Motorola stereo console

[Calboy]

Hi,
I have a 1959 Motorola tube console that worked well for the 15 years I used it.  I stopped using it a couple years ago and disconnected part of it.
I went to turn it on a couple days ago and it did not turn on.  I believe it was connected properly.  It has a power amp and separate preamp with an on/off switch.
I took a look at the power amp and preamp wiring and noticed no fuse.
The amp is high quality and very well built but I find it strange there is no fuse.
Does anyone know if this was standard practice on these types of consoles?
I am new here but I understand ac/dc safety and working mainly on guitar amps.
I am mainly curious about why the amp is not fused right now. 
Attached is a schematic I found on the Internet.

[sluckey]

That schematic shows a fuse.

[Calboy]

Thanks for the response sluckey.
I see E11 on the schematic, but I see no glass fuse in the actual power amp or preamp.  Could it be in the 8 pin plug that goes from preamp to power amp?  Any hints to where it could be or what it might look like? Thanks.

[shooter]

look at the PT schematic primary side shows 2 BLK? wires, find n follow

[Calboy]

Thanks shooter, I followed the black wires to an 8 pin receptacle and plug that goes to preamp.  I see no glass fuse in either power or preamp.

[sluckey]

Thanks for the response sluckey.
I see E11 on the schematic, but I see no glass fuse in the actual power amp or preamp.  Could it be in the 8 pin plug that goes from preamp to power amp?  Any hints to where it could be or what it might look like? Thanks.

Fuses come in a variety of shapes and sizes. Not all are in a common glass tube. The fuse is not going to be buried in that plug and certainly not on pin 8. The fuse is connected between pin 3 of the socket and a black wire on the power transformer. If you post a hi-rez pic that shows the socket and the PT and surround area we can probably identify the fuse .

I doubt the fuse is the issue. Recheck that you have all the component assemblies connected correctly. Which part of the console did you disconnect and why? Did you reconnect that part?

[sluckey]

The pic you posted is the preamp. The fuse is on the POWER AMP.

[Calboy]

The primary goes to pin3 and then another black wire connects back to AC outlet.
I removed the power amp to look at as I have done before, this time the amp did not turn on so I went looking for a fuse as well as checking all connections which I think are right but I will check again.  I just thought it was strange that I don't see a fuse.
I had to crop the pic so I could post it, I will try to get a better pic

[sluckey]

Since the PT black wire connects directly to the socket pin 3, rather than through a fuse as shown on your schematic, the fuse must be missing.

[Calboy]

Thanks for confirming there is no fuse.  I actually have 2 other consoles one which I'm using now.
I will either try and troubleshoot this one to get it to work or store it or sell it.
It worked great for me for 15 years mainly just the aux to cd/external music device.
If I get it to work I will update.

[AlNewman]

Well, before giving up, you could test the power transformer, and at least see if it COULD work...  Fuses are easy to install.
The fact that it "doesn't turn on", doesn't mean it can't be repaired.

Could be a bad switch...  You don't know until you dig into it.

[Calboy]

Good points Alnewman.  With the fuse question out of the way, next step is to dig in a bit.
My experience is mainly late 50's tweed style amp and this console amp is a bit out of my comfort zone.  I really don't want to pull out the solder iron yet. 
The PT seemed to meter out okay and I checked the rectifier in another amp and is okay, so I will probably check the on/off switch next.
I should mention I had a cracked screen resistor 3 years ago and replaced the power and cathode caps as well and the amp worked afterwards.
When I went to turn it on last week nothing happened, it just didn't turn on.  I suppose it could be a power supply cap as well.

[sluckey]

I'm betting the PT is not getting 120VAC from the power plug. Connect your AC voltmeter to pins 1 and 3 of the preamp power socket. What have you?

BTW, you do have the preamp power cable plugged in, right? And is the turntable connected?

[AlNewman]

If your switch is connected to a potentiometer (or any switch), they can just get dirty and stop working.  Easy fix, although it could be something you'd want to experiment with once you have a light bulb limiter, or at least a fuse in line.  Or, at the very very least, plugged into a ground fault outlet.

[Calboy]

With just the power amp plugged in(not connected to preamp) pin1 to pin3 is 0vac.
Pin1 to pin5 is 120vac and pin3 to pin5 is 120vac.
Right now power amp is not connected to anything but when it was connected the preamp plug was connected properly as well as the record changer plug.
Yes, I have a current limiter to use and I will fuse when the time comes.

[sluckey]

With just the power amp plugged in(not connected to preamp) pin1 to pin3 is 0vac.

THE PREAMP MUST BE PLUGGED IN TO GET VOLTAGE TO THE PT.

