B-52 AT100: No Output

[olddawg]

olddawg:
That is not a fair blanket statement. I ran the service department in a large independent music store for eight years, and I tried mightily to treat people fairly and ethically. We did all repairs in house in my shop. In fact we took in some repairs from smaller stores scattered around the middle of the state, when their level was not up to a repair. I left there to start my own shop, and located inside another music store. I had a cooperative agreement with them, but I was a separate business. I kept the same ethical standards. Like any other enterprise, some are great at it, some are total dogs, and many are soso. Some have their own techs, some don't, some have a shop in store, others serve as a drop off and pick up for a local tech based elsewhere. I kinda don't like being painted with the same brush as the kid who can;t put a new end on a guitar cord right.

My apologies Enzo.. I'm sure you were great. I should have said to ask if the repairs are done onsite and by who. If nothing else ... it effects turn around time if repairs are done off site. I can only speak for my experience. The last thing someone wants to do is drop off an amp for repair at a "big box" store unless it's warranty. Imho. Again that's my experience and opinion. I started my career as a tech for a large consumer electronics store. Their business model was to encourage customers to buy new gear if it was out of warranty. I know how ethical and competent you are, but I fear nowadays that's an exception to the norm.

[J M Fahey]

olddawg:
That is not a fair blanket statement. I ran the service department in a large independent music store for eight years, and I tried mightily to treat people fairly and ethically. We did all repairs in house in my shop. In fact we took in some repairs from smaller stores scattered around the middle of the state, when their level was not up to a repair. I left there to start my own shop, and located inside another music store. I had a cooperative agreement with them, but I was a separate business. I kept the same ethical standards. Like any other enterprise, some are great at it, some are total dogs, and many are soso. Some have their own techs, some don't, some have a shop in store, others serve as a drop off and pick up for a local tech based elsewhere. I kinda don't like being painted with the same brush as the kid who can;t put a new end on a guitar cord right.

Fully agree and add: many (most?) shops are **FORCED** to do what you despectively call "farm out" many repairs, for the very good reason that factories will *only* deal with AWSC (Authorized Warranty Service Centers) and NOT directly with shops.

Even if they have a fully qualified resident Tech, he will NOT get original parts, even less replacement boards, ROMs, Firmware, custom parts , etc. .... often not even schematics !!!!! so any XYZ brand product, although sold at a given shop, and left there to deal with Warranty repair, so it must be sent to the area AWSC ... which might be on another state; some brands (say MarkBass for example) have just one AWSC in all of the North American continent (yes, even users from Mexico or Canada must ship stuff to that lonely service shop).

And why does the shop not become AWSC?

Because itīs not easy, practical or plain possible: Tech must study Factory training courses, hve a certain kind of equipment, many times is required to purchase certain replacement parts for quick servicing without for the Factory to supply them.

"Farming out" indeed !!

[The Dude]

Getting off track here, but oh well. Why not while we wait for ZackyG to do some tests? I do repairs for some of the local stores and to my knowledge they don't mark up. I think they provide repair as a service to their customers as a sales tool.

As to warranty repair. I used to do it all. Now I do none. I have enough work without it. Assessing warranty repair, I found it was less profitable than regular repairs and I didn't need extra work for less money. There are certainly exceptions, but generally - 1) You had to order/stock their parts adding expense, 2) Many of them pay crap for repairs, 3) I was spending a lot of time doing book work, filing claims, making sure they got paid, keeping track of stock, etc. For some of the companies, you have to show a parts invoice from them to get paid for the parts. Once in a while I'll get a call from a company wanting me to do a warranty repair for a local customer. They usually try to get me setup as a warranty station. I tell them I'll fix it and bill them just like anybody else- they can take care of the warranty paperwork on their own. They, often times, accept my offer.

[Enzo]

As a warranty center, I got $50-60 an hour for the most part. I was OK with that. I used to pay for an ad in the yellow pages, but I got far more business from the OEMs. As a listed center, they referred warranty rpairs to me, and that made customers aware of me. Plus even non-warranty work, people still looked up authorized centers online, and there I'd be. I looked at it as advertising, well worth any discounts on my pay rate.

Many guys set up shops inside music stores. Stores see the value of having service on-site, and stores will give you or charge low rent for a room in the back for your shop.

WHen I set up my own storefront, I got business from other stores. The tech at one local music store occasionally brought me things he couldn;t fix. he was my "competitor." A store an hour away would bring me some warranty items.

