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RD50 Limiter Side Barely Working

Acquired an '82 blonde RD50 110 and 112 cab (both with EV speakers) from its original owner, all good except Limiter side has loud hum when turned up and barely passes any guitar signal. Tried all the obvious stuff, cleaned all pots and jacks, swapped tubes and ICs, did full electrolytic recap. Looks to be a Rev B board (dated 4-5-82, production label 6-82). All components I've measured so far are in spec. Voltages are correct. Limiter/Clean switch and footswitch check out fine. Owner said Limiter side worked when he bought it, but never used it. Thinking there is a bad ground/solder joint somewhere and/or some failed component blocking signal.

Using an audio probe, at the resistor and cap locations where you should get Clean or Limiter signal depending on switch setting, I am getting good Clean, but barely discernible Limiter. Locations include C17, C18, R23, R26, etc.

Suggestions welcome where to focus my attention vs checking every component.



Update - Voltages and Grounds

Took some voltage measurements of the troublesome '82 RD 50 vs my healthy '80 RD50. Clearly, something ain't right, Points 5 & 6 have no voltage and are showing as grounded.

No time to trace this back tonight, suggestions welcome.


Limiter side

is the 12ax7 ok?
did you verify IC2 4016B? I would try jumpers between 3&4 and between 8&9 to simulate limiter channel ON, verifying nothing between 10&11


Tubes and ICs

Swapped all tubes and ICs with my good '80 RD50. The bad '82 shows no change, the good '80 still works perfectly. Would that not be a definitive test on the health of the tubes and ICs?

The tubes are all original US (Sylvania and Phillips).

The mechanical switching parts seems fine (Limiter/Clean rocker switch on panel, footswitch). Since there is a volume dependent ground hum and very faint signal passed, I think it has something to do with board Points 5 & 6 measuring no voltage and grounded. I plan to trace back from those points to see if there is a shorted component upstream from them.

If there was an issue with the power transformer (internal short, etc.), wouldn't the Clean side not work either?


is the limiter chanel working?

if I were you I would like to know if the chanel itself is ok, similating proper ic2 switching with jumpers on its socket, if so looking around q3 & q4


Will try that tonight, thanks

I see your point, determine if the Limiter channel itself is bad, and/or if the switching is at fault.

Will try your suggestion tonight, thanks.



Did some testing and tracing last night.

Jumpered IC-2 as suggested, switching is working fine, the Limiter channel itself is the issue. Symptom is volume dependent hum and weak signal.

Checked all components in the circuit connecting to the orange wire board eyelets (identified as Point 5 Orange and Point 6 Brown in my previous voltage comparison chart). All were in spec and had correct DC voltages except at the eyelets themselves and one side of each of the four diodes (D12,13,14,15). Correct voltages should measure the -2.x DC volts.

I unsoldered the two orange power transformer wires from the board. They measure fine, about 32v AC each, with end to end continuity. However, there is a problem with Point 4, and the yellow wire from the power transformer. My understanding is that the yellow wire should provide ground from the power transformer to Point 4, its board eyelet. However this yellow wire is not grounded. More concerning is that the Point 4 eyelet is grounded without anything connected to it. So it looks like something in the circuit path connecting to Point 4 is (incorrectly) grounded.

With the orange wires reattached to the board, and the yellow wire detached, voltages are now the correct -2.x DC volts at the diodes and at Points 5 and 6 - which are are no longer showing a ground connection either.

I've seen the ungrounded yellow wire problem before, solved it with a separate chassis ground wire to Point 4. However, that did not improve things here. Nor would I expect it to, since Point 4 appears to be grounded somewhere in the circuit.

Goal now is to find the suspected shorted/grounded component in the circuit connected to the Point 4 eyelet.


Going down a dead end?

Forgot that my healthy '80 RD50 originally came to me DOA, and one of the issues was bad ground traces under the board. Fixed that with bridging wires and a new ground wire from a chassis bolt to the board Point 4 eyelet where the yellow Power transformer wire connects. So the Point 4 eyelet is supposed to be grounded, and provide ground to the Power transformer via the yellow wire. I pulled the board of the '82 patient and verified this for myself.

So the issue is still something I haven't yet found...


Problem Solved - Limiter Side Working Again!

Reconnected the yellow and two orange wires from the Power transformer to the board. All voltages now measure normal when compared with a stock (not yet recapped or modified) RD50. While checking voltages at the 12AX7 tube I immediately noticed Pin 1 was about 0.5 volts DC, nowhere near the 120v DC or so required. Traced the problem back to a bad R9 470K resistor. It was receiving about 260v DC but passing on only 0.5v DC to Pin 1 of the 12AX7. Replaced R9 and the Limiter side is now working correctly.

Will check the rest of the components in the circuit feeding the 12AX7 just to be thorough, and then I'm calling this one done.


Closing the Loop

Played the blonde 110 RD-50 EVM and 112X EVM cab at the NYC Marathon Sunday 11/3 for over four hours, performed flawlessly and sounded great.

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