Do Filter Caps in MRA-3 Amp Affect the Sound

[mauro]

Hello All,
I'm working on a friends MRA3 amplifier which has some issues, too much mids, good treble and not enough bass.
As a rule of thumb I always replace the large chassis mount aluminum can type capacitors, but in this amp whomever did the work left the originals in. There is no hum whatsoever, I was wondering if these caps affect the signal and the quality of sound put out by the amp as the rest of the rebuild seems ok?
Curious minds need to know!
Thank you,
Mauro

[Ron Rich]

Mauro,
Everything affects the sound-- It would be my guess that if the voltages are correct, that replacing these caps, would not make any difference. What cartridge, is being used, and is the mechanism and amp, set-up for it ?? Ron Rich

[robnyc]

Mauro, to "amplify" on Ron's comments, If the cart is using styli with low output you will be using a higher volume setting and that will reduce bass boost.

Does the bass control have any effect at lower settings of the volume control.

I've seen several reports of open bass choke coils and, not to be alarmist but shorted turns in the output tran will cut bass and often output level.

Using a blackhead cart the damping may be shot if it has not been recently replaced. This will result is added "sing" from the stylus cantilever and armature giving the appearance of too much treble.

Try a redhead with mono spikes if you have one.

RobNYC

[Ron Rich]

Mauro,Rob,
I would be careful using a Redhead with mono ("1 mil") needles in it ! The reasons being--The tone arm on the B,thru W models was set up for 7 "lbs" * pressure-- and unless you have an old mono record to test with--you may ruin the record due to both the needle size and pressure. Ron Rich
* OK--grams---

[mauro]

Thanks guys, we are using a 1970's cartridge with mono adapter in a Seeburg C. Tried this cartridge in other jukeboxes and it sounds good.
I put the amp in my model G with a Thorpe cartridge and got, pretty much same results.
The bass control setting does affect the bass. There is bass present just not as prominent as other amps, perhaps a bit overpowered by the midrange sounds.
In the past I've tried a couple different values for the input resistor but it seems to make little to no difference.
That's why asked about the caps, the rest have been replaced and resistors seem to be the correct values.

Mauro
Mississauga, Ontario

[Ron Rich]

Hi Mauro,
That brings up a whole 'nother, ball of wax. Who's "adaptor" ( series/parallel wired ?) are you using? Until very recently, the aftermarket needles sold for "mono" use were totally wrong ! What is the needle pressure setting ? There are now "improved" needles available for that cart. (see the needle sticky above). Do you have an AVC unit attached to that MRA-1 ? Ron Rich

[robnyc]

Ron, according to my schematics the MRA3 has AGC integrated.

Mauro,this gets into an area where having a test generator is a big help.

The idea here is to check the output stage first. Remove the 7 pin RVC plug and input the signal at Pin 7. Attach an AC meter ideally with DB scale across the speaker terminals. Start with 1KHz and set the output of the amp to some convenient level that does not overload the amp or deafen you. Then test 5000Hz,500Hz, 100Hz, 50Hz. The power stage should be essentially flat from around 50hz to just over 12KHz -though the later HFMA amps actually show a rise toward 20KHz due to a lot of feedback.

By essentially flat I mean within +-2db

If no generator you can use a phone or PC with the tones on sites like this:
www.audiocheck.net/ I have not tried it this way but suggest measuring the flatness of the output from whatever you use and make note of any deviation from flat. Turn off any tone controls on the PC or phone.

If you find noticeable reduction in bass (more than -2db) that would indicate a faulty tran, but also check for all capacitor values in this section.

Last, if you don't have a meter w/db scale (or any AC meter) but do have another machine with the same amp-speaker. Simply try inputting the signal at that same RVC pin (plug removed) and playing music directly into the power stage. It'll be a bit flat but you should be able to hear any difference.

RobNYC

[Ron Rich]

Rob, Maruo,
You are correct--the AVC is integrated on the MRA-3--don't know where my mind went on that--was thinking MRA-1 !
In which case--Mauro, is the squelch swith on the mechanism making good contact--is the
AVC section functioning as it should--if not, either of, or both of the above can affect the bass. Ron Rich