electronics, repairs, service manuals and datasheets discussion community for the repairs professionals and technicians
FAQSearchMembersGroupsRegisterProfilePM'sLogin/Out
Mystery Diodes

 
Post new topic   This topic is locked: you cannot edit posts or make replies.    Electronic equipment repair and service manual search discussion Forum Index -> Equipment Repairs
View previous topic :: View next topic  
Author Message
putter



Joined: 19 Jul 2005
Posts: 2

PostPosted: Tue Jul 19, 2005 6:31 pm    Post subject: Mystery Diodes Reply with quote

Well I'm new here and athough I know alot about tube equiptment I have limited knowledge of solid state. I have a Vector Research VR-7000 Receiver that has a burned out pair of power transistors of the 4 that exist and a burned driver transistor so I plan to replace all drivers and power transistors and have found replacements.My question is: there are two components mounted on the heatsink with the 4 power transistors that look like a small ball of epoxy with two long leads and no part numbers.Looking at the parts placement orientation drawings printed on the top of the board they look like they must be diodes and I assume that they must be some sort of protection circuit for the output.Looking in my Pioneer SA-7500 amp I can see they same type of parts mounted on it's heatsink but no part numbers on those either.Can someone Identify these for me so I might find replacements and also how do I test them.They test open with a digital meter set to diode test but so do the ones in the Pioneer amp that is workin fi Question ne. Thanks!
Back to top
View user's profile Send private message
absorbentgnome



Joined: 30 May 2005
Posts: 51
Location: Cardiff, UK

PostPosted: Tue Jul 19, 2005 8:55 pm    Post subject: Reply with quote

Bipolar amps usually need thermal compensation, to prevent thermal runaway. The particular component you are looking at may conduct above a certain temperature for amp protection, or be a linear device for thermal compensation.
It is fairly unlikely that it is damaged. Perhaps someone else can confirm?
Back to top
View user's profile Send private message
Ray
Guest





PostPosted: Wed Jul 20, 2005 2:01 am    Post subject: Reply with quote

Idea Wow, that is an oldie. as a matter of fact used to sell/Svc Vector research & liked them a lot. 'f course they been out of biz for some time.

K, the mystery part is actually a temperature compensating diode called Varistor & sometimes refered to as bias diode (absorbentgnome is correct)

Most if not all the amps of that era used the Sanken made SV series (SV02~SV06)diodes .Sanken's SV-02YS (silicon varistor) is essentially two silicon diodes in series, giving a nominal forward voltage drop of about 1.2V at 1 mA.( thats why diode test will show it open--it only applies .6V forward voltage) In Circuit, the exact voltage drop will vary with temperature, by roughly -4 mV per °C, as is typical for two silicon P-N junctions. Such devices are often used to compensate for changes in transistor characteristics, from variations in transistor Vbe diode voltage drops, with temperature. So i'd called them OK but,as a FYI;

It is always best to try using original, since it may be more than 2 diodes in a package. Possible substitutes might include ECG/NTE 605A, 1N4156, or Microsemi's MPD200

Not sure of the original's availability, but do remember using these as subs

http://www.nteinc.com/specs/600to699/pdf/nte601.pdf
http://www.nteinc.com/specs/600to699/pdf/nte605a.pdf
Back to top
putter



Joined: 19 Jul 2005
Posts: 2

PostPosted: Wed Jul 20, 2005 12:06 pm    Post subject: Vector Reserch Reply with quote

That's a lot of help Ray and Thanks for the help.I'll probably just proceed with the repairs as if the two varistors are good since it seems to be difficult to find replacements in the same package.Any chance someone out there knows where I could find a schematic for this Vector Research VR-7000 amp in case the repairs are more extensive? Also can anyone give me a quick primer on setting the bias on solid state amps.I know the importance of doing this from my experience with tube amps.This receiver has two trim pots and test pionts that seem to be for checking and setting the bias. Thanks again!
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   This topic is locked: you cannot edit posts or make replies.    Electronic equipment repair and service manual search discussion Forum Index -> Equipment Repairs All times are GMT
Page 1 of 1

 
Jump to:  
You can post new topics in this forum
You can reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Електроника София - Service Manuals Free Download - Prikachi - Service Diagrams

Free TV Schematics Download - HARDW.net - Pinout Master - Jabse Search - Jabse.InK  - Pulsator

Connector Pinout Forum - Новини - SnimkiOt.com - безплатен DNS - Whois разузнаване - Poblizo.com : пътешествия