Go Back   Home > Forums > >

Solid State Talk all about solid state amplification.

Peavey PA-200 Powerup With Dim Bulb Tester and Variac
Peavey PA-200 Powerup With Dim Bulb Tester and Variac
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools
Old 22nd November 2020, 05:09 AM   #1
Cheeto333 is offline Cheeto333
diyAudio Member
 
Join Date: Apr 2015
Default Peavey PA-200 Powerup With Dim Bulb Tester and Variac

Hi All,

I am slowly powering up an old Peavey PA-200 in unknown working condition. The boards and connections look ok (no signs of arcing or other damage), so I am slowly bringing up the mains power with a dim bulb tester (DBT) and Variac. I am starting with a 40 watt bulb, and my chain is wall -> Variax -> DBT -> amp. I have a speaker hooked up to the output as well.

Everything seems to be fine until around ~40VAC on the input, at which point there is a "pop" in the speaker and it starts to hum. The bulb is not lit at this point.

Is this expected? Should I keep cranking up the input AC, or is this hum indicative of an issue (bad filter caps)?

Any help is greatly appreciated.
  Reply With Quote
Old 22nd November 2020, 05:53 AM   #2
thimios is offline thimios  Greece
diyAudio Member
 
Join Date: Aug 2009
Location: Agrinion
Peavey PA-200 Powerup With Dim Bulb Tester and Variac
Don't use a load when test using a bulb tester.
Remove speaker and use a voltmeter, d.c scale, at the output.
Repeat test looking at the voltmeter and the bulb.
If you don't see high d.c (higher than 100mV)not a bright bulb, not something smell... remove the bulb and test again.Finaly remove the variac and repeat.
Now it is time for speaker attachments.
If a second voltmeter is available you can measure idle current at the same time.
Just huke up to one of the power transistors emitter resistor and you will have the idle current under control simultaneously wirh the output d.c.
Anything about 20mV expected for idle current.
I hope this will be useful.
Good luck!
  Reply With Quote
Old 22nd November 2020, 01:30 PM   #3
wg_ski is online now wg_ski  United States
diyAudio Member
 
Join Date: Oct 2007
Many amplifiers will not start up properly through a DBT or variac with a speaker connected. There is often a range of supply voltage below which the amplifier’s offset voltage is too high for the feedback to correct it, or perhaps even become unstable. It may or may not “snap out” of it as the supply is slowly increased - if it has a load. With no load it should - if it’s working properly. Bring it up without the speaker, and once you’ve verified that there is no significant DC offset, connect the speakers. Then you can run it at low volume to verify normal operation. If you can’t bring it up properly without the speaker, address that problem first.
  Reply With Quote
Old 22nd November 2020, 09:05 PM   #4
Cheeto333 is offline Cheeto333
diyAudio Member
 
Join Date: Apr 2015
Thanks for the tips everyone. I removed the speaker and added another meter to look for VDC across the output (white and green leads in attached picture, red and black are measuring VAC input from Variac). I measure >100mV DC pretty much instantly after turning up the Variac past a few VAC. Does this indicate an issue? If so, where should I start troubleshooting next?

See link here for schematic and here for manual. Thanks again!
Attached Images
File Type: jpg IMG_1352.jpg (564.4 KB, 182 views)
  Reply With Quote
Old 22nd November 2020, 09:27 PM   #5
indianajo is offline indianajo  United States
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
200 mv dc on speaker is kind of a a limit.
If it doesn't go over that as variac comes near line voltage, I'd plug a music source like a FM radio earphone jack in the input and start looking for AC volts on the output. That is if the light bulb in series with the AC line doesn't light up.
Keep the radio turned down, PA level is usually 1.6 or 2 vac input and a radio earphone will go to 7 maxed out. Don't use a cell phone, unless you can afford to throw it away if DC comes out the input of a dodgy amp and blows it. Used battery FM radios are about $4 at resale shops. Usually with a bad battery clip and maybe a dodgy volume pot. New 10k trimpots are $.50.
DVM produce random numbers on AC scale on music, except for the $160 RMS variety. Those RMS meters will totally miss an ultrasonic oscillation, so I never bought one. I use an analog voltmeter for music tracing, 2 vac scale in the front and 20 vac scale vas or after and nearer the speaker. Put a .047 uf cap series the negative probe with a clip lead to speaker ground. Analog AC voltmeter will read DC voltage on AC scales without the cap. Analog voltmeter can be bought for about $25-30. 20 kohm/volt is about standard, 5k for AC scales. Less sensitive VOM are only useful after the VAS, not near the input tone/control sections.
Otherwise, for music tracing use a scope, and the $50 probe which has to be replaced if you step on it. I fix my equipment without a scope, except for low level hum which an AC vom won't see.
High steady AC voltage on AC voltmeter that doesn't pulse with the beat of rock music, is probably ultrasonic oscillation. That can be checked with a 390 pf cap instead of the .047 uf one. Ultrasonic will blow right through a xxx pf cap, music will be blocked.
__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey PV-1.3K, MMA-81502, Herald RA88a mixer, Steinway console, Wurlitzer 4500, 4300

