Seeburg MRA4 L6

I am having some issues here with an amp and need some place to begin.....I knew nothing of the history of this machine I finally got it to go through its series of functions and now the amp needs attention .... the fuse is not blowing BUT the 5U4G is BLAZING hot as well as the little transformer mounted inside on the back, The smaller of the 2 transformers out front gives out smoke and the 5979 tube has no heater voltage... it was only on till i saw the smoke and that was it ... I have the amp home with me to go over it before it gets power again....... What would cause these symptoms or where should i begin ???

Thanks

Well, you've just illustrated why one does -not- power-up these old amps without replacing the capacitors, checking fuse value(s) and placing current limiting in-series with the AC input.

Most likely the main filter capacitors have shorted and some a-hole replaced the fuse with something too large.

Collateral damage:

- B+ transformer on amp chassis: Ruined. That is assuming the smoker is on the amp chassis.

- Small choke coil: possibly has shorted turns. this may have developed a short to chassis ground. Disconnect and test with ohmmeter.

- Filter caps: Likely culprits. Test for shorts...but replace anyway. the amp is sixty years old.

- 5U4: toast.

The amp can be rebuilt but a new power tran is inevitable. You can use one from an old TV or amplifier if you know what you are doing. Total capacitor replacement is essential.

You can get amp schematics here: http://www.verntisdale.com/Seeburgschem.htm He sells capacitor kits too.

A service manual for the mech ans selection system is a good idea if this is your first machine of this type or vintage:

Look up: Always jukin (magazine) or Victory Glass.

Rob-NYC

The smaller of the 2 Trans is the one smoking .... cap kit isnt as big an issue as finding the transformer... I do have another amp here that was in a machine that was in a fire Thas a MRA3 .... could that trans be used ?? Assuming it didnt roast itself to death I could possibly use that If i knew what i should read ohm wise across the leads...... I am leaning towards new caps, new trans , new 5u tube.... I should be ok with that YES ????.... what should I be reading for ohms on the choke ??

Mike, the tran on that donor amp should be OK as a sub

To be clear about which one was smoking, was it the one just below the two 6L6's and above the speaker power switch? That is the output tran and if the primary is shorted to chassis it will kill B+. The other one just under the 5U4 provides B+ for the amp.

You mentioned that there was no heater voltage for the 5879, were the other tubes lit (excluding the 5U4)? If not, check for the presence of 6 VAC at the mating plug that joins amp+receiver.

I don't know the proper reading for the choke, but the main thing is that it not be shorted to chassis. I pulled down an HFMA-2 which has similar current requirements, the choke measures 93 ohms.

Rob

It's the smaller of the 2 trans on the amp is the one that was smoking directly below the volume ......The preamp tube was not heating the rest appeared to be fine including the 5U but it was blazing hot !!! Deffinatly overloaded ..... Will it still be good ??

Mike, the 5U4 probably survived, but it may be weakened. I don't recommend using it.

Rob

I'm guessing a recap, trans n any weak resistors should get this up and going no ???
Have any outlets for that Tramsformer ??

Hammond # 158Q is a suitable replacement for that choke--I seem to remember the original being about 100 ohms--the new one is the same-- Ron Rich

I need the other one too ...... The smaller of the 2 big black ones

It it is the output tran, check these out:

http://www.hammondmfg.com/1608.htm

If you want the 70volt output as original use this one:
#1645 30Watt @4-8-16ohm or-70V

I used one of these and they are superior to the original in the amp. You may need to adapt the mounting.

I've later just used trans from junked Rowe amps that used 7868 outputs. Minor circuit changes are necessary for those.

Rob

the 1645 is showing max dc of 128.......my schematic for this is showing 395 coming off the input tubes......i may need some explanation as to how to tie this in where the other was ... the drawing for the 1645 hook up vs the amp schematic looks different to me ... any help i would appreciate

Thanks

You are looking at the milli-amps (ma) not voltage.

FWIW: It would not be possible to get 30watts out of those tubes @ only 128VDC.

The transformer is not an exact replacement in all aspects, other than an original there is no exact replacement

The primaries would be connected at per the Seeburg schematic.

For the secondaries things change slightly.

The machine speakers are an 8 ohm load.

So the tran's 8 ohm configuration goes to the "20watt" tap and chassis ground on the amp. At this configuration it will still be possible to use one 70 volt speaker tapped @ 4-5 watts.

If you want to use more 70 volt speakers wire the output in the 4 ohm configuration as this will reflect a 10 watt load on the amp and leave 15 watts for externals.

the 70 volt taps can no longer be floating with this transformer. On the amp's 70 volt terminals one must be connected to chassis ground and the other to -white- wire from the tran. The loss of a floating connection is of no consequence, this was just offered to reduce the chance that long external speaker lines might pick up interference in a commercial location (neon signs, etc).

If you get feedback (a loud howl) when the amp is first powered, simply swap the wires from one tube to the other. That is, move both wires from tube A to tube B. I always just tack solder the wires for the initial test and then solder them properly if no feedback occurs.

Rob

Does anyone have a cap list for this ??? It would be greatly appreciated and helpful since half are buried and I don't want to remove anything till I'm ready to immediatly replace it..... Also which need to be polarized ...... The service man I'm using has the mra3 in it which won't work for me....
Thanks guys

Mike,
The schematic and parts list are in the Service Manual. Also the late Vern Tisdale has a copy of the schematic on his site, and his son, Jason, still sells cap kits. See above "new/used parts--" post, in stickies.
With "new caps", only the lytics need to be polarized. Ron Rich

Ron I got the 1645 in .... theres a bit of confusion on the output side hopefully you can clear up for me........1st am I wiring this for a 4,8,16 ohm? and 2nd how exactly is that wired into the switch on the amp S3?? if you were able to give me color wires of the 1645 with locations on the amp that would be PERFECT but any help and insight I would greatly appreciate

Thanks