HR2510 Audio limiter location for restoration

[kc8mob]

Have a 2510 with inoperable ALC and SSB sounds awful

So I suspect the audio limiter has been removed or associated parts have failed.

C64, D129, Q138 etc

Have been looking for them to the point of irritation.

Where are they roughly located, physically on the board?

Wire bundle untied etc can't find them.

Thanks

 

[999]

Rogerbirds mic gain mod.

micgain

 

[nomadradio]

I suspect what you're looking for is Q114. It's the transistor that has its collector shunted across the mike audio feeding into the mike amp. It's just to the left and rear of IC104, the mike amp near the front-center edge of the main pcb.

There are other ways to disable Q114, but if it's missing, this is at least one of your culprits.

73

 

[brandon7861]

I colored a couple traces.

AMC:
Starting at the output network, audio is detected from the purple RF trace by D136 and D137. The negative-going portion of the audio waveform is compared with the base voltage at Q121. When the audio goes lower than the base, Q121 conducts and pulls down the pink trace. This turns on Q138 which turns on Q114 to reduce the mic audio.

ALC:
RF follows the blue trace where a capacitive voltage divider (C43 and C44) reduce the RF to a smaller RF voltage. The positive peaks (when they exceed the voltage at the emitter set by R117 and VR104) make Q119 conduct which pulls down the pink trace as above. If the ALC VR at Q119 fails open, the 8v supply will keep the emitter high (ALC detector off).

The orange 8v AM trace keeps Q120 off during AM. When the mode switch is set to sideband, that 8v disappears and Q120 turns on which ensures the AMC doesn't interfere in SSB (as Q121 will not turn on with its emitter always above its base).

 

[brandon7861]

Assuming Q114 hasn't been plucked out, here are some numbers. The service manual does not show the SSB ALC voltages, but if you are going to follow the ALC path trying to troubleshoot it, this is what I think you can expect.

12w peak into 50ohm is 24.5v RMS, or 34.6v peak. The capacitive voltage divider C43/C44 (5pF/100pF) samples this 12w signal and provides a smaller RF voltage to the base of Q119. 34.6v peak * 5/(100+5) = 1.65v peak. With 12w out, we know the base of the ALC transistor should see peaks up to 1.65v.

This means the emitter would have to be set at about .95v (base - .7v) so that the transistor starts to conduct as soon as the RF waveform exceeds 12w peak (or 1.65v at the base).

The emitter voltage is set by the voltage divider R117 (18k) and VR104 (10k). To get .95v at the emitter, the pot would, in theory, have to be set to about 2450ohm with 8v from the supply.

The voltage levels may be a little different, but that's the basics of how it works. Each RF peak, the ALC transistor will briefly pull down on that pink trace causing Q138 to pulse on, and the RC stuff at Q114 smooths out those pulses with the required time constant so Q114 isn't pulsing the audio to the individual RF peaks.

Good luck!

 

[Onelasttime]

I doubt that is the issue. These are low level modulation radio's and they really shine on SSB. AM is always underwhelming to your average AM jaw jacker.

So almost no one did much to AM audio maybe slow down the AMC and add a power mic but that is about it. That said there is no limit to what butchers will do even if it is pointless I guess.

A radio this old I would recap, de-glue, and run through alignment procedures as a form starting point to anything else. My President Lincoln is 1998 vintage and your HR2510 is likely 8-10 years older than that making it just under 40 years old.

These older radio's need a lot of patients especially if they have been road hard and put away wet.

 

[N8YX]

Neat old rigs.

I ran an HR-2510 or HR-2600 on my 10M packet node for years. The radios often saw 12+ hours continuous use and never broke a sweat.

Few years ago I got an original Lincoln and an HTX-100 (a kissing cousin) and all will eventually go in a rack on my vintage bench.

There is a local 10M net and every President's Day, a lot of those old Presidents are brought out of storage and put on the air for the evening. May be down an S unit from my regular rigs but everyone hears everyone else.