Finally hoisted it on my bench today. Opened the cover and right away I see that CR40 - 16V/5W zener, has burned to a crisp. Nothing else seems to have seen excessive heat in the PCB around it. I powered it up briefly and it did pass audio when I moved the board slightly.
Replacement zener is on order. I’ll change CR41 at the same time.
Once installed, I’ll verify if the LM300 series regulator is working correctly.
Replacement zener is on order. I’ll change CR41 at the same time.
Once installed, I’ll verify if the LM300 series regulator is working correctly.
Thanks for your input Hans, I greatly appreciate it. The drawings are not really clear, is the U2 are you referring to the opamp that looks like it’s configured as an integrator? It’s tough to discern anything from the drawings and which model they are relevant to.
I‘m beginning to wonder if I should take it in for service where hopefully, they have the pertinent data specifically for the 434.
I‘m beginning to wonder if I should take it in for service where hopefully, they have the pertinent data specifically for the 434.
U2 is a servo, keeping the output voltage at 0 Volt DC.
And K1 & K2 are 12Volt relays.
When CR40 blows, the 16Volt it should produce comes close to 40Volt, enough to roast the Relays and possibly U2.
You could try to insert a replacement for CR40 and check whether POS_16V is around 17Volt on TP5.
When everything still works and the output voltage is ca. 0Volt in the Amplifier On mode, you are lucky.
If not, it may become more complex to repair.
Hans
And K1 & K2 are 12Volt relays.
When CR40 blows, the 16Volt it should produce comes close to 40Volt, enough to roast the Relays and possibly U2.
You could try to insert a replacement for CR40 and check whether POS_16V is around 17Volt on TP5.
When everything still works and the output voltage is ca. 0Volt in the Amplifier On mode, you are lucky.
If not, it may become more complex to repair.
Hans
The voltage regulators should produce resp. +/- 113 volt adjustable with R2&R3..
If not possible, they are fried and should also be replaced.
But the original symptom you described of not getting output when turned on, points in the direction of relays K1 and K2 that are switched off when going from standby to on. That's why I asked you to see whether the power consumption made a big step, from 100Watt to 180Watt to be precise.
So I'm afraid that when CR40 died, K1 and K2 were also fried and maybe also K3 just because CR40 no longer did its job.
These are relatively cheap relays available from Mouser, but I don't know your skills to replace them.
Measure coil resistance with a multimeter from point 1 to point 10 and you will have to see 1K for K1 and 500R for K2//K3.
Hans
If not possible, they are fried and should also be replaced.
But the original symptom you described of not getting output when turned on, points in the direction of relays K1 and K2 that are switched off when going from standby to on. That's why I asked you to see whether the power consumption made a big step, from 100Watt to 180Watt to be precise.
So I'm afraid that when CR40 died, K1 and K2 were also fried and maybe also K3 just because CR40 no longer did its job.
These are relatively cheap relays available from Mouser, but I don't know your skills to replace them.
Measure coil resistance with a multimeter from point 1 to point 10 and you will have to see 1K for K1 and 500R for K2//K3.
Hans
Hans, I got a replacement CR40 installed. Unfortunately there was current flow severe enough to melt the upper PCB foil trace that ultimately connects to K3. I have it soldered together with a small wire to the zener lead for now.
I do measure a correct 16 Volts across the zener now. The amp still powers up, but won’t pass audio.
I measured resistance across CR33 (K3) and 34 (K1) I get 500 or 1K ohms depending on the lead polarity.
However across CR29(K2) I’m only reading about 45 ohms regardless of polarity. I’m guessing that K2 and the diode probably ought to be replaced based on this.
By the way, I have 97 Volts at TP1, but bearing in mind the very low DCR measured at K2, this might be stressed, hence the lower volts. While I at it, probably best to replace U2 as you suggested.
Nice to know I can still get parts from Mouser.
I don’t have an easy way of measuring the idle versus on current, although I could insert a low value resistor in line with the input AC power and monitor the voltage drop, if you recommend.
I can’t see CR29 on any of what is posted. Any idea what the part number is? The others look like 1N4036, but my old eyes find it kind of blurry.
I do measure a correct 16 Volts across the zener now. The amp still powers up, but won’t pass audio.
I measured resistance across CR33 (K3) and 34 (K1) I get 500 or 1K ohms depending on the lead polarity.
However across CR29(K2) I’m only reading about 45 ohms regardless of polarity. I’m guessing that K2 and the diode probably ought to be replaced based on this.
By the way, I have 97 Volts at TP1, but bearing in mind the very low DCR measured at K2, this might be stressed, hence the lower volts. While I at it, probably best to replace U2 as you suggested.
Nice to know I can still get parts from Mouser.
I don’t have an easy way of measuring the idle versus on current, although I could insert a low value resistor in line with the input AC power and monitor the voltage drop, if you recommend.
I can’t see CR29 on any of what is posted. Any idea what the part number is? The others look like 1N4036, but my old eyes find it kind of blurry.
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O.K. I would advise you to replace all three relays, they were all overloaded and K2 was fried.
The diode should be tested but is probably still functional.
One word of caution, switch the amp only to Standby and not to On as long as the relays are still malfunctioning.
About the 97Volt on TP1, that's probably O.K. More important to know is what TP4 and TP7 measure.
The 113 Volt that I mentioned was for the 436, but your Amp has of course a lower voltage.
Hans
The diode should be tested but is probably still functional.
One word of caution, switch the amp only to Standby and not to On as long as the relays are still malfunctioning.
About the 97Volt on TP1, that's probably O.K. More important to know is what TP4 and TP7 measure.
The 113 Volt that I mentioned was for the 436, but your Amp has of course a lower voltage.
Hans