Offset problem with TFM-55?

To add to the above:

What were the original large caps in the TFM-55's? I have Marcon caps 10000uF 100V. Where these cheap replacements? Does anyone know anything about this brand?

.....
I'm inside looking at the board now and see the Q103 and Q105. How do I test these to see if they're ok or not? or would it be a simple cheap replacement?

The best way to check them wouold be looking at the waveform on a scope with a 1KHz test signal, the top transistor would amplify the top half, the bottom the bottom. If you don't have an O'scope, you might be able to feed it a test tone (from a computer soundcard or test CD) and read the AC voltage on the top swing. It won't give you a visual, but whatever the top half's reading the bottom half should read the same, but negative.

.....The posts TP's can be seen from underneath.

I measured these again from underneath and above and really strangely I'm not getting any reading from either channel.

The other day when I got 1.85mV the speakers were connected I think.

I have everything disconnect now and get no reading from these posts

Hmmmmm....the adjustment is made with no input and nothing hooked up. I've watched it while playing music and hooked to speakers while I was playing around with the setting awhile back (and if you play music into her while reading AC voltage you should get a reading, so it shouldn't matter, you should get a reading either way; when it comes time to adjust it though, don't have any input.

If neither adjustment is changing anything then possibly the bias network is fubar on both channels.......although I thought previously the problem was on the right?

You caught me on the way to the airport (Brother's kid's baptism) so I can't look into this right now, sorry.

To add to the above:

What were the original large caps in the TFM-55's? I have Marcon caps 10000uF 100V. Where these cheap replacements? Does anyone know anything about this brand?

Yes, those are the correct values

Enjoy your baptism Rich and thanks for your help so far. I'm really enjoying getting my hands dirty with the electronics here.

If you don't have an O'scope, you might be able to feed it a test tone (from a computer soundcard or test CD) and read the AC voltage on the top swing. It won't give you a visual, but whatever the top half's reading the bottom half should read the same, but negative.

Can you give dummies advice on how to do this please? From step 1

If neither adjustment is changing anything then possibly the bias network is fubar on both channels.......although I thought previously the problem was on the right?

Left channel;
Speakers and source attached = 1.7mV
Speakers attached=1.7mV
source attached=No reading

Right channel;
no readings.

I see that there are some bad solder joints on OT Q219a/Q217a and Q218/Q220 on the right rail. These were very badly replaced when the mods were done. I'll try to get some pics once my other half returns with the camara. She's taking terabytes of photos of our new nephew just a week old.
Joints on the board (135) are making contact with the foil ok but it's messy.

Important: I see that the bias measuring points are connected to these OT's .

There is also discolouring around Q216/R244/R248. Don't know if this is from before the mods were done or after. Maybe another cause of the bias problem? Q216 burnt out??

What were the original large caps in the TFM-55's? I have Marcon caps 10000uF 100V. Where these cheap replacements? Does anyone know anything about this brand?
Yes, those are the correct values

What are Marcon like? Might be good to upgrade if they are cheap nasty ones done during the mods while I'm inside the amp. Also looking to change the speaker terminals.

Caps of that size are never cheap. I presume those are computer grade (screw terminal) caps. I just did a quick digi-key survey and found only one that meets that spec. It is THIS ONE

Caps of that size are never cheap. I presume those are computer grade (screw terminal) caps. I just did a quick digi-key survey and found only one that meets that spec. It is THIS ONE

Do you know what the original ones were?

I enjoy tweaking and wonder if these could be upgraded to improve sonics.

I'm guessing that those are the originals. Established in 1942 by the Toshiba Corporation, Marcon Electronics Co., Ltd. was acquired by Nippon Chemi-Con in 1995. This acquisition has enabled United Chemi-Con to expand its already extensive product line of aluminum electrolytic capacitors to include os-con, tantalum, film and multi-layer ceramic capacitors, as well as metal oxide varistors. As wholly owned subsidiary of Nippon Chemi-Con, United Chemi-Con is the largest manufacturer and supplier of aluminum electrolytic capacitors in North America. Established in the United States in 1970, United Chemi-Con distributes more than 8,000 products -- including products manufactured by United Chemi-Con's manufacturing plant located in Lansing, North Carolina, as well as other capacitors manufactured worldwide by NCC and its subsidiaries.

