• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Marantz MR255 / 1550 hi filter loud pop

runningdog

runningdog

AK Subscriber
Subscriber
Turning ON the Hi Filter causes a pop loud enough to make the DBT flash. Then it plays normally.

Turning OFF the Hi Filter causes a pop loud enough to make the protection circuit recycle. Then it plays normally.

It is independent of all controls and happens even with the board inputs disconnected. This is the last circuit before the main amps.

I think it started after replacing four electrolytic capacitors on the filter board with WIMA films. I'm not seeing any shorts or trace damage, the replacements went in smoothly. And the switch itself has been flushed with non-residue contact cleaner over several days followed by DeoxIT.

I'm not sure where to go next. Other components are testing good in-place. Suggestions on what to check next, please?

Here is the schematic and the board:

MR255 PS01 with highlights.jpg MR255 PS00 filter board labeled 2.jpg
 
Register to hide this ad
No change after replacing the two transistors in the circuit and replacing the switch. All the resistors on the board were tested and are within spec. I have not tested the two ceramic disc capacitors CS07 and CS08 that are in the circuit right after the switch so I'll pull those and test them next.
 
To wrap this up, I replaced the tone board and the board with the hi filter switch on it from a working 1550. I used electrolytic capacitors instead of the WIMA films, not that it should matter. The result? The new hi filter switch still pops when it is pressed. But not nearly as loud as the last board did and not enough to put it in protection.

If you have an MR255 or a 1550 I'd like to know if the hi filter switch on yours pops.
 
Reading through the thread I was thinking "poor design?". The thought has some merit after I saw that you replaced the board! I assume you measured the DC voltages but that's what I usually think when things pop loudly upon switching. I see that there are bleed resistors on the output coupling e-caps (470K) RS17&18. I wonder if the amp inputs have bleed down resistors?

Edit: It looks like it does and shouldn't really matter I'd think. The high filter switch only puts a 1500pF cap to ground before the buffer transistor base, that shouldn't make THAT much of a noise. I wonder if the circuit board was actually laid out correctly. I would next hook up a scope probe to the base of the buffer transistor(s) and watch what happens when the filter button is cycled. Firstly to see if the issue is on both channels or just one, even with the replacement board in place.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom