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Monarch STA 400X

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dave-g63

New Member
Hi all, I have a Monarch tube receiver I'm restoring for another AK member.
I figured since I work on tubes mostly, I should be on AK after being at Videokarma for 5 years now.

I proceeded to replace the electrolytic capacitors as SOP. An initial power-up was accompanied by fair sound from both channels.

But it revealed deeper problems as the four 6GW8 tubes began to glow in places they shouldn't after only 3 minutes. Fortunately I have nine to choose from and still have 4 that test "OK" after I raided my old hi fi tube stash.

While I normally would check and replace resistors that are out of tolerance, I was REALLY not prepared for what I measured. Most half-watters were measuring 50-100% too high! Examples;
Two 47K measures 59K, and 69K, 1K measured 1.9K, 15K measured 19K and 25K, 220K measured 290K
Four 2.2K measured 3.4K, 4.6K , 4.7k and 5.1K!!!!

I ordered all metal film 1 watt replacements and Ill be busy in the amp-pre-amp sections but....will the rest of them be OK?

Is it possible that extended operation destroyed these resistors causing them to drift so high? IMG_7877.JPG
 
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They're carbon composition resistors, which is what caused them to drift so high :-) That said, my intuition tells me that a resistor reading too high shouldn't cause the symptom you were seeing ... A too-high grid leak resistor on a power tube, perhaps... but on a pre-amp tube I would expect that, if anything, resistors with too high of a value would be limiting the current.
 
thorpej said:
They're carbon composition resistors, which is what caused them to drift so high :) That said, my intuition tells me that a resistor reading too high shouldn't cause the symptom you were seeing ... A too-high grid leak resistor on a power tube, perhaps... but on a pre-amp tube I would expect that, if anything, resistors with too high of a value would be limiting the current.
Click to expand...
I neglected to mention, I fired it up before replacing six paper coupling caps where the orange drops are in the photo. It was possible the caps were leaky, upsetting bias voltage.
Once I change the resistors, I'll try it again -with an ammeter on the cathodes while I do.. :cool:
 
it sounds like a bias supply problem,if fixed bias, or a bad cathode bypass cap if cathode biased.
Bad coupling caps will also cause the problem, especially the Elna grey oil caps
 
Just acquired Monarch STA-400X. First visual inspections show very clean top and bottom, no rust or bug residue, no signs of dripping from any capacitors or burned resistors, even the higher wattage power supply resistors, only one leaky paper capacitor in the preamp section.

This is a first tube project in about 30+ years, working on all solid state since the last BC-348 receiver conversion in late 80's so plan on taking it slow and easy. Once through inspecting and taking lots of close up photos, I'll bring up slowly on a variable power supply to check transformer and the low voltage DC rectifier. Was able to get a user manual with a schematic, so not shooting totally in the dark on this.

Comments, input and any suggestions welcome.
 
If it has any of the old Japanese grey wrapped oil capacitors, don't bother with powering up until they are all replaced. Most will probably have high DC leakage.
John
 
Hi all,
I also have one amazing STA-400x, with some not original tube. Can you help me in exactly which position the ECC83 (12AX7) tubes are in?
Thanks!
Ferenc
Hungary
 
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