Well shoot, I finally did it.

ringading

Well-Known Member
I know we all like to provide our success stories here to help others working on the same equipment. But here is one of my @#$%^%$^ mistakes.

Got a 2220B off CL with a cut power cord. New cord was installed and turns out it worked.

So I was recapping the main amplifier board and well, put a cap in backwards :eek:. And of course I wasn't using my DBT as I recently moved my work area and haven't hooked up everything yet. The cap I had backwards was C719 220uf/50v. It fried the R765 22ohms/1/4 watt resistor but nothing else that I can spot visually.

Replacing the resistor is easy but I am unsure of what else to check - maybe R757 and H723? I haven't had this happen before so am unsure of how pervasive the damage may be. It was on for only a couple of seconds but that probably doesn't matter as the damage can be done in fractions of a second.

When resistors go poof like this one did, I imagine it shorts the circuit but there could have been damage downstream in the brief time it took to surrender and put up its smoke signals.

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Usually when a resistor goes like that, it means it went to protect a transistor, or that the transistor also got fried.

To be safe, I woud also pull & test H723 ad output H002.
 
Thanks slimecity. That is what I was thinking. Usually they protect downstream components but enojugh voltage may have passed just long enough to do some damage.

I will check those two.
 
Well I powered her up and no white smoke or other alarming activity. However, now the relay wont click. Any ideas on what to check? I suck at reading schematics so I am not sure what in the path would be appropriate.
 
Bump anyone? I checked the DC offset via the SM but I think these points are after the relay so I get no reading and it doesn't move when adjusting the trimmer. Can I measure DC offset prior to the relay? I believe it should be across one of the emitter resistors but I am not sure which one to use.
 
If one of those is bad would it cause the relay not to engage? I ask because I have had units with bad transistors but they did not cause the relay not to engage.
 
Tested the outputs and all are okay. I went up the chain to check R757 which is in the path after the smoked resistor R765. Says it should be 10ohm and it reads 33 ohms. I checked H723 which is next and that one checks out okay. Not sure if I have a 10ohm resistor handy - it calls for 1/4 watt but would 1/2 watt be okay? I perused the threads here but there doesn't seem to be any single opinion.

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1/2 will work, but I would not trust the schematic. I would check r757 value of your working amp . Check the base voltage of h710 and h712 (feedback loop)
If you pull the outputs out, it would be safe to check ebc voltages of all transistors using mini clips.
work from the h701 and go down the line. when you find an e/b or c out of whack, trace it to its source, and you'll find your problem. or at least one of them.....
 
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Thanks Kutz. The resistor is 10ohm per the color code. I have some on order so will wait to get the new one in and then check the transistors if that doesn't solve the problem.
 
Still waiting on my resistors - seems all of the Ratshack stores around here are all out of business now so I can't go see if they have one.

I did replace the transistor that was measuring high and tested 701 thorough 704. If I am reading it right they should measure -28v off the collector They all measure 12v.

Am I doing that correctly?

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First of all, make sure your volt meter is accurate. dead batteries give odd readings. I compared about 6 dvm's I owned a few years ago, and threw 3 of them in the garbage can because they were so far off.

Is your base @ .6vdc???? The .06vdc base voltage shown on the schematic just does not sound right to me, but then i'm just a stupid mechanic
Accdng to the pic, yes it should be -28v, but I would trust the good(working) amp values more than the schematic, just as a rule.
Hey look , I have hit 3000 posts filled with lies and bad information:banana::banana::banana::)
 
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That base voltage is about right. At least it is if the common emitters are near 0.6-0.7Vdc as shown on the schem.

The collector voltage may be way off if R765 is still the old part or has been removed while you're waiting for your new one.
 
Thanks all. Waiting for some resistors to come in. Was going to go to Ratshack to see if they had some but they have closed all of the stores near me and now the closest one is now 20 miles away in Seattle traffic (ugh).

The VM should be okay - it has a pretty new battery in it and has been performing all other functions well.

In the meantime to pass the time I think I will start pulling transistors to test them out to make sure they weren't damaged.
 
Tested all transistors and they all look okay - all read around 600 in the two proper connections. I think I will start checking some of the resistors along the path from the cap that was backwards,

Replaced the 10 ohm resistor and still wont come out of protection (no relay click).
 
My apologies if this seems too obvious, but have you checked the coils on the relay itself? Anyone here know what the expected resistance for a coil like that would be?
 
My apologies if this seems too obvious, but have you checked the coils on the relay itself? Anyone here know what the expected resistance for a coil like that would be?

I did think about that - perhaps the bad capacitor damaged the relay itself. Reading posts it seems like there is no drop in replacement for this one unfortunately.

I am going to pull some of the other resistors in the path of the capacitor that was installed wrong to see if any of those are out of spec and will probably just replace them since I have to pull them out (or at least partially).
 
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