Help-WOPL 700b Lost Left Channel Bias

Vintage 700b

Chief Journeyman
Joined
Mar 13, 2019
Messages
741
Location
Wisconsin
Tagline
Fabricator
#1
Hello Guys,

I thought the 700b was ready for years of renewed life, but I am now dealing with a new issue. This amp was built as a present for my nephew who graduated from college with his Master’s Degree recently. The family presented the amp yesterday afternoon, I had it playing through the speakers when he and my brother and his wife arrived. My nephew was quite impressed, and then we “handed the keys” to him, and congratulated him on all the work he has put in. Of course he was shocked. He took it home and set it up, called me and said it sounded amazing, and he was so surprised, and grateful. About 9pm he called me and said that he shut it off about 8:30, then went to turn it on again about 8:50 and heard a pop, like a fuse had blown. He checked the fuses, and the left channel ground was blown (AGX-5). I told him to bring it over this morning and I would look at it. Early this morning, I had a chance to replace the fuse, and bring it up, with a DBT and Variac, and it came up as expected, relays kicked in as they should, and everything seemed okay. I checked the bias, and I have .360 in the right channel, and .008 in the left channel. DC offset in both channels is .001. I still have the +/- 15volts on both boards at those test points. I noticed that while I had the amp up, I had 86.8v DC at the caps separately, and 173.6 across both. I also have 173.6v DC across points 11, and 12 on the G1 board for both channels, no fuses blown on the backplane boards. Checking the collector case to ground, under power, I have 86.8v DC on all of the Right Channel outputs, and all 6 of the third row of 95’s on the Left channel. The row next to the transformer (96’s left channel) all show .003v DC when I tested them.

I pulled all of the 96’s in the row next to the transformer. They all test perfectly across Emitter/Base/Collector testing .514/.521 avg. and O/L as they should where appropriate. I did find one of these that had cut through the sil-pad ever so slightly, and there was evidence of arcing to the heat sink. This I figure is what blew the fuse. I checked all of the heat sinks carefully and chamfered accordingly before I initially installed the outputs. Some of the transistor case holes were quite sharp, and I took that edge off as well when I initially installed, to prevent cutting the sil-pads. I did observe the 5-inch pound final torque. So on that one I cleaned up the case arcing, and it tested fine. I reinstalled with a new sil-pad. The amp powers up fine with the DBT and Variac, but I still have the bias at .008 in the left channel.

I won’t have a chance to look at this until later this afternoon, or this evening. I was going to work my way through the Left backplane board and test as I go. You guys have much more experience in this area than I do. It looks like that output arced through the sil-pad, after a few days of working perfectly.

I am a master Tool and Die maker, so I was very careful checking burrs, and chamfering where appropriate, and base/emitter pin/screw alignment on the initial assembly , checking both transistor cases, and heat sinks. I did find several transistors that had the full burr on the punched holes, which surprised me, but as I said, I cleaned them up, before installing, and I checked each one for a short per the start up procedure. I was very careful not to tighten the SHCS to tight (5 in/lb), and I took my time as I assembled. The amp has worked perfectly for a couple days now, without even a hiccup.

Since the transistors all check okay on that offending row (LH channel next to the transformer) and I still have 86.8v dc on the cases of all 18 in the 3 other rows, but only .003v dc in row 4 by the transformer, any help on where to start troubleshooting? Nephew feels it is something he did, I am sure it was not. All he did was turn it on, and when he heard the “pop” of the fuse, he immediately shut it off.

Thank You
 

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
13,473
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#2
I think you mean columns, not rows. And you are saying that the 6 MJ21196 transistors in the column closest to the transformer has .003 volts on them. The feed that powers that column comes from one of the 4 fuseholders on the right channel side of the amplifier. The feed to the control board is separate and unfused as it enters the backplane board. The backplane boards contain the fusing for the control board. Check that you have your ~86V coming out of the respective fuseholder that runs to the wire that connects to F_B+1 of the left channel backplane board (upper left of each backplane board, labeled in silkscreen). Things that could be happening are bad fuseholder in that location, fuse that looks good but is not really (internal disconnect), cold solder joint at either end of the wire run to the backplane board.
 

