre-amp section.

 

[MikeGarofalo]

I'll give it a good cleaning and inspection, then re-flow all my solder joints. Appreciate your input and hopefully my next post will be touting success!
One more potentially odd question, when measuring DC offset at speaker terminals, I know the speakers should be disconnected, but what about volume pot. Does it matter what position it's in?

 

[sanjib]

supposed to be at a minimum.

 

[MikeGarofalo]

Thank you, that's normally what I do.

 

[Aiden T]

sounds like you probably have excess dc on the signal in the preamp check caps if you replaced any for polarity, and check for bridge solders or a bad capacitor.

 

[MikeGarofalo]

Thanks Aiden, will do and post if/when I find something.

 

[MikeGarofalo]

Success! A good cleaning of the Pre-out and Main-in connectors and problem solved. Keep forgetting on some Marantz units there are contacts in those connectors that need to be cleaned just like front panel pots and push buttons. Thanks again all...