On the other hand, the reason I left the first music store was that it was family run, and I considered Mr.Family to be a Nazi prick. They didn't like being a warranty center, so when they took on the Mackie line, they brought them over to me for warranty work, as I was a Loud warranty center. I had a hard time ethically doing anything to benefit Mr.Family. But I couldn't hold the innocent customers hostage.

[ZackyG]

She's open.
The filter caps are vertical. I had not prepared myself for this.

The fuses are pictured. They do not appear to be blown, but I'm not about to stick my fingers in there to pop them out and test them without getting a reading on the caps first.

It's been powered down (from a fully warmed up state) and unplugged with the power switch on OFF and the standby switch on OPERATE for about a week. If the bleeder circuit is working as it should, this amp should be colder than snow season, but like I said, I'm taking no chances.

Is it even possible for me to check the voltage in the capacitors in their current vertical configuration? Is this where I give up and sell the thing?

[g1]

I can see R137 there. Measure for DC volts between each side of it and ground.

[olddawg]

You can measure across fuses for continuity in circuit. Just have the amp unplugged and have insulated probes. A fuse should measure nearly zero ohms.

[ZackyG]

I can see R137 there. Measure for DC volts between each side of it and ground.

Both sides of R137 read absolutely minimal voltage (0.02 - 0.01) at the lowest setting (200mV). When I first probed it - before my most recent post here - it read 0.04, so I'm assuming whatever residual current was in there drained through the multimeter to the chassis.
I also probed some other resistors in the capacitor/bleeder circuit and was unable to detect any voltage worth writing home about. Does that mean it's safe to stick my hand in there or is it impossible to know without testing each of the eight filter capacitors individually?

I also checked the fuses - the ones I could find, anyway. Fuses F2 and F3 pass current - with the black lead still clipped to ground, I tested each fuse on continuity mode. I also measured resistance in F3 because it was the only one I could pop out without using my fingers. It measured in the single digits at the lowest resistance setting. F4 is playing hide-and-seek. The schematic says it's there but I can't find it for the life of me.

I also tested pin 1 of three of the four 6L6 power tubes (not the one closest to the AC input because it the only tube wired point-to-point, not PCB, and I wasn't sure which pin was pin 1). Each of the three tubes I tested passed current successfully.

I tried checking pin 1 of each of the preamp tubes as well but was not able to establish continuity through any of them.

[The Dude]

If you have no voltage at R137, we know the problem is before that. Check the DC voltage at the cathode (banded end) of the diodes D11 and D12 to ground and post it here. Also check the AC voltage from D9 anode to D10 anode (not to ground).

[g1]

Dude, he's checking voltage to ensure everything is discharged, not powering up yet.

I see the board marked 'to F4' up near the big caps, can you see where that leads to?

[The Dude]

Oh, sorry,...... I missed that! Ignore my post then. I thought we had no voltage while powered up. Thanks g1 and y'all carry on!

[The Dude]

I believe F4 is on the rear panel marked "POWER TUBES".

[52 Bill]

I also checked the fuses - the ones I could find, anyway. Fuses F2 and F3 pass current - with the black lead still clipped to ground, I tested each fuse on continuity mode. I also measured resistance in F3 because it was the only one I could pop out without using my fingers. It measured in the single digits at the lowest resistance setting. F4 is playing hide-and-seek. The schematic says it's there but I can't find it for the life of me.

To be certain, you should try to remove the fuses from the clips or at least pop one end out and read the resistance from one end to the other end of each fuse. You have safely tested to find there is no residual voltage on the filter caps, so don't worry about removing the fuses.

[Enzo]

And with the amp unplugged, but power and standby in operate position, pull a power tube, and measure voltage at the plate pin. Thast is where B+ would be, or not.

[ZackyG]

And with the amp unplugged, but power and standby in operate position, pull a power tube, and measure voltage at the plate pin. That is where B+ would be, or not.

I did this. I pulled V7, the second 6L6 in from the rectifier tube, set the amp to ON and OPERATE and tested pin 3. In the millivolt range. I've probed spots all over the circuit and I think it's pretty safe to say this amp is stone cold.

I removed and tested F2, F3 and F4.
All of them pass current with minimal resistance. None are burned or scorched.

So what and where is F1? And what should be my next steps? Individually testing tubes?
All this tells me is that the problem is going to be even more evasive than I'd hoped.