Resistance values

[Bingtou]

In testing the completed Eros tape amp, I've found that my resistance values don't match the manual. Here are those I've done so far, with the manual's required values in brackets. Switch toward output jacks. As they don't check out, it didn't seem worth going through the rest until I've asked for more information. As it says "roughly" for the values, I guess my question would be, isn't this too rough?:

T9: 46k (39k)
T10: 46k (39k)
T12: 184k (130k)
T16: 184k (130k)
T19: 99k (47k)
T27: 99k (47k)
T29: 186k (130k)
T30: 184k (130k)

 

[Paul Birkeland]

It's most important to not have 0 ohms where you should have something much higher, and that you have close to 0 ohms where you should.  The tape Eros is a pretty old kit that hasn't been sold in quite some time, so familiarity has faded a bit.  If you run into voltage issues during testing, be sure to report IA, IB, OA, OB, KregA, KregB, and Breg voltages on the populated areas of each board to help expedite the debugging process. 

 

[Bingtou]

Well that sounds good. Very good, in fact. I'll crack on with the resistance tests and if those are okay, I'll move on to voltages. If I remember right, the 0V resistances (@T1, T3, T5 etc.) were okay, but I'll do the whole lot from the beginning anyway, with switch in both positions.

Thanks for the help, as ever.

 

[Bingtou]

After retesting, most values check good.

With the switch toward output jack, the terminals I mentioned above retested to the same values. Basically, all 39k read as 46k, all 130k read at around 184, all 47k read at around 99k. The 280ks matched almost exactly.

The terminals that should read 0 all read to about 0.6 ohms.

With the switch away from the output jack, T17 and T29, which should be 130k, read as 99k (those were 184 with the switch the other way). I assume this is all okay.

The 5.2 megohm measurements (T2, 14, 16, 30, switch away from output jack) - well, I just realized my meter only reads to 2 megohms, but at least those all read out of range in the 2 Mohm setting and all lower settings. I'll take that as a good sign.

One error: the input and output jack centre pins checked good except for the centre pin of the left input jack, which read 0. I can't see it, but a continuity test of all jacks between the centre pin and the minus lug shows that the other three are good, but I have shorted the left input jack, so I'll resolder that. With luck, it'll then pass the resistance test!

 

[Bingtou]

Yep, the output jack had a solder bridge from the centre pin across the insulating plastic to the negative side. Used way too much. After removing and resoldering, the short is gone and resistance is 47k, as it should be. Progress!

 

[Bingtou]

Finished the terminal resistances and the pin resistances for A, B, and C tubes. All the resistances within my meter's capability checked out. I have a new meter on the way so that I can check the 5.2M ohm readings and the infinity for C4 before going on to the voltages.

All very encouraging so far.

 

[Bingtou]

I ran the voltage checks today. Local mains voltage is 100 @ 50Hz, so I used a transformer to step up to 110V for the tests. I will recheck with 100V in a week or so to see what happens without stepping up. If I do need to step up for the Eros in general use, I'd have to get a much smaller transformer: the one I have is large and has too much fan noise for use with audio equipment.

Everything read much as specified on the left and right boards. The largest difference was OA, which read 150 (instead of the spec. 170) on both boards. Interpreting from the intro to voltage testing in the manual, I am guessing that is still acceptable.

It'll be a few weeks before I actually hear this amp, despite my impatience, as I'll have to move on to the wiring modification from the tape heads. I'll probably do it with a Technics RS1500. That machine will ultimately need recapping and calibration, yet another project for this year (and inevitably, delayed by several years already), but it seems to perform rather well as it is.