I'm guessing that those are the originals. Established in 1942 by the Toshiba Corporation, Marcon Electronics Co., Ltd. was acquired by Nippon Chemi-Con in 1995.

If this is the case maybe it's time for a cap change as it's 15 years old now. Makes sense while I'm trying to sort out the bias problem also. Maybe I've got a leaky cap that could be a source of the 350mV reading on the left speaker terminal?

Rich: I'll wait for you to chime in on the bias problem when you return from the baptism.

Thanks a lot guys

...... Maybe I've got a leaky cap that could be a source of the 350mV reading on the left speaker terminal?......

The channels don't have seperate caps; if that were the problem you'd see it in both channels. When I get back home I'll look at the other things you mentioned (don't have a schematic with me here).

Hi Rich,

I'm going through every resistor now as I found a burned out one. R230 (1.5K) is burned in the middle reading 1K.
Also some others that are reading 20% more or less than there rating. These are 5% tolerance resistors with the gold band.

Looks like I'm going to get lots of electronics practice with this amp.

When you have a chance let me know what I should do with the differential transistors Q103 and Q105. Maybe just replace them and see if this improves things?


Thks a lot

Chris

..... I found a burned out one. R230 (1.5K) is burned in the middle .....Looks like I'm going to get lots of electronics practice with this amp.

Looks like whoever worked on it previously missed looking at the passive components and concentrated on transistors.

..... what I should do with the differential transistors Q103 and Q105. Maybe just replace them and see if this improves things?.....

The Collectors of each should be outputting a mirror image of each other. One carries the positive swing and another the negative. The collector is usually the center leg, which may be hard to look at without slipping the probe and shorting across something, unless you have clip leads, it might be safer to follow the circuit board trace from the center leg to the next resistor.

The Collectors of each should be outputting a mirror image of each other. One carries the positive swing and another the negative. The collector is usually the center leg, which may be hard to look at without slipping the probe and shorting across something, unless you have clip leads, it might be safer to follow the circuit board trace from the center leg to the next resistor.

Do I test these transistors with the amp powered up as you mentioned shorting?

. I found a burned out one. R230 (1.5K) is burned in the middle .....Looks like I'm going to get lots of electronics practice with this amp.
Looks like whoever worked on it previously missed looking at the passive components and concentrated on transistors.

Looking further some resistors have been doubled up in series with new ones but done so badly. I'm going to take my time with this one and remove and replace with correctly valued resistors.

But very busy for the next 3 weeks. SQL exam and project to complete!

Adding to the above: WTF! R284 is missing. Just 2 cut off ends!!! 5.6K 1/2 W resistor it is meant to be.

I'd say that piece has seen some bad work on it.

Adding to the above: WTF! R284 is missing. Just 2 cut off ends!!! 5.6K 1/2 W resistor it is meant to be.

My mistake: a resistor has been placed on the circuit side underneath. It has a jumper to R283.
I believe these resistors should read around 5.6K? I got about 5.6, which took some time to reach that with the voltmeter.
Is this normal with the voltmeter or is the battery low?
This resistor I see is part of the output circuitry to the left channel negative terminal.

photos attached.

R284 small top.JPG (65.25 KiB) Viewed 369 times

R284 small below.JPG (78.13 KiB) Viewed 369 times

Also a photo of the resistor R250 that I mentioned previously when I couldn't find the white rectangular one.

R250 small.JPG (102.2 KiB) Viewed 369 times

I'd say that piece has seen some bad work on it.

It's looking that way Bill. Some real cowboy has worked on this. Surprised any music comes out of it.
But I'm a bit of a tweaker and perfectionist so I aim to have this amp really cleaned up and working well again.
It's a good learning project for me also as I'm planning to build my first Aikido pre-amp soon.