• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

DC spike 9090

S

SF_art

AK Subscriber
Subscriber
Hello,

my 9090 when first turned on appears ok. But after I turn it off and back on again I noticed a DC voltage spike on both channels. DC offset is set at 0.3 mv on both channels. The DC spikes measures 145 mv left and 128 mv right channels. All caps have been replaced. All fusible resistors changed. The main caps are original. The relay is original. All output transistors are all original. Diff transistors and driver transistors replaced on F2436 board. I do not own a scope. My question is what is the cause of this voltage spike when the receiver is turned on? Is it something easy for me to fix. I hope someone who knows can chime in.

Regards, Art
 
Register to hide this ad
A couple of questions

1). Why are you turning it off and on again, after first turning it on?
2). the DC offset figure of 0.3mV is extremely low - when are you seeing this reading?

3). What is the effect of the 'spikes' you report, and what DC offset do you measure once the offset has stabilised again?
 
SF_art said:
Hello,

my 9090 when first turned on appears ok. But after I turn it off and back on again I noticed a DC voltage spike on both channels. DC offset is set at 0.3 mv on both channels. The DC spikes measures 145 mv left and 128 mv right channels. All caps have been replaced. All fusible resistors changed. The main caps are original. The relay is original. All output transistors are all original. Diff transistors and driver transistors replaced on F2436 board. I do not own a scope. My question is what is the cause of this voltage spike when the receiver is turned on? Is it something easy for me to fix. I hope someone who knows can chime in.

Regards, Art
Click to expand...

with out a scope it is impossible to see an actual spike !!!!!!.

if you are measuring it by a DMM... it is fooling you...

to get this out of your head, go get your hand on a good scope and re check it....
 
Thanks John, When I turn first turn on my 9090, the power meters both are steady. After turning the unit off and turning it on again the power meters show a voltage spike. I measure the DC offset at both speaker terminals. I have DC offset set at 0.3 mv. When I have my fluke connected to the speaker terminals after the unit is turned on again a DC voltage spike shows up but the settings go back to normal. I also see both of my speakers react to this voltage spike. If I turn the volume down i know it is one way of preventing damage to my speakers. What can be causing this DC voltage spike only on turn on? I can't afford to get a scope. Is there a layman's way to check what my receiver is doing. I got this 9090 barely working and was able to restore it with help from reading similar problems on the forum. I am not an electronic tech, but I have some basic electronic training back in high school.

Regards, Arthur
 
SF_art said:
Thanks John, When I turn first turn on my 9090, the power meters both are steady. After turning the unit off and turning it on again the power meters show a voltage spike. I measure the DC offset at both speaker terminals. I have DC offset set at 0.3 mv. When I have my fluke connected to the speaker terminals after the unit is turned on again a DC voltage spike shows up but the settings go back to normal. I also see both of my speakers react to this voltage spike. If I turn the volume down i know it is one way of preventing damage to my speakers. What can be causing this DC voltage spike only on turn on? I can't afford to get a scope. Is there a layman's way to check what my receiver is doing. I got this 9090 barely working and was able to restore it with help from reading similar problems on the forum. I am not an electronic tech, but I have some basic electronic training back in high school.

Regards, Arthur
Click to expand...

How long after turning on the first time, and turning off, do you wait before turning it on again?

I am still unclear about why you would want to turn it 'on' then 'off' then 'on' again?
 
Last edited:
I wait about 1 minute after turning the receiver on for the first time. My point is it should not matter whether I turn it on first then decide to turn it off and on again. The 9090 should not produce any kind of DC voltage spike if the DC offset voltage is set correctly. I have a 8080db that I compared it to and it does not have this problem of a DC spike. Can this be caused by transistors that maybe different in hfe values? I tried matching transistors on ones that I replaced on board F2436. As I mentioned the 9090 was in bad shape after I got it. It was on protection, no sound.

Diff transistors and drivers were replaced on F2436. The voltage spike only occurs during turn on. What should I check next. I tried to remove the jumpers and connect input to amp, problem stays.

Art
 
SF_art said:
The 9090 should not produce any kind of DC voltage spike if the DC offset voltage is set correctly
Click to expand...

That is actually incorrect, you will have a brief period of this until caps charge up and voltages stabilize. The relay should not engage until the DC has stabilized after turning on....
There will be some kind of crude delay circuit with a cap that will have to charge up before the transistor which switches the relay, turns on......
I would say something in the protection circuit is not working properly....

You do realize that before such things as protection circuits and relays and such were incorporated into amp design, that when you turned an amp on, you would always get the soft "pulse" of the amp turning on and voltages stabilizing.....I have a couple of old amps like that, it does not harm the speakers, because the DC is momentary....
 
Thanks, Kevzep. What component in the protection circuit do you think can be causing this problem. I only changed all caps on that board. The relay is original. You said a cap will have to charge up. I noticed this on my 8080db. The protection circuit does blink a little longer than my 9090 before I hear the relay engage. The 9090 protection switches faster therefore like you said It causes a DC pulse that I can see on the power meters. What can I check on the protection circuit of the 9090 to pin point the problem?
 
SF_art said:
Thanks, Kevzep. What component in the protection circuit do you think can be causing this problem. I only changed all caps on that board. The relay is original. You said a cap will have to charge up. I noticed this on my 8080db. The protection circuit does blink a little longer than my 9090 before I hear the relay engage. The 9090 protection switches faster therefore like you said It causes a DC pulse that I can see on the power meters. What can I check on the protection circuit of the 9090 to pin point the problem?
Click to expand...

Well, if you have re-capped the protection board, it is possible that the transistor which drives the relay could be turning on early, they spend their whole life in a completely saturated state, so they can do things like get trigger happy when they get old...
I just had a look at the schematic, it looks like the 6.3V 470µf cap charges up and allows the transistor TR01 to turn on...
I would replace TR01, 02 and D02 as well. Looks like its a very basic circuit, theres not much to it really. So hopefully its as simple as a the transistor, other than that, the only other thing it could be is the relay...
Just also check the value of the 6.3V 470µf cap, very important that it is the right value in this case, voltage included.
 
Ok Kevzep, I am looking at the schematic and F2547 board. What is a good replacement for 2sc945 for TR01, 02? I checked the value of the cap and it is correct. I have also replaced diodes already for D01 - D03

2sc945: V cbo- 60V; V ceo - 50V; V ebo- 5V. f t 150Mhz; hfe 600 max
to-92 case ECB
 
Thanks, I checked the transistor specs for fairchild ksc945 at Mouser. Is there another transistor that have better hfe values close to the original that I'm replacing.
 
BY George I think we fix it. Thank You Kevzep and everyone who chimed in regarding my earlier post on my 9090 receiver. TR01 - TR04 have all been replace by KSC945 ordered at Mouser. Original transistors were 2SC945. Kevzep was right regarding condition of old transistors and operating conditions changing due to age. I'm learning besides caps that have to be replace, transistors may also need changing. My receiver sounds great. The noise floor is much quieter listening to cd's and phono. Again thanks to all who participated.

Arthur:banana:
 
You must log in or register to reply here.
Back
Top Bottom