Vintage 700b

Chief Journeyman
Joined
Mar 13, 2019
Messages
741
Location
Wisconsin
Tagline
Fabricator
#4
Hello Mark,
I did check both (L and R) out right away, and they were fine, as well as the solder connections and Phoenix connections, all good.
Thank You.
 

Vintage 700b

Chief Journeyman
Joined
Mar 13, 2019
Messages
741
Location
Wisconsin
Tagline
Fabricator
#5
Hello,

I did check all fuses with a DMM, (4) AGX-5, AGC-10 main, (4) .5-amp fuses on the backplane and all are good.

I started to bring the amp up with both DBT and Variac. Normally when all is well, I barely see the filament glow in the 100w, clear glass bulb until I have a full 120v ac line input, then it is faint, of course.

I started to bring the Variac up, at 20v ac the dim bulb started to glow. I took that as a sign that things are not well with the ac input in the amp. I did however check the bias and had .180 at BOTH channels.

I reluctantly took the Variac up to 50v and the filament in the bulb was glowing about half as bright as it would unrestricted (I normally would only feel heat at 50v, with no visible glow at all, so again, this I took as a sign that things are not well, and did not intend to take the Variac any higher. At 50v ac at the Variac, I only had 1.69v ac at the transformer (120v legs). The DC at the power capacitors was 1.012 total dc across the capacitors, (.876v Top Cap, .136v Bottom Cap).

FB-1 Left was .116 dc

FB-1 Right was .134 dc with DMM set on 6 v dc.

UFB+ Left was .138 v dc

UFB- Right was .138 v dc

Bias was .210 in BOTH channels

Since the dim bulb was glowing at 50v ac, I shut everything down. I retested all of the above fuses, and all tested okay with the DMM.

I wanted to report back as it seems that the ac power is not getting to the transformer. I put a new power cord, new Bussman fuse holders in all 5 points. These are very odd readings, and I had nothing like this when I did all of the work bringing the amp up initially. I am looking at the power in now.

I may mention that the new fuse holders that I installed seem to be for 1.250 fuses, and the AGX-5 (Genuine Bussman, not “imports”) are 1.00” (actual .983) I installed 5 of the same fuse holders, and the AGC-10 fuses are 1.220” actual. I did not like these fuse holders when I put them in, and now that seems to be a possible issue? I put in (5) Mouser Part # 504-BK-HVP-R holders. There really was nothing wrong with the old ones (which fit the AGX fuses perfectly), and I broke 3 of those removing them.

I wanted to report back and will see what the issue is now not getting power to the transformer, which has always been fine before. Is there a better (correct?) fuse holder for the AGX-5’s?

Thank You
 

Vintage 700b

Chief Journeyman
Joined
Mar 13, 2019
Messages
741
Location
Wisconsin
Tagline
Fabricator
#6
Hello,
I am wondering if these fuses can be used instead of the AGX-5 (and keep these fuse holders):
Eaton AGC-5-R Mouser Part #504-AGC-5-R
They are 1.25" long (31.75mm). I remember when these were .15 apiece !
Thank You
 

George S.

Veteran and General Yakker
Joined
Feb 18, 2020
Messages
4,539
#7
With the unit unplugged, and storage caps discharged, carefully compare all wiring in the power section to the schematic including the wiring on the rectifier and polarity of the storage caps. Rectifiers must be wired a certain way. Make sure it is correct, no guessing. Then use your multimeter in ohm setting with one lead connected to a contact on the power cord plug and start probing with the other lead at the main fuse holder tip and further into the power section. Then do the same with the other side of the cords plug. Your looking for continuity from the cord plug contacts through the fuses and on into the power section.
Chances are those fuse holders have good spring pressure on the fuses for good contact. When you insert a fuse and press the cap down you should see the tip where the wire is soldered on move out a little, there's a spring in there pressing that contact against the fuse.
I'm thinking a wiring mistake in the power section or a flakey rectifier.
 

