Need some help troubleshooting a CR-2040

adpopulum said:

It seems that if I power off the unit for 20-30 mins (maybe less) when I power it back on it starts initially at or near 60v and then after a min (or less) it creeps back up to ~74v.

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Check for AC voltage riding on the 60 vdc rail. Sounds more like cap heating up in the P/S . I would replace all the small electrolytic capacitors on the power supply board. It might cost you 5-6 dollars and probably long over due.

I did check for AC on the 60v rails. I had ~1.09 VAC on the -60 and .02 VAC on the +60. I will try the recap suggestion and see where that takes me.

I did check for AC on the 60v rails. I had ~1.09 VAC on the -60

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There is your Hum..

I am currently putting together an order for all the small elecrolytics but isn't that I have AC on the ± 60 rail an indication that the large filter caps are also suspect?

Avionic, I see from another thread that you have recommended these as a good replacement for C905 & 906

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=P13288-ND

but they are now non-stocked. These look like a suitable replacement.

http://www.digikey.com/product-detail/en/382LX223M100B092VS/338-1990-ND/2256598

adpopulum said:

I am currently putting together an order for all the small elecrolytics but isn't that I have AC on the ± 60 rail an indication that the large filter caps are also suspect?

Avionic, I see from another thread that you have recommended these as a good replacement for C905 & 906

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=P13288-ND

but they are now non-stocked. These look like a suitable replacement.

http://www.digikey.com/product-detail/en/382LX223M100B092VS/338-1990-ND/2256598

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Yes . Those will be fine. They will take a little fabrication due to being snap in versus the lug originals lead spacing issues.

]I am currently putting together an order for all the small elecrolytics but isn't that I have AC on the ± 60 rail an indication that the large filter caps are also suspect?

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No . The big filters are for the +/- 56v unregulated supply.

The regulated +/- 60 is used in the front end of the amplifiers. The +/- 56 is applied directly to the collectors of the final outputs and driver transistors. Any noise induced in the front end will be amplified by the finals.

Not sure who Avionic is, but any person that can diagnose electronics without an eyeball on it is damn good..........Have read many of your posts and you always amaze me. Just saying............

avionic said:

Yes . Those will be fine. They will take a little fabrication due to being snap in versus the lug originals lead spacing issues.

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OK, great.

avionic said:

No . The big filters are for the +/- 56v unregulated supply.

The regulated +/- 60 is used in the front end of the amplifiers. The +/- 56 is applied directly to the collectors of the final outputs and driver transistors. Any noise induced in the front end will be amplified by the finals.

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Good to know. I have got to get better at studying those damn schematics :stupid:. Anyhow, will report back once I have the caps done.

Cheers Avionic, you've been a big help this far!

Well, a long time has passed since my last post... sufficed to say sometimes life gets in the way of the important stuff! Anyhow, I am hoping to pick this up where I left off. To bring folks up to speed, I have a 2040 with an issue with the regulated power supply. My 60vdc test points were reading over 70vdc. I have replaced H904 & 905, a bunch of zeners and then I put the receiver away.

On Avionic's advice I finally ordered the PS caps (all but the big cans) and they arrived yesterday. Got the unit out, was stored in the basement which gets cold in the winter and humid in the summer. Tried to fire her up (slowish on a variac) and no sound. The relay appeared to not be engaging (before I put it away it was). I went ahead and recapped the board. Still no sound but at this point I was more interested where my 60v supply was at. Initially as it warmed up it was slightly over 60v (~63v). I thought that was good news. I am finding however that it is all over the place - it can be up to 78v and down to 59v. Adjusting the pot (901) moves it but won't stabilize it.

Checked my voltages for TR905, 905 (and 906 because it is easy to reach) and I am getting very strange readings. With the ground lead of my DMM on the chassis I get 32vdc on E, 80 on C and 32 on B, for TR 904 similar negative voltages, on TR905. On TR906 I get 10, 27 & 10vdc respectively.

Also when checking 60v at the amp boards with ground to chassis I get roughy half whatever the 60v supply happens to be at at the time of measuring (similar to the emitter and base reading on tr904 & 5). Measuring across +60 and -60v on the board gives me the full reading.

Whats more DC offset at RO & E, and LO & E is at 27vdc (not mv but v!) Idle voltage is fine at -10mv.

I have rechecked my cap job and don't see any soldering issues, bridges, etc and the caps are in the correct polarity.

I am thinking I will need to get the PS out of the receiver and start pulling checking every diode, resistor and transistor one by one and I am not looking forward to this. Before I do that any other ideas?

Is dc offset 27V on both channels or one?

Both

I'd start by working from the outputs back- test transistors.

adpopulum said:

Both

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Both channels. Only thing common to both channels is the power supplys.

Measuring across +60 and -60v on the board gives me the full reading.

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What is the full reading ? 120vdc ? or 60vdc ?
Measuring from +60 to -60 should yield 120 vdc.

avionic said:

What is the full reading ? 120vdc ? or 60vdc ?
Measuring from +60 to -60 should yield 120 vdc.

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Sorry, should be more clear. The full reading would be 60vdc. Measuring from +60 to - 60 gives me something between 78v and 59v.

adpopulum said:

Sorry, should be more clear. The full reading would be 60vdc. Measuring from +60 to - 60 gives me something between 78v and 59v.

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That isn't right.Sounds like you have a dead or low rail. Post a photo of the PS include the 2 large caps as well.

I have attached a few pics or the PS.

Just wanted to also clarify, when I initially measured and reported readings at the +60vdc test point

adpopulum said:

Initially as it warmed up it was slightly over 60v (~63v). I thought that was good news. I am finding however that it is all over the place - it can be up to 78v and down to 59v. Adjusting the pot (901) moves it but won't stabilize it.

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I was checking across the +60/-60 tabs in the center of power supply board. If I measure +60vdc with the neg lead to ground I get ~ 30vdc

When making voltage checks for the +/- 60 vdc regulated PS. For your negative meter test lead use a testing point (E) somewhere on the circuit board rather than chassis. Looking for a stable + and - 58-62vdc.

Makes no difference, testing +60 (neg to E on PS and Amp board) yields 30vdc - 35vdc at the amp boards and the PS. I think I will need to remove the PS and start pulling and testing all the components. Ugh!

PS is fixed!

Thought I'd take one last close look at the PS board today and check my work before seriously contemplating major surgery only to discover that I had forgotten to replace a pair of 63v 4.7uf caps (c913 & 14). I though that this would be too good to be true but I popped the new ones in, connected my 8ohm dummy load, turned it on and heard the magical click of the relay. Measured my +60 and found it stable at 50vdc. I quick adjustment of vr901 brought her up to a stable 60vdc

Next up will be a complete recap.

Thanks again to all of you who offered advice on this (especially Avionic!)