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 25th November 2020, 07:01 PM   #21
indianajo is online now indianajo  United States
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
Well nothing is going to pretend to work right until all 4 dead components, the diodes and resistors, are replaced.
IMHO 1n4003 may not have enough current rating for these positions. I'd use 2 or 3 amp rated ordinary diodes. Not schottky, but UF might be okay here. MR752 like in the bridge would do it but those might be pricey. You want at least 100 piv rating diodes.
Something took those diodes out, I'd check the .02 uf cap to the preamp power supply is not shorted. I'd also pull the base of each output transistor and make sure it is not shorted b to e or b to c.
Well, seeing Mr. Fahey's post, I see my prejudice against 30 year old rubber sealed wet capacitors is justified.
I hope the interstage transformer is not damaged. It has a dark spot on it.
Happy hunting.
__________________
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; 25th November 2020 at 07:04 PM.
  Reply With Quote
Old 25th November 2020, 07:19 PM   #22
wg_ski is online now wg_ski  United States
diyAudio Member
 
Join Date: Oct 2007
Those dead bias diodes are probably 1N4001. You don’t want/need high current, ultrafast, or highly overrated voltage ones in there. Any of the above would result in the output stage bias being wrong. Too high a voltage drop and you get excessive bias. Too low and you get crossover distortion. There doesn’t appear to be a pot anywhere to adjust this. Fried output transistors are the likely culprits if the diodes are blown.

Let’s hope that interstate transformer isn’t damaged. You will likely have to MAKE one if it needs to be replaced. Highly unlikely, though.
  Reply With Quote
Old 25th November 2020, 07:33 PM   #23
indianajo is online now indianajo  United States
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
Quote:
Let’s hope that interstate transformer isn’t damaged. You will likely have to MAKE one if it needs to be replaced. Highly unlikely, though.
Lots of these old monaural Peavey devices show up on ebay $20-30. consumer electronic & musical instrument category both. The freight to ship it in is probably more than the purchase price. They are cheaper than a new power transformer, case, pots, power switch, fuseholder all that stuff.
__________________
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 26th November 2020, 04:22 AM   #24
Cheeto333 is offline Cheeto333
diyAudio Member
 
Join Date: Apr 2015
Thanks all, I'll replace the damaged components and report back. Should I be removing the output transistors from the circuit before measuring voltage from bottom emitters to ground?
  Reply With Quote
Old 26th November 2020, 05:28 AM   #25
indianajo is online now indianajo  United States
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
The TO3 output transistors, looking at the bottom from the screw in the long direction, the base is the one on the left. Emitter is the one on the right. Collector is the screw on the end, unless it is insulated from case by a plastic ferrule.
With power off, with the diode scale of your meter, should read 600 to 700 mv b to e and b to c on the output transistors. Higher backwards. If zero, remove & recheck.
With power on, as shown on the print, lower output transistors e should be .05 v and b .6 v. Upper transistor e could be 37 + .05 above emitter resistor, 37.6 on the base. This does not show on the print. Whatever, the upper bases should be ~.6 v higher than the input of the output cap. The current across the upper emitter resistor should be minimal, also. ie .05 v.
Note shorted output transistors often take the emitter resistors with them, which go open.
Note also, if mica washers are under the power transistors, they are 30 year life parts and should be replaced. If you use mica again, you'll also need heat sink compound. If you use silicon rubber insulators, those can be used without heat sink compound. If you get modern TO3 output transistors, the holes on the ends won't fit the old #6 screws; you'll need new #4 screws & nuts or 3 mm screws. Try to get those in the same box for the $9 freight charge, too.
In replacement On output transistors, from cheapest to most expensive is probably MJ15015, MJ21194, MJ15003, 2n3773, MJ15024. If other brand do not use 2n3055, the 75 v rail requires specially selected ones available in 1970 (Like 40406) and not now. In other brands 2n5629 or 2n5630 are probably good enough. Other brands 2n6338 39 40 41 are probably good enough.
__________________
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; 26th November 2020 at 05:56 AM.
  Reply With Quote
Old 29th November 2020, 07:51 PM   #26
Cheeto333 is offline Cheeto333
diyAudio Member
 
Join Date: Apr 2015
Progress update: I replaced the 2x diodes and 15 ohm resistors with new parts (1N4001 for the diode). Only the burnt diode measured as faulty once removed from the circuit (other diode and resistors were ok). I also replaced the 470uf and 50 uf caps whose values had drifted far out of original range. The 3500uf, 1000uf, 100uf, and 25uf caps measured ok, so I left them in.

Measuring the output transistors showed that 3 out of 4 were continuous (0V drop in diode test) from base (green wire) to emitter (black wire). I'll remove the transistors from the circuit and retest. The emitter resistors also seem to be much higher value than expected (but not open), so I'll remove them from circuit and retest as well.

Thanks to all for the help so far! I'm glad this amp is so easy to work on
Attached Images
File Type: jpg IMG_1377.jpg (822.3 KB, 44 views)

Last edited by Cheeto333; 29th November 2020 at 07:53 PM.
  Reply With Quote
Old 29th November 2020, 10:42 PM   #27
Cheeto333 is offline Cheeto333
diyAudio Member
 
Join Date: Apr 2015
Ok, I removed the transistors and emitter resistors from the circuit with the attached results: it looks like 2x transistors and 2x resistors are dead. Interestingly, the dead parts are not from the same pairs (a resistor from dead transistor is ok, and vice versa).

Would this transistor be a suitable replacement? I'll replace the mica washer with rubber or mica+heat transfer compound as well. Is it good practice to replace all transistors such that they all are from the same series?
Attached Images
File Type: jpg IMG_1377.jpg (795.4 KB, 40 views)
  Reply With Quote
Old 30th November 2020, 12:31 AM   #28
indianajo is online now indianajo  United States
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
Yes, zero voltage drop across b-e or c-e on a silicon transistor on diode scale is the wrong number.
Yes, MJ15024 is a superior npn transistor that will replace these. All 4 transistors should be changed at the same time, with the same part number, from the same batch. MJ21194 may be cheaper, and would be entirely adequate. MJ15015 is also adequate and very inexpensive, if they still stock it. I got my 15015 at $1.40 each years ago.
About the "good" capacitors. What goes wrong on electrolytic capacitors is ESR, which can only be measured by an ESR meter. I measure the things with a calender (>20 years is wrong), but I prefer my equipment to not be broken all the time as the rubber capacitors fail one by one.
I replace the screw terminal caps with snap-in caps, mounted through holes on a piece of bare board, with the board mounted to the case with a couple of screws and a stand off. Much cheaper. Also multi-gang can caps, I replace with several single snap in or leaded caps, mounted on a solder type cinch terminal strip. Which terminal strip can be mounted to the case with screws.
__________________
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; 30th November 2020 at 12:54 AM.
  Reply With Quote
Old 30th November 2020, 12:36 AM   #29
Cheeto333 is offline Cheeto333
diyAudio Member
 
Join Date: Apr 2015
Ok, I'll replace the other electrolytics. Should I replace the big cap can as well?
  Reply With Quote
Old 30th November 2020, 12:49 AM   #30
wg_ski is online now wg_ski  United States
diyAudio Member
 
Join Date: Oct 2007
I’d only replace the big can if it needs it. They generally have decent rubber seals on them compared to the little PCB mount caps. They can be pricey, too. If you get full output power, it’s fine for now. What you might do is keep your eyes peeled for similar units on the surplus market - they show up from time to time.
  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 08:54 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