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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Fri Sep 07, 2012 6:39 am
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Rock Star
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BMW2002Ti wrote:
Blue, think you have some kinda parasitic oscillation going on in the power section?

If you, try ferret beads around some suspect wires and see what happens.


I'm sorry, but I fail to understand why you think that. The only wires in the power supply are the wires from the transformer to the PCB. Everything else is on the PCB. How/why would a parasitic oscillation cause the power supply voltages to drop as the load on the power supply increases? Exactly what problem do you think a parasitic oscillation is causing and why? What wires should I suspect? What measurements (using a scope or DVM) can I make to verify a parasitic oscillation? This is a stock, unmodified Fender Blues Deluxe Reissue, not some custom, hand wired amp.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Fri Sep 07, 2012 8:35 am
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Blue, Ok you got me. You can turn me over, this side is done.


Good luck with your issues.


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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Fri Sep 07, 2012 8:58 am
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Rock Star
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I'm an engineer. I don't accept things on faith. I ask questions and look for the answers.

Thanks.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Sun Sep 09, 2012 6:16 pm
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From Bob Latino's Dynaco site, May help...

"1.Low B+ (HT) voltage from rectifier diodes or tube cathode. (Turn amplifier off before performing tests.)

This indicates either a restriction in the power supply (open or high resistance windings in transformer or choke, bad rectifier tube) or excessive current draw (caused by leaky filter capacitors, or shorts in tubes or output transformers) pulling down the voltage.

A. Test or substitute rectifier tube or check continuity of diodes. Replace if necessary.

B. Check continuity between the rectifier plate leads (pins 4 and 6) and between these leads and ground.If open at any point, check for broken connections in the leads. If high resistance found on one sideof the winding, check for cold solder joints, inside the transformer as well if necessary. If high resistance condition exists after repairs, probable defective power transformer.

C. Discharge all filter capacitors, by turning off amplifier, allowing the several minutes for the capacitors to discharge. Check voltage on each capacitor lug terminal, if a sufficiently low value (not over 100V) short the terminals to ground (you may get a little spark), then test with ohmmeter, observing polarity. Meter should first indicate close to dead short then rise slowly to a high value. If one or more sections will not rise or is much lower than the others indicates defective capacitor.

Unfortunately some capacitors will pass a low voltage ohmmeter test but leak enough to cause trouble at high voltages. Use of a high voltage capacitor tester would be preferable, or a milliammeter in series with a high voltage power supply. Any current over a few ma after charging up would be suspect. Filter capacitors becoming excessively warm or hot during operation indicates a leaky condition also.

D. Pull output tubes, and disconnect the leads running from the filter capacitors to the output transformers. Check with ohmmeter resistance between transformer leads and ground (chassis). Resistance should be very high. If not indicates possible defective transformer. If suspected, try reconnecting the good transformer, installing the output tubes on that side, then operating the amplifier.

If condition does not recur, but recurs when other transformer is hooked up (try without output tubes, so a short tube will not give you a false indication), then that transformer is the culprit . "


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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Sun Sep 09, 2012 6:37 pm
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Rock Star
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My response posted within your post in RED

BMW2002Ti wrote:
From Bob Latino's Dynaco site, May help...

"1.Low B+ (HT) voltage from rectifier diodes or tube cathode. (Turn amplifier off before performing tests.)

This indicates either a restriction in the power supply (open or high resistance windings in transformer or choke, bad rectifier tube) or excessive current draw (caused by leaky filter capacitors, or shorts in tubes or output transformers) pulling down the voltage.

A. Test or substitute rectifier tube or check continuity of diodes. Replace if necessary. All diodes test OK as documented in my thread.

B. Check continuity between the rectifier plate leads (pins 4 and 6) and between these leads and ground.If open at any point, check for broken connections in the leads. If high resistance found on one sideof the winding, check for cold solder joints, inside the transformer as well if necessary. If high resistance condition exists after repairs, probable defective power transformer. N/A. Solid state rectifier.

C. Discharge all filter capacitors, by turning off amplifier, allowing the several minutes for the capacitors to discharge. Check voltage on each capacitor lug terminal, if a sufficiently low value (not over 100V) short the terminals to ground (you may get a little spark), then test with ohmmeter, observing polarity. Meter should first indicate close to dead short then rise slowly to a high value. If one or more sections will not rise or is much lower than the others indicates defective capacitor. All capacitors test good as documented in my thread.

