Sanity check of A470 Output Xfmrs

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Sanity check of A470 Output Xfmrs

Postby joeztech » Wed Aug 20, 2014 8:50 am

A Dynakit Stereo-70 I acquired recently has me concerned about the output transformers. I have made some measurements using the equipment I have on hand which consists of a Knight KG-625 VTVM and a Heathkit IB 2A Impedance Bridge.

DC Resistance Measurements:
Test Conditions:
Center tap disconnected from power supply electrolytic
Allied KG-625 VTVM
Output tubes removed from sockets
Left Channel:
Center Tap to front plate lead = 92 ohms
Center Tap to rear plate lead = 103 ohms
Center Tap to front screen lead = 32 ohms
Center Tap to rear screen lead = 36 ohms
Plate to Plate = 192 ohms
Right channel:
Center Tap to front plate lead = 180 ohms
Center Tap to rear plate lead = 178 ohms
Center Tap to front screen lead = 58 ohms
Center Tap to rear screen lead = 36 ohms
Plate to Plate = 390 ohms.
I am concerned that the transformers do not measure close to the same in the plate circuit measurements.

My inductrance measurements of the two transformers with their center taps disconnected from the power supply electrolytic were:

I set up the Heathkit bridge to operate using its internal 1kHz generator frequency.
Following the Heathkit instructions for the bridge the setup of the bridge that gave the lowest left hand 100uA reading of the AC level when the range switch set to the 10H range gave the lowest set-up reading after the AC set the meter to 100uA at left end of scale. Then the other dials are adjusted for best null reading as the generator level is gradually increased to maximum. The controls are basically adjusted for the best deflection back toward the left end of the scale on the meter.

DQ dial adjusted to read 1.0
Final readings of the CRL Dial were:
Left channel transformer 1.57 X Range dial reading of 10, so 15.7 Henries
Right channel transformer 1.42 X Range dial reading of 10, so 14.2 Henries.

A post on Antique Radio Forum said that he tested a known good transformer measured 192 ohms plate to plate and 300 Henries at 20Hz. My inductance readings are so far off from those posted on the forum that I am not sure which inductance readings are to be trusted. What are your thoughts? What is a normal inductance reading plate to plate of the A470 transformers?

Do you think the right channel transformer has a partially open winding that is being bridged somewhat by a carbon trace? I suspect it is bad.

Joe
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Re: Sanity check of A470 Output Xfmrs

Postby skidave » Fri Aug 22, 2014 6:33 am

Here is a video of Dave from El Paso Tube Amps checking the impedance of an audio output transformer using his General Radio impedance bridge. I know it is a different model than your bridge, but it probably will not hurt to watch this video. https://www.youtube.com/watch?v=96G_Vjk ... hKMxrpzqAQ

Also, here is a link to a very simple circuit to interface to a computer (with software) to measure resistance, capacitance and inductance. In the article, it talks about measuring an audio output transformer. http://www.marucchi.it/ZRLC_web/ZRLC/St ... Bridge.pdf

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Re: Sanity check of A470 Output Xfmrs

Postby joeztech » Fri Aug 22, 2014 12:05 pm

As a follow-up, I received another response from the person who posted the inductance reading at 20Hz. He had also done data points at various frequencies up to 60 or 70kHz. Typical transformers exhibit varying inductance over audio frequencies and beyond. He found the inductance at the low frequency end was a good predictor of how good the bass response of the transformer would be. He also stated that the A470 transformer becomes mostly a capacitor above its self-resonant frequency of ~950Hz. I need to go back and make multiple data points of inductance/capacitance using the external signal generator to see if I get essentially the same curve that he did for the A470 transformers. He also had data on the transformers used in the MK-III and the SCA-35 amps plus a comparison to a Triode Electronics new transformer.

When I used the Heathkit IB-2A impedance bridge, I only used the internal 1kHz oscillator. The bridge does allow for connection of an external oscillator/signal generator, so I could do the same thing using a separate signal generator I have that goes from almost DC to 1mHz. I should have realized that a transformer needs to be measured at multiple data points as I have also seen impedance curves for various speakers and speaker system components.

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Re: Sanity check of A470 Output Xfmrs

Postby Shannon Parks » Fri Aug 22, 2014 1:42 pm

From my mid 70s Japanese Dynaco A470 (out of circuit, on my bench):
Measured with Wavetek Meterman 37XR:

Center Tap to front plate lead = 87 ohms
Center Tap to rear plate lead = 104 ohms
Center Tap to front screen lead = 29 ohms
Center Tap to rear screen lead = 34 ohms

I measure 10H from center tap to either plate connection.

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Re: Sanity check of A470 Output Xfmrs

Postby joeztech » Fri Aug 22, 2014 7:53 pm

Shannon;

OK thanks for the DC resistance checks. Your transformer measures very close to the one in the ST-70 I have with the lowest readings. I still believe the one that measures nearly twice as much is defective. I have a new one coming from Heyboer Transformer Co. Should be here next week some time.

What test frequency did you use for your inductance test?