Last edited by indianajo; 22nd November 2020 at 09:36 PM.
  Reply With Quote
Old 22nd November 2020, 09:34 PM   #6
Cheeto333 is offline Cheeto333
diyAudio Member
 
Join Date: Apr 2015
I was getting over 1 VDC across the output around 40VAC on the input, so I'm not sure I can trace an input signal yet... What does a large DC voltage across the output normally mean? Bad output transistor?
  Reply With Quote
Old 22nd November 2020, 10:08 PM   #7
wg_ski is online now wg_ski  United States
diyAudio Member
 
Join Date: Oct 2007
A 1 volt offset can be caused by a number of things. What does it do when you turn up the AC to normal or near-normal (still using the bulb limiter)? The amp may behave poorly at only 40V.
  Reply With Quote
Old 22nd November 2020, 11:22 PM   #8
indianajo is offline indianajo  United States
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
DC on out can be bad transistor, but 1 v indicates something marginal, not a huge meltdown.
Your schematic is missing the output stage page.
I'd look at the line out jack which is at the sum of the inputs, with the output putting out 1 v dc. If there is substantial DC on line out, one of the many electrolytic coupler caps from input stages may be leaking.
At this age, all rubber sealed wet caps (electrolytic) are suspect. Could also be a bad solder joint in the output stage, or a resistor burnt up or something. If there is a speaker coupler cap, a 2200 or 3300 uf or something, it could be leaking DC current.
__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey PV-1.3K, MMA-81502, Herald RA88a mixer, Steinway console, Wurlitzer 4500, 4300
  Reply With Quote
Old 23rd November 2020, 02:38 AM   #9
Cheeto333 is offline Cheeto333
diyAudio Member
 
Join Date: Apr 2015
I was able to bring the amp up to 107 VAC on the input, at which I maxed out the Variac (probably due to the bulb in series?). The voltage across the output was about 10 VDC @ 107 VAC input, but was as high as ~12.5 VDC @ ~92 VAC input. The DC voltage slowly drops over time, but this is still abnormally high, right? The bulb is glowing, but I wouldn't necessarily call it "brightly lit".

What should I try next? Should I just replace all electrolytic caps to rule them out? There are only ~5 or so, although there is that big cap can as well...

Also, haven't been able to find a schematic of the output section, does anyone have one per chance?
Attached Images
File Type: jpg IMG_1354.jpg (940.1 KB, 165 views)
File Type: jpg IMG_1353.jpg (950.1 KB, 159 views)
  Reply With Quote
Old 23rd November 2020, 03:26 AM   #10
wg_ski is online now wg_ski  United States
diyAudio Member
 
Join Date: Oct 2007
Yeah, I’d start with electrolytics - if you have an output coupling cap that may be part of the problem. If there is an output cap you need to verify what the DC is on the other side of it. It should be about half the supply voltage. If there is no output cap, you need to dig to see what is causing the offset.

The bulb is glowing brighter than I would be comfortable with for an amp operating normally. That’s what keeping the voltage down to 107. Can you measure the bias in the output stage? Do that by measuring voltage across the emitter resistors and ohms law. Once you verify there is not excessive current you can move on from there.
  Reply With Quote

Reply


Peavey PA-200 Powerup With Dim Bulb Tester and VariacHide this!Advertise here!
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Could someone confirm than my multiple bulb Dim Bulb Tester schematic is correct? jazzzman Pass Labs 12 20th October 2020 11:51 PM
Dim bulb tester... nelsondog Swap Meet 9 26th January 2018 07:22 PM
Dim Bulb Tester bloodtypw Instruments and Amps 30 26th October 2015 10:18 AM
dim bulb tester question gary h Construction Tips 5 10th December 2014 04:10 PM
Dim bulb tester wiring for SP switch Hogwild Construction Tips 7 2nd August 2014 04:35 AM


New To Site? Need Help?

All times are GMT. The time now is 09:11 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Copyright ©1999-2021 diyAudio
Wiki