The Monarch SA-60

[The Fuxtor]

Howdy all, started cleaning up one of these units , thought I'd post some pics and ask a few questions...
First off I believe this unit dates back to around 1965 or so and uses 7189 power tubes. The guy I bought it from says he replaced outputs probably 20-25 years ago and basically never used it.... I noticed the replacement tubes are GE 7189A.
Questions.... What does this unit look like as far as biasing goes? From first glances I see pin3 tied to pin3 and then grounded for each channel....
As I am going thru listing caps for replacement, I am noticing those little hockey puck diodes and wonder if they are reliable??
Some pics!

 

[primosounds]

What i usually do is to first check to make sure that all the transformers are ok. Once that is determined then you need to check the coupling caps. Those Japanese i believe Suzuki coupling caps are probably shot and you should change them out. Once they get changed out you can assess sound quality and decide if you need to address other problems. I have never run across those plastic disc diodes going bad but changing them out to freds or schottkys is never a bad idea. You notice that there is a full wave bridge rectifier (the block shaped thingy), which i believe is for the bias supply. That is definitely a replacement part. It looks like the bias is fixed negative type which needs to be checked and upgraded if you wish. Also the carbon comp resistors all have to be checked for tolerance. My big caveat with Japanese amps from this era is that the tube lugs are very fragile and can only stand so much pushing and pulling (about 3 or 4 before breaking). It looks like the power supply caps have been previously changed (usually ELNA was stock) so maybe they are good. So it depends on how much you want to invest in good parts or premium parts.

 

[maxhifi]

You're lucky it came with those tubes, they are now quite expensive!

It looks to be cathode biased, so you don't really need to do anything.


Agree about replacing the suzuki caps, and also about the filters looking non original.

Make sure to clean and tighten the output tube sockets if necessary. The 6BQ5 packs a lot of heat into a small package, and the thermal cycling can be hard on tube sockets

 

[The Fuxtor]

Ok, a full wave bridge rectifier aside from those disc resistors indicates the bias supply?? Cool, my last project used cathode biasing, so my thoughts of cooling down the bias a bit was worthless.
Is it worthwhile to beef up the caps in ps? Only using 2 - double 40uf / 500v
And a 20uf/ 300v...
These caps even if they have been swapped out at one point are still 25+ years so I'll just replace I think

 

[primosounds]

It looks to be cathode biased, so you don't really need to do anything.

I dunno, there are 2 rectifier circuits. The disc caps and the FWBR block that you can see above the green ww resistor. So unless the round disc caps are not in circuit it suggests that there is also bias voltage supply.
Fuxtor, whatever the case just trace the cathode on the power tubes if it is directly connected to ground then it is negative fixed bias. If the 7189 cathodes are connected to a large wattage resistor that is bypassed with a capacitor then the amp is cathode biased and no adjustments are needed. If it is fixed bias with no "bias sensing resistor" then it is a good idea to install some 10 or 1 ohm 1/2 watt resistor to ground so you can check the bias current.

 

[primosounds]

Ok, a full wave bridge rectifier aside from those disc resistors indicates the bias supply?? Cool, my last project used cathode biasing, so my thoughts of cooling down the bias a bit was worthless.
Is it worthwhile to beef up the caps in ps? Only using 2 - double 40uf / 500v
And a 20uf/ 300v...

With SS rectifiers, the size of the caps can be much larger. you could go to 100uf for each. What is the 20uf x 300v cap for? If it is the preamp cap? You can probably double that value with no problems.

 

[arts]

When I saw the reference to the ''hockey puck'' diodes,I just had to check your location; yep,why am I not surprised

They rarely fail,except when overloaded by failing filter caps.While one can has clearly been replaced,the others appear to be original.If one has already failed,the others probably aren't far behind.Change all of the other electrolytics, as well as the grey-cased coupling caps,as these are notorious for leakage and failure.The resistors of the non carbon-comp type,unless of the overheated power variety,are extremely quiet and reliable.

Those are really sweet sounding little amps,and with those GE 7189's,you scored big. Hook them up to some efficient speakers and they are amazing.

 

[The Fuxtor]

With SS rectifiers, the size of the caps can be much larger. you could go to 100uf for each. What is the 20uf x 300v cap for? If it is the preamp cap? You can probably double that value with no problems.

That can is connected to V3....
V1&V2 appear to be phono...

 

[primosounds]

Well, if the cap only supplies the one tube, a larger value is not so important unless the B+ is poorly filtered and additional filtering will help. But for me using a film cap sounds best in that position.

 

[arts]

Does the manual have a schematic?

 

[The Fuxtor]

Does the manual have a schematic?

Unfortunately no... Can't find one anywhere...

 

[arts]

Unfortunately no... Can't find one anywhere...

Damn.I couldn't find one either,that's why I was hoping for the manual

A lot of this stuff was just re-badged ''something else'',and I'm sure that your machine falls somewhere into the fiasco of Trio/Kenwood/Pioneer/Lafayette/Radio Shack etc etc. You just have to figure which manufacturer this unit crosses over to. Not essential,but it is certainly nice to have a schematic on hand to reference while working.

 

[gadget73]

might be worth making at least a scribble diagram of what the power supply setup is so you have an idea what you're looking at.


also thinking the bridge is original, probably bias supply and possibly phono tube heater supply as well. Its a big enough bridge that I expect it has more load than just a bias supply needs.