Vintage 700b

Chief Journeyman
Joined
Mar 13, 2019
Messages
741
Location
Wisconsin
Tagline
Fabricator
#8
Hello George,
Those are great suggestions, and I had just gotten done doing exactly that check, through the power cord to the fuse holder tips, rectifier, and transformer leads. All of that is brand new. I put a new bridge rectifier in for this build with the snubber caps. Tested the AC and DC sides and all was well, and seems to have maintained that way. I am really suspecting the fuse holders for the (4) AGX-5 fuses. They do make contact within the fuse holder, so I get continuity across the fuse when it is installed, but the amp was moved around a lot (bouncing those 1.000 (25.1mm) fuses around a bit inside a fuse holder that seems to be designed for a .25 X 1.25 fuse (AGC). The AGC-10 I have in the power in position fits well, I did not like how the AGX-5's fit from day 1. I will see if an AGC-5 will work. It will definitely fit the new fuse holder better. They are .25 X 1.25 (31.75mm)
I wired the hot and neutral lines per the schematic, incorporating the thermal cutouts on the back of the amp as well. I'm leaving no stone unturned however, and will go through your suggestions again.
Thank You, I really appreciate the help.
 

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
13,473
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#9
If you put in AGC fuse holders which you did according the the part number, you cannot use AGX fuses in them for the reasons that you cite. If you replaced the fuse holders with AGC type then you have to use AGC5 fast blow fuses instead of the AGX originals.
 

Vintage 700b

Chief Journeyman
Joined
Mar 13, 2019
Messages
741
Location
Wisconsin
Tagline
Fabricator
#10
Hello,
I think this was a problem that was bound to rear it's ugly head. I am really sorry that it happened
to my nephew, instead of me. The Eaton AGC-5-R , Mouser Part #504-AGC-5-R are a fast blow 5 amp AGC.
I am really surprised that I was even able to get the amp working. I will see what I have in the shop tomorrow,
I may have some of these already. As always, thank You for all the help! I am guilty once again of not seeing the forest for the trees!
Thank You
 

Vintage 700b

Chief Journeyman
Joined
Mar 13, 2019
Messages
741
Location
Wisconsin
Tagline
Fabricator
#14
Hello Guys,
Here is what I did today when I got time to take another look at the amp. I did have the AGC-5 fuses in the shop. I replaced one fuse holder that was new two weeks ago, but seemed to be the one that always blew, and I even though it always seemed to check out fine, I replaced it with another new one. I installed the AGC-5 fuses, and went back to our basic "bring up" protocol, removing the chassis ground from chassis to buss bar between the power caps. I then tested for shorts, tested for continuity at the fuse holders, A/C power plug to transformer, thermal relay connections, made sure A/C switch was functioning, checked continuity between FB-1 on backplane respective channels and fuse holders. All checked out. Looked and checked for cold soldered joints as well. I left the ground strap from Buss to chassis off, and brought the amp up with the DBT and Variac. All was well. At 124.5v A/C (my line voltage here) I had 86v D/C at each power capacitor, and 172v D/C across B- to B+. I also had 86v D/C at F_B+1 on both boards, and 172v D/C on points 11 and 12 on the G1 board, on each channel. Bias was .360 Left and .360 Right. I had +15v and -15v at those test points, both Left and Right. I switched the DBT to direct, from test, shut the amp power switch off, waited a few seconds, turned it on, and had bright bulb, to dim, as it should.
I took the Variac down, and bled off the power caps. I repeated the test, same protocol, chassis ground removed from the chassis point, and checked the chassis ground (or Buss between the caps) and here is where I was concerned. When I was up to 124.5v A/C using the dim bulb tester, and the Variac, I tested the chassis ground for A/C.....A/C and I had 57.45v A/C at the chassis ground ( and Buss as well, of course) . Using the neutral A/C point from the transformer, I had the same number on the white wire on the relay board, and on Pad 1 and Pad 16 on the front board (Red and White wires from the OUT on the Backplane boards. Also same on the speaker outputs (yellow wires to relay board and binding posts) This did not look right. At this point, no fuses had blown, and the dim bulb was behaving as it should, again, Variac at 124.5v A/C in.
I checked all fuses, and they were fine, and I replaced the ground connection, and brought the amp up again, and the same situation occurred as yesterday. Left channel (Hot) fuse (AGC-5) blew, and no bias in the left channel. Right channel was fine.
Everything comes up with the chassis ground wire from the Buss unhooked. Those A/C voltages ( the 57.45v A/C to the speakers essentially) were alarming, and then I am blowing that Left fuse with the chassis ground hooked up. It is as if I have taken a few steps backwards from several days ago, when all worked as it should. I checked the bridge rectifier (which was new), and closely inspected solder joints. I replaced the one fuse holder (again, same fuse blew tonight) and checked all of those connections again when I was in there.
Any ideas, what may have happened, or why that chassis ground seems to have gone haywire? I am getting to that "ghost in the machine" phase.
Thank You
 

