I just watched a video of a C64 which had been left in the weather for a dozen years with pine needles INSIDE it and rat crap.. turned on and worked with minimal work.. A marantz should handle some dust and rust in stride
Thanks for the vote of confidence Mike. I need all I can get. Do you have any advice on post repair start-up procedures? Anything I should try before firing her up?I just watched a video of a C64 which had been left in the weather for a dozen years with pine needles INSIDE it and rat crap.. turned on and worked with minimal work.. A marantz should handle some dust and rust in stride
Thanks for the vote of confidence Mike. I need all I can get. Do you have any advice on post repair start-up procedures? Anything I should try before firing her up?
Woah I'd replace those trimmers first, they're known to be corroded and blow up outputs. And the burnt to heck resistor next to the white 5W one in the top right. Maybe even caps while you're in there as they may have seen some wild current or voltage when things went down.Here's the repaired main amp board. Had a little trouble with pads lifting in previously repaired areas but otherwise no issues.
Im going to put it back together and see if it will fire up. Any tips? Should I try headphones first? Connect up to sacrificial speakers? Measure speaker output with a DMM?
Good catch. I actually ordered the trimpots and forgot to swap them out. Also good catch on that resistor. It measured within spec but upon visual comparison to other 1090's its definitely changed colors. Is there any way to test the unit out without blowing the output transformers (in addition to the dim bulb)? Perhaps remove them for testing? I'd rather not have to source those again if possible so I'd like to take all reasonable steps to protect them.Woah I'd replace those trimmers first, they're known to be corroded and blow up outputs. And the burnt to heck resistor next to the white 5W one in the top right. Maybe even caps while you're in there as they may have seen some wild current or voltage when things went down.
Yup you should be good running it without the output transistors installed, however, the DBT will protect them. Your call, I totally get being extra careful.Good catch. I actually ordered the trimpots and forgot to swap them out. Also good catch on that resistor. It measured within spec but upon visual comparison to other 1090's its definitely changed colors. Is there any way to test the unit out without blowing the output transformers (in addition to the dim bulb)? Perhaps remove them for testing? I'd rather not have to source those again if possible so I'd like to take all reasonable steps to protect them.
Headphones did sound OK.So with the outputs removed, your headphones sounded OK? Did you use mica to insulate the transistors from the heatsink? Was the bias adjustment turned down? If that was turned up, it would pop the outputs and cause a short.
I appreciate the offer Wolfman. I've downloaded a copy of the SM. The service manual has a section for "Adjustment of Idling Current" that goes like this:Would having the service manual help you out? If so, I have one that my tech used to fix mine 1090. It's to big for me to open post it here. You will need to send me your email and I can send it to you. Check my profile page.
Something is still amiss. The bias should be able to be full blast on DBT and nothing bad will happen like things smoking.I appreciate the offer Wolfman. I've downloaded a copy of the SM. The service manual has a section for "Adjustment of Idling Current" that goes like this:
Connect a vacuum voltmeter to between emitters Q721 and Q723. Adjust R771 until 12mA is reached. Likewise, adjust Q724, Q722 and R772
This is what I was attempting to do when the unit smoked.
Unfortunately, the problem lies with me. I'm really too green and I've had to make some assumptions that seem to be incorrect.
Something is still amiss. The bias should be able to be full blast on DBT and nothing bad will happen like things smoking.
How do I know? I've done it.
Bias - constant current running through the output transistors, measured usually at the emitter resistor.
DC offset - measured at the speaker terminals, adjustable, or when not it is controlled by how well the input pair is matched in gain.
For sure. You don't even need to pull them, a short will appear in circuit.I agree, something is definitely amiss. I've ordered new output transistors and they should be here next week. In the meantime I'm going to start replacing the caps in the power amp board just to be safe.
Should I pull the smaller transistors from circuit and test those as well?
I've ordered new output transistors and they should be here next week. In the meantime I'm going to start replacing the caps in the power amp board just to be safe.
I believe these are original replacements. I bought these specifically:Did you order the original style "Bat Wing" output transistors? - Chris