Are there 3 "pill" diodes or 4? I see 3 but that doesn't make a lot of sense. Two in series (four total) or four in a bridge I'd expect. Unless it has both a HV setup using two, a bias supply using one, and a DC heater using the bridge. That seems overly complex though.

 

[The Fuxtor]

might be worth making at least a scribble diagram of what the power supply setup is so you have an idea what you're looking at.


also thinking the bridge is original, probably bias supply and possibly phono tube heater supply as well. Its a big enough bridge that I expect it has more load than just a bias supply needs.

Are there 3 "pill" diodes or 4? I see 3 but that doesn't make a lot of sense. Two in series (four total) or four in a bridge I'd expect. Unless it has both a HV setup using two, a bias supply using one, and a DC heater using the bridge. That seems overly complex though.

It has 4 of those diodes....
Maybe I will attempt a crude schematic, I have been searching for its doppleganger but haven't any luck yet...

 

[The Fuxtor]

Update!
Got this all recapped and testing today.
After getting the controls all cleaned this rig is sounding very good!!
Now not having a schematic I thought I would run some of the voltages to the community to desipher if everything is looking good? Here goes:

First can cap - ( +218 dcv )
Second can - ( +323 dcv )
Third can cap- ( +441 dcv)

Outputs: 7819a x 4
Pin1 - ( -12.3 dcv)
Pin 2 .( -12.3 dcv)
Pin3. ( +1.6 mv)
Pin 4 .(+ 2.5 acv)
Pin 5 .( +2.9 acv)
Pin 6. (+439 dcv)
Pin 7. (+437 dcv)
Pin 8. ( 0 )
Pin 9. ( +439 dcv)
I do not have any cathode resistors installed yet, but I will get on that soon...

 

[gadget73]

so its fixed bias, and it seems that it internally connects pin 1 to pin 2. 2 is the grid, 1 is listed as "internal connection"

Lots of plate volts, but doesn't seem like enough grid voltage to go with it. Datasheet conditions show -15v with 400 on the plate and 300 on the screen. You're at basically 440 plate and screen, which is beyond ratings. Datasheet gives 440 plate and 400 on the screen maximum. I might be tempted to re-wire the screen supply to the 323v tap on the power supply.

I would not keep using this without at least installing the cathode resistors.

 

[The Fuxtor]

so its fixed bias, and it seems that it internally connects pin 1 to pin 2. 2 is the grid, 1 is listed as "internal connection"

Lots of plate volts, but doesn't seem like enough grid voltage to go with it. Datasheet conditions show -15v with 400 on the plate and 300 on the screen. You're at basically 440 plate and screen, which is beyond ratings. Datasheet gives 440 plate and 400 on the screen maximum. I might be tempted to re-wire the screen supply to the 323v tap on the power supply.

I would not keep using this without at least installing the cathode resistors.

What causes too much voltage at screen or grid?? That first can was originally only 10mf or maybe 20?? I bumped it up to 50, but it was only feeding the preamp tube, also at the can caps were 3 big resistors, I haven't checked those for values, possibly they are whack and buggering up the power supply??
10 ohm 1/4 resistors tied to pin3 and ground will give me a bias number correct?
Also where/how do I install screen stability resistors ? 100 ohm 1/2 watt goes where to where??
Thanks once again,!
EDIT: that first smaller ps cap actually feeds the phono tubes! It also ties into that next can cap section, I guess my options here are to either lower the capacitance back to the original 20uf , or add some more resistance with a bigger resistor where it ties into the next can?? The big 3 resistors as t the ps cans are all in tolerance...

 

[gadget73]

probably just the design. I doubt it indicates a problem in the amp or with your repair. Changing the cap values won't actually change the voltages all that much. Some amps just didn't follow the tube datasheets very well. Based on voltages I'll take a guess and say the output transformers and the screens connect to the same point in the power supply.

cathode resistor would be 3 to ground, or between 3 and whatevever it goes to now.

Screen resistor goes between 6 (or 6 and 9 if it has a jumper) and whatever it feeds from.

 

[The Fuxtor]

probably just the design. I doubt it indicates a problem in the amp or with your repair. Changing the cap values won't actually change the voltages all that much. Some amps just didn't follow the tube datasheets very well. Based on voltages I'll take a guess and say the output transformers and the screens connect to the same point in the power supply.

cathode resistor would be 3 to ground, or between 3 and whatevever it goes to now.

Screen resistor goes between 6 (or 6 and 9 if it has a jumper) and whatever it feeds from.

Ok, nice... Just needed some clarity on those resistors....
So as far as the screen voltage goes , think I should try to move that wire to the other can, essentially dropping it's voltage from 440 down to 323v, any issues doing this? I figure lowering voltage at one point will tend to raise voltage somewhere else....

 

[primosounds]

Ok, nice... Just needed some clarity on those resistors....
So as far as the screen voltage goes , think I should try to move that wire to the other can, essentially dropping it's voltage from 440 down to 323v, any issues doing this? I figure lowering voltage at one point will tend to raise voltage somewhere else....

The relation ship between the screen grid and plate voltage is important. Since the amp is fixed bias that voltage should be very close to stock, what ever that was. So, while lowering the voltage from stock will not hurt the tubes it may change the sound quality for the worse. But 440 is way to high.