Unfortunately some capacitors will pass a low voltage ohmmeter test but leak enough to cause trouble at high voltages. Use of a high voltage capacitor tester would be preferable, or a milliammeter in series with a high voltage power supply. Any current over a few ma after charging up would be suspect. Filter capacitors becoming excessively warm or hot during operation indicates a leaky condition also. Capacitors do not get hot under operating conditions.

D. Pull output tubes, and disconnect the leads running from the filter capacitors to the output transformers. Check with ohmmeter resistance between transformer leads and ground (chassis). Resistance should be very high. If not indicates possible defective transformer. If suspected, try reconnecting the good transformer, installing the output tubes on that side, then operating the amplifier. Based on the minor difference between B+ and power tube plate voltage, I do not believe there is a problem with the OT.

If condition does not recur, but recurs when other transformer is hooked up (try without output tubes, so a short tube will not give you a false indication), then that transformer is the culprit . As documented in my thread, the amp sounds great. It just will not bias as hot as I would prefer.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Sun Sep 09, 2012 9:11 pm
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Prolly asked already. Wrong PT (or mislabeled one) sent to you?


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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Sun Sep 09, 2012 10:49 pm
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BMW2002Ti wrote:
Prolly asked already. Wrong PT (or mislabeled one) sent to you?


I have not checked to see. I'd have to pull the entire chassis and I'm not inclined to do that right now. I do know that the output of my PT is 8 volts AC lower than the one in shimmilou's Hot Rod Deluxe (they use the same PT) without a load and drops 10 more volts (just like shimmilou's does) under full load. So it is still only about 8 volts lower than his.

Question: Shimmilou claims to be able to measure the power supply caps while in circuit. I cannot. I have to remove them from the circuit. Our power supplies are identical except for the bias supplies. Should I be able to read them in circuit?

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Tue Sep 11, 2012 2:51 am
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Blue, the only way that I can accurately measure capacitance and resistance of these components is to lift one leg of the component I'm measuring. Isn't there other circuit interaction and time-constant (esp in pi-filter circuits) issues that would change the nominal value?

I'd imagine that PT should have values around the 290UV or VX...

http://www.hammondmfg.com/pdf/EDB290UX.pdf

http://www.hammondmfg.com/pdf/EDB290VX.pdf

Good luck! These things are so vexxing. But, once you can get the thing working, next best thing to Hoochie-Coochie. A good incentive, no?


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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Tue Sep 11, 2012 8:47 am
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Rock Star
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Location: Natural Bridge, Virginia
BMW2002Ti wrote:
Blue, the only way that I can accurately measure capacitance and resistance of these components is to lift one leg of the component I'm measuring. Isn't there other circuit interaction and time-constant (esp in pi-filter circuits) issues that would change the nominal value?

I'd imagine that PT should have values around the 290UV or VX...

http://www.hammondmfg.com/pdf/EDB290UX.pdf

http://www.hammondmfg.com/pdf/EDB290VX.pdf

Good luck! These things are so vexxing. But, once you can get the thing working, next best thing to Hoochie-Coochie. A good incentive, no?


Regarding measureing in circuit vs out of circuit, yes, I too feel that you need to break the circuit for accurate measurement. To measure the 47 mfd cap for the B+, I disconnected the choke from one side, leaving only the rectifier on the other side. The cap read 49 mfd. Close enough. The other three caps are effectively in parallel with a 4.7K or 10K ohm resistor between cap pairs. I lifted the positive lead to get an accurate reading.

I have a couple more ideas to try. If they don't work, I'm done. The amp works fine and sounds great, so I don't HAVE to fix anything. I just want to find out why it is behaving the way it is.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Tue Sep 11, 2012 4:44 pm
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BMW,

I got it working. Check out the last couple of posts:

viewtopic.php?f=13&t=74557&p=887035#p887035

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Wed Sep 12, 2012 10:26 pm
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Bluesky, VERY COOL! I know how vexing and frustrating working on amps can be. It's not a very scientific explanation, but these things can take on a life. And become as difficult as understanding women.

Good you got it going... Persistence pays.


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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 13, 2012 6:50 am
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Rock Star
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Thanks. The tone has noticably improved. :D

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