Montgomery Ward Model 450 with dead signal strength meter

Ordered parts but my buddy pushed pause.

Right side bias adjusted ok, but the left side tops out at 3.54mV, way below the 30mV +/-10mV spec. IIRC this is similar to my 730 Twin where I replaced the trimmers and it solved the bias situation.

FMI, what is the consequence of bias this low? Would it account for the low output?
 
JJCalvillo said:
Ordered parts but my buddy pushed pause.

Right side bias adjusted ok, but the left side tops out at 3.54mV, way below the 30mV +/-10mV spec. IIRC this is similar to my 730 Twin where I replaced the trimmers and it solved the bias situation.

FMI, what is the consequence of bias this low? Would it account for the low output?
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Not usually. But you will have some crossover distortion. The BIAS current setting is the idle current. Much like a car where you set the idle speed. But it will still go full throttle once the RPMs come up via the throttle (volume) position.
Something is keeping the idle current low. Which could be anything in the circuit such as a driver transistor, biasing transistor, diode, resistor, bum solder joint, etc. Yes, these can be a pain with anything soldered under the output sockets. I always tried to get those components done and out of the way first, and leave the easy stuff for the home stretch. And I always rebuilt the entire board instead of having to repeatedly separate the heat sink/outputs to go back in. It saves mucho time in the long run, not to mention the nerves. :rant: :rflmao:
 
Convinced my friend to finish this. Took all the 2SC-1400s out and replaced the caps on the amp board. Now seeing 35 wpc. Also replaced the trimmers and the left side still doesn't want to bias past 4 or 5 mV.

Questions:
• There's a combo resistor/diode where the 1S2076 is marked on the board. Anyone know if the resistor should be there, and can I sub a 1N4148 for the 1S2076?
Post #8 KA-9100 - overhaul

• SM says 1S1211 is the "varistor/bias compensation" and I see peeps also replacing that with 1N4148. Can you use a diode as a varistor, if not what do I replace the 1S1211 with? Post #2 Restoring a Harman Kardon A402
 
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D405 in series with R401 (150k), normal, clearly visible on the schematic. You should have approximately 330mV across R423. If so, the current source (Q411) for the voltage gain stage (Q407) is working properly. As you can see, the bias voltage source is the sum of the forward voltage of D403, and the B-E voltage of Q409. You may consider to replace D403 (1S1211) by a diode which has a larger forward voltage drop (you just need 30mV more), or add a 10 Ohm resistor in series with D403.
 
@ecluser on the left side there is 244 mVdc and on the right side there is 299 mVdc (across R423/424). So I have a problem upstream somewhere.
The only transistors replaced were the six 2SC-1400s at Q401-406 so I double checked and they are all pinned correctly.
Do you think one of the other transistors is bad?
 
So, the current source in the left channel, provided by Q411, is only 7.4mA vs 9.1mA on the right channel. That means there is less voltage drop in D403 and the B-E junction of Q409. That's why you have a low bias at the output stage.

Could you tell us what voltage drop you have in R409, R410, R413 and R414?

Confirm that you have 5.6V at D401 and D402.
 
ecluser said:
So, the current source in the left channel, provided by Q411, is only 7.4mA vs 9.1mA on the right channel. That means there is less voltage drop in D403 and the B-E junction of Q409. That's why you have a low bias at the output stage.

Could you tell us what voltage drop you have in R409, R410, R413 and R414?

Confirm that you have 5.6V at D401 and D402. Negative side was -33.7Vdc and -33.3Vdc. Am I supposed to be measuring across?
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ecluser said:
Yes, across D401 and D402
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Let me know if this sounds plausible:
D401 = 5.61 Vdc and D402 = 5.73 Vdc
Voltage drop for:
R409 = 1.5 Vdc, tests as 42.5K ohm resistor.
R410 = 1.6 Vdc. Tests as 42.4K ohm resistor.
R413 = .8 Vdc, tests as 389 ohm resistor.
R414 = .9 Vdc. Tests as 391 ohm resistor.

Edited to correct R414 test value.
 
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I want to rule out one source of confusion between the left and right channels. On the HK A-402 service manual, odd numbers for the components correspond to the right channel but it is usually the even numbers for the right channel. That said, I will talk about the "low side" vs the "high side"

On the "low side", the voltage at the zener diode is 0.1V less than on the "high side", and as a consequence the voltages at the base of the current source for the input differential stage, and the base of the current source for the voltage gain stage is approximately 0.1V less. That last one makes a big difference in the emitter (and collector) current of that current source, when you subtract the usual 0.6V for the B-E junction. But that's not all. If we compute the voltage at the base of the first current source from the voltage at the zener, and the resistive voltage divider network before the transistor, the voltage on the "high side" is coherent with your measurements (neglecting the base current of the transistor), but is 0.1V less on the "low side".

My point is that on the "low side" R405 is too high or/and R409 is too low in regards to their nominal values. The cheapest way to correct the situation of the "low side" would be by reducing the resistance of R405 from 120k to 115k or 117k. Remark that it is within the 5% tolerance range of the 120k resistor. If you do that you will increase the current in the bias stage and that will increase the bias current in the output stage.
 
Replaced R405 with a 68K + 47K for 115K total. It a little more than doubled the mV to 12 mV. Need to get to at least 20 mV as the spec is 30 +/-10 mV.
Do I take some more ohms off R405?
 
Yes, or increase R409, just a few kOhm

Don't forget to readjust the offset when you change R405 or R409 because it will change the current in the differential stage as well.
 
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ecluser said:
Yes, or increase R409, just a few kOhm

Don't forget to readjust the offset when you change R405 or R409 because it will change the current in the differential stage as well.
Click to expand...
How do you adjust offset, the only VRs are for bias, and the SM doesn’t show a procedure for DC offset.
 
So I increased R409 to 51.7K ohms from 47K ohms and both channels are 27mV and change. Plus when I put a wave in and calculated power it was 20.8 volts rms and if I got my math right that's 54 wpc. (up from 35 previously. Listening test next and possibly a trying to find out why the SS meter doesn't go past 2 of 5.

DC offset is 0 on one side and .25mV on the other.
 
da_kube said:
just lurking on this one ahead of doing my HK. Can you just briefly explain how changing ohms at either of these adjust offset? Thanks!
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I didn't have to mess with them because my offset was nearly perfect after my other resistor replacements, but I guessing an increase in resistance would keep unwanted DC away from the speakers. Will wait to see if @ecluser checks in.
 
ecluser said:
Do you mean that the bias changes by itself, unstable like, or you can adjust around 27mV?
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It's stable, but takes a little time to rise and stop. I waited until the left side peaked at about 27.3mV and then brought the right side down to very close to the the same mV. So it's good.

Now I just have the get the SS meter to push past 2. Pretty sure the signal is stronger than the meter is indicating. Tried the VR but that just got me from 1 to 2.
 
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