Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Equipment & Tools From test equipment to hand tools

R&S UPL + Fisher 400 Receiver = Oscillation!
R&S UPL + Fisher 400 Receiver = Oscillation!
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 Search this Thread
Old 15th April 2018, 08:14 PM   #1
ampexperts is offline ampexperts  United States
diyAudio Member
 
ampexperts's Avatar
 
Join Date: Feb 2014
Location: New Milford, CT
Default R&S UPL + Fisher 400 Receiver = Oscillation!

Had an odd situation today:

Customer sent in his Fisher 400 due to 'distortion from the left speaker'.

A typical problem with these amps is that the output tubes will get weak over time and sometimes one channel will degrade more than the other to the point where one channel is clipping at 6W while the other can do 20 or more unclipped.

So I fed the AUX inputs and measured the amp. 20 wpc with both driven without clipping. I call the customer and tell him that the output stage is working fine and that it may be the speaker or wiring to the speak after all.

He tells me he's using the phono input, so I go back and test the phono input. I discover that I can't go past 12 o'clock on the volume before the amp starts to oscillate at a high frequency at first, followed by a subsonic rate so the output looks like a gated oscillator.

It does this even with the output of the UPL turned off (no signal).
Odder still, with the volume at 11 o'clock, a RIAA sweep reveals a relatively flat right channel, but a large rise in response near 20Hz and a huge -14dB dip around 100-200hz, then gradually rising again to a large peak at 5000Hz. At 20Hz, the L and R channels are out of phase, but in phase at higher frequencies.

I measured the phono stage again at the Tape Out jacks and got the expected nearly flat RIAA (compensated) curve (+/- 1dB or less deviation.)

I note that if I disconnect the phono input from the UPL, the amplifier does not oscillate even at max volume setting. This got me thinking to try a separate generator, so I used an Tektronics oscillator to drive the preamp and that worked without any instability.

So now I'm wondering what about the Fisher and the UPL that causes some sort of positive feedback loop? Is it because the Fisher output isn't ground referenced? But I've measured bridged amplifiers without this sort of instability. And it's only through the phono inputs that it oscillates.

I've come to the conclusion that nothing is wrong with the Fisher. It is a quirk in the UPL that makes the Fisher unstable. Any thoughts on this from other UPL owners?
__________________
Mark A. Weiss, P.E.
www.ampexperts.com
  Reply With Quote
Old 17th April 2018, 03:53 AM   #2
1audio is offline 1audio  United States
diyAudio Member
 
Join Date: Mar 2004
Location: SF Bay Area
R&S UPL + Fisher 400 Receiver = Oscillation!
Check that the low side of the speaker connections are ground or not. Some amps of that period did stuff on the low side of the amp output.

There are several potential feedback gotchas on those older receivers where the grounds are shared but not everywhere. The RCA input's low side may also be a culprit. If the tonearm shield were connected to ground in more than one place you could have issues. Lots of gain from phono to speaker in those guys, especially internally on the plates of the out;put tubes.
__________________
Demian Martin
Product Design Services
  Reply With Quote
Old 17th April 2018, 06:26 AM   #3
udok is offline udok  Austria
diyAudio Member
 
Join Date: Jul 2011
Location: Austria
The UPL generator is floating. The capacitance between output and GND is about 1 nF. Try to ground the UPL generator - there is a setting in the menu.
  Reply With Quote
Old 17th April 2018, 02:08 PM   #4
ampexperts is offline ampexperts  United States
diyAudio Member
 
ampexperts's Avatar
 
Join Date: Feb 2014
Location: New Milford, CT
At first, I thought that it was like some Fisher amps where the 4Ω tap is ground and the 0 tap is the low side of speaker out, but this is not the case here. This amplifier has speaker out referenced to chassis. The phono section is insulated and probably tied to the power supply ground somewhere.

I tried setting the generator to ground referenced and then lift, but neither had any effect.
I noticed that the threshold of oscillation was lower if the generator was set to 600Ω and much higher if set to 5Ω.

Pretty peculiar issue. I thought maybe the ground of the generator might be connected to the ground of the analyzer, forming a secondary path.

One thing I checked just now is to see if grounds between left and right are separate. Apparently my dummy load has the ground side of each resistor tied together. Maybe that is an issue, but given that speaker grounds are chassis common on the Fisher, I can't see how.
__________________
Mark A. Weiss, P.E.
www.ampexperts.com
  Reply With Quote
Old 17th April 2018, 07:09 PM   #5
jackinnj is offline jackinnj  United States
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
R&S UPL + Fisher 400 Receiver = Oscillation!
Any issues with the 10 ohm resistors on the input selector switch? You probably know already that the Fisher 400 phono inputs aren't directly connected to ground.

Using the strategy of fixing it until it's broken:

1) Cartridge loading seems just a tad under-damped. The paralleled value of R3||(R5+R7)|| R9 is ~52k, proper value would be 47k. If you change R3 (and R4) to 150k the damping should be adequate.

2) The RIAA network values on PC1 and PC2 are just a tad off. One could fashion a new SMT circuit card with the 270k value resistor changed to 243k.

WRT the UPL, I will sometimes encounter issues using the AP SYS2722 in floating mode.
Attached Images
File Type: png Fisher400_RIAA.png (17.3 KB, 28 views)
  Reply With Quote

Reply


R&S UPL + Fisher 400 Receiver = Oscillation!Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

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
Sears by Fisher Receiver won't power up freeisforme Solid State 59 28th March 2018 02:37 AM
Fisher RS-9005 Stereo Receiver jomi Solid State 0 29th July 2017 10:34 PM
FS: Fisher 400 FM Receiver - Fine Example tlarwa Swap Meet 0 6th January 2016 07:14 PM
need diagram fisher receiver 600T november Solid State 1 2nd July 2009 03:59 AM
Fisher 220 Receiver bcrescen Solid State 2 8th October 2007 12:23 AM


New To Site? Need Help?

All times are GMT. The time now is 09:44 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki