Monarch STA 100X weird?

[Dumbledog]

So I have started to work on recapping the receiver that I had purchased a couple months ago and stumble upon this resistor that is not connected to anything at one end. It sticks out of the enclosed metal cage that houses most of the circuitry, as seen in the pic. I found a picture of the same amp on eBay with that same resistor arrangement. What might this be for?

 

[llwhtt]

It's part of the tuner, anytime you see a length of wire sticking up and going no where it's called a gimmick. It's cut to length as part of aligning the tuner,
leave it be.

Craig

 

[Dumbledog]

I see. Thanks for the info!

 

[Dumbledog]

So I got it recapped and replaced the bulbs with LEDs. It sounds fantastic... except for on phono. Would something like this have a built in phono preamp and RIAA equilization? Or back in the 60's would that be a separate piece of equipment? Right now it is playing but sounds like there is no preamp.

 

[Eli Duttman]

This thread on Audio Karma shows some pics of the receiver. It could easily be the case that the unit was designed to be used with piezoelectric, not magnetic, carts., as it looks like most of the tubes are for the tuners.

Please post clear pics of the front and rear panels. I want a good look at the source selector switch options. Also, that pot. on the rear panel interests me.

You probably have to manually trace the circuit out and draw a schematic. Obviously, the audio, not RF, area is of greatest interest. If you do that, please post the schematic. Perhaps, like some Sansui units, the receiver has a SS phono section. However, given your description of almost complete inaudability, I would not get my hopes up.

 

[wrenchone]

I may have one of these - at least, I know it's a Monarch. I believe it has SE 6BM8 outputs, though I might be getting it mixed up with another old receiver.

Edit - I just searched the forums, and I have the same receiver - it has SE 6BQ5 outputs, ripe for some sort of partial feedback setup with a pentode front end.

 

[Eli Duttman]

I just searched the forums, and I have the same receiver - it has SE 6BQ5 outputs, ripe for some sort of partial feedback setup with a pentode front end.

IIRC, the A/K thread mentioned 7189s, 6BQ5s on steroids.

Do you have a schematic?

The "iron" didn't look crappy and, I believe, there's a choke too. How about a mix of short loop (Schade) NFB nested inside a GNFB loop of only a few dB. The "gotcha" is that single Noval tube, almost certainly a 12AX7. Perhaps hybrid cascodes, with 6922 sections "downstairs" and ZVN0545As "upstairs", would work for the small signal complement.

 

[wrenchone]

Whatever - that receiver has sat in a pile of some other stuff for some time. It'll be a while (quite) until I get to it. Wimpy 'AX7s begone!

BTW - still no schematic, though I wouldn't be surprised if the receiver was set up for ceramic phono, as it looks like a budget deal. The actual tubes in the sockets were 6BQ5s, though they might get replaced with 6P14P-EV, since that's what I've got on hand.

 

[TheGimp]

I've got the STA400X, and it has two phono inputs, ceramic and magnetic. If you only have one input, it is more likely ceramic. If so, you could change it to magnetic input.

 

[Dumbledog]

Here are some pics of the top side of the amp and the front panel and rear.

 

[Dumbledog]

And some pics of the rats nest of point to point wiring that lies below

 

[Dumbledog]

I've got the STA400X, and it has two phono inputs, ceramic and magnetic. If you only have one input, it is more likely ceramic. If so, you could change it to magnetic input.

How would i change it to a magnetic input?

I also noticed that on the phono input there seems to be a resistor/capacitor setup that the signal connection runs through. Could this just be some impedance matching?

 

[DF96]

The resistor sticking out of the tuner could be a test point. Unlikely to be a gimmick capacitor, as that would not normally have a resistor attached.

 

[TheGimp]

First you need to find a schematic, or trace the circuit out and draw your own schematic.

Once you have a schematic the circuit can be analyzed to determine how to proceed.

 

[wrenchone]

I would definitely give that source selector switch a shot of cleaner and a few rotations. I would also replace the RCA connectors so that the low-level signals have a nice, shiny metal interface rather than 50 years of oxidixed crud.

 

[Dumbledog]

I got everything cleaned off. I can also say that there is nothing solid state in the receiver. I think it's designed for ceramic cartridges. There are no circuits that the phono input feeds through that the aux input doesn't feed through- there is no noticeable preamp section.

What does changing it to a magnetic cartridge input entail? Just from a general standpoint. Like adding a whole preamp and RIAA equalization circuit?

 

[wrenchone]

If what you say is true about the inputs, a dedicated phono preamp may be the most sensible way to go. Two extra tubes would do it - the question would be whether the power transformer can handle the extra load. In this situation, I might consider going against my grain and using a 12AX7-based circuit to keep the power draw down.

 

[TheGimp]

Or, you could dro pin a sand RIAA preamp which could be powered off the heater supply since It would draw so little current.

I'll try to get to my STA-400X tonight and look at it to see how it is done. I think it has an RIAA with 12AX7s, and for crystal input they use a 10:1 divider to the same ckt as the magnetic input.

 

[Eli Duttman]

How would i change it to a magnetic input?

I also noticed that on the phono input there seems to be a resistor/capacitor setup that the signal connection runs through. Could this just be some impedance matching?

That R/C combo is probably for frequency shaping. You want the phono I/P to be exactly like the aux I/P, which is line level.

Neither space nor filament current is available to install a tubed phono section in the unit. A SS setup needs regulated low voltage DC, which is not present.

Regardless of whether you chose tubes or SS, an external phono preamp is (IMO) needed. How much do you want to spend and how much DIY are you prepared to cope with?

Jim Hagerman's "Bugle" may be the easiest way to get the phono section out of the way. While it is opamp based, RIAA EQ is passive and comments are favorable.

If you want a tubed setup, I'll immodestly suggest the tweaked RCA setup I'm associated with. Courtesy of the ZVN0545A MOSFET, wrenchone's objection to the WIMPY 12AX7 is overcome. If this setup is of interest to you, contact member Jeff Yourison and get some 1st hand builder feedback.

 

[wrenchone]

Unless you just hate 12AX7s - I'm stubborn like that.