[Calboy]

Okay, connected pre and power amp.  No AC voltage from pin1 to pin3 after I flipped power switch.
Further testing will have to wait til tomorrow or Wednesday.  I have to double check everything is connected right and set up a better test station.
Please continue any suggestions and I will get to them. Thanks.

[sluckey]

Will continue when you come back.

[sluckey]

Okay, connected pre and power amp.  No AC voltage from pin1 to pin3 after I flipped power switch.

There are two switches on the preamp that control the AC voltage switching. One switch is on the balance control (R25). The other is part of the input selector switch. If the selector is in phono mode, there will NOT be any AC voltage between pin 1 and pin 3 unless the turntable is turned on.

I've attached a simplified drawing that clearly shows the AC path through the preamp chassis. So, two switches, wire, and solder joints are the only items that would prevent AC voltage between pin 1 and pin 3. Should be a no brainer to find the fault. Hopefully it's not one of the switches, but if it is, you could just put a jumper across the switch contacts.

I've also attached a complete schematic that is easier to look at.   

[Calboy]

Okay, the above post was helpful along with having the time to set up the console and making sure everything is connected including speakers.
The console weighs 100lbs and the power amp is at floor level so awkward to connect everything.
The problem turned out to be the selecter switch was on riaa instead of tape.(I think). Im sure I checked this earlier but who knows.
New issue is no sound out of the bass speaker.  The hookup is fairly straightforward so hopefully it's not the OT.
All the tubes light up and I did a bit of tube swapping, may need to do more.
The schematic shows red to 8ohm but I couldn't tell from outside which tab was red or common but  not sure it matters. I tried both ways. 

[shooter]

on the schematic Sluckey posted, RED is the CT of the Primary OT, B+ for the BASS PA section, green n black look like speaker out, OT secondary.


did you swap in a new speaker just for giggles?

[Calboy]

on the schematic Sluckey posted, RED is the CT of the Primary OT, B+ for the BASS PA section, green n black look like speaker out, OT secondary.


did you swap in a new speaker just for giggles?

Sorry, I meant the Jensen speaker has a red dot. It has 2 tabs with wires that connects to 2 chassis screw jacks.  I have to remove the power amp to get a look at which one is common and 8ohm.
When I did the test I didn't use the other  console speakers with the crossover network and tweeter, I just used temporary speakers that were easier to hook up. 
Does the crossover network have to be connected to get sound to bass speaker?
I will try to do that today as well as check something else.
I tested the bass speaker a week ago, but will check that as well. Thanks.

[AlNewman]

I would think now would be a good time to make a voltage chart.  It's a good schematic, all the info is there.

[shooter]

speaker polarity doesn't matter for testing
the Crossover looks like it's only used for mid n tweeter

[Calboy]

I hooked up the console speakers with the crossovers and no change.(I thought the crossover was only for the mid + tweeter but wasn't sure).
I will check tubes tomorrow and  probably do a voltage chart although I don't want to commit to that. It seems the preamp and power amp( as well as speakers) have to be connected to take voltage measurements so it seems that will be tricky and awkward.

[shooter]

Swap V3 with V4
Swap V5 with V6
This "moves" the Bass PA tubes to the L n R channel PA


IF no joy;
with original tubes in original sockets, power up, verify the L/R work, then;


measure the VDC across R22, then R24, then R25
those are the bias resistors for Bass PA, L n R PA

[Calboy]

Took some el84 voltages.
For bass channel v3,v4,r22
R22 149ohm(130) voltage 10.4vdc
Pin7  323vdc
Pin9 296vdc
.070ma x 313 is 22w /2 = 11w per tube

V5.
R24. 150ohm. (150).    7.6vdc. Pin3
Pin7.  282vdc
Pin9. 259vdc
.050ma x 275 is ~ 13.5w

V6
R25 187ohm (200).  7.8 vdc.  Pin3
Pin7 283vdc
Pin9. 258vdc
.041ma x 275 is ~ 11w

These voltages seem fine. 
Edit to add that I checked the sound again and it seems like the bass channel / OT work.  I'm not sure what happened.  To take voltage readings I had to lean the chassis up, so maybe that had something to do with it. 
Edit update -  I put the chassis back in the console laying down and the bass channel does not seem to work again.  So it looks like an intermittent issue.  I put a new cathode cap for v3/4 in about 3 years ago.  This is my guess, either cold solder or something.  I will figure it out. 
I will do more testing when I get a chance over the next few weeks.  This console will go back on the shelf until I decide what to do with it.
Thanks for all the help straightening out the condition of the amp.