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Re: Sanity check of A470 Output Xfmrs

Postby Shannon Parks » Sat Aug 23, 2014 5:57 am

Yes, the wildly off DCR of your one A470 certainly points to a problem with it. Tried to make sense of the data in case it was just a damaged external lead, but nothing leaped out at me. You may as well pull it from the circuit and redo that test - can't hurt and it's coming out anyway.

joeztech wrote:What test frequency did you use for your inductance test?


200Hz.

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Re: Sanity check of A470 Output Xfmrs

Postby joeztech » Sat Aug 23, 2014 7:13 am

5 Shannon;

OK, thanks for your input on this.

I just finished making cuts and jumpers to convert the original PC board to use 6GH8A tubes. This morning I went back over my work, rechecking the connections from each contact of the pentode section to the appropriate components and eyelet connections on the board. Everything appears to be OK and no shorts to ground etc. I decided to do a check of the various resistors on the board, and as suspected most of the multi-K ohm resistors and the 1.5meg ohm resistors will have to be replaced with new parts. All of them are beyond their 5% or 10% specification. I am not surprised at this. I have no attachment to the 7199 tube type and since 6GH8A tubes are readily available and far less expensive I will never go back to using 7199 tubes.

The new A470 transformer has been ordered from Heyboer Transformer Co. In the meantime, I will be ordering new resistors for the PC board. I am leaning toward getting Joe Curcio's power supply board and installing all new electrolytics and other power supply parts. The original voltage ratings on the can electrolytic were marginal in my opinion. A new fuse holder is also needed since the old one was damaged at some point in its life. I found a fuse holder at Radio Shack, but unfortunately installing it would require filing the chassis as it is a metric size that is just slightly larger OD than the original fuse holder, so I will find an OEM type to install and make life easier.

Keep up the good work!

Joe
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Re: Sanity check of A470 Output Xfmrs

Postby joeztech » Sat Aug 30, 2014 2:07 pm

I thought I would provide a link to some pictures of the Dynakit Stereo 70 I am restoring. It was made about 1968 time frame based on date codes on some of the parts. I removed, sanded and repainted the transformer bells to make them look good and used some "almost chrome" Krylon paint. I learned on Friday that Heyboer is making a new transformer for me. At first I thought I was getting an off-the-shelf item - my mistake. In the meantime I have replaced parts on the PC board and plan to work on the power supply area next to improve capacitance and voltage rating of the first two stages of filtering the B+ sees following the rectifier tube. The end result will have about 68mFd@over 600VDC capability at both locations. These will be installed under the chassis on terminal strips to be added.

See: http://s1213.photobucket.com/user/rubal ... t=6&page=1

Joe
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Re: Sanity check of A470 Output Xfmrs

Postby joeztech » Mon Sep 08, 2014 4:12 pm

As a follow-up I decided to do an autopsy on the OEM Dynaco A470 transformer that measured nearly twice the resistance plate to plate as the good one.

I disassembled the transformer winding/bobbin assembly from the E and I leaf stack-up. I carefully removed the outer paper wrapping and another single wrap of Mylar tape. Some additional paper tape and Mylar tape over the primary winding lead terminations came off next. Looking things over carefully, I noticed that near one blue/white wire that one of two magnet wire leads was broken in two about 3/8 of an inch from where it exited the winding assembly. I carefully re-tinned the end with some solder and using my pencil iron I unsoldered the remaining portion of the wire that was attached to the end of the blue/white wire so that I could get a little extra length to work with. It was just enough to be able to tin the end where it had broken and tack solder the end to the pre-tinned end coming out of the winding. I slid a small OD short piece of Teflon sleeving over the repair area and secured it with a combination of clear flexible plastic hobby glue and some 3M electrical tape. Then the rest of the wire was reattached to the end of the blue/white wire and following that all the wire ends were repositioned as originally and secured using the Mylar tape, some of the paper tape outer wrapping and some more 3M electrical tape.

The magnet wire that was used appears to be in the range of 34AWG to 36AWG depending on the thickness of the insulation film used (single, double or triple film). There are 130 E leaves and 128 I leaves. There are also two wooden wedge shims to help hold the winding/bobbin assembly firmly against the leaf stack-up. Four rectangular wax impregnated card stock pieces help insulate the edge of the winding/bobbin assembly from the leaf stack-up.

Outer insulation wrappings were put back in place and an outer layer of new 3M electrical tape made a final securing wrap. The is a thick Mylar sheet that covers the area where the wires exit the winding/bobbin area just before the end bells are put back on.

I stopped and made DC ohm resistance measurements to compare to the one good transformer I had. Now both transformers measure the same plate to plate and center tap to plate or center tap to screen grid leads. I have a new transformer coming from DynakitParts.com along with a new chassis and mounting hardware kit. I will keep this repaired transformer on hand as a replacement in case the other OEM transformer ever fails.

Both OEM transformers had the cloth insulated leads. I am sure the new replacement from DynakitParts.com will have plastic insulated wires. I am not that concerned about the cosmetics of plastic versus cloth leads. The main thing is to make sure the amplifier works properly.

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