LDR 144-070 for MC2200/2205

[Matt's Dad]

Great Picture! Thanks!, did you do a recapping job on the electrolytics?, they sort of look new.

The actual component layout is different on the board (your picture) than what the schematic depicts. Specifically the C503, C504 & C505 are shown on the photograph but on the schematic it's different. The schematic shows C503 & C504 & R555 with the leg of C503 tied to R555.

My 2255 (SN# C2592) has one of the 045864 boards like the picture you sent (C503, 504 & 505 all on the card and no R555) and the other board is like the schematic (my picture attached). I dont have a repair history (if there was any) and don't know how one board is correct and the other one isn't).

Does anyone know what the modification actually remediated or improved?

I also noted on your picture that the LDRs were different. The LDR (LDR301) for the Power Guard is smaller than the mutting LDR (LDR 501). On my unit both the PG and Muting LDRs are the same (identical part #).

 

[jeffs79]

Great Picture! Thanks!, did you do a recapping job on the electrolytics?, they sort of look new.

The actual component layout is different on the board (your picture) than what the schematic depicts. Specifically the C503, C504 & C505 are shown on the photograph but on the schematic it's different. The schematic shows C503 & C504 & R555 with the leg of C503 tied to R555.

My 2255 (SN# C2592) has one of the 045864 boards like the picture you sent (C503, 504 & 505 all on the card and no R555) and the other board is like the schematic (my picture attached). I dont have a repair history (if there was any) and don't know how one board is correct and the other one isn't).

Does anyone know what the modification actually remediated or improved?

I also noted on your picture that the LDRs were different. The LDR (LDR301) for the Power Guard is smaller than the mutting LDR (LDR 501). On my unit both the PG and Muting LDRs are the same (identical part #).

The picture came from AMP 8 repair studio, so I cannot confirm that it is stock - an LDR may have been replaced at some time. The electrolytic caps appear to be stock.

According to the service manual, use of C505 stopped at serial number CT2400, however it is pictured on the V1 driver boards. The service manual indicates that V1 was used until S/N CT2449.

I could speculate as to why your driver boards are different, and to why the 045864 board that I linked to is different than what the service manual but it would just be speculation.

V1 driver boards originally didn't have Q525 for the Current Source SW. These parts were tacked onto the board. The V2 board was changed to accommodate the components for this. This is also when we see the removal of C505, the addition of R555 and the change to 2 different LDR's . One end of C505 goes to one of the differential amplifier transistors and IC501 (power guard comp. IC) . The other end I believe goes to signal ground.

To the best of my knowledge it was not a common practice to update an earlier unit with the latest revision driver board. V1 driver boards that I've come across tend to have discoloration due to heat but without knowing the units total history - operating condition, ect - it's hard to know if this is circuit related or not.

FWIW, I've bench tested a 2255 with V1 driver cards and V3 with no noticeable difference in specs. This leads me to believe that the circuit changes were done to better efficiency / reliability.

 

[c_dk]

Are we trying to solve a perceived problem or trying to discover what the problem was that engineering needed to add the - current mirror switch back 35 years ago. Have a feeling I am witnessing the stalking of windmills.....

If you examine the specs of the original CLM6500 (muting) CLM6000 (PG) the biggest difference is the "dark" resistance and a slightly different ramp up if memory serves me.

 

[Matt's Dad]

No Senior Quxiote, not looking for windmills here....

I just noticed that the two 045864's are different in this particular unit and that the LDRs are the same for muting and PG circuits.

These discrepancies shouldn't be there. I just want to have some preventive maintenance performed so these will live for a few more decades. In operation the amp is dead silent.

 

[handyguy]

An old thread, but wth... Wondering what the symptoms would be of failing LDR's?

 

[handyguy]

An old thread, but wth... Wondering what the symptoms would be of failing LDR's, specifically the muting LDR.

 

[Bob the Tech]

An old thread, but wth... Wondering what the symptoms would be of failing LDR's, specifically the muting LDR.

Hi good morning,

I recently serviced a C29 preamp with the LDR muting circuit in series with the output signal path. It was the original incandescent LDR and the lamp had opened up. No light = high resistance and the symptom of the bad LDR was simply no output.

I was able to replace with a modern LED LDR and a dropping resistor w/o modifying the driver circuit, simplifying things a bit.

Good luck,

Bob the Tech

 

[handyguy]

thanks for the update.Wondering if a failing muting ldr might be the cause of the turn on pop on my mc2255? If it's failing to completely mute before the light switches on? I guess I can test that, in circuit?

 

[Bob the Tech]

thanks for the update.Wondering if a failing muting ldr might be the cause of the turn on pop on my mc2255? If it's failing to completely mute before the light switches on? I guess I can test that, in circuit?

The MC2255 mute uses an LDR in series followed by a FET in shunt. When it's trying to mute it's a 0V signal which should turn the FET on and the LDR off, muting the channel. Even if the LDR was passing a signal the FET should shunt it to ground. The -22V signal for "unmute" should light the LED in the LDR while the FET gets turned off, so it's going to pass a signal. I'm sort of thinking that if the LDR is "bad" it'd probably mean high resistance, not low, so it'd probably fail "quietly" and not thump. Also, even if the LDR is bad and somehow passing the signal, the FET should still mute the channel.

I wonder if you might have something broken in the mute drive circuit. The same drive circuit affects both channel's mutes. Are you hearing a thump in one channel (in which case could be bad LDR or FET in that channel)? Or do you hear it in both channels (maybe something wrong with the common mute drive signal).

Of course, a thump on power up could be a lot of other things too ...

Bob the Tech

 

[handyguy]

I was trying to further diagnose the thump, as it does not happen on every turn on. It's fairly minor, occurs with the speaker switch off or on, and occurs immediately upon turn on (as the self check counts down). Mostly seems to be the right channel. The bias and dc offset have been checked and set recently.

 

[handyguy]

The next time I have my 2255 on the bench, I'll test the circuits and report back.
It's so heavy, I don't like to move (and remove the glass) more than necessary.
Thanks!