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
13,473
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#15
A guess:
Some foreign object rattled under the left backplane or the Fused B+ lead soldered into the backplane is not trimmed short enough and is touching the chassis. You have some form of B+ short to chassis. That only shows up when you complete the ground connection
 

Vintage 700b

Chief Journeyman
Joined
Mar 13, 2019
Messages
741
Location
Wisconsin
Tagline
Fabricator
#16
Thank You,
I have been so careful about errant anything floating around in the chassis, and I quadruple checked those backplane boards, but that sure is what it acts like. All was working like I said, even when my nephew got the amp home, he hooked it up, sounded great for a couple hours, then he shut it off......and did move it, hooked it back up and when he turned it on, the left fuse blew and this all happened. The moving may have shook something foreign loose and then it shorted.
It may be a tiny booger of solder, or who knows, I will take it apart tomorrow. To be thorough, it seems best to pull all TO's, heat sinks, and lift the backplane board up, to try and shake it loose. Is there an easier way? Obviously not if something was not trimmed short enough, the backplane needs to come completely out to inspect again. If that was the case though, I don't think it would have operated at all. Are those strange A/C numbers explained by the chassis ground short as well?
I will explore further.
Thank You
 

Gepetto

Administrator
Staff member
Joined
May 15, 2011
Messages
13,473
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#17
Those strange AC reading could well be. If you have the B+ rail shorted to the chassis, you will essentially have the full wave rectified signal of the transformer secondary connected to the chassis.
 

Vintage 700b

Chief Journeyman
Joined
Mar 13, 2019
Messages
741
Location
Wisconsin
Tagline
Fabricator
#18
Thank You,
I will pull that backplane board carefully and see what's going on.
Thanks again, for all of your help!
 

laatsch55

Administrator,
Staff member
Joined
Jan 14, 2011
Messages
74,124
Location
Gillette, Wyo.
Tagline
Halfbiass...Electron Herder and Backass Woof
#19
Thank You,
I have been so careful about errant anything floating around in the chassis, and I quadruple checked those backplane boards, but that sure is what it acts like. All was working like I said, even when my nephew got the amp home, he hooked it up, sounded great for a couple hours, then he shut it off......and did move it, hooked it back up and when he turned it on, the left fuse blew and this all happened. The moving may have shook something foreign loose and then it shorted.
It may be a tiny booger of solder, or who knows, I will take it apart tomorrow. To be thorough, it seems best to pull all TO's, heat sinks, and lift the backplane board up, to try and shake it loose. Is there an easier way? Obviously not if something was not trimmed short enough, the backplane needs to come completely out to inspect again. If that was the case though, I don't think it would have operated at all. Are those strange A/C numbers explained by the chassis ground short as well?
I will explore further.
Thank You

Those small star washers can be tricky...
 

George S.

Veteran and General Yakker
Joined
Feb 18, 2020
Messages
4,539
#20
Last edited:
Top