Dynaco Z565 OPT - DC Resistance Readings

mdye

Active Member
I recently acquired a clean Dynaco SCA-35 amp that has the original Z565 OPTs with cloth leads. I haven't fired it up yet; I've just been taking some initial readings with a multimeter of the resistors, caps and transformers...looking for anything out of spec, etc.

I'm getting some interesting readings when measuring the DC resistance of both OPT's primaries (all readings are in ohms, measured from the B+ tap):

Transformer #1
Blue: 174
Green: 44
Blue/White: 201
Green/White: 51

Transformer #2
Blue: 375
Green: 92
Blue/White: 197
Green/White: 50

Transformer #1 seems to be in line with other people's measurements of the Z565 that I've found online. As you can see, Transformer #2 is reading a little more than twice as much resistance on one half of it's primary.

I know that DC resistance measurements are by no means the ultimate test of a transformer (unless you're getting readings that are badly off), but does it make sense that two identical, healthy transformers would give results like the above? Do I have something to be concerned about, or is this much ado about nothing?
 
Here's a tech. tip I learned. DMM resistance readings and transformers don't always get along. The meter uses a frequency to measure resistance. The transformer and its' associated circuitry can cause an error. Try lifting the wires from their connections and re-measure. I have seen unequal readings before yet everything was OK and worked properly.
 
I'm not sure about the Z565, but the ST-70 transformer primary was wound with two parallel wires. If one of the connections goes open, resistance is doubled. It'll still work, but symmetry and coupling (mainly affecting high frequencies) are reduced.
 
Thanks for the info, both. I did lift the leads and take measurements out of circuit, and I got the same results. Tom--your note about the parallel wires on the OPT of an ST-70 and the affect of doubling resistance should one go open, certainly does sound awfully similar to what I'm seeing here. Anyone else experience this specifically with the Z565?

I was excited about installing EFB and finally listening to a pair of these OPTs, given all of the very positive experiences other have had, but it sounds like it's possible that one of my transformers won't be performing to spec. I guess I'll just have to bring it up slowly tomorrow and see if I can hear any audible differences between the left and right channels.
 
I'm not sure about the Z565, but the ST-70 transformer primary was wound with two parallel wires. If one of the connections goes open, resistance is doubled. It'll still work, but symmetry and coupling (mainly affecting high frequencies) are reduced.

Interesting. Bet that’s the basis of their OPTs that were available as catalog items being described as “paracoupled”.
 
I recently acquired a clean Dynaco SCA-35 amp that has the original Z565 OPTs with cloth leads. I haven't fired it up yet; I've just been taking some initial readings with a multimeter of the resistors, caps and transformers...looking for anything out of spec, etc.

I'm getting some interesting readings when measuring the DC resistance of both OPT's primaries (all readings are in ohms, measured from the B+ tap):

Transformer #1
Blue: 174
Green: 44
Blue/White: 201
Green/White: 51

Transformer #2
Blue: 375
Green: 92
Blue/White: 197
Green/White: 50

Transformer #1 seems to be in line with other people's measurements of the Z565 that I've found online. As you can see, Transformer #2 is reading a little more than twice as much resistance on one half of it's primary.

I know that DC resistance measurements are by no means the ultimate test of a transformer (unless you're getting readings that are badly off), but does it make sense that two identical, healthy transformers would give results like the above? Do I have something to be concerned about, or is this much ado about nothing?
If you have a variac you can test with voltage. Since the sca is SSR, just check the voltages on the power tube pins. An initial test at 10vac from the variac will show most any circuit problem that is a short or bad component.
 
Slowly brought the amp up using a variac over a period of a couple hours this morning. First took some measurements with the variac at 10VAC--voltages looked stable and balanced throughout. When I got the variac to about 100VAC one of the EL84s started to red plate. Shut it down and checked the wiring to that particular EL84's socket. Its the very same socket as the one with the OPT leads that are showing double the amount of resistance from post #1. Coincidence? I tested all the tubes for shorts and leaks before putting them in the amp. Although I don't have a good may of determining whether or not they are matched, they all tested strong for emissions and good for shorts/leaks.

Maybe I should try another tube in that spot to eliminate the possibility of a bad tube? :dunno:
 
Slowly brought the amp up using a variac over a period of a couple hours this morning. First took some measurements with the variac at 10VAC--voltages looked stable and balanced throughout. When I got the variac to about 100VAC one of the EL84s started to red plate. Shut it down and checked the wiring to that particular EL84's socket. Its the very same socket as the one with the OPT leads that are showing double the amount of resistance from post #1. Coincidence? I tested all the tubes for shorts and leaks before putting them in the amp. Although I don't have a good may of determining whether or not they are matched, they all tested strong for emissions and good for shorts/leaks.

Maybe I should try another tube in that spot to eliminate the possibility of a bad tube? :dunno:
Yes, could be. Did you check the grid voltage on the el84? If it is a steady DC not fluctuating, probably leaky coupling cap.
 
I would change out the coupling caps on the bad side for a start. A quick cheap check.
 
Yep, I did that a few hours ago with some positive results. I replaced two of the coupling caps on the bad channel with some new orange drops that I had on hand. No red plating that I could see and I did get some music out that side. After a few minutes the opposite channel started to distort (the one that was previously good and still had the old caps) and one of the tubes started to red plate. All of these old sea blue caps must be toast or getting close to it. They are literally everywhere--on all the boards, all the tone controls, etc.

The good news is that I'm feeling more positive about the OPTs. Seems that I can pass music through both of them. Now I'm left with the decision to just recap the existing boards or start fresh. I was planning on ordering the EFB board from Dave's Store. Maybe I'll grab some new power and preamp boards as well.
 
I've not found those sea-blue caps to be bad usually and would suspect old solder joints too, esp on the PCBs. Why not just see if replacing the coupling caps on the other side stops the redplating there?
 
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